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  • richardmitnick 1:54 pm on February 22, 2018 Permalink | Reply
    Tags: , , Ethan Siegel, , Lisa will change everything   

    From Ethan Siegel: “Black Hole Mergers To Be Predicted Years In Advance By The 2030s” 

    From Ethan Siegel
    Feb 22, 2018

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    Although we’ve seen black holes directly merging three separate times in the Universe, we know many more exist. When supermassive black holes merge together, LISA will allow us to predict, up to years in advance, exactly when the critical event will occur.
    LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet)

    ESA/NASA eLISA space based the future of gravitational wave research

    Across the Universe, innumerable masses are locked in an inevitable death spiral. As white dwarfs, neutron stars, and black holes orbit each other, they travel through the curved spacetime that the other one’s mass creates. Accelerating through this has an inevitable consequence in General Relativity: the emission of gravitational radiation, also known as gravitational waves. Since these waves carry energy away, these orbits eventually decay, leading to an inspiral and merger. Over the past 2-3 years, LIGO has directly detected the very first mergers of black holes and neutron stars, with many more to come. But even with optimal technology, we’ll never get a signal more than seconds in advance of the actual merger.

    UC Santa Cruz

    UC Santa Cruz

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    A UC Santa Cruz special report

    Tim Stephens

    Astronomer Ryan Foley says “observing the explosion of two colliding neutron stars” [see https://sciencesprings.wordpress.com/2017/10/17/from-ucsc-first-observations-of-merging-neutron-stars-mark-a-new-era-in-astronomy ]–the first visible event ever linked to gravitational waves–is probably the biggest discovery he’ll make in his lifetime. That’s saying a lot for a young assistant professor who presumably has a long career still ahead of him.

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    The first optical image of a gravitational wave source was taken by a team led by Ryan Foley of UC Santa Cruz using the Swope Telescope at the Carnegie Institution’s Las Campanas Observatory in Chile. This image of Swope Supernova Survey 2017a (SSS17a, indicated by arrow) shows the light emitted from the cataclysmic merger of two neutron stars. (Image credit: 1M2H Team/UC Santa Cruz & Carnegie Observatories/Ryan Foley)

    Carnegie Institution Swope telescope at Las Campanas, Chile, 100 kilometres (62 mi) northeast of the city of La Serena. near the north end of a 7 km (4.3 mi) long mountain ridge. Cerro Las Campanas, near the southern end and over 2,500 m (8,200 ft) high, at Las Campanas, Chile

    A neutron star forms when a massive star runs out of fuel and explodes as a supernova, throwing off its outer layers and leaving behind a collapsed core composed almost entirely of neutrons. Neutrons are the uncharged particles in the nucleus of an atom, where they are bound together with positively charged protons. In a neutron star, they are packed together just as densely as in the nucleus of an atom, resulting in an object with one to three times the mass of our sun but only about 12 miles wide.

    “Basically, a neutron star is a gigantic atom with the mass of the sun and the size of a city like San Francisco or Manhattan,” said Foley, an assistant professor of astronomy and astrophysics at UC Santa Cruz.

    These objects are so dense, a cup of neutron star material would weigh as much as Mount Everest, and a teaspoon would weigh a billion tons. It’s as dense as matter can get without collapsing into a black hole.

    THE MERGER

    Like other stars, neutron stars sometimes occur in pairs, orbiting each other and gradually spiraling inward. Eventually, they come together in a catastrophic merger that distorts space and time (creating gravitational waves) and emits a brilliant flare of electromagnetic radiation, including visible, infrared, and ultraviolet light, x-rays, gamma rays, and radio waves. Merging black holes also create gravitational waves, but there’s nothing to be seen because no light can escape from a black hole.

    Foley’s team was the first to observe the light from a neutron star merger that took place on August 17, 2017, and was detected by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO).


    VIRGO Gravitational Wave interferometer, near Pisa, Italy

    Caltech/MIT Advanced aLigo Hanford, WA, USA installation


    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

    Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-Zib

    ESA/eLISA the future of gravitational wave research

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    Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)

    Now, for the first time, scientists can study both the gravitational waves (ripples in the fabric of space-time), and the radiation emitted from the violent merger of the densest objects in the universe.

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    The UC Santa Cruz team found SSS17a by comparing a new image of the galaxy N4993 (right) with images taken four months earlier by the Hubble Space Telescope (left). The arrows indicate where SSS17a was absent from the Hubble image and visible in the new image from the Swope Telescope. (Image credits: Left, Hubble/STScI; Right, 1M2H Team/UC Santa Cruz & Carnegie Observatories/Ryan Foley)

    It’s that combination of data, and all that can be learned from it, that has astronomers and physicists so excited. The observations of this one event are keeping hundreds of scientists busy exploring its implications for everything from fundamental physics and cosmology to the origins of gold and other heavy elements.


    A small team of UC Santa Cruz astronomers were the first team to observe light from two neutron stars merging in August. The implications are huge.

    ALL THE GOLD IN THE UNIVERSE

    It turns out that the origins of the heaviest elements, such as gold, platinum, uranium—pretty much everything heavier than iron—has been an enduring conundrum. All the lighter elements have well-explained origins in the nuclear fusion reactions that make stars shine or in the explosions of stars (supernovae). Initially, astrophysicists thought supernovae could account for the heavy elements, too, but there have always been problems with that theory, says Enrico Ramirez-Ruiz, professor and chair of astronomy and astrophysics at UC Santa Cruz.

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    The violent merger of two neutron stars is thought to involve three main energy-transfer processes, shown in this diagram, that give rise to the different types of radiation seen by astronomers, including a gamma-ray burst and a kilonova explosion seen in visible light. (Image credit: Murguia-Berthier et al., Science)

    A theoretical astrophysicist, Ramirez-Ruiz has been a leading proponent of the idea that neutron star mergers are the source of the heavy elements. Building a heavy atomic nucleus means adding a lot of neutrons to it. This process is called rapid neutron capture, or the r-process, and it requires some of the most extreme conditions in the universe: extreme temperatures, extreme densities, and a massive flow of neutrons. A neutron star merger fits the bill.

    Ramirez-Ruiz and other theoretical astrophysicists use supercomputers to simulate the physics of extreme events like supernovae and neutron star mergers. This work always goes hand in hand with observational astronomy. Theoretical predictions tell observers what signatures to look for to identify these events, and observations tell theorists if they got the physics right or if they need to tweak their models. The observations by Foley and others of the neutron star merger now known as SSS17a are giving theorists, for the first time, a full set of observational data to compare with their theoretical models.

    According to Ramirez-Ruiz, the observations support the theory that neutron star mergers can account for all the gold in the universe, as well as about half of all the other elements heavier than iron.

    RIPPLES IN THE FABRIC OF SPACE-TIME

    Einstein predicted the existence of gravitational waves in 1916 in his general theory of relativity, but until recently they were impossible to observe. LIGO’s extraordinarily sensitive detectors achieved the first direct detection of gravitational waves, from the collision of two black holes, in 2015. Gravitational waves are created by any massive accelerating object, but the strongest waves (and the only ones we have any chance of detecting) are produced by the most extreme phenomena.

    Two massive compact objects—such as black holes, neutron stars, or white dwarfs—orbiting around each other faster and faster as they draw closer together are just the kind of system that should radiate strong gravitational waves. Like ripples spreading in a pond, the waves get smaller as they spread outward from the source. By the time they reached Earth, the ripples detected by LIGO caused distortions of space-time thousands of times smaller than the nucleus of an atom.

    The rarefied signals recorded by LIGO’s detectors not only prove the existence of gravitational waves, they also provide crucial information about the events that produced them. Combined with the telescope observations of the neutron star merger, it’s an incredibly rich set of data.

    LIGO can tell scientists the masses of the merging objects and the mass of the new object created in the merger, which reveals whether the merger produced another neutron star or a more massive object that collapsed into a black hole. To calculate how much mass was ejected in the explosion, and how much mass was converted to energy, scientists also need the optical observations from telescopes. That’s especially important for quantifying the nucleosynthesis of heavy elements during the merger.

    LIGO can also provide a measure of the distance to the merging neutron stars, which can now be compared with the distance measurement based on the light from the merger. That’s important to cosmologists studying the expansion of the universe, because the two measurements are based on different fundamental forces (gravity and electromagnetism), giving completely independent results.

    “This is a huge step forward in astronomy,” Foley said. “Having done it once, we now know we can do it again, and it opens up a whole new world of what we call ‘multi-messenger’ astronomy, viewing the universe through different fundamental forces.”

    IN THIS REPORT

    Neutron stars
    A team from UC Santa Cruz was the first to observe the light from a neutron star merger that took place on August 17, 2017 and was detected by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO)

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    Graduate students and post-doctoral scholars at UC Santa Cruz played key roles in the dramatic discovery and analysis of colliding neutron stars.Astronomer Ryan Foley leads a team of young graduate students and postdoctoral scholars who have pulled off an extraordinary coup. Following up on the detection of gravitational waves from the violent merger of two neutron stars, Foley’s team was the first to find the source with a telescope and take images of the light from this cataclysmic event. In so doing, they beat much larger and more senior teams with much more powerful telescopes at their disposal.

    “We’re sort of the scrappy young upstarts who worked hard and got the job done,” said Foley, an untenured assistant professor of astronomy and astrophysics at UC Santa Cruz.

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    David Coulter, graduate student

    The discovery on August 17, 2017, has been a scientific bonanza, yielding over 100 scientific papers from numerous teams investigating the new observations. Foley’s team is publishing seven papers, each of which has a graduate student or postdoc as the first author.

    “I think it speaks to Ryan’s generosity and how seriously he takes his role as a mentor that he is not putting himself front and center, but has gone out of his way to highlight the roles played by his students and postdocs,” said Enrico Ramirez-Ruiz, professor and chair of astronomy and astrophysics at UC Santa Cruz and the most senior member of Foley’s team.

    “Our team is by far the youngest and most diverse of all of the teams involved in the follow-up observations of this neutron star merger,” Ramirez-Ruiz added.

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    Charles Kilpatrick, postdoctoral scholar

    Charles Kilpatrick, a 29-year-old postdoctoral scholar, was the first person in the world to see an image of the light from colliding neutron stars. He was sitting in an office at UC Santa Cruz, working with first-year graduate student Cesar Rojas-Bravo to process image data as it came in from the Swope Telescope in Chile. To see if the Swope images showed anything new, he had also downloaded “template” images taken in the past of the same galaxies the team was searching.

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    Ariadna Murguia-Berthier, graduate student

    “In one image I saw something there that was not in the template image,” Kilpatrick said. “It took me a while to realize the ramifications of what I was seeing. This opens up so much new science, it really marks the beginning of something that will continue to be studied for years down the road.”

    At the time, Foley and most of the others in his team were at a meeting in Copenhagen. When they found out about the gravitational wave detection, they quickly got together to plan their search strategy. From Copenhagen, the team sent instructions to the telescope operators in Chile telling them where to point the telescope. Graduate student David Coulter played a key role in prioritizing the galaxies they would search to find the source, and he is the first author of the discovery paper published in Science.

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    Matthew Siebert, graduate student

    “It’s still a little unreal when I think about what we’ve accomplished,” Coulter said. “For me, despite the euphoria of recognizing what we were seeing at the moment, we were all incredibly focused on the task at hand. Only afterward did the significance really sink in.”

    Just as Coulter finished writing his paper about the discovery, his wife went into labor, giving birth to a baby girl on September 30. “I was doing revisions to the paper at the hospital,” he said.

    It’s been a wild ride for the whole team, first in the rush to find the source, and then under pressure to quickly analyze the data and write up their findings for publication. “It was really an all-hands-on-deck moment when we all had to pull together and work quickly to exploit this opportunity,” said Kilpatrick, who is first author of a paper comparing the observations with theoretical models.

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    César Rojas Bravo, graduate student

    Graduate student Matthew Siebert led a paper analyzing the unusual properties of the light emitted by the merger. Astronomers have observed thousands of supernovae (exploding stars) and other “transients” that appear suddenly in the sky and then fade away, but never before have they observed anything that looks like this neutron star merger. Siebert’s paper concluded that there is only a one in 100,000 chance that the transient they observed is not related to the gravitational waves.

    Ariadna Murguia-Berthier, a graduate student working with Ramirez-Ruiz, is first author of a paper synthesizing data from a range of sources to provide a coherent theoretical framework for understanding the observations.

    Another aspect of the discovery of great interest to astronomers is the nature of the galaxy and the galactic environment in which the merger occurred. Postdoctoral scholar Yen-Chen Pan led a paper analyzing the properties of the host galaxy. Enia Xhakaj, a new graduate student who had just joined the group in August, got the opportunity to help with the analysis and be a coauthor on the paper.

