Tagged: Ethan Siegel Toggle Comment Threads | Keyboard Shortcuts

  • richardmitnick 1:53 pm on May 28, 2017 Permalink | Reply
    Tags: , , B;ack holes, , , Ethan Siegel,   

    From Ethan Siegel: “Ask Ethan: What happens when a black hole’s singularity evaporates?” 

    Ethan Siegel
    May 27, 2017

    1
    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.

    If even black holes won’t last forever, what will happen when the last one goes?

    “My discovery that black holes emit radiation raised serious problems of consistency with the rest of physics. I have now resolved these problems, but the answer turned out to be not what I expected.” -Stephen Hawking

    It’s hard to imagine, given the full diversity of forms that matter takes in this Universe, that for millions of years, there were only neutral atoms of hydrogen and helium gas. It’s perhaps equally hard to imagine that someday, quadrillions of years from now, all the stars will have gone dark. Only the remnants of our now-vibrant Universe will be left, including some of the most spectacular objects of all: black holes. But even they won’t last forever. David Weber wants to know how that happens for this week’s Ask Ethan, inquiring:

    What happens when a black hole has lost enough energy due to hawking radiation that its energy density no longer supports a singularity with an event horizon? Put another way, what happens when a black hole ceases to be a black hole due to hawking radiation?

    In order to answer this question, it’s important to understand what a black hole actually is.

    2
    The anatomy of a very massive star throughout its life, culminating in a Type II Supernova when the core runs out of nuclear fuel. Image credit: Nicole Rager Fuller/NSF.

    Black holes generally form during the collapse of a massive star’s core, where the spent nuclear fuel ceases to fuse into heavier elements. As fusion slows and ceases, the core experiences a severe drop in radiation pressure, which was the only thing holding the star up against gravitational collapse. While the outer layers often experience a runaway fusion reaction, blowing the progenitor star apart in a supernova, the core first collapses into a single atomic nucleus — a neutron star — but if the mass is too great, the neutrons themselves compress and collapse to such a dense state that a black hole forms. (A black hole can also form if a neutron star accretes enough mass from a companion star, crossing the threshold necessary to become a black hole.)

    3
    When a neutron star accretes enough matter, it can collapse to a black hole. When a black hole accretes matter, it grows an accretion disk and will increase its mass as matter gets funneled into the event horizon. Image credit: NASA/ESA Hubble Space Telescope collaboration.

    NASA/ESA Hubble Telescope

    From a gravitational point of view, all it takes to become a black hole is to gather enough mass in a small enough volume of space that light cannot escape from within a certain region. Every mass, including planet Earth, has an escape velocity: the speed you’d need to achieve to completely escape from the gravitational pull at a given distance (e.g., the distance from Earth’s center to its surface) from its center-of-mass. But if there’s enough mass so that the speed you’d need to achieve at a certain distance from the center of mass is the speed of light or greater, then nothing can escape from it, since nothing can exceed the speed of light [in a vacuum].

    4
    The mass of a black hole is the sole determining factor of the radius of the event horizon, for a non-rotating, isolated black hole. Image credit: Cornell SXS team; Bohn et al 2015.

    That distance from the center of mass where the escape velocity equals the speed of light — let’s call it R — defines the size of the black hole’s event horizon. But the fact that there’s matter inside under these conditions has another consequence that’s less-well appreciated: this matter must collapse down to a singularity. You might think there could be a state of matter that’s stable and has a finite volume within the event horizon, but that’s not physically possible.

    In order to exert an outward force, an interior particle would have to send a force-carrying particle away from the center-of-mass and closer to the event horizon. But that force-carrying particle is also limited by the speed of light, and no matter where you are inside the event horizon, all light-like curves wind up at the center. The situation is even worse for slower, massive particles. Once you form a black hole with an event horizon, all the matter inside gets crunched into a singularity.

    Singularity.singularityweblog.com

    And since nothing can escape, you might think a black hole would remain a black hole forever. If it weren’t for quantum physics, this would be exactly what happens. But in quantum physics, there’s a non-zero amount of energy inherent to space itself: the quantum vacuum. In curved space, the quantum vacuum takes on slightly different properties than in flat space, and there are no regions where the curvature is greater than near the singularity of a black hole. Combining these two laws of nature — quantum physics and the General Relativistic spacetime around a black hole — gives us the phenomenon of Hawking radiation.

    5
    A visualization of QCD illustrates how particle/antiparticle pairs pop out of the quantum vacuum for very small amounts of time as a consequence of Heisenberg uncertainty. Image credit: Derek B. Leinweber.

    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. If our Sun were a black hole, the temperature of the Hawking radiation would be about 62 nanokelvin; if you took the black hole at the center of our galaxy, 4,000,000 times as massive, the temperature would be about 15 femtokelvin, or just 0.000025% the temperature of the less massive one.

    6
    An X-ray / Infrared composite image of the black hole at the center of our galaxy: Sagittarius A*. It has a mass of about four million Suns, and is found surrounded by hot, X-ray emitting gas. However, it also emits (undetectable) Hawking radiation, at much, much lower temperatures. Image credit: X-ray: NASA/UMass/D.Wang et al., IR: NASA/STScI.

    NASA/Chandra Telescope

    This means the smallest black holes decay the fastest, and the largest ones live the longest. Doing the math, a solar mass black hole would live for about 10⁶⁷ years before evaporating, but the black hole at the center of our galaxy would live for 10²⁰ times as long before decaying. The crazy thing about it all is that right up until the final fraction-of-a-second, the black hole still has an event horizon. Once you form a singularity, you remain a singularity — and you retain an event horizon — right up until the moment your mass goes to zero.

    7
    Hawking radiation is what inevitably results from the predictions of quantum physics in the curved spacetime surrounding a black hole’s event horizon. Image credit: E. Siegel.

    That final second of a black hole’s life, however, will result in a very specific and very large release of energy. When the mass drops down to 228 metric tonnes, that’s the signal that exactly one second remains. The event horizon size at the time will be 340 yoctometers, or 3.4 × 10^-22 meters: the size of one wavelength of a photon with an energy greater than any particle the LHC has ever produced. But in that final second, a total of 2.05 × 10²² Joules of energy, the equivalent of five million megatons of TNT, will be released. It’s as though a million nuclear fusion bombs went off all at once in a tiny region of space; that’s the final stage of black hole evaporation.

    8
    As a black hole shrinks in mass and radius, the Hawking radiation emanating from it becomes greater and greater in temperature and power. Image credit: NASA.

    What’s left? Just outgoing radiation. Whereas previously, there was a singularity in space where mass, and possibly charge and angular momentum existed in an infinitesimally small volume, now there is none. Space has been restored to its previously non-singular state, after what must have seemed like an eternity: enough time for the Universe to have done all it’s done to date trillions upon trillions of times over. There will be no other stars or sources of light left when this occurs for the first time in our Universe; there will be no one to witness this spectacular explosion. But there’s no “threshold” where this occurs. Rather, the black hole needs to evaporate completely. When it does, to the best of our knowledge, there will be nothing left behind at all but outgoing radiation.

    87
    Against a seemingly eternal backdrop of everlasting darkness, a single flash of light will emerge: the evaporation of the final black hole in the Universe. Image credit: ortega-pictures / pixabay.

    In other words, if you were to watch the last black hole in our Universe evaporate, you would see an empty void of space, that displayed no light or signs of activity for perhaps 10¹⁰⁰ years or more. All of a sudden, a tremendous outrush of radiation of a very particular spectrum and magnitude would appear, leaving a single point in space at 300,000 km/s. For the last time in our observable Universe, an event would have occurred to bathe the Universe in radiation. The last black hole evaporation of all would, in a poetic way, be the final time that the Universe would ever say, “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 9:01 am on May 21, 2017 Permalink | Reply
    Tags: Ask Ethan: Can the Universe still end in a Big Crunch?, , , , , Ethan Siegel   

    From Ethan Siegel: “Ask Ethan: Can the Universe still end in a Big Crunch?” 

    Ethan Siegel
    May 20, 2017

    1
    A ‘Big Bounce’ requires a recollapsing phase (i.e., a Big Crunch) followed by an expanding phase (which looks like a new Big Bang). Image credit: E. Siegel, derivative from Ævar Arnfjörð Bjarmason.

    Dark energy may be real and the Universe may be accelerating, but does that mean a Big Freeze is inevitable?

    “It’s everywhere, really. It’s between the galaxies. It is in this room. We believe that everywhere that you have space, empty space, that you cannot avoid having some of this dark energy.” -Adam Riess, Johns Hopkins University and the Space Telescope Science Institute

    One of the biggest advances of the 20th century has been to identify exactly how rich, expansive, and massive our Universe actually is. With approximately two trillion galaxies contained in a volume some 46 billion light years in radius centered on us, our Observable Universe allows us to reconstruct the entire tale of our cosmic history, stretching all the way back to the Big Bang and even, perhaps, slightly before. But what about the future? What about the fate of the Universe? Is that a certainty? That’s what Andy Moss wants to know, as he asks:

    “You [wrote] that the Universe is expanding at a decreasing rate. I thought a Nobel Prize was awarded for the “discovery” that the Universe was expanding at an increasing rate. Can you please clarify the leading theories? Is the “Big Crunch” still a possibility?”

    The best predictor of future behavior is past behavior, it’s true. But just as people can sometimes surprise us, the Universe might, too.

    Inflationary Universe. NASA/WMAP

    The expansion rate of the Universe, at any moment in time, is only dependent on two things: the total energy density present within spacetime and the amount of spatial curvature present. If we understand the laws of gravitation and how the different types of energy evolve over time, we can reconstruct what the expansion rate should have been at any moment in the past. We can also look out at a variety of distant objects at various distances, and measure how that light has been stretched due to the expansion of space. Every galaxy, supernova, molecular gas cloud, etc. — everything that absorbs or emits light — will tell the cosmic history of how the expansion of space has stretched it from the moment it was emitted until we observe it.

    3
    The farther a galaxy is, the faster it expands away from us, and the more its light gets redshifted, necessitating that we look at longer and longer wavelengths. Image credit: Larry McNish of RASC Calgary Center.

    We’ve been able to conclude, from a variety of independent lines of observation, exactly what the Universe is made out of. The three big, independent lines of observation are:

    The temperature fluctuations present in the cosmic microwave background, which encode information about the Universe’s curvature, normal matter, dark matter, neutrino, and total density contents.
    The correlations between galaxies on the largest scales — known as baryon acoustic oscillations — which give very strict measurements on the total matter density, the normal matter to dark matter ratio, and the expansion rate throughout time.
    And the most distant, luminous standard candles in the Universe, type Ia supernova, which tell us about the expansion rate and dark energy as it evolved over time.

    4
    Standard candles (L) and standard rulers (R) are two different techniques astronomers use to measure the expansion of space at various times/distances in the past. Image credit: NASA/JPL-Caltech.