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    Yen-Chen Pan, postdoctoral scholar

    “There are so many interesting things to learn from this,” Foley said. “It’s a great experience for all of us to be part of such an important discovery.”

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    Enia Xhakaj, graduate student

    IN THIS REPORT

    Scientific Papers from the 1M2H Collaboration

    Coulter et al., Science, Swope Supernova Survey 2017a (SSS17a), the Optical Counterpart to a Gravitational Wave Source

    Drout et al., Science, Light Curves of the Neutron Star Merger GW170817/SSS17a: Implications for R-Process Nucleosynthesis

    Shappee et al., Science, Early Spectra of the Gravitational Wave Source GW170817: Evolution of a Neutron Star Merger

    Kilpatrick et al., Science, Electromagnetic Evidence that SSS17a is the Result of a Binary Neutron Star Merger

    Siebert et al., ApJL, The Unprecedented Properties of the First Electromagnetic Counterpart to a Gravitational-wave Source

    Pan et al., ApJL, The Old Host-galaxy Environment of SSS17a, the First Electromagnetic Counterpart to a Gravitational-wave Source

    Murguia-Berthier et al., ApJL, A Neutron Star Binary Merger Model for GW170817/GRB170817a/SSS17a

    Kasen et al., Nature, Origin of the heavy elements in binary neutron star mergers from a gravitational wave event

    Abbott et al., Nature, A gravitational-wave standard siren measurement of the Hubble constant (The LIGO Scientific Collaboration and The Virgo Collaboration, The 1M2H Collaboration, The Dark Energy Camera GW-EM Collaboration and the DES Collaboration, The DLT40 Collaboration, The Las Cumbres Observatory Collaboration, The VINROUGE Collaboration & The MASTER Collaboration)

    Abbott et al., ApJL, Multi-messenger Observations of a Binary Neutron Star Merger

    PRESS RELEASES AND MEDIA COVERAGE


    Watch Ryan Foley tell the story of how his team found the neutron star merger in the video below. 2.5 HOURS.

    Press releases:

    UC Santa Cruz Press Release

    UC Berkeley Press Release

    Carnegie Institution of Science Press Release

    LIGO Collaboration Press Release

    National Science Foundation Press Release

    Media coverage:

    The Atlantic – The Slack Chat That Changed Astronomy

    Washington Post – Scientists detect gravitational waves from a new kind of nova, sparking a new era in astronomy

    New York Times – LIGO Detects Fierce Collision of Neutron Stars for the First Time

    Science – Merging neutron stars generate gravitational waves and a celestial light show

    CBS News – Gravitational waves – and light – seen in neutron star collision

    CBC News – Astronomers see source of gravitational waves for 1st time

    San Jose Mercury News – A bright light seen across the universe, proving Einstein right

    Popular Science – Gravitational waves just showed us something even cooler than black holes

    Scientific American – Gravitational Wave Astronomers Hit Mother Lode

    Nature – Colliding stars spark rush to solve cosmic mysteries

    National Geographic – In a First, Gravitational Waves Linked to Neutron Star Crash

    Associated Press – Astronomers witness huge cosmic crash, find origins of gold

    Science News – Neutron star collision showers the universe with a wealth of discoveries

    UCSC press release
    First observations of merging neutron stars mark a new era in astronomy

    Credits

    Writing: Tim Stephens
    Video: Nick Gonzales
    Photos: Carolyn Lagattuta
    Header image: Illustration by Robin Dienel courtesy of the Carnegie Institution for Science
    Design and development: Rob Knight
    Project managers: Sherry Main, Scott Hernandez-Jason, Tim Stephens

    Dark Energy Survey


    Dark Energy Camera [DECam], built at FNAL


    NOAO/CTIO Victor M Blanco 4m Telescope which houses the DECam at Cerro Tololo, Chile, housing DECam at an altitude of 7200 feet

    Gemini South telescope, Cerro Tololo Inter-American Observatory (CTIO) campus near La Serena, Chile, at an altitude of 7200 feet

    Noted in the vdeo but not in te article:

    NASA/Chandra Telescope

    NASA/SWIFT Telescope

    NRAO/Karl V Jansky VLA, on the Plains of San Agustin fifty miles west of Socorro, NM, USA

    Prompt telescope CTIO Chile

    NASA NuSTAR X-ray telescope

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    This figure shows reconstructions of the four confident and one candidate (LVT151012) gravitational wave signals detected by LIGO and Virgo to date for black holes, including the most recent black hole detection GW170814 (which was observed in all three detectors). Note the duration of the merger is paltry: from hundreds of milliseconds up to approximately 2 seconds at the greatest. LIGO/Virgo/B. Farr (University of Oregon)

    With the launch of LISA, the Laser Interferometer Space Antenna, scheduled for the 2030s, however, all of that is set to change. For the first time, we’ll be able to know exactly when and where to point our telescopes to watch the fireworks from the very start. Here’s the story of how.

    In our Universe, all sorts of astrophysical phenomena take place that generate gravitational waves. Whenever there’s a large mass that either:

    accelerates through a strongly curved region of space,
    rapidly rearranges its shape,
    causes another enormous mass to accelerate-and-fall onto it,

    or otherwise alters the fabric of spacetime from its pre-existing state, gravitational energy is radiated away. These ripples travel through space at the speed of light, carrying energy away. The way that energy gets conserved is that the original masses must wind up more tightly bound than they were before: gravitational potential energy gets converted into these gravitational waves.

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    Any object or shape, physical or non-physical, would be distorted as gravitational waves passed through it. Whenever one large mass is accelerated through a region of curved spacetime, gravitational wave emission is an inevitable consequence. NASA/Ames Research Center/C. Henze.

    The strongest amplitude signals come from the strongest changes in gravitational fields. This means that large masses accelerating at extremely short distances are the best candidates. Things like neutron star pairs, black hole binaries, supernovae, glitching pulsars, or neutron star-black hole systems are the best candidate systems for a detector like LIGO. These aren’t, however, the strongest signals in the entire Universe; they’re simply the strongest signals at the frequencies LIGO is sensitive to. These gravitational wave signals truly are waves: they have a wavelength and a frequency, depending on, for example, the orbital period of a binary system.

    LIGO, with its 4-kilometer arms that reflects light back-and-forth around a few thousand times, is sensitive to phenomena that generate waves with periods of milliseconds. The reason is that light travels thousands of kilometers in just a few milliseconds, so anything with a longer-period orbit will generate waves that are simply too large for LIGO to detect. Supernovae, merging neutron stars, and inspiraling black holes are processes that take minuscule fractions-of-a-second to complete, and hence they’re ideally suited for these relatively small gravitational wave detectors. However, there are plenty of other massive systems — in some cases, far more massive than the ones LIGO can see — that take far longer to complete a period.

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    The five black hole-black hole mergers discovered by LIGO (and Virgo), along with a sixth, insufficiently significant signal. The most massive black hole seen by LIGO, thus far, was 36 solar masses, pre-merger. However, galaxies contain supermassive black holes millions or even billions of times the mass of the Sun, and while LIGO isn’t sensitive to them, LISA will be. LIGO/Caltech/Sonoma State (Aurore Simonnet)

    The black holes we’ve seen are only a few tens of times the mass of the Sun; we know there are black holes out there with millions or even billions of times the Sun’s mass. At the centers of practically every galaxy are these supermassive behemoths, and they routinely devour asteroids, planets, stars, or even other massive black holes. However, with such large masses, they have enormous event horizons, so large that even an object revolving at the very edge would take many seconds or even minutes to complete a revolution. LIGO could never be sensitive to such a long-period gravitational wave, as its arms are too short. To see that, we’d need a gravitational wave detector in space: exactly what LISA is going to be.

    With three spacecraft orbiting one another far away from the Earth, LISA will be sensitive to inspirals and mergers of objects around supermassive black holes: the most reliable and expected source of gravitational waves out there. Mergers or collisions involving two supermassive black holes, as well as smaller objects merging or inspiraling into a lone supermassive black hole, are guaranteed to create gravitational waves with wavelengths many millions of kilometers in size. With an orbiting space antenna and comparably-sized laser arms, however, LISA will be able to see these objects. All of a sudden, objects with periods of minutes-to-hours are within reach.

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    The core of galaxy NGC 4261, like the core of a great many galaxies, show signs of a supermassive black hole in both infrared and X-ray observations. When a planet, star, black hole, or other massive object spirals into the central supermassive black hole, gravitational waves will be emitted, and the electromagnetic counterpart should be visible to our other great observatories, if we know where and when to look. NASA / Hubble and ESA

    When we detect black hole-black hole events with LIGO, it’s only the last few orbits that have a large enough amplitude to be seen above the background noise. The entirety of the signal’s duration lasts from a few hundred milliseconds to only a couple of seconds. By time a signal is collected, identified, processed, and localized, the critical merger event has already passed. There’s no way to point your telescopes — the ones that could find an electromagnetic counterpart to the signal — quickly enough to catch them from birth. Even inspiraling and merging neutron stars could only last tens of seconds before the critical “chirp” moment arrives. Processing time, even under ideal conditions, makes predicting the particular when-and-where a signal will occur a practical impossibility. But all of this will change with LISA.

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    For the past 2+ years, gravitational waves have been detected on Earth, from merging neutron stars and merging black holes. By building a gravitational wave observatory in space, we may be able to reach the sensitivities necessary to predict when a merger involving a supermassive black hole will occur.
    ESA / NASA and the LISA collaboration

    These extreme masses can generate signals of a much greater amplitude at a much lower frequency, meaning that they’ll be detectable in an instrument like LISA not seconds, but weeks, months, or even years in advance. Rather than looking at your data after-the-fact and concluding, “hey, we had a gravitational wave event here a few minutes ago,” you could look at your data and know, “in 2 years, 1 month, 21 days, 4 hours, 13 minutes and 56 seconds, we should point our telescopes at this location on the sky.” It will mean we can make these predictions way in advance, and the era of real-time, predictive, multi-messenger astronomy will have truly arrived.

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    Active galaxies both devour, as well as accelerate and eject infalling matter, that gets close to their central, supermassive black hole. With the localization and timing capabilities of LISA, we should know exactly when and where to point our telescopes to see the action unfold from the outset.

    Gravitational wave astronomy, as a science, is still only in its infancy, but it provides a whole new way to look at and study the entire Universe. While LIGO may only be sensitive to millisecond-period events, LISA will extend that to minutes-and-hours, while other techniques like pulsar timing and polarization measurements of the Big Bang’s leftover glow could capture events that take years or decades, or even billions of years, respectively. With LIGO, we have no realistic hope of collecting, processing, and analyzing the data fast enough to tell our telescopes where to point in advance of the critical event; optical astronomy is destined to remain a follow-up only. But with the advent of LISA, we’ll be able to know exact when and where to point our telescopes to get the ultimate cosmic show from the moment an event begins. For the first time, we won’t be reacting to the Universe; we’ll have a bona fide way to predict its most spectacular events ahead of time.

    See the full article here .

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    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

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  • richardmitnick 1:16 pm on February 22, 2018 Permalink | Reply
    Tags: , , , , , Ethan Siegel,   

    From Ethan Siegel: “Black Holes Must Have Singularities, Says Einstein’s Relativity” 

    From Ethan Siegel
    Feb 21, 2018

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    Inside a black hole, the spacetime curvature is so large that light cannot escape, nor can particles, under any circumstances. A singularity, based on our current laws of physics, must be an inevitability. Image credit: Pixabay user JohnsonMartin.

    Unless you can make a force that travels faster than the speed of light, a singularity is inevitable.

    The more mass you place into a small volume of space, the stronger the gravitational pull gets. According to Einstein’s general theory of relativity, there’s an astrophysical limit to how dense something can get and still remain a macroscopic, three-dimensional object. Exceed that critical value, and you’re destined to become a black hole: a region of space where gravitation is so strong that you create an event horizon, and a region from within which nothing can escape. No matter how fast you move, how quickly you accelerate, or even if you move at the ultimate speed limit of the Universe — the speed of light — you can’t get out. People have often wondered whether there might be a stable form of ultra-dense matter inside that event horizon that will hold up against gravitational collapse, and whether a singularity is truly inevitable. But if you apply the laws of physics as we know them today, you cannot avoid a singularity. Here’s the science behind why.

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    The very slowly-rotating neutron star at the core of the supernova remnant RCW 103 is also a magnetar. In 2016, new data from a variety of satellites confirmed this as the slowest-rotating neutron star ever found. More massive supernovae can create a black hole, but neutron stars may be the densest physical objects nature can create without a singularity. Image credit: X-ray: NASA/CXC/University of Amsterdam/N.Rea et al; Optical: DSS.

    Imagine the densest object you can make that isn’t yet a black hole. When massive stars go supernova, they can make either a black hole (if they’re above a critical threshold), but more commonly will see their cores collapse to form a neutron star.

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    Neutron star http://factslegend.org/20-interesting-neutron-star-facts/

    A neutron star is basically an enormous atomic nucleus: a bound-together collection of neutrons more massive than the Sun, but contained in a region of space just a few kilometers across. It’s conceivable that if you exceed the allowed density at the core of a neutron star, it might move on to an even more concentrated state of matter: a quark-gluon plasma, where densities are so great that it no longer makes sense to consider the matter in there as individual, bound structures.

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    MIT

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    A white dwarf, a neutron star or even a strange quark star are all still made of fermions. The Pauli degeneracy pressure helps hold up the stellar remnant against gravitational collapse, preventing a black hole from forming. Image credit: CXC/M. Weiss.

    Why can we have matter at all, though, inside the core of such a dense object? Because something must be exerting an outward force, holding the center up against gravitational collapse. For a low-density object like Earth, the electromagnetic force is enough to do it. The atoms that we have are made of nuclei and electrons, and the electron shells push against each other. Because we have the quantum rule of the Pauli Exclusion Principle, which prevents any two identical fermions (like electrons) from occupying the same quantum state. This holds for matter as dense as a white dwarf star, where a stellar-mass object can exist in a volume no greater than the size of Earth.

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    An accurate size/color comparison of a white dwarf (L), Earth reflecting our Sun’s light (middle), and a black dwarf (R). When white dwarfs finally radiate the last of their energy away, they will all eventually become black dwarfs. The degeneracy pressure between the electrons within the white/black dwarf, however, will always be great enough, so long as it doesn’t accrue too much mass, to prevent it from collapsing further. Image credit: BBC / GCSE (L) / SunflowerCosmos (R).

    If you place too much mass on a white dwarf star, however, the individual nuclei themselves will undergo a runaway fusion reaction; there’s a limit to how massive a white dwarf star can get. In a neutron star, there are no atoms at the core, but rather one enormous atomic nucleus, made almost exclusively of neutrons. Neutrons also act as fermions — despite being composite particles — and quantum forces also work to hold them up against gravitational collapse. It’s possible, beyond that, to imagine another, even denser state: a quark star, where individual quarks (and free gluons) interact with each other, obeying the rule that no two identical quantum particles can occupy the same quantum state.

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    The electron energy states for the lowest possible energy configuration of a neutral oxygen atom. Because electrons are fermions, not bosons, they cannot all exist in the ground (1s) state, even at arbitrarily low temperatures. This is the physics that prevents any two fermions from occupying the same quantum state, and holds most objects up against gravitational collapse. Image credit: CK-12 Foundation and Adrignola of Wikimedia Commons.

    But there’s a key realization in the mechanism that prevents matter from collapsing down to a singularity: forces must be exchanged. What this means, if you attempt to visualize it, is that force carrying particles (like photons, gluons, etc.) must be exchanged between the various fermions in the interior of the object.

    7
    The force exchanges inside a proton, mediated by colored quarks, can only move at the speed of light; no faster. Inside a black hole’s event horizon, these light-like geodesics are inevitably drawn to the central singularity. Image credit: Wikimedia Commons user Qashqaiilove.

    The thing is, there’s a speed limit to how fast these force-carriers can go: the speed of light. If you want an interaction to work by having an interior particle exert an outward force on an exterior particle, there needs to be some way for a particle to travel along that outward path. If the spacetime containing your particles is below the density threshold necessary to create a black hole, that’s no problem: moving at the speed of light will allow you to take that outward trajectory.

    But what if your spacetime crosses that threshold? What if you create an event horizon, and have a region of space where gravity is so intense that even if you moved at the speed of light, you couldn’t escape?

    8
    Anything that find itself inside the event horizon that surrounds a black hole, no matter what else is going on in the Universe, will find itself sucked into the central singularity. Image credit: Bob Gardner / ETSU.

    All of a sudden, there’s no path at all that will work! The gravitational force will work to pull that exterior particle inwards, but under these conditions the force-carrying particle coming from the interior particle simply cannot move outwards. Inside a dense-enough region, even massless particles have nowhere to go except towards the most interior points possible; they cannot influence exterior points. So the exterior particles have no choice but to fall in, closer to the central region. No matter how you set it up, every single particle inside the event horizon inevitably winds up at a singular location: the singularity at the black hole’s center.

    9
    Once you cross the threshold to form a black hole, everything inside the event horizon crunches down to a singularity that is, at most, one-dimensional. No 3D structures can survive intact. Image credit: Ask The Van / PHYSICS ILLINOIS – University of Illinois at Urbana-Champaign.

    So long as particles — including force-carrying particles — are limited by the speed of light, there’s no way to have a stable, non-singular structure inside a black hole. If you can invent a tachyonic force, which is to say a force mediated by particles that move faster-than-light, you might be able to create one, but so far no real, tachyon-like particles have been shown to physically exist. Without that, the best you can do is “smear out” your singularity into a one-dimensional, ring-like object (due to angular momentum), but that still won’t get you a three-dimensional structure. So long as your particles are either massive or massless, and obey the rules of physics we know, a singularity is an inevitability. There can be no real particles, structures, or composite entities that survive a journey into a black hole. Within seconds, all you cam ever have is a singularity.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 11:51 am on February 21, 2018 Permalink | Reply
    Tags: , , Ethan Siegel   

    From Ethan Siegel: “Chaos Theory, The Butterfly Effect, And The Computer Glitch That Started It All” 

    From Ethan Siegel
    Feb 20, 2018

    By Paul Halpern

    1
    A chaotic system is one where extraordinarily slight changes in initial conditions (blue and yellow) lead to similar behavior for a while, but that behavior then diverges after a relatively short amount of time. (Hellisp of Wikimedia Commons / Created by XaosBits using Mathematica and POV-Ray)

    For centuries, we thought that the Universe was completely deterministic. But even if you know all the rules, you can’t get rid of chaos.

    As Bob Dylan famously sang, “You don’t need a weatherman to know which way the wind blows.” Yet if you do have enough wind velocity information, combined with an array of readings from barometers, thermometers, and such, you might ask a weatherman, particularly a trained meteorologist with access to state-of-the-art computers and software, to make a sound forecast. We often plan our outdoor activities these days with the help of newscasts, websites, apps, and voice assistants that provide reasonable forecasts hours or days in advance. It is rather amazing that meteorology can perform such a feat.

    On the other hand, if we happen to rely on a sunny forecast to schedule a picnic, and it rains instead, we don’t condemn the entire field of meteorology, or dismiss it as useless guessing. We recognize that it is an imperfect science. Moreover, we recognize that it can only give us probabilities of a particular outcome, not a definitive prediction for what must come to pass. While compared to decades ago, forecasts are so much better, they’re far from flawless. And even with advances in technology, the theory of deterministic chaos shows that they’ll never be perfect.

    2
    Even with all the advances we’ve made in predictive modeling, a complex system like the Earth’s atmosphere only allows us to pick a slew of probabilistic outcomes, not any one particular outcome with any certainty.

    Everyone knows that quantum theory embodies randomness — or, as Einstein famously put it, “dice-rolling.” But the weather is a large scale effect, which Newtonian physics should be able to handle. Indeed, it does, and quite well. However, chaos theory points to the limitations of prediction for even deterministic, Newtonian physics.

    Newton’s second law of motion, the net force on an object equals its mass times its acceleration, embodies the type of mathematical relationship known as a differential equation. That equation acts as a kind of machine for processing the raw data of initial conditions for a system of particles — its precise set of positions and velocities at any given moment, along with the forces of interaction — and churning out location and speed coordinates indefinitely into the future.

    In his 1814 treatise, “A philosophical essay on probabilities,” French mathematician Pierre Laplace speculated that Newtonian mechanics heralded a rigid determinism that would theoretical enable the successful prediction of the entire future of the universe, given absolute knowledge of its complete state at any given time. The only catch is that the prognosticator would somehow need to step outside of the universe and obtain a complete snapshot at once of all the particles in it and their instantaneous trajectories. In philosophical discussions such a hypothetical being has been dubbed Laplace’s Demon. As Laplace wrote:

    “We may regard the present state of the universe as the effect of its past and the cause of its future. An intellect which at a certain moment would know all forces that set nature in motion, and all positions of all items of which nature is composed, if this intellect were also vast enough to submit these data to analysis, it would embrace in a single formula the movements of the greatest bodies of the universe and those of the tiniest atom; for such an intellect nothing would be uncertain and the future just like the past would be present before its eyes.”

    3
    Artist’s logarithmic scale conception of the observable universe. According to Laplace, if you knew all the positions and momenta of all the particles in the Universe at once, you’d be able to determine everything, far into the future, with arbitrary precision. (Wikipedia user Pablo Carlos Budassi).

    In the same essay, Laplace argued that any need to invoke probability in nature stemmed from ignorance, including uncertainty in weather forecasts. Someday, he suggested, weather forecasts would be perfectly accurate — as predictable as the orbits of planets — with nothing left to chance. Yet even if it weren’t for quantum phenomena like Heisenberg’s uncertainty principle, this wouldn’t be the case. No matter how well you know the initial conditions, determinism doesn’t rule the Universe.

    In the early 1960s, MIT meteorology professor Edward Lorenz was convinced that the mainframe computers used to great effect in planning weapons tests and launching satellites into orbit would help yield accurate weather forecasts. Given that weather is determined by a set of measurable factors, such as temperature, pressure, and wind velocity, conventional wisdom at the time was that a solid model, complete set of data, and a powerful number-crunching device, could, in principle, predict the weather conditions well into the future. With that goal in mind, Lorenz constructed a simple set of equations for air convection and programmed them into his cabinet-sized, vacuum-tube-based Royal-McBee computer.

    4
    Two systems starting from an identical configuration, but with imperceptibly small differences in initial conditions (smaller than a single atom), will keep to the same behavior for a while, but over time, chaos will cause them to diverge. After enough time have gone by, their behavior will appear completely unrelated to one another. (Larry Bradley).

    He input an initial set of data, switched the computer on, and waited for the printout. Placing the output next to the machine, he decided to re-enter some of the data and run the program longer. Typing it in meticulously, he was astonished to find that the program yielded a radically different forecast. Finally, he realized that the computer printout had rounded the data, and what he had input was slightly different the second time than the first. Somehow, even for a straightforward, deterministic set of equations, a minute change in initial conditions yielded radically different behavior.

    As he would later note, in what was dubbed the ‘butterfly effect,’ the extreme sensitivity to initial conditions meant that the flapping of a butterfly’s wings over the Amazon could influence the weather in China. This phenomenon, pioneered by Lorenz and others, has found widespread application as deterministic chaos.

    5
    The Butterfly Effect, also known as deterministic chaos, is a phenomenon where equations with no uncertainty will still yield uncertain outcomes, no matter how precisely the computations are performed (Public Domain).

    Lorenz not only discovered chaos, he also identified its key mechanism. When he graphed his data along several axes, he noted the strange property that iterating (plotting the trajectory over time) any two nearby points resulted in their separation. The gap would grow greater and greater with each iteration until the mathematical “offspring” of the two points would be so widely separated that they be in completely different regions of the cloud of information. On the other hand, points off the cloud, if iterated, would quickly approach it. Thus the dynamics of Lorenz’s equations served two contradictory purposes: repulsion of trajectories within the data set and attraction beyond it. Such a complex system is called a “strange attractor,” with the specific dynamics discovered by Lorenz called the “Lorenz attractor.”

    6
    Multiple chaotic pathways mean that, at any instant, the particle’s location and trajectory is completely indeterminable, no matter how precisely all the previous conditions were known. (Wikimol / Wikimedia Commons.)

    Other strange attractors were discovered soon thereafter, notably the Hénon attractor, identified in 1976 by French mathematican Michel Hénon. Strange attractors possess a peculiar self-similar structure, dubbed “fractals” by French-Polish mathematician Benoit Mandelbrot. If you map out a strange attractor and “blow up” any given region, that smaller region appears similar in structure to the whole thing. Similarly, enlarging any tiny section of the region reveals a similar pattern to the region itself, and so on. Mathematically, that implies a fractional dimensionality, hence the term “fractal.”

    7
    The Mandelbrot set is an example of a Fractal, where the same structure and behavior appears on a variety of scales. In many chaotic systems, that same behavior emerges. (Wolfgangbeyer / Wikimedia Commons).