    6
    Dr. Saul Perlmutter

    [Time to introduce Saul Perlmutter, a U.S. astrophysicist at the Lawrence Berkeley National Laboratory and a professor of physics at the University of California, Berkeley. He is a member of the American Academy of Arts & Sciences, and was elected a Fellow of the American Association for the Advancement of Science in 2003. He is also a member of the National Academy of Sciences. Perlmutter shared the 2006 Shaw Prize in Astronomy, the 2011 Nobel Prize in Physics, and the 2015 Breakthrough Prize in Fundamental Physics with Brian P. Schmidt and Adam Riess for providing evidence that the expansion of the universe is accelerating.]

    These lines of evidence, combined, all point to one consistent picture of the Universe. They tell us what’s in the Universe today, and give us a cosmology where:

    4.9% of the Universe’s energy is in normal matter (like protons, neutrons and electrons),
    0.1% of the Universe’s energy is in the form of massive neutrinos (which act like matter at late times and radiation at early times),
    0.01% of the Universe’s energy is in the form of radiation (like photons),
    27% of the Universe’s energy is in the form of dark matter, and
    68% is in the form of energy inherent to space itself: dark energy.

    They give us a flat Universe (with 0% curvature), a Universe with no topological defects (magnetic monopoles, cosmic strings, domain walls, or cosmic textures), and a Universe whose past expansion history is known.

    54
    The relative importance of different energy components in the Universe at various times in the past. In the future, dark energy will approach 100% importance. Image credit: E. Siegel.

    The equations governing General Relativity are very deterministic in this sense: if we know what the Universe is made of today and the laws of gravity, we know exactly how important each component was at every juncture in the past. Early on, radiation and neutrinos dominated. For billions of years, dark matter and normal matter were the most important pieces. And for the past few billion years — and this will get more severe as time goes on — dark energy is the dominant factor in the Universe’s expansion. It’s causing the Universe to accelerate, and this is where the confusion (for most people) begins.

    5
    Possible fates of the expanding Universe. Notice the differences of different models in the past. Image credit: The Cosmic Perspective / Jeffrey O. Bennett, Megan O. Donahue, Nicholas Schneider and Mark Voit.

    There are two things we can measure when it comes to the Universe’s expansion: the expansion rate and the speed at which an individual galaxy appears to recede from our perspective. These are related, but they are not the same. The expansion rate, on one hand, talks about how the fabric of space itself stretches over time. It’s always quantified as a speed-per-unit-distance, which is typically given in kilometers-per-second (the speed) per Megaparsec (the distance), where a Megaparsec is about 3.26 million light years.

    6
    How matter (top), radiation (middle), and a cosmological constant (bottom) all evolve with time in an expanding Universe. Image credit: E. Siegel / Beyond the Galaxy.

    If there were no dark energy, the expansion rate would drop over time, approaching zero, since the matter-and-radiation density would drop to zero as the volume expands. But with dark energy, that expansion rate approaches whatever energy density dark energy has. If dark energy, for example, is a cosmological constant, then the expansion rate asymptotes to a constant value. But if that’s what the expansion rate does, then individual galaxies receding from us will see their speeds accelerate.

    7
    Optical image [no telescope(s) credited, except for the VLT] of the distant galaxy Markarian 1018, with an overlay of VLT (radio) data. Image credit: ESO/CARS Survey.

    ESO/VLT at Cerro Paranal, with an elevation of 2,635 metres (8,645 ft) above sea level

    Imagine the expansion rate is some value: 50 km/s/Mpc. If a galaxy is 20 Mpc away, then it appears to recede from us at 1,000 km/s. But give it time; as the fabric of space expands, this galaxy will eventually be farther from us. By time it’s twice as distant, 40 Mpc away from us, it will appear to recede at 2,000 km/s. Over even more time, it will be ten times as far as it began: 200 Mpc, where it now recedes at 10,000 km/s. By time it gets to a distance of 6,000 Mpc from us, it will appear to recede at 300,000 km/s, which is faster than the speed of light. But this goes on and on; the more time passes, the faster the galaxy appears to move away from us. This is what’s “accelerating” about the Universe: the expansion rate goes down, but the speed an individual galaxy moves away from us just rises and rises over time.

    8
    The full UV-visible-IR composite of the Hubble eXtreme Deep Field; the greatest image ever released of the distant Universe. Image credit: NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI).

    NASA/ESA Hubble Telescope

    All of this is consistent with our best measurements: that dark energy represents a constant energy density inherent to space itself. As space stretches, the dark energy density remains constant, and the Universe will end in this “Big Freeze” fate, where everything that isn’t gravitationally bound together (like our local group, galaxy, solar system, etc.) winds up being pushed apart from one another. If dark energy is truly a cosmological constant, then the expansion will continue indefinitely, giving rise to a cold, empty Universe.

    9
    When astronomers first realized the universe was accelerating, the conventional wisdom was that it would expand forever. However, until we better understand the nature of dark energy other scenarios for the fate of the universe are possible. This diagram outlines these possible fates. Image credit: NASA/ESA and A. Riess (STScI).

    But if dark energy is dynamical — something theoretically possible but observationally without support — it could yet end in a Big Crunch or a Big Rip. In a Big Crunch, dark energy would weaken and reverse sign, causing the Universe to reach a maximum size, turn around, and contract. It could even give rise to a cyclical Universe, where the “crunch” gives rise to another Big Bang. If dark energy continues to strengthen, however, the opposite fate occurs, where bound structures eventually get torn apart by the increasing expansion rate. The evidence we have today, however, overwhelmingly supports a “Big Freeze,” the condition of expansion continuing at a constant rate forever.

    The major science goals of upcoming observatories like the ESA’s Euclid, NASA’s WFIRST, and the ground-based LSST include measuring whether dark energy is truly a cosmological constant or not.

    ESA/Euclid spacecraft

    NASA/WFIRST


    LSST Camera, built at SLAC



    LSST telescope, currently under construction at Cerro Pachón Chile, a 2,682-meter-high mountain in Coquimbo Region, in northern Chile, alongside the existing Gemini South and Southern Astrophysical Research Telescopes.

    Although the leading theoretical idea is, in fact, in favor of constant dark energy, it’s important to entertain all the possibilities not ruled out by our measurements and observations. As far fetched as it may seem, a Big Crunch still isn’t ruled out. With more and better data, we may yet find a compelling hint that reality is even stranger than most of us have imagined!

    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 3:04 pm on May 20, 2017 Permalink | Reply
    Tags: , , , , , Ethan Siegel,   

    From Ethan Siegel: “Ask Ethan: What Happens When A Black Hole’s Singularity Evaporates?” 

    Ethan Siegel
    May 20, 2017

    1
    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. NASA; Jörn Wilms (Tübingen) et al.; ESA

    It’s hard to imagine, given the full diversity of forms that matter takes in this Universe, that for millions of years, there were only neutral atoms of hydrogen and helium gas. It’s perhaps equally hard to imagine that someday, quadrillions of years from now, all the stars will have gone dark. Only the remnants of our now-vibrant Universe will be left, including some of the most spectacular objects of all: black holes. But even they won’t last forever. David Weber wants to know how that happens for this week’s Ask Ethan, inquiring:

    What happens when a black hole has lost enough energy due to hawking radiation that its energy density no longer supports a singularity with an event horizon? Put another way, what happens when a black hole ceases to be a black hole due to hawking radiation?

    In order to answer this question, it’s important to understand what a black hole actually is.

    2
    The anatomy of a very massive star throughout its life, culminating in a Type II Supernova when the core runs out of nuclear fuel. Nicole Rager Fuller/NSF

    Black holes generally form during the collapse of a massive star’s core, where the spent nuclear fuel ceases to fuse into heavier elements. As fusion slows and ceases, the core experiences a severe drop in radiation pressure, which was the only thing holding the star up against gravitational collapse. While the outer layers often experience a runaway fusion reaction, blowing the progenitor star apart in a supernova, the core first collapses into a single atomic nucleus — a neutron star — but if the mass is too great, the neutrons themselves compress and collapse to such a dense state that a black hole forms. (A black hole can also form if a neutron star accretes enough mass from a companion star, crossing the threshold necessary to become a black hole.)

    3
    When a neutron star accretes enough matter, it can collapse to a black hole. When a black hole accretes matter, it grows an accretion disk and will increase its mass as matter gets funneled into the event horizon. NASA/ESA Hubble Space Telescope collaboration

    NASA/ESA Hubble Telescope

    From a gravitational point of view, all it takes to become a black hole is to gather enough mass in a small enough volume of space that light cannot escape from within a certain region. Every mass, including planet Earth, has an escape velocity: the speed you’d need to achieve to completely escape from the gravitational pull at a given distance (e.g., the distance from Earth’s center to its surface) from its center-of-mass. But if there’s enough mass so that the speed you’d need to achieve at a certain distance from the center of mass is the speed of light or greater, then nothing can escape from it, since nothing can exceed the speed of light.

    4
    Cornell SXS team; Bohn et al 2015

    That distance from the center of mass where the escape velocity equals the speed of light — let’s call it R — defines the size of the black hole’s event horizon. But the fact that there’s matter inside under these conditions has another consequence that’s less-well appreciated: this matter must collapse down to a singularity. You might think there could be a state of matter that’s stable and has a finite volume within the event horizon, but that’s not physically possible.

    In order to exert an outward force, an interior particle would have to send a force-carrying particle away from the center-of-mass and closer to the event horizon. But that force-carrying particle is also limited by the speed of light, and no matter where you are inside the event horizon, all light-like curves wind up at the center. The situation is even worse for slower, massive particles. Once you form a black hole with an event horizon, all the matter inside gets crunched into a singularity.

    5
    http://www.speed-light.info/miracles_of_quran/singularity.htm

    6
    The exterior spacetime to a Schwarzschild black hole, known as Flamm’s Paraboloid, is easily calculable. But inside an event horizons, all geodesics lead to the central singularity. Wikimedia Commons user AllenMcC

    And since nothing can escape, you might think a black hole would remain a black hole forever. If it weren’t for quantum physics, this would be exactly what happens. But in quantum physics, there’s a non-zero amount of energy inherent to space itself: the quantum vacuum. In curved space, the quantum vacuum takes on slightly different properties than in flat space, and there are no regions where the curvature is greater than near the singularity of a black hole. Combining these two laws of nature — quantum physics and the General Relativistic spacetime around a black hole — gives us the phenomenon of Hawking radiation.

    6
    Ethan Siegel

    8
    A visualization of QCD illustrates how particle/antiparticle pairs pop out of the quantum vacuum for very small amounts of time as a consequence of Heisenberg uncertainty. Derek B. Leinweber

    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. If our Sun were a black hole, the temperature of the Hawking radiation would be about 62 nanokelvin; if you took the black hole at the center of our galaxy, 4,000,000 times as massive, the temperature would be about 15 femtokelvin, or just 0.000025% the temperature of the less massive one.