    We owe Lorenz a debt for finding a key flaw in Laplacean determinism. Even in Newtonian classical mechanics, with its clockwork regularity, some systems are so sensitive to initial conditions that they are effectively impossible to predict. Unless you know every data point with perfect precision — next to impossible with realistic measuring devices — such chaotic systems act as randomly as a series of coin tosses. Thus along with randomness in quantum systems, effective randomness in some classical systems, such as the weather, seems a key feature of nature. God plays dice in more ways than one.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 12:54 pm on February 15, 2018 Permalink | Reply
    Tags: , , , , Ethan Siegel, What Separates A Good Scientific Theory From A Bad One?   

    From Ethan Siegel: “What Separates A Good Scientific Theory From A Bad One?” 

    From Ethan Siegel
    Feb 15, 2018

    1
    There is a large suite of scientific evidence that supports the picture of the expanding Universe and the Big Bang. The small number of input parameters and the large number of observational successes and predictions that have been subsequently verified are among the hallmarks of a successful scientific theory.
    NASA / GSFC.

    When you look at any phenomenon in the Universe, one of the major goals of scientific investigation is to understand its cause. If we see something occur, we want to know what made it happen. Quantitatively, we want to understand what processes were at play, and how they caused the effect of the exact magnitude that we observed. And finally, we want to know what to expect for systems we have not yet observed, and to make predictions about what behavior we’re likely to see in novel situations we may encounter in the future. You can find dime-a-dozen ideas, from professional physicists to philosophers to amateur enthusiasts, but most of them make lousy scientific theories. The reason? Because they simply assume too much and predict too little. There’s a science to how this all works.

    2
    The planets of the Solar System, shown to the scale of their physical sizes, all orbit according to certain specific rules. Figuring out those rules was something that was progressively improved upon, with each step forward explaining more with fewer rules and free parameters. NASA.

    Consider our Solar System. The planets orbit through the skies, taking the complex paths that we see from our location on Earth. Throughout history, there have been many explanations put forth to explain their behavior, depending on how you look at the problem. While Ptolemy and Copernicus are perhaps famous for putting forth the two major concepts for models, geocentric and heliocentric, these are only models, not theories. Why’s that? Because for every planet you introduce, there’s no rule or law governing how it orbits. These models simply state that if you supply the correct parameters, like equants, deferents, and epicycles for Ptolemy, you can describe a planet’s motion through the skies.

    3
    One of the great puzzles of the 1500s was how planets moved in an apparently retrograde fashion. This could either be explained through Ptolemy’s geocentric model (L), or Copernicus’ heliocentric one (R). However, getting the details right to arbitrary precision was something neither one could do. As interesting as both of these models are, neither one would have very much to say if another, new planet were discovered. Ethan Siegel / Beyond The Galaxy

    A descriptive model is an important step, but it isn’t a full-fledged scientific theory. Principles are a great starting point, but they won’t get you all the way to the end. For that, you have to go a step further: you need a rule, a law, and/or a quantitative set of equations that give you the ability to make predictions about things you have not yet measured. Kepler’s laws were the first leap in that direction. They didn’t simply accurately prescribe the orbits’ shapes and paths (ellipses, with the Sun at one focus), but begin to quantitatively describe it. Kepler’s second law gives the relationship between the orbital speed and the relative distance from the Sun, while the third law gives the relationship between the orbital period and the semimajor axis. For the first time, it wasn’t just that a behavior could be described through a set of parameters, but predicted.

    4
    Kepler’s three laws, that planets move in ellipses with the Sun at one focus, that they sweep out equal areas in equal times, and that the square of their periods is proportional to the cube of their semimajor axes, apply just as well to any gravitational system as they do to our own Solar System. RJHall / Paint Shop Pro.

    Newton’s law of universal gravitation went even further, allowing all of Kepler’s laws to be derived from just a single equation: the law of gravity. Suddenly, if you knew the masses and positions of the objects you had, you could predict how their motions would change arbitrarily far into the future. It represented a huge leap forward: you could give someone who understood the theory just a few parameters, like masses and positions, and they could calculate anything you wanted to know about the future behavior of any mass in the Universe. It was, in short, a good scientific theory.

    5
    The orbit of the Earth around the Sun generates gravitational waves, as do all masses moving and accelerating in the presence of a gravitational source. The bending of spacetime by matter and energy is the hallmark of Einstein’s General Theory of Relativity. T. Pyle/Caltech/MIT/LIGO Lab

    Einstein’s General Relativity went one better: given the same set of information as you’d give someone under Newton’s gravity, it would replicate all the successes of the prior theory, plus it made a suite of predictions that were observably different. These included:

    the orbits of Mercury and all the inner planets,
    the time-delay of light traveling through a gravitational field,
    the gravitational redshifting of light,
    the bending of starlight due to intervening foreground masses,
    frame-dragging effects,
    and the existence and properties of gravitational waves,

    to name just a few. In every single regime where it’s been put to the test, Einstein’s General Relativity has been demonstrated to be successful.

    6
    As ripples through space arising from distant gravitational waves pass through our Solar System, including Earth, they ever-so-slightly compress and expand the space around them. Alternatives can be constrained incredibly tightly thanks to our measurements in this regime. European Gravitational Observatory, Lionel BRET/EUROLIOS

    The key feature of what made the later theories not only successful, but more successful than their predecessors, can be boiled down to this:

    By adding the fewest number of new, free parameters, the greatest number of hitherto unexplained phenomena can be explained and accurately predicted.

    This is why Einstein’s General Relativity is so successful, and why so many of our greatest theories are accepted the way they are. The great power of a scientific theory is in its ability to quantitatively make predictions that can be verified or refuted by experiment or observations.

    7
    Four colliding galaxy clusters, showing the separation between X-rays (pink) and gravitation (blue), indicative of dark matter. On large scales, CDM is necessary, but on small scales, it isn’t as successful on its own as we like.

    X-ray: NASA/CXC/UVic./A.Mahdavi et al. Optical/Lensing: CFHT/UVic./A. Mahdavi et al. (top left); X-ray: NASA/CXC/UCDavis/W.Dawson et al.; Optical: NASA/ STScI/UCDavis/ W.Dawson et al. (top right); ESA/XMM-Newton/F. Gastaldello (INAF/ IASF, Milano, Italy)/CFHTLS (bottom left); X-ray: NASA, ESA, CXC, M. Bradac (University of California, Santa Barbara), and S. Allen (Stanford University) (bottom right)X-ray: NASA/CXC/UVic./A.Mahdavi et al. Optical/Lensing: CFHT/UVic./A. Mahdavi et al. (top left); X-ray: NASA/CXC/UCDavis/W.Dawson et al.; Optical: NASA/ STScI/UCDavis/ W.Dawson et al. (top right); ESA/XMM-Newton/F. Gastaldello (INAF/ IASF, Milano, Italy)/CFHTLS (bottom left); X-ray: NASA, ESA, CXC, M. Bradac (University of California, Santa Barbara), and S. Allen (Stanford University) (bottom right)

    NASA/Chandra Telescope

    NASA/ESA Hubble Telescope

    ESA/XMM Newton X-ray telescope


    CFHT, at Maunakea, Hawaii, USA,4,207 m (13,802 ft) above sea level

    It’s why an idea like dark matter is so powerful. By adding just a single new species of particle — something that’s cold, collisionless, and transparent to light and normal matter — you can explain everything from rotating galaxies to the cosmic web, the fluctuations in the microwave background, galaxy correlations, colliding galaxy clusters, and much, much more. It’s why ideas with a huge number of free parameters that must be tuned to get the right results are less satisfying and less predictively powerful. If we can model dark energy, for instance, with just one constant, why would we invent multi-field models with many parameters that are no more successful?

    8
    A detailed look at the Universe reveals that it’s made of matter and not antimatter, that dark matter and dark energy are required, and that we don’t know the origin of any of these mysteries. If you could explain, for example, dark energy with just a single new parameter, there’s no advantage to using a more complicated model. Chris Blake and Sam Moorfield.

    There are all sorts of scientific-sounding ideas, like the recently promoted-by-Aeon cosmopsychism, that start with a grand idea, but that require a whole slew of new physics (and new free parameters) to explain very little. In general, the number of new free parameters your idea introduces should be far smaller than the number of new things it purports to explain. Most people who invoke Occam’s razor fail to evaluate it based on this near-universal criterion.

    9
    A Universe with dark energy (red), a Universe with large inhomogeneity energy (blue), and a critical, dark-energy-free Universe (green). Note that the blue line behaves differently from dark energy. New ideas should make different, observable predictions from the other leading ideas. Gábor Rácz et al., 2017.

    The next time you encounter a bold new idea, ask yourself how many new free parameters are in there, compared to the leading theory it’s seeking to replace or extend: call that number X. Then ask yourself how many hitherto unexplained phenomena it claims to explain: call that number Y. If Y is significantly greater than X, you might have something worth investigating. You might be dealing with a good idea. But if not, you’re almost certainly dealing with a bad scientific idea at best, or an unscientific idea at worst. The great power of science is in its ability to predict and explain what we see in the Universe. The key is to do it as simply as possible, but not to oversimplify it any further than that.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 11:03 am on February 14, 2018 Permalink | Reply
    Tags: , , , , , , Ethan Siegel,   

    From Ethan Siegel: “Black Holes Must Have Singularities, Says Einstein’s Relativity” 

    From Ethan Siegel
    Feb 14, 2018

    1
    Inside a black hole, the spacetime curvature is so large that light cannot escape, nor can particles, under any circumstances. A singularity, based on our current laws of physics, must be an inevitability. Pixabay user JohnsonMartin.

    The more mass you place into a small volume of space, the stronger the gravitational pull gets. According to Einstein’s general theory of relativity, there’s an astrophysical limit to how dense something can get and still remain a macroscopic, three-dimensional object. Exceed that critical value, and you’re destined to become a black hole: a region of space where gravitation is so strong that you create an event horizon, and a region from within which nothing can escape. No matter how fast you move, how quickly you accelerate, or even if you move at the ultimate speed limit of the Universe — the speed of light — you can’t get out. People have often wondered whether there might be a stable form of ultra-dense matter inside that event horizon that will hold up against gravitational collapse, and whether a singularity is truly inevitable. But if you apply the laws of physics as we know them today, you cannot avoid a singularity. Here’s the science behind why.

    1
    The very slowly-rotating neutron star at the core of the supernova remnant RCW 103 is also a magnetar. In 2016, new data from a variety of satellites confirmed this as the slowest-rotating neutron star ever found. More massive supernovae can create a black hole, but neutron stars may be the densest physical objects nature can create without a singularity. X-ray: NASA/CXC/University of Amsterdam/N.Rea et al; Optical: DSS

    Imagine the densest object you can make that isn’t yet a black hole. When massive stars go supernova, they can make either a black hole (if they’re above a critical threshold), but more commonly will see their cores collapse to form a neutron star. A neutron star is basically an enormous atomic nucleus: a bound-together collection of neutrons more massive than the Sun, but contained in a region of space just a few kilometers across. It’s conceivable that if you exceed the allowed density at the core of a neutron star, it might move on to an even more concentrated state of matter: a quark-gluon plasma, where densities are so great that it no longer makes sense to consider the matter in there as individual, bound structures.

    2
    A white dwarf, a neutron star or even a strange quark star are all still made of fermions. The Pauli degeneracy pressure helps hold up the stellar remnant against gravitational collapse, preventing a black hole from forming. CXC/M. Weiss

    Why can we have matter at all, though, inside the core of such a dense object? Because something must be exerting an outward force, holding the center up against gravitational collapse. For a low-density object like Earth, the electromagnetic force is enough to do it. The atoms that we have are made of nuclei and electrons, and the electron shells push against each other. Because we have the quantum rule of the Pauli Exclusion Principle, which prevents any two identical fermions (like electrons) from occupying the same quantum state. This holds for matter as dense as a white dwarf star, where a stellar-mass object can exist in a volume no greater than the size of Earth.

    3
    An accurate size/color comparison of a white dwarf (L), Earth reflecting our Sun’s light (middle), and a black dwarf (R). When white dwarfs finally radiate the last of their energy away, they will all eventually become black dwarfs. The degeneracy pressure between the electrons within the white/black dwarf, however, will always be great enough, so long as it doesn’t accrue too much mass, to prevent it from collapsing further. BBC / GCSE (L) / SunflowerCosmos (R)

    If you place too much mass on a white dwarf star, however, the individual nuclei themselves will undergo a runaway fusion reaction; there’s a limit to how massive a white dwarf star can get. In a neutron star, there are no atoms at the core, but rather one enormous atomic nucleus, made almost exclusively of neutrons. Neutrons also act as fermions — despite being composite particles — and quantum forces also work to hold them up against gravitational collapse. It’s possible, beyond that, to imagine another, even denser state: a quark star, where individual quarks (and free gluons) interact with each other, obeying the rule that no two identical quantum particles can occupy the same quantum state.

    4
    The electron energy states for the lowest possible energy configuration of a neutral oxygen atom. Because electrons are fermions, not bosons, they cannot all exist in the ground (1s) state, even at arbitrarily low temperatures. This is the physics that prevents any two fermions from occupying the same quantum state, and holds most objects up against gravitational collapse. CK-12 Foundation and Adrignola of Wikimedia Commons.

    But there’s a key realization in the mechanism that prevents matter from collapsing down to a singularity: forces must be exchanged. What this means, if you attempt to visualize it, is that force carrying particles (like photons, gluons, etc.) must be exchanged between the various fermions in the interior of the object.

    5
    The force exchanges inside a proton, mediated by colored quarks, can only move at the speed of light; no faster. Inside a black hole’s event horizon, these light-like geodesics are inevitably drawn to the central singularity. Wikimedia Commons user Qashqaiilove.

    The thing is, there’s a speed limit to how fast these force-carriers can go: the speed of light. If you want an interaction to work by having an interior particle exert an outward force on an exterior particle, there needs to be some way for a particle to travel along that outward path. If the spacetime containing your particles is below the density threshold necessary to create a black hole, that’s no problem: moving at the speed of light will allow you to take that outward trajectory.

    But what if your spacetime crosses that threshold? What if you create an event horizon, and have a region of space where gravity is so intense that even if you moved at the speed of light, you couldn’t escape?

    6
    Anything that find itself inside the event horizon that surrounds a black hole, no matter what else is going on in the Universe, will find itself sucked into the central singularity. Bob Gardner / ETSU

    All of a sudden, there’s no path at all that will work! The gravitational force will work to pull that exterior particle inwards, but under these conditions the force-carrying particle coming from the interior particle simply cannot move outwards. Inside a dense-enough region, even massless particles have nowhere to go except towards the most interior points possible; they cannot influence exterior points. So the exterior particles have no choice but to fall in, closer to the central region. No matter how you set it up, every single particle inside the event horizon inevitably winds up at a singular location: the singularity at the black hole’s center.

    7
    Once you cross the threshold to form a black hole, everything inside the event horizon crunches down to a singularity that is, at most, one-dimensional. No 3D structures can survive intact. Ask The Van / UIUC Physics Department.

    So long as particles — including force-carrying particles — are limited by the speed of light, there’s no way to have a stable, non-singular structure inside a black hole. If you can invent a tachyonic force, which is to say a force mediated by particles that move faster-than-light, you might be able to create one, but so far no real, tachyon-like particles have been shown to physically exist. Without that, the best you can do is “smear out” your singularity into a one-dimensional, ring-like object (due to angular momentum), but that still won’t get you a three-dimensional structure. So long as your particles are either massive or massless, and obey the rules of physics we know, a singularity is an inevitability. There can be no real particles, structures, or composite entities that survive a journey into a black hole. Within seconds, all you cam ever have is a singularity.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 2:18 pm on February 3, 2018 Permalink | Reply
    Tags: Antimatter and Matter, , Ethan Siegel, ,   

    From Ethan Siegel: “What’s So ‘Anti’ About Antimatter?” 

    From Ethan Siegel
    Feb 3. 2018

    1
    High-energy collisions of particles can create matter-antimatter pairs or photons, while matter-antimatter pairs annihilate to produce photons as well, as these bubble chamber tracks show. But what determines whether a particle is matter or antimatter? Image credit: Fermilab.

    There are lots of properties inherent to particles, and while everyone has an antiparticle, not everyone is matter or antimatter.

    For every particle of matter that’s known to exist in the Universe, there’s an antimatter counterpart. Antimatter has many of the same properties as normal matter, including the types of interaction it undergoes, its mass, the magnitude of its electric charge, and so on. But there are a few fundamental differences as well. Yet two things are certain about matter-antimatter interactions: if you collide a matter particle with an antimatter counterpart, they both immediately annihilate away to pure energy, and if you undergo any interaction in the Universe that creates a matter particle, you must also create its antimatter counterpart. So what makes antimatter so “anti,” anyway? That’s what Robert Nagle wants to know, as he asks:

    On a fundamental level, what is the difference between matter and its counterpart antimatter? Is there some sort of intrinsic property that causes a particle to be matter or antimatter? Is there some intrinsic property (like spin) that distinguishes quarks and antiquarks? What what puts the ‘anti’ in anti matter?

    To understand the answer, we need to take a look at all the particles (and antiparticles) that exist.

    2
    The particles and antiparticles of the Standard Model obey all sorts of conservation laws, but there are fundamental differences between fermionic particles and antiparticles and bosonic ones. Image credit: E. Siegel / Beyond The Galaxy.

    This is the Standard Model of elementary particles: the full suite of discovered particles in the known Universe. There are generally two classes of these particles, the bosons, which have integer spins (…, -2, -1, 0, +1, +2, …) and are neither matter nor antimatter, and the fermions, which have half-integer spins (…, -3/2, -1/2, +1/2, +3/2, …) and must either be “matter-type” or “antimatter-type” particles. For any particle you can think about creating, there are going to be a slew of inherent properties to it, defined by what we call quantum numbers. For an individual particle in isolation, this includes a number of traits you’re likely familiar with, as well as some that you may not be familiar with.

    3
    These possible configurations for an electron in a hydrogen atom are extraordinarily different from one another, yet all represent the same exact particle in a slightly different quantum state. Particles (and antiparticles) also have intrinsic quantum numbers that cannot be changed, and those numbers are key in defining whether a particle is matter, antimatter, or neither. Image credit: PoorLeno / Wikimedia Commons.

    The easy ones are things like mass and electric charge. An electron, for example, has a rest mass of 9.11 × 10^–31 kg, and an electric charge of -1.6 × 10^–19 C. Electrons can also bind together with protons to produce a hydrogen atom, with a series of spectral lines and emission/absorption features based on the electromagnetic force between them. Electrons have a spin of either +1/2 or -1/2, a lepton number of +1, and a lepton family number of +1 for the first (electron) of the three (electron, mu, tau) lepton families. (We’re going to ignore numbers like weak isospin and weak hypercharge, for simplicity.)

    Given these properties of an electron, we can ask ourselves what the antimatter counterpart of the electron would need to look like, based on the rules governing elementary particles.

    4
    In a simple hydrogen atom a single electron orbits a single proton. In an antihydrogen atom a single positron (anti-electron) orbits a single antiproton. Positrons and antiprotons are the antimatter counterparts of electrons and protons, respectively. Image credit: Lawrence Berkeley Labs.

    The magnitudes of all the quantum numbers must remain the same. But for antiparticles, the signs of these quantum numbers must be reversed. For an anti-electron, that means it should have the following quantum numbers:

    a rest mass of 9.11 × 10^–31 kg,
    an electric charge of +1.6 × 10^–19 C,
    a spin of (respectively) either -1/2 or +1/2,
    a lepton number of -1,
    and a lepton family number of -1 for the first (electron) lepton family.

    And when you bind it together with an antiproton, it should produce exactly the same series of spectral lines and emission/absorption features that the electron/proton system produced.

    5
    Electron transitions in the hydrogen atom, along with the wavelengths of the resultant photons, showcase the effect of binding energy and the relationship between the electron and the proton in quantum physics. The spectral lines between positrons and antiprotons have been verified to be exactly the same. Image credit: Wikimedia Commons users Szdori and OrangeDog.

    All of these facts have been verified experimentally. The particle matching this exact description of the anti-electron is the particle known as a positron! The reason why this is necessary comes when you consider how you make matter and antimatter: you typically make them from nothing. Which is to say, if you collide two particles together at a high enough energy, you can often create an extra “particle-antiparticle” pair out of the excess energy (from Einstein’s E = mc2), which conserves energy.

    6
    Whenever you collide a particle with its antiparticle, it can annihilate away into pure energy. This means if you collide any two particles at all with enough energy, you can create a matter-antimatter pair. Image credit: Andrew Deniszczyc, 2017.

    But you don’t just need to conserve energy; there are a slew of quantum numbers you also have to conserve! And these include all of the following:

    electric charge,
    angular momentum (which combines “spin” and “orbital” angular momentum; for individual, unbound particles, that’s only “spin”),
    lepton number,
    baryon number,
    lepton family number,
    and color charge.

    Of these intrinsic properties, there are two that define you as either “matter” or “antimatter,” and those are “baryon number” and “lepton number.”

    7
    In the early Universe, the full suite of particles and their antimatter particles were extraordinarily abundant, but as they Universe cooled, the majority annihilated away. All the conventional matter we have left over today is from the quarks and leptons, with positive baryon and lepton numbers, that outnumbered their antiquark and antilepton counterparts. (Only quarks and antiquarks are shown here.) Image credit: E. Siegel / Beyond The Galaxy.

    If either of those numbers are positive, you’re matter. That’s why quarks (which each have baryon number of +1/3), electrons, muons, taus, and neutrinos (which each have lepton number of +1) are all matter, while antiquarks, positrons, anti-muons, anti-taus, and anti-neutrinos are all antimatter. These are all the fermions and antifermions, and every fermion is a matter particle while every antifermion is an antimatter particle.

    8
    The particles of the standard model, with masses (in MeV) in the upper right. The Fermions make up the left three columns; the bosons populate the right two columns. While all particles have a corresponding antiparticle, only the fermions can be matter or antimatter. Image credit: Wikimedia Commons user MissMJ, PBS NOVA, Fermilab, Office of Science, United States Department of Energy, Particle Data Group.

    But there are also the bosons. There are gluons which have for their antiparticles the gluons of the opposite color combinations; there is the W+ which is the antiparticle of the W- (with opposite electric charge), and there are the Z0, the Higgs boson, and the photon, which are their own antiparticles. However, bosons are neither matter nor antimatter. Without a lepton number or baryon number, these particles may have electric charges, color charges, spins, etc., but no one can rightfully call themselves either “matter” or “antimatter” and their antiparticle counterpart the other one. In this case, bosons are simply bosons, and if they have no charges, then they’re simply their own antiparticles.

    9
    On all scales in the Universe, from our local neighborhood to the interstellar medium to individual galaxies to clusters to filaments and the great cosmic web, everything we observe appears to be made out of normal matter and not antimatter. This is an unexplained mystery. Image credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA).

    So what puts the “anti” in antimatter? If you’re an individual particle, then your antiparticle is the same mass as you with all the opposite conserved quantum numbers: it’s the particle that’s capable of annihilating with you back to pure energy if ever the two of you meet. But if you want to be matter, you need to have either positive baryon or positive lepton number; if you want to be antimatter, you must have either negative baryon or negative lepton number. Beyond that, there’s no known fundamental reason for our Universe to have favored matter over antimatter; we still don’t know how that symmetry was broken. (Although we have ideas.) If things had turned out differently, we’d probably call whatever we were made of “matter” and its opposite “antimatter,” but who gets which name is completely arbitrary. As in all things, the Universe is biased towards the survivors.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 1:29 pm on February 3, 2018 Permalink | Reply
    Tags: , , Ethan Siegel, ,   

    From Ethan Siegel: “Ask Ethan: Can A Laser Really Rip Apart Empty Space?” 

    From Ethan Siegel

    Feb 3, 2018

    1
    Tabletop laser experiments might not have the highest energy output for lasers, but they can out-compete even the lasers used to ignite nuclear fusion in terms of power. Could the quantum vacuum finally yield? United States Air Force.

    Empty space, as it turns out, isn’t so empty. The fluctuations in the vacuum of space itself mean that even if you take all the matter and radiation out of a region of space, there’s still a finite amount of energy there, inherent to space itself. If you fire a powerful enough laser at it, can you, as a Science magazine story called it, break the vacuum and rip apart empty space? That’s what our Patreon supporter Malcolm Schongalla wants to know, as he asks:

    Science Magazine recently reported that Chinese physicists will start building a 100-petawatt(!!!) laser this year. Can you please explain how they plan to achieve this, and what unique phenomenon this will help physicists explore? Such as, what exactly is “breaking the vacuum?”

    The story is real, verified, and a little bit exaggerated in terms of claims that it can break the vacuum, as though such a thing were possible. Let’s dive into the real science to find out what’s really happening.

    2
    A set of Q-line laser pointers showcase the diverse colors and compact size that now are commonplace for lasers. The continuously-operating lasers shown here are very low power, measuring just watts or fractions of watts, while the record is in petawatts. Wikimedia Commons user Netweb01.