    9
    An X-ray / Infrared composite image of the black hole at the center of our galaxy: Sagittarius A*. It has a mass of about four million Suns, and is found surrounded by hot, X-ray emitting gas. However, it also emits (undetectable) Hawking radiation, at much, much lower temperatures. X-ray: NASA/UMass/D.Wang et al., IR: NASA/STScI

    NASA/Chandra Telescope

    NASA Infrared Telescope facility Mauna Kea, Hawaii, USA

    This means the smallest black holes decay the fastest, and the largest ones live the longest. Doing the math, a solar mass black hole would live for about 10^67 years before evaporating, but the black hole at the center of our galaxy would live for 10^20 times as long before decaying. The crazy thing about it all is that right up until the final fraction-of-a-second, the black hole still has an event horizon. Once you form a singularity, you remain a singularity — and you retain an event horizon — right up until the moment your mass goes to zero.

    That final second of a black hole’s life, however, will result in a very specific and very large release of energy. When the mass drops down to 228 metric tonnes, that’s the signal that exactly one second remains. The event horizon size at the time will be 340 yoctometers, or 3.4 × 10^-22 meters: the size of one wavelength of a photon with an energy greater than any particle the LHC has ever produced. But in that final second, a total of 2.05 × 10^22 Joules of energy, the equivalent of five million megatons of TNT, will be released. It’s as though a million nuclear fusion bombs went off all at once in a tiny region of space; that’s the final stage of black hole evaporation.

    10
    As a black hole shrinks in mass and radius, the Hawking radiation emanating from it becomes greater and greater in temperature and power. NASA

    What’s left? Just outgoing radiation. Whereas previously, there was a singularity in space where mass, and possibly charge and angular momentum existed in an infinitesimally small volume, now there is none. Space has been restored to its previously non-singular state, after what must have seemed like an eternity: enough time for the Universe to have done all it’s done to date trillions upon trillions of times over. There will be no other stars or sources of light left when this occurs for the first time in our Universe; there will be no one to witness this spectacular explosion. But there’s no “threshold” where this occurs. Rather, the black hole needs to evaporate completely. When it does, to the best of our knowledge, there will be nothing left behind at all but outgoing radiation.

    11
    Against a seemingly eternal backdrop of everlasting darkness, a single flash of light will emerge: the evaporation of the final black hole in the Universe. ortega-pictures / pixabay

    In other words, if you were to watch the last black hole in our Universe evaporate, you would see an empty void of space, that displayed no light or signs of activity for perhaps 10^100 years or more. All of a sudden, a tremendous outrush of radiation of a very particular spectrum and magnitude would appear, leaving a single point in space at 300,000 km/s. For the last time in our observable Universe, an event would have occurred to bathe the Universe in radiation. The last black hole evaporation of all would, in a poetic way, be the final time that the Universe would ever say, “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 8:29 pm on May 18, 2017 Permalink | Reply
    Tags: , , , , Ethan Siegel, ,   

    From Ethan Siegel: “What if cosmic inflation is wrong?” 

    Ethan Siegel
    May 18, 2017

    1
    The earliest stages of the Universe, before the Big Bang, are what set up the initial conditions that everything we see today has evolved from. Image credit: E. Siegel, with images derived from ESA/Planck and the DoE/NASA/ NSF interagency task force on CMB research.

    One of inflation’s co-founders lashes out against the community. But is there a scientific leg to stand on?

    “…an understanding of the infinite tree of universes seems to be needed in order to make statistical predictions about the properties of our own universe, which is assumed to be a typical “branch” on the tree.” –Alan Guth

    9
    Dr. Alan Guth, Highland Park, NJ High School, M.I.T.

    HPHS Owls

    All scientific ideas, no matter how accepted or widespread they are, are susceptible to being overturned. For all the successes any idea may have, it only takes one experiment or observation to falsify it, invalidate it, or necessitate that it be revised. Beyond that, every scientific idea or model has a limitation to its range of validity: Newtonian mechanics breaks down close to the speed of light; General Relativity breaks down at singularities; evolution breaks down when you reach the origin of life. Even the Big Bang has its limitations, as there’s only so far back we can extrapolate the hot, dense, expanding state that gave rise to what we see today. Since 1980, the leading idea for describing what came before it has been cosmic inflation, for many compelling reasons. But recently, a spate of public statements has shown a deeper controversy:

    In February, a group of theorists, including one of inflation’s co-founders, claimed that inflation had failed.
    The mainstream group of inflationary cosmologists, including inflation’s inventor, Alan Guth, wrote a rebuttal.
    This prompted the original group to dig in further, denouncing the rebuttal.
    And earlier this week, a major publication and one of the rebuttal’s co-signers highlighted and gave their perspective on the debate.

    3
    The expanding Universe, full of galaxies and complex structure we see today, arose from a smaller, hotter, denser, more uniform state. Image credit: C. Faucher-Giguère, A. Lidz, and L. Hernquist, Science 319, 5859 (47).

    There are three things going on here: the problems with the Big Bang that led to the development of cosmic inflation, the solution(s) that cosmic inflation provides and generic behavior, and subsequent developments, consequences, and difficulties with the idea. Is that enough to cast doubt on the entire enterprise? Let’s lay it all out for you to see.

    Ever since we first recognized that there are galaxies beyond our own Milky Way, all the indications have shown us that our Universe is expanding.

    Universe map Sloan Digital Sky Survey (SDSS) 2dF Galaxy Redshift Survey

    Because the wavelength of light is what determines its energy and temperature, then the fabric of expanding space stretches those wavelengths to be longer, causing the Universe to cool. If the Universe is expanding and cooling as we head into the future, then that means it was closer together, denser, and hotter in the past. As we extrapolate farther and farther back, the hot, dense, uniform Universe tells us a story about its past.

    4
    The stars and galaxies we see today didn’t always exist, and the farther back we go, the closer to an apparent singularity the Universe gets, but there is a limit to that extrapolation. Image credit: NASA, ESA, and A. Feild (STScI).

    We arrive at a point where galaxy clusters, individual galaxies or even stars haven’t had time to form due to the influence of gravity. We can go even earlier, where the amount of energy in particles and radiation make it impossible for neutral atoms to form; they’d immediately be blasted apart. Even earlier, and atomic nuclei are blasted apart, preventing anything more complex than a proton or neutron from forming. Even earlier, and we begin creating matter/antimatter pairs spontaneously, due to the high energies present. And if you go all the way back, as far as your equations can take you, you’d arrive at a singularity, where all the matter and energy in the entire Universe were condensed into a single point: a singular event in spacetime. That was the original idea of the Big Bang.

    7
    http://www.speed-light.info/miracles_of_quran/singularity.htm

    5
    If these three different regions of space never had time to thermalize, share information or transmit signals to one another, then why are they all the same temperature? Image credit: E. Siegel.

    If that were the way things worked, there would be a number of puzzles based on the observations we had.

    1. Why would the Universe be the same temperature everywhere? The different regions of space from different directions wouldn’t have had time to exchange information and thermalize; there’s no reason for them to be the same temperature. Yet the Universe, everywhere we looked, had the same background 2.73 K temperature.
    2. Why would the Universe be perfectly spatially flat? The expansion rate and the energy density are two completely independent quantities, yet they must be equal to one part in 1024 in order to produce the flat Universe we have today.
    3. Why are there no leftover high-energy relics, as practically every high-energy theory predicts? There are no magnetic monopoles, no heavy, right-handed neutrinos, no relics from grand unification, etc. Why not?

    In 1979, Alan Guth had the idea that an early phase of exponential expansion preceding the hot Big Bang could solve all of these problems, and would make additional predictions about the Universe that we could go and look for. This was the big idea of cosmic inflation.

    6
    In 1979, Alan Guth had a revelation that a period of exponential expansion in the Universe’s past could set up and provide the initial conditions for the Big Bang. Image credit: Alan Guth’s 1979 notebook, tweeted via @SLAClab.

    This type of expansion, exponential expansion, is different from what happened for the majority of the Universe’s history. When your Universe is full of matter and radiation, the energy density drops as the Universe expands. As the volume expands, the density goes down, and so the expansion rate goes down, too. But during inflation, the Universe is filled with energy inherent to space itself, so as the Universe expands, it simply creates more space, and that keeps the density the same, and prevents the expansion rate from dropping. This, all at once, solves the three puzzles as follows:

    1.The Universe is the same temperature everywhere today because disparate, distant regions were once connected in the distant past, before the exponential expansion drove them apart.
    2.The Universe is flat because inflation stretched it to be indistinguishable from flat; the part of the Universe that’s observable to us is so small relative to how much inflation stretched it that it’s unlikely to be any other way.
    3. And the reason there are no high-energy relics is because inflation pushed them away via the exponential expansion, and then when inflation ended and the Universe got hot again, it never achieved the ultra-high temperatures necessary to create them again.

    By the early 1980s, not only did inflation solve those puzzles, but we also began coming up with models that successfully recovered a Universe that was isotropic (the same in all directions) and homogeneous (the same in all location), consistent with all our observations.

    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. Image credit: ESA and the Planck Collaboration.

    NASA/COBE

    NASA WMAP satellite

    ESA/Planck

    These predictions are interesting, but not enough, of course. For a physical theory to go from interesting to compelling to validated, it needs to make new predictions that can then be tested. It’s important not to gloss over the fact that these early models of inflation did exactly that, making six important predictions:

    1.The Universe should be perfectly flat. Yes, that was one of the original motivations for it, but at the time, we had very weak constraints. 100% of the Universe could be in matter and 0% in curvature; 5% could be matter and 95% could be curvature, or anywhere in between. Inflation, quite generically, predicted that 100% needed to be “matter plus whatever else,” but curvature should be 0%. This prediction has been validated by our ΛCDM model, where 5% is matter, 27% is dark matter and 68% is dark energy; curvature is still 0%.

    2.There should be an almost scale-invariant spectrum of fluctuations. If quantum physics is real, then the Universe should have experienced quantum fluctuations even during inflation. These fluctuations should be stretched, exponentially, across the Universe. When inflation ends, these fluctuations should get turned into matter and radiation, giving rise to overdense and underdense regions that grow into stars and galaxies, or great cosmic voids. Because of how inflation proceeds in the final stages, the fluctuations should be slightly greater on either small scales or large scales, depending on the model of inflation. For perfect scale invariance, a parameter we call n_s would equal 1 exactly; n_s is observed to be 0.96.

    3.There should be fluctuations on scales larger than light could have traveled since the Big Bang. This is another consequence of inflation, but there’s no way to get a coherent set of fluctuations on large scales like this without something stretching them across cosmic distances. The fact that we see these fluctuations in the cosmic microwave background and in the large-scale structure of the Universe — and didn’t know about them in the early 1980s — further validates inflation.