    The very idea of a laser itself is still relatively novel, despite how widespread they are. Originally an acronym standing for Light Amplification by Stimulated Emission of Radiation, lasers are a bit of a misnomer. In truth, nothing is really being amplified. You know that, in normal matter, you have an atomic nucleus and various energy levels for an electron; in molecules, crystals, and other bound structures, the particular separations between an electron’s energy levels dictate which transitions are allow. In a laser, the electrons oscillate between two allowable states, emitting a photon of a very particular energy when they drop from the higher-energy state to the lower one. These oscillations are what produce the light, but for some reason, no one wanted the acronym Light Oscillation by Stimulated Emission of Radiation.

    3
    By ‘pumping’ electrons into an excited state and stimulating them with a photon of the desired wavelength, you can cause the emission of another photon of exactly the same energy and wavelength. This action is how the light for a laser is first created. Wikimedia Commons user V1adis1av.

    If you can produce either multiple atoms-or-molecules in the same excited state and stimulate their spontaneous jump to the ground state, they’ll emit the same energy photon. These transitions are extremely fast (but not infinitely so), and so there is a theoretical limit to how quickly you can make a single atom-or-molecule hop up to the excited state and spontaneously emit a photon. Normally, some type of gas, molecular compound or crystal is used inside a resonant-or-reflective cavity to create a laser, but you can also make one out of free electrons, semiconductors, optical fibers, and, in theory, even positronium.

    4
    The ALICE free-electron laser is an example of an exotic laser that doesn’t rely on conventional atomic or molecular transitions, but still produces narrowly-focused, coherent light. STFC.

    The amount of energy that comes out of a laser is limited by the amount you put in, so the only way to achieve extremely high power in your laser is to shorten the timescale of the emitted laser pulse. You might hear the term petawatt, which is 1015 W, and think this is a tremendous amount of energy. But “petawatts” aren’t energy, but power, which is an energy over a time. A petawatt laser could either be a laser that emits 1015 J of energy (the amount released by about 200 kilotons of TNT) every second, or could just be a laser that emits one joule of energy (the amount released by burning 60 micrograms of sugar) over femtosecond (10-15 second) timescales. In terms of energy, these two scenarios are vastly different, even though their power is the same.

    5
    Amplifiers for the University of Rochester’s OMEGA-EP, lit up by flash lamps, could drive a U.S. high-power laser that works on very short timescales.
    University of Rochester, Laboratory for laser energetics / Eugene Kowaluk.

    The 100 petawatt laser in question hasn’t been built yet, but is rather the next enormous threshold that researchers plan to cross in the 2020s. The hypothesized project is known as the Station of Extreme Light, and is set to be constructed at the Shanghai Superintense Ultrafast Laser Facility in China. An external pump, which is usually light from a different wavelength, excites the electrons in the lasing material, causing the characteristic transition that creates the laser light. The photons then all emege in a tightly packed stream, or a pulse, at a very narrow set of wavelengths. To the surprise of many, the 1 petawatt threshold was crossed way back in 1996; it’s taken nearly two decades to cross the 10 petawatt mark.

    6
    The preamplifiers of the National Ignition Facility are the first step in increasing the energy of laser beams as they make their way toward the target chamber. In 2012, NIF achieved a 0.5 petawatt shot, reaching a peak of 1,000 times more power than the United States uses at any instant in time. Damien Jemison/LLNL.


    LLNL/NIF

    The National Ignition Facility in the United States may be what we first think of when we envision high-powered lasers, but this is a bit of a red herring. This array of 192 lasers, focusing on a single point to compress a hydrogen pellet and ignite nuclear fusion, hovers right around the 1 PW mark, but isn’t the most powerful one around. It has a high amount of energy at over a million joules, but its pulses are, comparatively, very long-duration. To set the power record, you need to deliver the greatest amount of energy in the shortest amount of time.

    The current record-holder, instead, uses a sapphire crystal doped with titanium, pumps hundreds of joules of energy into it, bounces the light back-and-forth until destructive interference cancels out most of the pulse length, and the output is compressed into a single pulse just tens of femtoseconds long. That’s how we can reach output powers in the ballpark of 10 PW.

    7
    Part of a Ti-sapphire laser; the bright red light on the left is the Ti:sapphire crystal; the bright green light is scattered pump light from a mirror.
    Wikimedia Commons user Hankwang.

    In order to go higher — to reach that next order-of-magnitude milestone — we’ll have to either increase the energy we input into the laser, from hundreds of joules to thousands, or decrease the pulse time. The first one is problematic for the materials we presently use. Small titanium-sapphire crystals won’t hold up to that kind of energy, while larger ones tend to emit light in the wrong direction: at right angles to the desired pathway. The three main approaches, therefore, that researchers are considering at the present time are:

    To take the original, 10 PW pulse, stretch it out over a grating, and combine it into an artificial crystal, where you can pump it again, raising its power.
    To combine multiple pulses from a series of different high-powered lasers to create the right level of overlap: a challenge for pulses just tens of femtoseconds (3-15 microns) long that move at the speed of light.
    Or, to add a second round of pulse compression, squeezing them to as little as a couple of femtoseconds.

    8
    Bending light and focusing it to a point, regardless of wavelength or where it’s incident on your surface, is one key step towards maximizing the intensity of your light at a single location in space. M. Khorasaninejad et al., Nano Lett., 2017, 17 (3), pp 1819–1824.

    The pulses must then be brought to a tight focus, raising not just the power, but the intensity, or the power concentrated at a single point. As the Science article states:

    If a 100-PW pulse can be focused to a spot measuring just 3 micrometers across […] the intensity in that tiny area will be an astonishing 1024 watts per square centimeter (W/cm2)—some 25 orders of magnitude, or 10 trillion trillion times, more intense than the sunlight striking Earth.

    This opens the door to a long-sought-after opportunity to create particle-antiparticle pairs where there were none before, but it’s hardly “breaking the quantum vacuum.”

    9
    Visualization of a quantum field theory calculation showing virtual particles in the quantum vacuum. Even in empty space, this vacuum energy is non-zero.
    Derek Leinweber.

    According to the theory of quantum electrodynamics, the zero-point energy of empty space isn’t zero, but some positive, finite value. Although we visualize it as particles and antiparticles popping in-and-out of existence, a better depiction is to recognize that, with enough energy, you can — through physics — use these electromagnetic properties of empty space to generate real particle/antiparticle pairs [Matter and Radiation at Extremes]. This is based on the simple Einsteinian physics of E = mc2, but requires a strong enough electric field to build those particles: around 1016 volts per meter. Light, since it’s an electromagnetic wave, carries with it both electric and magnetic fields, and will reach that critical threshold with a laser intensity of 1029 W/cm2.

    10
    Zetawatt lasers, reaching an intensity of 10^29 W/cm^2, should be sufficient to create real electron/positron pairs from the quantum vacuum itself. This will require additional energy, shorter pulses, and/or increased focusing over what we even envision for the future. Wikimedia Commons user Slashme.

    You ought to notice, right away, that even the dream scenario of the science article gives intensities that are still 100,000 times too small to reach this threshold, and whenever you’re below that threshold, your ability to produce particle/antiparticle pairs is exponentially suppressed. The mechanism at play is quite different than simply the reverse of pair production, where instead of an electron and positron annihilating to create two photons, two photons interact to produce an electron/positron pair. (That process was first experimentally demonstrated way back in 1997.) In the laser setup, no individual photons have enough energy to produce new particles, but rather their combined effects on the vacuum of space causes particle/antiparticle pairs to pop into existence with a particular probability. Unless, however, those intensities approach that critical 1029 W/cm^2 threshold, that probability might as well be zero.

    11
    A laser in Shanghai, China, has set power records yet fits on tabletops. The most powerful lasers aren’t the most energetic, but are often the ones with the shortest laser pulses. Kan Zhan.

    The ability to generate matter/antimatter pairs of particles from empty space alone will be an important test of quantum electrodynamics, and will also be a remarkable demonstration of the power of lasers and our ability to control them. It may not take reaching that critical threshold to generate the first particle/antiparticle pairs from this mechanism, but you’ll have to either get close, get lucky, or have some sort of mechanism to enhance your production over what you naively expect. In any case, the quantum vacuum never breaks, but rather does exactly what you expect of it: responds to matter and energy in accordance with the laws of physics. It might not be intuitive, but it’s something even more powerful: it’s predictable. The art of doing that prediction and doing the experiments to verify or refute them is what science is all about! We may not be there yet, but every leap upwards in power and intensity is another step closer to this “holy grail” in laser physics.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 11:49 am on January 28, 2018 Permalink | Reply
    Tags: , , , , Ethan Siegel, How Do Hawking Radiation And Relativistic Jets Escape From A Black Hole?   

    From Ethan Siegel: “Ask Ethan: How Do Hawking Radiation And Relativistic Jets Escape From A Black Hole?” 

    Ethan Siegel
    1.27.18

    1
    Concept art of an accretion ring and jet around a supermassive black hole. Image credit: NASA / JPL-Caltech.

    If nothing can escape from beneath the event horizon, where do these phenomena come from?

    The most important feature of a black hole is that it has an event horizon: a region of space where the gravitational field is so strong that nothing, not even light, can escape from it. How, then, do we explain the matter and radiation that we both see and predict should come from them? That’s what Russell Sisson wants to know, as he asks:

    “Everything you read about a black indicates that ‘nothing, not even light, can escape them’. Then you read that there is Hawking radiation, which ‘is blackbody radiation that is predicted to be released by black holes’. Then there are relativistic jets that “shoot out of black holes at close to the speed of light”. Obviously, something does come out of black holes, right?”

    Matter and radiation can definitely come towards us, originating from the black hole’s location. But does that mean something escapes from a black hole? Let’s find out!

    2
    While distant host galaxies for quasars and active galactic nuclei can often be imaged in visible/infrared light, the jets themselves and the surrounding emission is best viewed in both the X-ray and the radio, as illustrated here for the galaxy Hercules A. It takes a black hole to power an engine such as this, but that doesn’t necessarily mean that this is matter/radiation escaping from inside the event horizon. Image credit: NASA, ESA, S. Baum and C. O’Dea (RIT), R. Perley and W. Cotton (NRAO/AUI/NSF), and the Hubble Heritage Team (STScI/AURA).

    NASA/ESA Hubble Telescope

    NRAO/Karl V Jansky VLA, on the Plains of San Agustin fifty miles west of Socorro, NM, USA, at an elevation of 6970 ft (2124 m)

    When we talk about a black hole, it’s important to recognize what we mean. If you put enough mass together in a small enough volume of space, the curvature of spacetime will become so large that a ray of light, no matter what direction it propagates in, will inevitably arrive back at the central singularity. The escape velocity — or the speed at which you’d need to move to overcome the black hole’s gravitational pull — is greater than the speed of light. A consequence of this is that there’s a critical region, or an event horizon, where once you cross inside of it, you can never get out. Things that are inside the event horizon always hit the singularity; things that are outside can either escape or fall in, dependent on their properties.

    3
    As viewed with our most powerful telescopes, such as Hubble, advances in camera technology and imaging techniques have enabled us to better probe and understand the physics and properties of distant quasars, including their central black hole properties. Image credit: NASA and J. Bahcall (IAS) (L); NASA, A. Martel (JHU), H. Ford (JHU), M. Clampin (STScI), G. Hartig (STScI), G. Illingworth (UCO/Lick Observatory), the ACS Science Team and ESA (R).

    NASA/ESA Hubble ACS

    UCSC Lick Observatory, Mt Hamilton, in San Jose, California, Altitude 1,283 m (4,209 ft)

    There are, though, real particles and radiation, both observed and theorized, that do originate from a black hole. Accretion disks are a spectacular example. Imagine you’re a particle outside of a black hole’s event horizon, but gravitationally bound to it. The strong gravitational pull will cause you to move in an elliptical orbit, where your fastest speed corresponds to your closest approach to the black hole. So long as you don’t cross the event horizon, you shouldn’t ever fall in. Occasionally, if there are enough particles in orbit, you’ll interact with the other ones, experiencing inelastic collisions and friction. You’ll heat up, be compelled to move in a more circular orbit, and eventually emit radiation.

    This radiation doesn’t come from inside the black hole, but from the matter orbiting outside the event horizon.

    4
    An illustration of an active black hole, one that accretes matter and accelerates a portion of it outwards in two perpendicular jets, may describe the black hole in our galaxy and, in particular, more active ones in many regards. Image credit: Mark A. Garlick.

    Sure, some of the matter will eventually lose enough energy that it will cross over to the inside of the event horizon, arriving at the singularity and increasing the mass of the black hole. But there’s a lot going on in the vicinity of the black hole. There are charged particles of different signs and magnitudes traveling very rapidly: moving close to the speed of light. Charged objects in motion create magnetic fields, and that causes many of the ionized matter particles to be accelerated in a helix-shape, away from the plane of the accretion disk. These accelerating particles are the origin of relativistic jets, producing showers of particles and radiation when they collide with the material farther away from the black hole.

    5
    The galaxy Centaurus A, shown in a composite of visible light, infrared (submillimeter) light and in the X-ray. Image credit: ESO/WFI (Optical); MPIfR/ESO/APEX/A.Weiss et al. (Submillimetre); NASA/CXC/CfA/R.Kraft et al. (X-ray).

    ESO WFI LaSilla 2.2-m MPG/ESO telescope at La Silla, 600 km north of Santiago de Chile at an altitude of 2400 metres

    MPG/ESO 2.2 meter telescope at Cerro La Silla, Chile, 600 km north of Santiago de Chile at an altitude of 2400 metres

    ESO/APEX high on the Chajnantor plateau in Chile’s Atacama region, at an altitude of over 4,800 m (15,700 ft)

    NASA/Chandra Telescope

    Relativistic jets are a remarkable sight, and in some cases, are so brilliant that they actually appear in visible light. The galaxy Centaurus A [abve] has a jet in both directions that becomes large, diffuse and spectacular; the galaxy Messier 87 has a single, collimated jet that extends for over 5,000 light years.

    6
    The second-largest black hole as seen from Earth, the one at the center of the galaxy Messier 87, is around 1000 times larger than the Milky Way’s black hole, but is over 2000 times farther away. The relativistic jet emanating from its central core is one of the largest, most collimated ones ever observed. Image credit: NASA/ESA Hubble.

    Both of these are caused by an active, supermassive black hole that’s many times larger than even the four-million-solar-mass monstrosity at the center of the Milky Way.

    For accretion disks and relativistic jets, these are phenomena that are observable around black holes, but nothing is coming from inside the black hole and getting out. For Hawking radiation, however, things get a little more complicated. In theory, you can imagine a black hole that was truly in the vacuum of space, with no matter, radiation, or other masses around it. If the black hole weren’t there, all you’d have was the vacuum of flat, uncurved space governed by the fundamental laws of the Universe. But if you put the black hole there, you have curved space, an event horizon, and the laws of physics. And a consequence of that is that you get omnidirectional radiation with a blackbody spectrum to it: Hawking radiation.

    7
    The event horizon of a black hole is a spherical or spheroidal region from which nothing, not even light, can escape. But outside the event horizon, the black hole is predicted to emit radiation. Image credit: NASA; Jörn Wilms (Tübingen) et al.; ESA.

    The problem with conceptualizing Hawking radiation is the following: all of the radiation originates from outside the event horizon, but the only place to draw energy from is the mass inside the black hole itself. For every quantum of energy (E) released in the form of Hawking radiation, the mass of the black hole (m) has to decrease by an equivalent amount. How much is that? By exactly the amount that Einstein’s most famous equation predicts, E = mc2. But how, then, can radiation from outside a black hole be caused by mass that’s inside a black hole, particularly if nothing can escape the event horizon?

    8
    A visualization of what a black hole silhouetted against the backdrop of the Milky Way would look like. The event horizon is the dark region from which no light can escape. Image credit: SXS team; Bohn et al. 2015.

    The most common explanation — given by Hawking himself — is also the most wrong. One of the ways you can visualize vacuum energy, or the energy inherent to space itself, is with particle-antiparticle pairs. Empty space, because its zero-point energy is a positive value (rather than zero), can’t be visualized as altogether empty; you need something to occupy it. Combining this fact with the Heisenberg uncertainty principle, you arrive at a picture where matter-and-antimatter pairs pop into existence for a very brief amount of time, before annihilating away back into the nothingness of empty space. When one member is outside the event horizon but the other falls in, the “outside” one can escape, carrying energy away, while the “inside” one carries negative energy and decreases the mass of the black hole.

    9
    Particle-antiparticles pairs pop in-and-out of existence continuously, both inside and outside the event horizon of a black hole. When an outside-created pair has one of its members fall in, that’s when things get interesting. Image credit: Ulf Leonhardt of the University of St. Andrews.

    First off, this visualization is not for real particles, but virtual ones. They are calculational tools only, not physically observable entities. Second, the Hawking radiation that leaves a black hole is almost exclusively photons, not matter or antimatter particles. And third, most of the Hawking radiation doesn’t come from the edge of the event horizon, but from a very large region surrounding the black hole. If you must adhere to the particle-antiparticle pairs explanation, it’s better to try and view it as a series of four types of pairs:

    out-out,
    out-in,
    in-out, and
    in-in,

    where it’s the out-in and in-out pairs that virtually interact, producing photons that carry energy away, where the missing energy comes from the curvature of space, and that in turn decreases the mass of the central black hole.

    10
    Hawking radiation is what inevitably results from the predictions of quantum physics in the curved spacetime surrounding a black hole’s event horizon. This diagram shows that it’s the energy from outside the event horizon that creates the radiation, meaning that the black hole must lose mass to compensate. Image credit: E. Siegel.

    But the true explanation doesn’t lend itself very well to a visualization, and that troubles a lot of people. What you must calculate is how the quantum field theory of empty space behaves in the highly-curved region around a black hole. Not necessarily right by the event horizon, but over a large, spherical region outside of it. Performing the quantum field theory calculation in curved space yields a surprising solution: that thermal, blackbody radiation is emitted in the space surrounding a black hole’s event horizon. And the smaller the event horizon is, the greater the curvature of space near the event horizon is, and thus the greater the rate of Hawking radiation.

    11
    As a black hole shrinks in mass and radius, the Hawking radiation emanating from it becomes greater and greater in temperature and power. Once the decay rate exceeds the growth rate, Hawking radiation only increases in temperature and power. Image credit: NASA.

    Under no circumstances, however, can we conclude that anything crosses the event horizon from inside to out. Hawking radiation comes from the space outside of the event horizon, and propagates away from the black hole. The loss of energy lowers the mass of the central black hole, eventually leading to total evaporation. Hawking radiation is an incredibly slow process, where a black hole the mass of our Sun would take 10⁶⁷ years to evaporate; the one at the Milky Way’s center would require 10⁸⁷ years, and the most massive ones in the Universe could take up to 10¹⁰⁰ years! And whenever a black hole decays, the last thing you see is a brilliant, energetic flash of radiation and high-energy particles.

    These final decay steps, which won’t occur until long after the final star has burned out, are the last gasps of energy the Universe has to give off. In it’s own way, it’s the Universe itself trying, one final time, to create an energy imbalance and an opportunity for the creation of complex structures. When the last black hole decays, it will be the Universe’s final attempt to say the same thing it said at the start of the hot Big Bang, “Let there be light!”

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 12:06 pm on January 19, 2018 Permalink | Reply
    Tags: , , , , Ethan Siegel, Why NASA’s Kepler Mission Is Toast   

    From Ethan Siegel: “Why NASA’s Kepler Mission Is Toast” 

    Ethan Siegel
    Dec 22, 2017 [Just showed up in social media]

    1
    The Kepler-90 planets have a similar configuration to our solar system with small planets found orbiting close to their star, and the larger planets found farther away. However, the eighth planet discovered orbits only at the Earth-Sun distance, so we have no idea what lies beyond, including if there are smaller worlds still left to discover. Planets are shown to correct relative size, but not to correct orbital scales. Image credit: NASA/Ames Research Center/Wendy Stenzel.

    The ‘big discovery’ is nothing of the kind. We need a new planet-finding mission to probe the next frontier.

    Last week, NASA dropped a bombshell that it’s Kepler mission — the greatest planet-finding mission in history — had teamed up with Google’s AI to make a groundbreaking new discovery. Speculation ran rampant as to what it might be. An Earth-like twin? A signal unlike anything else we’d ever seen? Even a hint of alien intelligence, or life beyond our Solar System? Nope. Yesterday’s big reveal was an incredibly mundane announcement: Kepler-90, a star system previously known to have seven planets, was now found to have eight. While this makes Kepler-90 the only star system known to have as many planets as our Solar System, this mundane announcement highlights just how thoroughly the data from Kepler has been examined. Despite the headlines you’ll likely see in the future, bet on this: all of Kepler’s major discoveries are in the past.

    2
    Today, as shown in figure 10, we know of over 3,500 confirmed exoplanets, with more than 2,500 of those found in the Kepler data. These planets range in size from larger than Jupiter to smaller than Earth. In just a couple decades, thanks largely to Kepler, we have gone from suspecting exoplanets existed to knowing that there are more exoplanets than stars in our galaxy. Image credit: NASA/Ames Research Center/Jessie Dotson and Wendy Stenzel.

    The Kepler-90 system, even before the discovery of the eighth planet, was an objectively interesting one. First, it was a G-class star, the same class as our own. The orientation of this system is nearly perfectly edge-on to our own line-of-sight, with the plane of its solar system oriented towards us at a precision of less than one degree. When we look at the main star, each time a planet makes a complete orbital revolution, we see the star dim due to the transit effect. This is how Kepler goes about finding planets: it looks for periodic dips of equal magnitude in the brightness of a star, corresponding to a planet of a certain distance and size. Follow-up observations are then undertaken to confirm the planet’s existence, moving the robust Kepler discoveries from the category of candidate planet to planet.

    The three innermost worlds we found were comparable to the physical size of Earth, ranging between 18% to 32% larger than our world. All of them are incredibly close to their star, completing an orbit in two weeks or under, with the closest one orbiting at just 7% of the Earth-Sun distance. The next three planets are all closer to Neptune-sized worlds, approximately 2½ to 3 times the radius of Earth. They surely have gas envelopes around them, and orbit at distances comparable to that of Mercury or slightly beyond. The seventh planet, discovered by a rarer and more sophisticated transit-timing variations method, is actually enormous: 8 times the radius of Earth, or nearly as large as Saturn, orbiting at the distance of Venus.

    4
    Kepler-90 is a Sun-like star, but all of its eight planets are scrunched into the equivalent distance of Earth to the Sun. The inner planets have extremely tight orbits with a “year” on Kepler-90i lasting only 14.4 days. In comparison, Mercury’s orbit is 88 days. There is much left to discover, still, about this system. Image credit: NASA/Ames Research Center/Wendy Stenzel.

    And through the use of machine learning techniques pioneered by Google, they were able to extract evidence for one more influential world, one larger than Jupiter and orbiting at the distance of Earth. Applying the same technique to the Kepler-80 system, they found another world there as well, bringing that system’s total up to five.

    But this is it. This is what counts as a “major advance” for Kepler: the application of an extremely sophisticated, novel technique to extract one additional data point at the absolute limits of what the full suite of data can offer. The truth is that Kepler acquired, during its main mission lifetime, approximately three years of data concerning 150,000 stars. Well over 90% of these stars show no evidence for planets, since their alignments aren’t right to cause transits. Those that do are predominantly close to their star, because those are both the ones most likely to transit, and also the ones that have more transits, meaning more data, more signal, and a greater chance of being seen. We’re basically limited to seeing, even in the best case scenario, planets that orbit no further than Earth or Mars from their star.

    5
    This graphic shows that a small area around the Kepler-90 system, on the left, has been searched by the Kepler space telescope. Compared to our solar system, where we know of planets farther out, it is possible that Kepler-90 has even more planets. If planets (in the blue area) do exist, they probably would not have transited enough times while Kepler was watching for us to know they were there. Image credit: NASA/Ames Research Center/Wendy Stenzel.

    Kepler can’t really measure anything other than the orbital parameters (like semimajor axis and period) and the radius of the planet, either; that’s the only information it was designed to detect. In concert with other observatories, we can learn some other things, such as mass or orbital eccentricity, but that’s about it. We can’t measure atmospheric content, temperature, or search for signs of life with it. And even though the K2 mission is ongoing with what’s still working aboard Kepler, there’s no new data being collected on those 150,000 stars that were part of the original mission. What this latest announcement showcases, more than anything else, is that the development of new techniques to extract the tiniest amount of signal left buried in the data are the only Kepler advances left.

    6
    This figure shows the number of systems with one, two, three, planets, etc. Each dot represents one known planetary system. We know of more than 2,000 one-planet systems, and progressively fewer systems with many planets. The discovery of Kepler-90i, the first known exoplanet system with eight planets, is a hint of more highly populated systems to come. Image credit: NASA/Ames Research Center/Wendy Stenzel and The University of Texas at Austin/Andrew Vanderburg.