    4. These quantum fluctuations, which translate into density fluctuations, should be adiabatic. Fluctuations could have come in different types: adiabatic, isocurvature, or a mixture of the two. Inflation predicted that these fluctuations should have been 100% adiabatic, which should leave unique signatures in both the cosmic microwave background and the Universe’s large-scale structure. Observations bear out that yes, in fact, the fluctuations were adiabatic: of constant entropy everywhere.

    5.There should be an upper limit, smaller than the Planck scale, to the temperature of the Universe in the distant past. This is also a signature that shows up in the cosmic microwave background: how high a temperature the Universe reached at its hottest. Remember, if there were no inflation, the Universe should have gone up to arbitrarily high temperatures at early times, approaching a singularity. But with inflation, there’s a maximum temperature that must be at energies lower than the Planck scale (~1019 GeV). What we see, from our observations, is that the Universe achieved temperatures no higher than about 0.1% of that (~1016GeV) at any point, further confirming inflation.

    6. And finally, there should be a set of primordial gravitational waves, with a particular spectrum. Just as we had an almost perfectly scale-invariant spectrum of density fluctuations, inflation predicts a spectrum of tensor fluctuations in General Relativity, which translate into gravitational waves. The magnitude of these fluctuations are model-dependent on inflation, but the spectrum has a set of unique predictions. This sixth prediction is the only one that has not been verified observationally.
    See the full article here .

    7
    The final prediction of cosmic inflation is the existence of primordial gravitational waves. It is the only prediction to not be verified by observation… yet. Image credit: National Science Foundation (NASA, JPL, Keck Foundation, Moore Foundation, related) — Funded BICEP2 Program; modifications by E. Siegel.

    So inflation has a tremendous number of successes to its name. But since the late 1980s, theorists have spent a lot of time cooking up a variety of inflationary models. They’ve found some incredibly odd, non-generic behavior in some of them, including exceptions that break some of the predictive rules, above. In general, the simplest inflationary models are based on a potential: you draw a line with a trough or well at the bottom, the inflationary field starts off at some point away from that bottom, and it slowly rolls down towards the bottom, resulting in inflation until it settles at its minimum. Quantum effects play a role in the field, but eventually, inflation ends, converting that field energy into matter and radiation, resulting in the Big Bang.

    7
    The Universe we see today is based on the initial conditions it began with, which are dictated, predictively, by which model of cosmic inflation you choose. Image credit: Sloan Digital Sky Survey (SDSS).

    SDSS Telescope at Apache Point Observatory, NM, USA

    But you can make multi-field models, fast-roll models instead of slow-roll models, contrived models that have large departures from flatness, and so on. In other words, if you can make the models as complex as you want, you can find one that gives departures from the generic behavior described above, sometimes even resulting in departures from one or more of these six predictions.

    8
    The fluctuations in the CMB are based on primordial fluctuations produced by inflation. In particular, the ‘flat part’ on large scales (at left) have no explanation without inflation. Image credit: NASA / WMAP Science Team.

    This is what the current controversy is all about! One side goes so far as to claim that because you can contrive models that can give you almost arbitrary behavior, inflation fails to rise to the standard of a scientific theory. The other side claims that inflation makes these generic, successful predictions, and that the better we measure these parameters of the Universe, the more we constrain which models are viable, and the closer we come to understanding which one(s) best describe our physical reality.

    8
    The shape of gravitational wave fluctuations is indisputable from inflation, but the magnitude of the spectrum is entirely model-dependent. Measuring this will put the debate over inflation to rest, but if the magnitude is too low to be detected over the next 25 years or so, the argument may never be settled. Image credit: Planck science team.

    The facts that no one disputes are that without inflation, or something else that’s very much like inflation (stretching the Universe flat, preventing it from reaching high energies, creating the density fluctuations we see today, causing the Universe to begin at the same temperatures everywhere, etc.), there’s no explanation for the initial conditions the Universe starts off with. Alternatives to inflation have that hurdle to overcome, and right now there is no alternative that has displayed the same predictive power that the inflationary paradigm brings. That doesn’t mean that inflation is necessarily right, but there sure is a lot of good evidence for it, and many of the “possible” models that can be concocted have already been ruled out. Until an alternative model can achieve all of inflation’s successes, cosmic inflation will remain the leading idea for where our hot Big Bang came from.

    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 10:56 am on May 17, 2017 Permalink | Reply
    Tags: , , , Closest Supernova In Years Brings Cosmic Fireworks To Earth's Skies, , Ethan Siegel   

    From Ethan Siegel: “Closest Supernova In Years Brings Cosmic Fireworks To Earth’s Skies” 

    Ethan Siegel
    May 17, 2017

    1
    Patrick Wiggins
    The discoverer of this supernova, Patrick Wiggins, took these two images of the Fireworks galaxy on May 12th and May 14th, with the additional light in the more recent image showing the location of the night sky’s newest supernova.

    After burning bright for millions of years, the brightest stars of all are destined to explode in one final blaze of glory: a supernova.

    Supernova remnant Crab nebula. NASA/ESA Hubble

    Capable of shining as bright as many billions of stars put together, the light from a single outburst can be seen with the naked eye if it occurs in our own galaxy, and through a modest telescope from even tens of millions of light years away. On May 14th, a brand new supernova was discovered just 22 million light years away in a prolific object known as the Fireworks galaxy, making it the closest supernova to grace the skies in three years.

    2
    Fireworks galaxy. NASA

    With warm weather and summer approaching, this is the best chance you’ll likely have to see one for yourself all year.

    2
    This image of the center of NGC 6946, constructed from multiple Hubble instruments, filters and observations, showcases the dusty arms present around the galactic core. This galaxy is a hotbed of new star formation. ESA/Hubble and NASA / Judy Schmidt

    When a star is born, it’s destined to shine brightly under the power of its own internal, nuclear reaction for anywhere from millions to trillions of years. But the most massive stars have a uniquely spectacular fate in store as their destiny: after burning through all the fusible material in its central region, its core will collapse under its own gravity. With radiation and even the pressure of the atoms and atomic nuclei themselves unable to resist the intense forces at play, the core implodes, triggering a runaway fusion reaction. The result is a Type II supernova explosion, which occurs only once per century in a galaxy like the Milky Way.

    3
    Cas A Type II Supernova Remnant. NASA Chandra

    4
    Two images of NGC 6946: one from 2011 and a similar one from May 14, 2017, which shows the new and brightening supernova, SN 2017eaw. Gianluca Masi / Virtual Telescope Project / Tenagra Observatories, Ltd

    5
    Tenagra Observatories, Ltd, Rio Rico, Arizona, USA

    But in a nearby spiral galaxy that has only half the stars of our Milky Way, these cosmic fireworks occur ten times as frequently. In fact, the galaxy in question, NGC 6946, is nicknamed the Fireworks galaxy [above]for exactly this reason. Located just 22 million light years away, on the border of the constellations Cygnus and Cepheus, an amateur astronomer named Patrick Wiggins discovered a new point of light on May 14th where none had been seen previously, including just two days prior, on May 12th.

    6
    This side-by-side image shows a ground-based view of the relevant region of the Fireworks galaxy (L), along with that same region as imaged with Hubble data years ago (R). The progenitor star can clearly be identified, suggesting a massive, core-collapse supernova as the origin of this new light. Las Cumbres Observatory (L); ESA/Hubble and NASA (R)

    NASA/ESA Hubble Telescope

    Follow-up observations confirmed that this is, in fact, a Type II supernova, one that continues to brighten as the days pass by. This very region of the galaxy happened to be previously imaged by the Hubble Space Telescope, which confirms that there was a progenitor star there, faint but clearly visible, despite the incredible cosmic distance separating us. It’s a spectacular find, and one you can see with yourself through a backyard telescope, so long as you know where to look.

    7
    The brightest star in Cepheus, Alderamin, is relatively close to the Fireworks galaxy. By tracing an imaginary line back towards Vega, you can arrive at NGC 6946’s approximate location. E. Siegel / Stellarium

    8

    The bright stars Vega and Deneb make up two-thirds of the Summer Triangle, which begins rising in the northeast skies after sunset as summer approaches. Moving away from Deneb in the east and heading towards Polaris, the north star, you’ll encounter the brightest star in Cepheus: Alderamin. If you then trace an imaginary line back towards Vega, the unmistakably bright, blue star you started at back at the beginning, and move about the width of three fingers held at arm’s length, you’ll want to point your telescope there.

    9
    The galaxy NGC 6946, supernova SN 2017eaw (denoted with red markings) and the open star cluster NGC 6939. Although the galaxy and the cluster take up the same approximate area on the sky, the galaxy is 22 million light years away, while the star cluster is within our own galaxy at a mere 3860 light years distant. Gianluca Masi / virtualtelescope.eu

    10
    virtualtelescope.eu, Bellatrix Astronomical Observatory, Italy

    A bright star cluster, NGC 6939 (top right, above) and a faint galaxy, NGC 6946 (lower left, above) should appear in the same frame. If you can see the bright “star” located at about the 1 o’clock position of the Fireworks galaxy, that’s no star at all; that’s the latest firework! What you’re seeing is the tenth supernova discovered in the Fireworks galaxy since 1917, where the first one was discovered exactly 100 years ago. No other galaxy has had as many supernovae over this period, which might surprise you, considering that it’s less than a third the extent of the Milky Way.

    11
    This infrared observation of the Fireworks galaxy from NASA’s Spitzer space telescope showcases the warm gas that will become a part of the next generation of stars yet to form in NGC 6946. NASA / JPL-Caltech / SSC / R. Kennicutt et al.

    NASA/Spitzer Telescope

    Size isn’t everything, though! The key to a supernova factory is to have formed a large number of massive stars very recently. In almost every case, the place to look for that is inside an incredibly large star-forming region. When giant clouds of molecular gas collapse, triggered by a supernova, collision, or a major gravitational merger, large amounts of stars form. If the region is massive enough, a huge number of high-mass stars form as well. In the case of this galaxy, practically the entire thing has become a star-forming region.

    12
    The unmistakable pink color along the spiral arms traces out regions of ionized hydrogen, caused by the formation of hot, young stars in this galaxy, many of which will eventually go supernova. AURA/Gemini Observatory

    Gemini/North telescope at Mauna Kea, Hawaii, USA


    Gemini South telescope, Cerro Tololo Inter-American Observatory (CTIO) campus near La Serena, Chile

    Galaxies exhibiting a huge rate of star formation throughout are known as starburst galaxies, and are usually triggered by a major merger or galactic interaction. The pink regions seen throughout, above, are indicative of new and current star formation, and help explain why Type II supernovae are so common in this galaxy. With 10 supernovae in the past century, including four already in the new millennium, you’d expect to see a large number of X-ray sources as a result of these cosmic fireworks.

    13
    This X-ray image of the Fireworks galaxy reveals a barred spiral structure, extended regions of intense star formation, and a large number of X-ray point sources, indicative of supernovae that occurred millions of years in the past but whose light is just reaching us now. NASA/CXC/MSSL/R.Soria et al.

    NASA/Chandra Telescope

    Even from 22 million light years away, the Chandra X-ray observatory can see exactly that. The gas within the spiral arms is heated to such a degree that there’s a diffuse glow extending throughout the star-forming portion, but it’s the bright point sources that are most interesting. Representing both active black holes and recent supernova remnants, this data revealed three of the oldest supernovas ever detected in X-rays. When you combine the optical and X-ray images, you can see just what makes this galaxy so spectacular.

    14
    A composite of X-ray (Chandra, above) and optical (Gemini, above) data show the extent of the star forming regions of the Fireworks galaxy, NGC 6946. X-ray: NASA/CXC/MSSL/R.Soria et al, Optical: AURA/Gemini OBs

    Cosmic fireworks like these don’t truly happen at random; they are clustered in time and space around the most massive, intense star-forming regions of all. You can’t have a bigger star-forming region than one that includes the entire galaxy, and the sweeping, grand, irregular arms of the Fireworks galaxy are as good as they come. Based on what we see, we expect this elevated rate to continue for more than a million years. Keep an eye on this galaxy for the appearance of a “new star,” and if you find one, you just might discover the Universe’s newest supernova!

    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 12:25 pm on May 13, 2017 Permalink | Reply
    Tags: , , , Can The Universe Still End In A Big Crunch?, , Ethan Siegel   

    From Ethan Siegel: “Ask Ethan: Can The Universe Still End In A Big Crunch?” 

    Ethan Siegel
    May 13, 2017

    One of the biggest advances of the 20th century has been to identify exactly how rich, expansive, and massive our Universe actually is. With approximately two trillion galaxies contained in a volume some 46 billion light years in radius centered on us, our Observable Universe allows us to reconstruct the entire tale of our cosmic history, stretching all the way back to the Big Bang and even, perhaps, slightly before. But what about the future? What about the fate of the Universe? Is that a certainty? That’s what Andy Moss wants to know, as he asks:

    “You [wrote] that the Universe is expanding at a decreasing rate. I thought a Nobel Prize was awarded for the “discovery” that the Universe was expanding at an increasing rate. Can you please clarify the leading theories? Is the “Big Crunch” still a possibility?”

    The best predictor of future behavior is past behavior, it’s true. But just as people can sometimes surprise us, the Universe might, too.

    2
    After the Big Bang, the Universe was almost perfectly uniform, and full of matter, energy and radiation in a rapidly expanding state. The Universe’s evolution at all times is determined by the energy density of what’s inside it. NASA / WMAP science team

    The expansion rate of the Universe, at any moment in time, is only dependent on two things: the total energy density present within spacetime and the amount of spatial curvature present. If we understand the laws of gravitation and how the different types of energy evolve over time, we can reconstruct what the expansion rate should have been at any moment in the past. We can also look out at a variety of distant objects at various distances, and measure how that light has been stretched due to the expansion of space. Every galaxy, supernova, molecular gas cloud, etc. — everything that absorbs or emits light — will tell the cosmic history of how the expansion of space has stretched it from the moment it was emitted until we observe it.

    3
    The farther a galaxy is, the faster it expands away from us, and the more its light gets redshifted, necessitating that we look at longer and longer wavelengths. Larry McNish of RASC Calgary Center

    We’ve been able to conclude, from a variety of independent lines of observation, exactly what the Universe is made out of. The three big, independent lines of observation are:

    The temperature fluctuations present in the cosmic microwave background, which encode information about the Universe’s curvature, normal matter, dark matter, neutrino, and total density contents.
    The correlations between galaxies on the largest scales — known as baryon acoustic oscillations — which give very strict measurements on the total matter density, the normal matter to dark matter ratio, and the expansion rate throughout time.
    And the most distant, luminous standard candles in the Universe, type Ia supernova, which tell us about the expansion rate and dark energy as it evolved over time.

    4
    Standard candles (L) and standard rulers (R) are two different techniques astronomers use to measure the expansion of space at various times/distances in the past. NASA/JPL-Caltech

    These lines of evidence, combined, all point to one consistent picture of the Universe. They tell us what’s in the Universe today, and give us a cosmology where:

    4.9% of the Universe’s energy is in normal matter (like protons, neutrons and electrons),
    0.1% of the Universe’s energy is in the form of massive neutrinos (which act like matter at late times and radiation at early times),
    0.01% of the Universe’s energy is in the form of radiation (like photons),
    27% of the Universe’s energy is in the form of dark matter, and
    68% is in the form of energy inherent to space itself: dark energy.

    They give us a flat Universe (with 0% curvature), a Universe with no topological defects (magnetic monopoles, cosmic strings, domain walls, or cosmic textures), and a Universe whose past expansion history is known.

    5
    The relative importance of different energy components in the Universe at various times in the past. In the future, dark energy will approach 100% importance. E. Siegel

    The equations governing General Relativity are very deterministic in this sense: if we know what the Universe is made of today and the laws of gravity, we know exactly how important each component was at every juncture in the past. Early on, radiation and neutrinos dominated. For billions of years, dark matter and normal matter were the most important pieces. And for the past few billion years — and this will get more severe as time goes on — dark energy is the dominant factor in the Universe’s expansion. It’s causing the Universe to accelerate, and this is where the confusion (for most people) begins.

    6
    Possible fates of the expanding Universe. Notice the differences of different models in the past. The Cosmic Perspective / Jeffrey O. Bennett, Megan O. Donahue, Nicholas Schneider and Mark Voit.

    There are two things we can measure when it comes to the Universe’s expansion: the expansion rate and the speed at which an individual galaxy appears to recede from our perspective. These are related, but they are not the same. The expansion rate, on one hand, talks about how the fabric of space itself stretches over time. It’s always quantified as a speed-per-unit-distance, which is typically given in kilometers-per-second (the speed) per Megaparsec (the distance), where a Megaparsec is about 3.26 million light years.

    7
    How matter (top), radiation (middle), and a cosmological constant (bottom) all evolve with time in an expanding Universe. E. Siegel / Beyond the Galaxy

    If there were no dark energy, the expansion rate would drop over time, approaching zero, since the matter-and-radiation density would drop to zero as the volume expands. But with dark energy, that expansion rate approaches whatever energy density dark energy has. If dark energy, for example, is a cosmological constant, then the expansion rate asymptotes to a constant value. But if that’s what the expansion rate does, then individual galaxies receding from us will see their speeds accelerate.

    8
    Optical image of the distant galaxy Markarian 1018, with an overlay of VLT (radio) data. ESO/CARS Survey

    ESO/VLT at Cerro Paranal, with an elevation of 2,635 metres (8,645 ft) above sea level

    Imagine the expansion rate is some value: 50 km/s/Mpc. If a galaxy is 20 Mpc away, then it appears to recede from us at 1,000 km/s. But give it time; as the fabric of space expands, this galaxy will eventually be farther from us. By time it’s twice as distant, 40 Mpc away from us, it will appear to recede at 2,000 km/s. Over even more time, it will be ten times as far as it began: 200 Mpc, where it now recedes at 10,000 km/s. By time it gets to a distance of 6,000 Mpc from us, it will appear to recede at 300,000 km/s, which is faster than the speed of light. But this goes on and on; the more time passes, the faster the galaxy appears to move away from us. This is what’s “accelerating” about the Universe: the expansion rate goes down, but the speed an individual galaxy moves away from us just rises and rises over time.

    9
    The full UV-visible-IR composite of the Hubble eXtreme Deep Field; the greatest image ever released of the distant Universe. NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI)

    NASA/ESA Hubble Telescope

    All of this is consistent with our best measurements: that dark energy represents a constant energy density inherent to space itself. As space stretches, the dark energy density remains constant, and the Universe will end in this “Big Freeze” fate, where everything that isn’t gravitationally bound together (like our local group, galaxy, solar system, etc.) winds up being pushed apart from one another. If dark energy is truly a cosmological constant, then the expansion will continue indefinitely, giving rise to a cold, empty Universe.

    10
    When astronomers first realized the universe was accelerating, the conventional wisdom was that it would expand forever. However, until we better understand the nature of dark energy other scenarios for the fate of the universe are possible. This diagram outlines these possible fates. NASA/ESA and A. Riess (STScI)

    But if dark energy is dynamic – something theoretically possible but observationally without support – it could yet end in a Big Crunch or a Big Rip. In a Big Crunch, dark energy would weaken and reverse sign, causing the Universe to reach a maximum size, turn around, and contract. It could even give rise to a cyclical Universe, where the “crunch” gives rise to another Big Bang. If dark energy continues to strengthen, however, the opposite fate occurs, where bound structures eventually get torn apart by the increasing expansion rate. The evidence we have today, however, overwhelmingly supports a “Big Freeze,” the condition of expansion continuing at a constant rate forever.

    The major science goals of upcoming observatories like the ESA’s Euclid, NASA’s WFIRST, and the ground-based LSST include measuring whether dark energy is truly a cosmological constant or not.

    ESA/Euclid spacecraft

    NASA/WFIRST


    LSST Camera, built at SLAC



    LSST telescope, currently under construction at Cerro Pachón Chile, a 2,682-meter-high mountain in Coquimbo Region, in northern Chile, alongside the existing Gemini South and Southern Astrophysical Research Telescopes.

    Although the leading theoretical idea is, in fact, in favor of constant dark energy, it’s important to entertain all the possibilities not ruled out by our measurements and observations. As far fetched as it may seem, a Big Crunch still isn’t ruled out. With more and better data, we may yet find a compelling hint that reality is even stranger than most of us have imagined!

    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 10:04 am on May 13, 2017 Permalink | Reply
    Tags: , , , , Ethan Siegel, When will the first star go dark?   

    From Ethan Siegel: “When will the first star go dark?” 

    Ethan Siegel
    May 12, 2017

    It hasn’t happened yet in the entire Universe, not even once.

    1
    This is the Milky Way from Concordia Camp, in Pakistan’s Karakoram Range. While many of the stars seen here may have already died, their stellar remnants continue to shine on. Image credit: Anne Dirkse / http://www.annedirkse.com.

    “End? No, the journey doesn’t end here. Death is just another path, one that we all must take. The grey rain-curtain of this world rolls back, and all turns to silver glass, and then you see it.” -J.R.R. Tolkien

    Ever since the first star in the Universe ignited some 13.7 billion years ago, the Universe has been flooded with light. When enough matter — mostly hydrogen and helium gas — gravitates together into a single, compact object, nuclear fusion will take place inside the core, giving rise to a true star. But as time goes on and fusion continues, eventually that star will run out of fuel. Sometimes, the star is massive enough that additional fusion reactions will take place, but at some point, it all must stop. When those stars finally die, however, their remnants shine on. In fact, the Universe hasn’t been around long enough for even a single remnant to stop shining. Here’s the story of how long we’ll need to wait for the first star to go dark.