    It was a great mission. The scientists working to extract the last usable pieces of data — both on an ongoing basis from K2 and from the archival data of the original mission — are honestly doing great work. But if you think Kepler-90 is anything like our Solar System, or has eight planets like ours does, you’ve fallen for the NASA hype train.

    It’s very likely that there are solar systems out their like ours, and that where the alignment has been good, NASA’s Kepler would have detected perhaps the Venus-like and Earth-like worlds, with Mercury being too small and all the other worlds being too distant. The idea that a solar system would simply “end” where Earth’s orbit exists is absurd; there are certainly additional worlds beyond the ones Kepler was sensitive to. In order to see them, we’ll either need longer observing periods or the technology to do direct imaging, both of which are quite far off given the current funding situation. Based on what we’ve seen so far, it’s likely that Kepler-90 is

    a far younger system than our own,

    less evolved than our Sun and our Solar System,

    contains many more planets that are too distant for us to see (I’d guess around a total of 20),

    and that, just like our own Solar System, it constitutes a very different example of what’s also “normal” in this Universe.

    7
    This is an illustration of the different elements in NASA’s exoplanet program, including ground-based observatories, like the W. M. Keck Observatory, and space-based observatories, like Hubble, Spitzer, Kepler, Transiting Exoplanet Survey Satellite, James Webb Space Telescope, Wide Field Infrared Survey Telescope and future missions. Image credit: NASA.

    There are future missions in the pipeline that are poised to take the next great leap in planet-finding and in learning more about these planets. James Webb will allow direct imaging of large, distant exoplanets, and will potentially measure the atmospheric contents of worlds only twice the diameter of Earth. WFIRST will be even more successful at planet-hunting than Kepler was, and if it launches with a starshade, could look for organic signs on Earth-sized planets that are close enough to our own world. There are big breakthroughs ahead, but they’re on a relatively distant horizon.

    NASA/ESA/CSA Webb Telescope annotated

    NASA/WFIRST

    8
    The Starshade concept could enable direct exoplanet imaging as early as the 2020s. This concept drawing illustrates a telescope using a star shade. Image credit: NASA and Northrop Grumman.

    It’s been almost a decade since NASA’s Kepler began operations, and while it’s revolutionized our knowledge of planets in the Universe, we sorely need new equipment and better data to push forward in our understanding. Unfortunately, the next planet-finding mission, TESS, will be inexpensive (under $100 million; very inexpensive for a satellite mission) and unambitious: just a wide-field and low-depth version of Kepler, measuring the light from about three times as many stars for just a period of two years. There are great advances to come, but Kepler has already seen almost all of what it’s going to, and practically all of the great science we’d hoped for has already been extracted. It’s time for the next step. It’s no longer satisfying to speculate about what could be out there. It’s time to know.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 2:47 pm on January 17, 2018 Permalink | Reply
    Tags: , , , , Ethan Siegel, New Dark Matter Physics Could Solve The Expanding Universe Controversy   

    From Ethan Siegel: “New Dark Matter Physics Could Solve The Expanding Universe Controversy” 

    From Ethan Siegel
    1.17.18

    1
    The leftover glow from the Big Bang, as well as the galaxies that exist today, give us a way to measure the expanding Universe that’s very different from the standard cosmic distance ladder. Their results are mutually incompatible. Image credit: E.M. Huff, the SDSS-III team and the South Pole Telescope team; graphic by Zosia Rostomian.

    2
    SDSS III Galaxy Map

    SDSS Telescope at Apache Point Observatory, near Sunspot NM, USA, Altitude 2,788 meters (9,147 ft)

    South Pole Telescope SPTPOL. The SPT collaboration is made up of over a dozen (mostly North American) institutions, including the University of Chicago, the University of California, Berkeley, Case Western Reserve University, Harvard/Smithsonian Astrophysical Observatory, the University of Colorado Boulder, McGill University, The University of Illinois at Urbana-Champaign, University of California, Davis, Ludwig Maximilian University of Munich, Argonne National Laboratory, and the National Institute for Standards and Technology. It is funded by the National Science Foundation.

    Multiple teams of scientists can’t agree on how fast the Universe expands. Dark matter may unlock why.

    There’s an enormous controversy in astrophysics today over how quickly the Universe is expanding. One camp of scientists, the same camp that won the Nobel Prize for discovering dark energy, measured the expansion rate to be 73 km/s/Mpc, with an uncertainty of only 2.4%. But a second method, based on the leftover relics from the Big Bang, reveals an answer that’s incompatibly lower at 67 km/s/Mpc, with an uncertainty of only 1%. It’s possible that one of the teams has an unidentified error that’s causing this discrepancy, but independent checks have failed to show any cracks in either analysis. Instead, new physics might be the culprit. If so, we just might have our first real clue to how dark matter might be detected.

    3
    The expanding Universe, full of galaxies and the complex structure we observe today, arose from a smaller, hotter, denser, more uniform state. Why the Universe expands at the rate it shows when you ask with different methods is hitherto unexplained. Image credit: C. Faucher-Giguère, A. Lidz, and L. Hernquist, Science 319, 5859 (47).

    The expanding Universe has been one of the most important discoveries of the past 100 years, and it’s brought with it a revolution in how we conceive of the Universe. It was the key observation that led to the formulation of the Big Bang; it allowed us to discover how stars and galaxies came to exist; it taught us the age of the Universe. Most recently, it led to the discovery of the accelerating Universe, whose cause we commonly call “dark energy.”

    4
    Possible fates of the expanding Universe. Notice the differences of different models in the past; only a Universe with dark energy matches our observations. Image credit: The Cosmic Perspective / Jeffrey O. Bennett, Megan O. Donahue, Nicholas Schneider and Mark Voit.

    It’s now 20 years since dark energy was first uncovered, though, and we still only have three main classes of possibilities for why the Universe appears to accelerate:

    1. Vacuum energy, like a cosmological constant, is energy inherent to space itself, and drives the Universe’s expansion.
    2. Dynamical dark energy, driven by some kind of field that changes over time, could lead to differences in the Universe’s expansion rate depending on when/how you measure it.
    3. General Relativity could be wrong, and a modification to gravity might explain what appears to us as an apparent acceleration.

    The evidence, from everything we’ve gathered, strongly points to that first case, where dark energy is a cosmological constant.

    5
    The matter and energy content in the Universe at the present time (left) and at earlier times (right). Note the presence of dark energy, dark matter, and the prevalence of normal matter and radiation. Image credit: NASA, modified by Wikimedia Commons user 老陳, modified further by E. Siegel.

    t the dawn of 2018, however, the controversy over the expanding Universe might threaten that picture. Our Universe, made up of 68% dark energy, 27% dark matter, and just 5% of all the “normal” stuff (including stars, planets, gas, dust, plasma, black holes, etc.), should be expanding at the same rate regardless of the method you use to measure it. At least, that would be the case if dark energy were truly a cosmological constant, and if dark matter were truly cold and collisionless, interacting only gravitationally. If everyone measured the same rate for the expanding Universe, there would be nothing to challenge this picture, known as standard (or “vanilla”) ΛCDM.

    But everyone doesn’t measure the same rate.

    The standard (and oldest) method of measuring the Hubble rate is through a method known as the cosmic distance ladder. Today, the simplest version only has three rungs. First, you measure the distances to nearby stars directly, through parallax, and specifically you measure the distance to the long-period Cepheid stars like this. Second, you then measure other properties of those same types of Cepheid stars in nearby galaxies, learning how far away those galaxies are. And lastly, in some of those galaxies, you’ll have a specific class of supernovae known as Type Ia supernovae, which you can then observe both nearby as well as many of billions of light years away. With just three steps, you can measure the expanding Universe, arriving at a result of 73.24 ± 1.74 km/s/Mpc.

    6
    The fluctuations in the Cosmic Microwave Background were first measured accurately by COBE in the 1990s, then more accurately by WMAP in the 2000s and Planck (above) in the 2010s. This image encodes a huge amount of information about the early Universe, including its composition, age, and history. Image credit: ESA and the Planck Collaboration.

    ESA/Planck

    NASA/COBE

    NASA/WMAP

    But if you look at the early Universe, before there were stars and galaxies, all you had was the ionized plasma of normal matter, the hot mix of neutrinos and photons which act as radiation, and the cold, slow-moving mass of dark matter. Based on the physics of gravitation, trying to pull the matter together, and radiation, which smooths out overdense regions, we should get a specific pattern of density and temperature fluctuations. This not only shows up in the Cosmic Microwave Background, which is the Big Bang’s leftover glow, but also sets a distance scale for galaxy correlations. These methods of measuring the Hubble rate give a vastly different result: 66.9 ± 0.6 km/s/Mpc.

    7
    Modern measurement tensions from the distance ladder (red) with CMB (green) and BAO (blue) data. The red points are from the distance ladder method; the green and blue are from ‘leftover relic’ methods. Image credit: Aubourg, Éric et al. Phys.Rev. D92 (2015) no.12, 123516.

    Many new physics explanations have been floated to attempt to explain this, yet all have run into tremendous difficulties.

    Dark energy might not be a cosmological constant, with a specific balance between outward (accelerating) pressure and inward (gravitating) energy density, but might have a different balance.

    Dark energy could change over time, where it was stronger (or weaker) in the past. This would represent a change in the dark energy equation-of-state over time.

    There could be a contribution of spatial curvature, which represents an additional component affecting the Universe’s expansion rate at various scales.

    There could be an extra species of radiation (or neutrino) in the early Universe, which would alter the pattern of density-and-temperature fluctuations that we see.

    Or we could add in a new type of interaction, either between dark matter and radiation, or by mixing in a new type of “dark radiation” into the Universe, to change the physics of the early Universe.

    8
    It’s believed that the interactions between dark matter and radiation are understood, but the possibility that there are additional interactions, or a new type of radiation, could change the story tremendously. Image credit: NASA/Sonoma State University/Aurore Simonnet.

    That last possibility doesn’t have the problem of the other suggestions, which are all tightly constrained by a variety of observations. Because we know so little about dark matter, and yet because dark matter is so important to the formation of large-scale structure in our Universe, any interaction that affects it could affect the density fluctuations that we see. This could impact both the scale of the Cosmic Microwave Background and also of the galaxies that form much later.

    9
    The density fluctuations in the cosmic microwave background provide the seeds for modern cosmic structure to form, including stars, galaxies, clusters of galaxies, filaments, and large-scale cosmic voids. Image credit: Chris Blake and Sam Moorfield.

    If either photons, neutrinos, or some new type of dark radiation (that interacts with dark matter but not any of the normal particles) has a non-zero cross-section with dark matter, it could bias measurements of the Hubble rate to an artificially low value, but only for one type of measurement: the kind that you get from measuring these leftover relics. If interactions between dark matter and radiation are real, they might not only explain this cosmic controversy, but could be our first hint of how dark matter might directly interact with other particles. If we’re lucky, it could even give us a clue to how to finally see dark matter directly.

    10
    An illustration of clustering patterns due to Baryon Acoustic Oscillations, where the likelihood of finding a galaxy at a certain distance from any other galaxy is governed by the relationship between dark matter and normal matter. As the Universe expands, this characteristic distance expands as well, allowing us to measure the Hubble constant. If there’s a new interaction between dark matter and radiation, the greatest cosmic controversy about the expanding Universe might have an incredible resolution. Image credit: Zosia Rostomian.

    Currently, the fact that distance ladder measurements say the Universe expands 9% faster than the leftover relic method is one of the greatest puzzles in modern cosmology. Whether that’s because there’s a systematic error in one of the two methods used to measure the expansion rate or because there’s new physics afoot is still undetermined, but it’s vital to remain open-minded to both possibilities. As improvements are made to parallax data, as more Cepheids are found, and as we come to better understand the rungs of the distance ladder, it becomes harder and harder to justify blaming systematics. The resolution to this paradox may be new physics, after all. And if it is, it just might teach us something about the dark side of the Universe.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
    • Mike Cavedon 2:23 pm on January 18, 2018 Permalink | Reply

      Dark energy is the outflow of dark matter associated with our universal black hole.

      Our visible universe is in the outflow of a super-supermassive black hole. As ordinary matter falls toward the super-supermassive black hole it evaporates into dark matter. It is the dark matter outflow which pushes the galaxy clusters, causing them to move outward and away from us. The dark matter outflow is dark energy.

      The galaxy clusters which have been pushed for longer than we have are accelerating outward and away from us. We are accelerating outward and away from the galaxy clusters which have been pushed for less time than we have. From our perspective most of the galaxy clusters are accelerating away from us.

      Like

    • Marco Pereira 8:05 pm on January 18, 2018 Permalink | Reply

      I wrote a response to this article in quora. Please feel free to ask questions.

      Like

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