    It all begins from a cloud of gas. When a cloud of molecular gas collapses under its own gravity, there are always a few regions that start off just a little bit denser than others. Every location with matter in it does its best to attract more and more matter towards it, but these overdense regions attract matter more efficiently than all the others. Because gravitational collapse is a runaway process, the more matter you attract to your vicinity, the faster additional matter accelerates to join you.

    2
    Dark, dusty molecular clouds, like this one within our Milky Way, will collapse over time and give rise to new stars, with the densest regions within forming the most massive stars. Image credit: ESO.

    While it can take millions to tens of millions of years for a molecular cloud to go from a large, diffuse state to a relatively collapsed one, the process of going from a collapsed state of dense gas to a new cluster of stars — where the densest regions ignite fusion in their cores — takes only a few hundred thousand years.

    Stars come in a huge variety of colors, brightnesses and masses, all of which are predestined from the moment of the star’s birth. When you create a new cluster of stars, the easiest ones to notice are the brightest ones, which also happen to be the most massive. These are the brightest, bluest, hottest stars in existence, with up to hundreds of times the mass of our Sun and with millions of times the luminosity. But despite the fact that these are the stars that appear the most spectacular, these are also the rarest stars, making up far less than 1% of all the known, total stars, and also the shortest-lived stars, as they burn through all the nuclear fuel (in all the various stages) in their cores in as little as 1–2 million years.

    3
    Hubble space telescope of the merging star clusters at the heart of the Tarantula Nebula, the largest star-forming region known in the local group. The hottest, bluest stars are over 200 times the mass of our Sun. Image credit: NASA, ESA, and E. Sabbi (ESA/STScI); Acknowledgment: R. O’Connell (University of Virginia) and the Wide Field Camera 3 Science Oversight Committee.

    NASA/ESA Hubble Telescope

    Local Group. Andrew Z. Colvin 3 March 2011

    When these brightest stars run out of fuel, they die in a spectacular type II supernova explosion.

    11
    Type II supernova http://cse.ssl.berkeley.edu/bmendez/ay10/2000/cycle/snII.html

    As this occurs, the inner core implodes, collapsing all the way down to a neutron star (for the low-mass cores) or even to a black hole (for the high-mass cores), while expelling the outer layers back into the interstellar medium.

    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

    There, these enriched gases will contribute to future generations of stars, providing them with the heavy elements necessary to create rocky planets, organic molecules, and in rare, wonderful cases, life.

    4
    Crab Nebula. When the most massive stars die, their outer layers, enriched with heavy elements from the result of nuclear fusion and neutron capture, are blown off into the interstellar medium, where they can help future generations of stars by providing them with the raw ingredients for rocky planets and, potentially, life. Image credit: NASA, ESA, J. Hester, A. Loll (ASU).

    You don’t have to wait long for a black hole to go dark. In fact, by definition, black holes go “black” immediately. Once the core collapses sufficiently to form an event horizon, everything inside collapses down to a singularity in a fraction of a second. Any remnant heat, light, temperature, or energy in any form in the core simply gets converted into the mass of the singularity. No light will ever emanate from it again, except in the form of Hawking radiation, when the black hole decays, and in the accretion disk surrounding the black hole, which is constantly fed and refueled from the surrounding matter.

    But neutron stars are a different story.

    5
    Forming from the remnant of a massive star that’s gone supernova, a neutron star is the collapsed core that remains behind. Image credit: NASA.

    You see, a neutron star takes all the energy in a star’s core and collapses incredibly rapidly. When you take anything and compress it quickly, you cause the temperature within it to rise: this is how a piston works in a diesel engine. Well, collapsing from a stellar core all the way down to a neutron star is maybe the ultimate example of rapid compression. In the span of seconds-to-minutes, a core of iron, nickel, cobalt, silicon and sulfur many hundreds-of-thousands of miles (kilometers) in diameter has collapsed down to a ball just around 10 miles (16 km) in size or smaller. Its density has increased by around a factor of a quadrillion (10¹⁵), and its temperature has grown tremendously: to some 10¹² K in the core and all the way up to around 10⁶ K at the surface. And herein lies the problem.

    6
    A neutron star is very small and low in overall luminosity, but it’s very hot, and takes a long time to cool down. If your eyes were good enough, you’d see it shine for millions of times the present age of the Universe. Image credit: ESO/L. Calçada.

    You have all this energy stored within a collapsed star like this, and its surface is so tremendously hot that it not only glows bluish-white in the visible portion of the spectrum, but most of the energy isn’t visible or even ultraviolet: it’s X-ray energy! There is an insanely large amount of energy stored within this object, but the only way it can release it out into the Universe is through its surface, and its surface area is very small. The big question, of course, is how long will it take a neutron star to cool?

    The answer depends on a piece of physics that practically isn’t well-understood for neutron stars: neutrino cooling! You see, while photons (radiation) are soundly trapped by the normal, baryonic matter, neutrinos, when generated, can pass right through the entire neutron star unimpeded. On the fast end, neutron stars might cool down, out of the visible portion of the spectrum, after as little as 10¹⁶ years, or “only” a million times the age of the Universe. But if things are slower, it might take 10²⁰-to-10²² years, which means you’ll be waiting for some time.

    7
    When lower-mass, Sun-like stars run out of fuel, they blow off their outer layers in a planetary nebula, but the center contracts down to form a white dwarf, which takes a very long time to fade to darkness. Image credit: NASA/ESA and The Hubble Heritage Team (AURA/STScI).

    But other stars will go dark much more quickly. You see, the vast majority of stars — the other 99+% — don’t go supernova, but rather, at the end of their lives, contract (slowly) down into a white dwarf star. The “slow” timescale is only slow compared to a supernova: it takes tens-to-hundreds of thousands of years rather than mere seconds-to-minutes, but that’s still fast enough to trap almost all the heat from the star’s core inside. The big difference is that instead of trapping it inside of a sphere with a diameter of only 10 miles or so, the heat is trapped in an object “only” about the size of Earth, or around a thousand times larger than a neutron star. This means that while the temperatures of these white dwarfs can be very high — over 20,000 K, or more than three times hotter than our Sun — they cool down much faster than neutron stars.

    8
    An accurate size/color comparison of a white dwarf (L), Earth reflecting our Sun’s light (middle), and a black dwarf (R). Image credit: BBC / GCSE (L) / SunflowerCosmos (R).

    Neutrino escape is negligible in white dwarfs, meaning that radiation through the surface is the only effect that matters.

    12
    White dwarf star. http://sciencewala.blogspot.com/2015/07/white-dwarf.html

    When we calculate how quickly heat can escape by radiating away, it leads to a cooling timescale for a white dwarf (like the kind the Sun will produce) of around 10¹⁴-to-10¹⁵ years. And that’s to get all the way down to just a few degrees above absolute zero! This means that after around 10 trillion years, or “only” around 1,000 times the present age of the Universe, the surface of a white dwarf will have dropped in temperature so that it’s out of the visible light regime. When this much time has passed, the Universe will possess a brand new type of object: a black dwarf star.

    9
    The Universe is not yet old enough for a stellar remnant to have cooled enough to become invisible to human eyes, much less to cool all the way to just a few degrees above absolute zero. Image credit: NASA / JPL-Caltech.

    I’m sorry to disappoint you, but there aren’t any black dwarfs around today. The Universe is simply far too young for it. In fact, the coolest white dwarfs have, to the best of our estimates, lost less than 0.2% of their total heat since the very first ones were created in this Universe.

    For a white dwarf created at 20,000 K, that means its temperature is still at least 19,960 K, telling us we’ve got a terribly long way to go, if we’re waiting for a true dark star.

    We currently conceive of our Universe as littered with stars, which cluster together into galaxies, which are separated by vast distances. But by time the first black dwarf comes to be, our local group will have merged into a single galaxy (Milkdromeda), most of the stars that will ever live will have long since burned out, with the surviving ones being exclusively the lowest-mass, reddest and dimmest stars of all.

    13
    Milkdromeda. https://futurism.com/galactic-collisions-milky-way-has-4-billion-years-of-life-left/

    And beyond that? Only darkness, as dark energy will have long since pushed away all the other galaxies, making them unreachable and practically unmeasureable by any physical means.

    10
    It will take hundreds of trillions of years for the first stellar remnant to cool completely, fading from a white dwarf through red, infrared and all the way down to a true black dwarf. By that point, the Universe will hardly be forming any new stars at all, and space will be mostly black. Image credit: user Toma/Space Engine; E. Siegel.

    And yet, amidst it all, a new type of object will come to be for the very first time. Even though we’ll never see or experience one, we know enough of nature to know not only that they’ll exist, but how and when they’ll come to be. And that, in itself, is one of the most amazing parts of science of all!

    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 4:42 pm on May 12, 2017 Permalink | Reply
    Tags: , , , , , , Ethan Siegel,   

    From Ethan Siegel: “What If Cosmic Inflation Is Wrong?” 

    Ethan Siegel
    May 11, 2017

    1
    The earliest stages of the Universe, before the Big Bang, are what set up the initial conditions that everything we see today has evolved from. E. Siegel, with images derived from ESA/Planck and the DoE/NASA/ NSF interagency task force on CMB research.

    All scientific ideas, no matter how accepted or widespread they are, are susceptible to being overturned. For all the successes any idea may have, it only takes one experiment or observation to falsify it, invalidate it, or necessitate that it be revised. Beyond that, every scientific idea or model has a limitation to its range of validity: Newtonian mechanics breaks down close to the speed of light; General Relativity breaks down at singularities; evolution breaks down when you reach the origin of life. Even the Big Bang has its limitations, as there’s only so far back we can extrapolate the hot, dense, expanding state that gave rise to what we see today. Since 1980, the leading idea for describing what came before it has been cosmic inflation, for many compelling reasons. But recently, a spate of public statements has shown a deeper controversy:

    In February, a group of theorists, including one of inflation’s co-founders, claimed that inflation had failed.
    The mainstream group of inflationary cosmologists, including inflation’s inventor, Alan Guth, wrote a rebuttal.
    This prompted the original group to dig in further, denouncing the rebuttal.
    And earlier this week, a major publication and one of the rebuttal’s co-signers highlighted and gave their perspective on the debate.

    2
    The expanding Universe, full of galaxies and complex structure we see today, arose from a smaller, hotter, denser, more uniform state. C. Faucher-Giguère, A. Lidz, and L. Hernquist, Science 319, 5859 (47)

    There are three things going on here: the problems with the Big Bang that led to the development of cosmic inflation, the solution(s) that cosmic inflation provides and generic behavior, and subsequent developments, consequences, and difficulties with the idea. Is that enough to cast doubt on the entire enterprise? Let’s lay it all out for you to see.

    Ever since we first recognized that there are galaxies beyond our own Milky Way, all the indications have shown us that our Universe is expanding. Because the wavelength of light is what determines its energy and temperature, then the fabric of expanding space stretches those wavelengths to be longer, causing the Universe to cool. If the Universe is expanding and cooling as we head into the future, then that means it was closer together, denser, and hotter in the past. As we extrapolate farther and farther back, the hot, dense, uniform Universe tells us a story about its past.

    3
    The stars and galaxies we see today didn’t always exist, and the farther back we go, the closer to an apparent singularity the Universe gets, but there is a limit to that extrapolation. NASA, ESA, and A. Feild (STScI)

    We arrive at a point where galaxy clusters, individual galaxies or even stars haven’t had time to form due to the influence of gravity. We can go even earlier, where the amount of energy in particles and radiation make it impossible for neutral atoms to form; they’d immediately be blasted apart. Even earlier, and atomic nuclei are blasted apart, preventing anything more complex than a proton or neutron from forming. Even earlier, and we begin creating matter/antimatter pairs spontaneously, due to the high energies present. And if you go all the way back, as far as your equations can take you, you’d arrive at a singularity, where all the matter and energy in the entire Universe were condensed into a single point: a singular event in spacetime. That was the original idea of the Big Bang.

    4
    If these three different regions of space never had time to thermalize, share information or transmit signals to one another, then why are they all the same temperature? E. Siegel

    If that were the way things worked, there would be a number of puzzles based on the observations we had.

    Why would the Universe be the same temperature everywhere? The different regions of space from different directions wouldn’t have had time to exchange information and thermalize; there’s no reason for them to be the same temperature. Yet the Universe, everywhere we looked, had the same background 2.73 K temperature.
    Why would the Universe be perfectly spatially flat? The expansion rate and the energy density are two completely independent quantities, yet they must be equal to one part in 1024 in order to produce the flat Universe we have today.
    Why are there no leftover high-energy relics, as practically every high-energy theory predicts? There are no magnetic monopoles, no heavy, right-handed neutrinos, no relics from grand unification, etc. Why not?

    In 1979, Alan Guth had the idea that an early phase of exponential expansion preceding the hot Big Bang could solve all of these problems, and would make additional predictions about the Universe that we could go and look for. This was the big idea of cosmic inflation.

    6
    Alan Guth

    7
    In 1979, Alan Guth had a revelation that a period of exponential expansion in the Universe’s past could set up and provide the initial conditions for the Big Bang. Alan Guth’s 1979 notebook, tweeted via @SLAClab

    This type of expansion, exponential expansion, is different from what happened for the majority of the Universe’s history. When your Universe is full of matter and radiation, the energy density drops as the Universe expands. As the volume expands, the density goes down, and so the expansion rate goes down, too. But during inflation, the Universe is filled with energy inherent to space itself, so as the Universe expands, it simply creates more space, and that keeps the density the same, and prevents the expansion rate from dropping. This, all at once, solves the three puzzles as follows:

    The Universe is the same temperature everywhere today because disparate, distant regions were once connected in the distant past, before the exponential expansion drove them apart.
    The Universe is flat because inflation stretched it to be indistinguishable from flat; the part of the Universe that’s observable to us is so small relative to how much inflation stretched it that it’s unlikely to be any other way.
    And the reason there are no high-energy relics is because inflation pushed them away via the exponential expansion, and then when inflation ended and the Universe got hot again, it never achieved the ultra-high temperatures necessary to create them again.

    By the early 1980s, not only did inflation solve those puzzles, but we also began coming up with models that successfully recovered a Universe that was isotropic (the same in all directions) and homogeneous (the same in all location), consistent with all our observations.

    8
    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. ESA and the Planck Collaboration

    NASA WMAP satellite

    ESA/Planck

    These predictions are interesting, but not enough, of course. For a physical theory to go from interesting to compelling to validated, it needs to make new predictions that can then be tested. It’s important not to gloss over the fact that these early models of inflation did exactly that, making six important predictions:

    The Universe should be perfectly flat. Yes, that was one of the original motivations for it, but at the time, we had very weak constraints. 100% of the Universe could be in matter and 0% in curvature; 5% could be matter and 95% could be curvature, or anywhere in between. Inflation, quite generically, predicted that 100% needed to be “matter plus whatever else,” but curvature should be 0%. This prediction has been validated by our ΛCDM model, where 5% is matter, 27% is dark matter and 68% is dark energy; curvature is still 0%.

    9
    Lambda-Cold Dark Matter, Accelerated Expansion of the Universe, Big Bang-Inflation (timeline of the universe)
    Date 2010
    Author User:Coldcreation

    There should be an almost scale-invariant spectrum of fluctuations. If quantum physics is real, then the Universe should have experienced quantum fluctuations even during inflation. These fluctuations should be stretched, exponentially, across the Universe. When inflation ends, these fluctuations should get turned into matter and radiation, giving rise to overdense and underdense regions that grow into stars and galaxies, or great cosmic voids. Because of how inflation proceeds in the final stages, the fluctuations should be slightly greater on either small scales or large scales, depending on the model of inflation. For perfect scale invariance, a parameter we call n_s would equal 1 exactly; n_s is observed to be 0.96.

    There should be fluctuations on scales larger than light could have traveled since the Big Bang. This is another consequence of inflation, but there’s no way to get a coherent set of fluctuations on large scales like this without something stretching them across cosmic distances. The fact that we see these fluctuations in the cosmic microwave background and in the large-scale structure of the Universe — and didn’t know about them in the early 1980s — further validates inflation.

    These quantum fluctuations, which translate into density fluctuations, should be adiabatic. Fluctuations could have come in different types: adiabatic, isocurvature, or a mixture of the two. Inflation predicted that these fluctuations should have been 100% adiabatic, which should leave unique signatures in both the cosmic microwave background and the Universe’s large-scale structure. Observations bear out that yes, in fact, the fluctuations were adiabatic: of constant entropy everywhere.

    There should be an upper limit, smaller than the Planck scale, to the temperature of the Universe in the distant past. This is also a signature that shows up in the cosmic microwave background: how high a temperature the Universe reached at its hottest. Remember, if there were no inflation, the Universe should have gone up to arbitrarily high temperatures at early times, approaching a singularity. But with inflation, there’s a maximum temperature that must be at energies lower than the Planck scale (~1019 GeV). What we see, from our observations, is that the Universe achieved temperatures no higher than about 0.1% of that (~1016 GeV) at any point, further confirming inflation.

    And finally, there should be a set of primordial gravitational waves, with a particular spectrum. Just as we had an almost perfectly scale-invariant spectrum of density fluctuations, inflation predicts a spectrum of tensor fluctuations in General Relativity, which translate into gravitational waves. The magnitude of these fluctuations are model-dependent on inflation, but the spectrum has a set of unique predictions. This sixth prediction is the only one that has not been verified observationally.

    9
    The final prediction of cosmic inflation is the existence of primordial gravitational waves. It is the only prediction to not be verified by observation… yet. National Science Foundation (NASA, JPL, Keck Foundation, Moore Foundation, related) — Funded BICEP2 Program; modifications by E. Siegel

    So inflation has a tremendous number of successes to its name. But since the late 1980s, theorists have spent a lot of time cooking up a variety of inflationary models. They’ve found some incredibly odd, non-generic behavior in some of them, including exceptions that break some of the predictive rules, above. In general, the simplest inflationary models are based on a potential: you draw a line with a trough or well at the bottom, the inflationary field starts off at some point away from that bottom, and it slowly rolls down towards the bottom, resulting in inflation until it settles at its minimum. Quantum effects play a role in the field, but eventually, inflation ends, converting that field energy into matter and radiation, resulting in the Big Bang.

    10
    The Universe we see today is based on the initial conditions it began with, which are dictated, predictively, by which model of cosmic inflation you choose. Sloan Digital Sky Survey (SDSS)

    SDSS Telescope at Apache Point Observatory, NM, USA

    But you can make multi-field models, fast-roll models instead of slow-roll models, contrived models that have large departures from flatness, and so on. In other words, if you can make the models as complex as you want, you can find one that gives departures from the generic behavior described above, sometimes even resulting in departures from one or more of these six predictions.

    11
    The fluctuations in the CMB are based on primordial fluctuations produced by inflation. In particular, the ‘flat part’ on large scales (at left) have no explanation without inflation. NASA / WMAP Science Team.

    This is what the current controversy is all about! One side goes so far as to claim that because you can contrive models that can give you almost arbitrary behavior, inflation fails to rise to the standard of a scientific theory. The other side claims that inflation makes these generic, successful predictions, and that the better we measure these parameters of the Universe, the more we constrain which models are viable, and the closer we come to understanding which one(s) best describe our physical reality.

    12
    The shape of gravitational wave fluctuations is indisputable from inflation, but the magnitude of the spectrum is entirely model-dependent. Measuring this will put the debate over inflation to rest, but if the magnitude is too low to be detected over the next 25 years or so, the argument may never be settled. Planck science team.

    The facts that no one disputes are that without inflation, or something else that’s very much like inflation (stretching the Universe flat, preventing it from reaching high energies, creating the density fluctuations we see today, causing the Universe to begin at the same temperatures everywhere, etc.), there’s no explanation for the initial conditions the Universe starts off with. Alternatives to inflation have that hurdle to overcome, and right now there is no alternative that has displayed the same predictive power that the inflationary paradigm brings. That doesn’t mean that inflation is necessarily right, but there sure is a lot of good evidence for it, and many of the “possible” models that can be concocted have already been ruled out. Until an alternative model can achieve all of inflation’s successes, cosmic inflation will remain the leading idea for where our hot Big Bang came from.

    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 10:10 am on May 8, 2017 Permalink | Reply
    Tags: , Ethan Siegel, , Hubble Views The Final Frontier For Dark Matter,   

    From Ethan Siegel: “Hubble Views The Final Frontier For Dark Matter” 

    Ethan Siegel
    May 8, 2017

    1
    The streaks and arcs present in Abell 370, a distant galaxy cluster some 5-6 billion light years away, are some of the strongest evidence for gravitational lensing and dark matter that we have. NASA, ESA/Hubble, HST Frontier Fields

    When you look out into the distant Universe, in most locations, you’ll find a field of faint, distant galaxies: beautiful, but nothing special.

    2
    The ‘parallel field’ of Abell 370 showcases a deep view of a region of space with no particularly massive or significant structure inside. This is what most of the Universe looks like, when imaged deeply enough. NASA, ESA/Hubble, HST Frontier Fields

    Six billion light years away, Abell 370 is one of the most massive, dense ones discovered so far, but one galaxy, noticed early on, provided a hint of something more.

    4
    The distorted galaxy shown here is actually two images of a single galaxy located twice as far away as the rest of the galaxy; it is the effects of gravitational lensing that cause the odd appearance and multiple images. NASA, ESA/Hubble, HST Frontier Fields

    The “stretched-out” galaxy you see here isn’t a distorted cluster member, but is instead two images of a single galaxy, twice as far away as the cluster itself.

    5
    An illustration of gravitational lensing showcases how background galaxies — or any light path — is distorted by the presence of an intervening mass, such as a foreground galaxy cluster. NASA/ESA

    This phenomenon of gravitational lensing stretches galaxies into streaks and arcs, magnifying them, and creating multiple images.

    6
    The streaks of galaxies shown here are not representative of the actual shapes of the galaxies themselves, but rather the galaxies subject to the effects of the gravitational lens they pass through. Undistorted galaxies, like the one at the top left, are most likely in the foreground of the lens. NASA, ESA/Hubble, HST Frontier Fields

    It also enables us to reconstruct the mass distribution of the cluster, revealing that it’s mostly due to dark matter.

    7
    The mass distribution of cluster Abell 370. reconstructed through gravitational lensing, shows two large, diffuse halos of mass, consistent with dark matter with two merging clusters to create what we see here. NASA, ESA, D. Harvey (École Polytechnique Fédérale de Lausanne, Switzerland), R. Massey (Durham University, UK), the Hubble SM4 ERO Team and ST-ECF

    There are two separate clumps present, showing that this is likely two clusters merging together.

    8
    Despite the presence of large, elliptical galaxies, the location where the mass density is greatest, indicated by the dotted circle, corresponds to no known massive galaxy or other structure based in normal matter. The only explanation for this is the presence of an invisible source of mass: dark matter. NASA, ESA/Hubble, HST Frontier Fields / E. Siegel (annotation)

    Most importantly, dark matter must be present — and present outside of the individual galaxies themselves — to explain these gravitational effects.

    9
    A 2009 image, based on only a fraction of the Hubble data available today, revealed some of the incredible structure in Abell 370. The current data, benefitting from 8 extra years, showcases even more information about the distant, massive Universe. NASA/ESA Hubble

    Additional observations from 2009-2017 reveal unprecedented details about the massive, distant 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

     
  • richardmitnick 1:59 pm on May 6, 2017 Permalink | Reply
    Tags: , , , , Ethan Siegel, How Far Is The Edge Of The Universe From The Farthest Galaxy?   

    From Ethan Siegel: “How Far Is The Edge Of The Universe From The Farthest Galaxy?” 

    Ethan Siegel
    May 6, 2017

    1
    Our deepest galaxy surveys can reveal objects tens of billions of light years away, but even with ideal technology, there will be a large distance gap between the farthest galaxy and the Big Bang. Sloan Digital Sky Survey (SDSS)

    SDSS Telescope at Apache Point Observatory, NM, USA

    When we look out into the Universe, there’s light everywhere we can see, for as far as our telescopes are capable of looking. But at some point, there’s a limit to what we’ll encounter. One limit is set by the cosmic structure that forms in the Universe: we can only see the stars, galaxies, etc., as long as they emit light. Without that ingredient, our telescopes can’t detect anything. But another limit, if we can use astronomy to go beyond starlight, is the limit of how much of the Universe is accessible to us since the Big Bang. These two values might not have much to do with one another, and that’s what Oleg Pestovsky wants to know!

    Why is the redshift of CMB … around 1,000, while the highest redshift for any galaxy we have observed is 11?

    The first thing we need to think about is exactly what happens in our Universe, moving forward, from the moment of the Big Bang.

    2
    The observable Universe might be 46 billion light years in all directions from our point of view, but there’s certainly more, unobservable Universe, perhaps even an infinite amount, just like ours beyond that. Frédéric MICHEL and Andrew Z. Colvin, annotated by E. Siegel

    The full suite of all we know, see, observe and interact with is what we’ll call the “Observable Universe.” Beyond what we can see, there’s very likely more Universe out there, and as time goes on, we’ll be able to see more and more of it, as light from more distant objects finally reaches us after a cosmic journey taking billions of years. Seeing what we do in the Universe (and not more, and not less) is possible because of a combination of three things:

    The fact that it’s been a finite amount of time, 13.8 billion years, since the Big Bang,
    The fact that the speed of light, the maximum speed that any signal or particle can travel in the Universe, is finite and constant,
    And the fact that the fabric of space itself has been stretching and expanding ever since the Big Bang occurred.

    3
    The timeline of our observable Universe’s history. NASA / WMAP science team

    What we see today is the result of those three conditions, combined with the initial distribution of matter and energy, operating under the laws of physics for the entire history of our Universe. If we want to know what the Universe was like at any earlier time, all we need to do is observe what the Universe is like today, measure all the relevant parameters, and calculate what it was like in the past. There’s a lot we have to observe and measure to get there, but Einstein’s equations, difficult though they are, are at least straightforward. (The derived results are two equations known as the Friedmann equations, and solving them is a task every graduate student in cosmology becomes intimately familiar with.) And, quite honestly, we’ve made some incredible measurements about the Universe.

    4
    Looking towards the north pole of the Milky Way galaxy, we can see out into the depths of space. What’s mapped in this image are hundreds of thousands of galaxies, where each pixel in the image is a unique galaxy. SDSS III, data release 8

    We know how fast it’s expanding today. We know what the matter density is everywhere we look. We know how much structure forms on all different scales, from globular clusters to dwarf galaxies to larger galaxies to groups and clusters and large-scale filaments. We know how much of the Universe is normal matter, dark matter, dark energy, as well as much smaller components like neutrinos, radiation and even black holes. And just from that information, extrapolating backwards in time, we can decipher both how big the Universe was and how fast it was expanding at any point in its cosmic history.

    5
    A graph of the size/scale of the observable Universe vs. the passage of cosmic time. This is displayed on a log-log scale, with a few major size/time milestones identified. E. Siegel

    Today, our observable Universe extends for approximately 46.1 billion light years in all directions from where we are. That’s the distance that if, at the instant of the Big Bang, the original location-in-space of an imaginary particle traveling at the speed of light would be at today if it were to reach us right now, 13.8 billion years later. In principle, that’s where any gravitational waves left over from cosmic inflation — the state prior to the Big Bang that set it up and provided its initial conditions — would originate from.

    6
    Gravitational waves generated by cosmic inflation are the farthest signal back in time humanity can conceive of potentially detecting, which originate from the end of cosmic inflation and the very beginning of the hot Big Bang. National Science Foundation (NASA, JPL, Keck Foundation, Moore Foundation, related) — Funded BICEP2 Program; modifications by E. Siegel

    But there are other signals left over from the Universe as well. When the Universe was about 380,000 years old, the leftover radiation from the Big Bang stopped scattering off of free, charged particles as they formed neutral atoms. These photons, once neutral atoms form, continue to redshift with the expanding Universe, and can be seen with a microwave or radio telescope/antenna today. But because of how rapidly the Universe expanded back in the earliest stages, the “surface” we see this leftover glow at — the cosmic microwave background — is already only 45.2 billion light years away. The distance from the beginning of the Universe to where the Universe is at 380,000 years of age is already 900 million light years!

    7
    The light we perceive as the cosmic microwave background is actually leftover photons from the Big Bang, released at the instant they last scattered off of free electrons. Although that light travels for 13.8 billion years before reaching us, the expansion of space causes that location to be, at present, 45.2 billion light years away. E.M. Huff, the SDSS-III team and the South Pole Telescope team; graphic by Zosia Rostomian

    South Pole Telescope SPTPOL

    It’s much, much longer than that until we find the most distant galaxy ever discovered in the Universe. While simulations and calculations indicate that the very first stars may have formed when the Universe was between 50 and 100 million years old, and the very first galaxies at around 200 million years, we haven’t been able to see back that far just yet. (Although, hopefully, with the James Webb Space Telescope launching next year, we soon will!).

    NASA/ESA/CSA Webb Telescope annotated

    The current cosmic record-holder, shown below, is a galaxy from when the Universe was 400 million years old: just 3% of its present age. However, that galaxy, GN-z11, is only located 32 billion light years away: some 14 billion light years from the “edge” of the observable Universe.

    8
    The most distant galaxy ever found: GN-z11, in the GOODS-N field as imaged deeply (but not the deepest-ever) by Hubble. NASA, ESA, and P. Oesch (Yale University)

    NASA/ESA Hubble Telescope

    The reason for this? The expansion rate has been dropping in a tremendous fashion over time. At the time galaxy Gz-11 existed in the state we see it, the Universe was expanding 20 times faster than it is today. When the cosmic microwave background was emitted, the Universe was expanding 20,000 times faster than it is today. And at the moment of the Big Bang, to the best of our knowledge, the Universe was expanding some 10^36 times faster, or 1,000,000,000,000,000,000,000,000,000,000,000,000 times faster than it is today. The Universe’s expansion rate has been slowing down tremendously over time.

    This is incredibly good for us! The balance between the initial expansion rate and the total amount of energy in the Universe in all its forms is perfectly balanced, to the limits of the quality of our observations. If the Universe had even slightly too much matter or radiation in the early stages, it would have recollapsed billions of years ago, and we wouldn’t exist. If the Universe had slightly too little matter or radiation early on, it would have expanded too quickly for particles to find one another and even form atoms, much less complex structures like galaxies, stars, planets and humans. The cosmic story that the Universe tells to us is one of extraordinary balance, and one where we actually get to exist.

    9
    The intricate balance between the expansion rate and the total density in the Universe is so precarious that even a 0.00000000001% difference in either direction would render the Universe completely inhospitable to any life, stars, or potentially even molecules existing at any point in time. Ned Wright’s Cosmology tutorial.

    If our current best theories are correct, the first true galaxies will have formed at some point between around 120 and 210 million years of age. That corresponds to a distance from us of between 37 and 35 billion light years, placing the distance from the farthest galaxy of all to the edge of the observable Universe at 9-to-11 billion light years today. That’s incredibly far, and points to one incredible fact: the Universe was expanding extremely rapidly in the early stages, and expands at a much slower rate today. That first 1% of the Universe’s age is responsible for approximately 20% of the Universe’s total expansion!

    10
    The history of our Universe is filled with a number of fantastic events, but since inflation ended and the hot Big Bang occurred, the expansion rate has been dropping precipitously, and slowing its rate of descent as the density continues to drop. Bock et al. (2006, astro-ph/0604101); modifications by E. Siegel

    The expansion of the Universe is what’s stretched the light’s wavelength (and caused the “redshift” we see), and that rapid expansion is why there’s such a difference between the cosmic microwave background and the farthest galaxy. But the size of the Universe today is evidence of something else incredible: the incredible effects that the progression of time has. As time goes on, the Universe will continue to expand farther and farther, and by time it’s around ten times its current age, distances will have expanded so much that no galaxies beyond our local group will be visible, even with the equivalent of the Hubble Space Telescope. Enjoy all we can see today about the great variety of what’s present on all cosmic scales. It won’t be around forever!

    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

     
c
Compose new post
j
Next post/Next comment
k
Previous post/Previous comment
r
Reply
e
Edit
o
Show/Hide comments
t
Go to top
l
Go to login
h
Show/Hide help
shift + esc
Cancel
%d bloggers like this: