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  • richardmitnick 12:16 pm on June 10, 2016 Permalink | Reply
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    From Ethan Siegel: “NASA’s big mistake: LIGO’s merging black holes were invisible after all” 

    Ethan Siegel
    6.10.16

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    Image Credit: SXS, the Simulating eXtreme Spacetimes (SXS) project (http://www.black-holes.org).

    The gravitational waves were real. But earlier announcements that X-rays and gamma-rays were detected, too? Not so much.

    What’s really exciting is what comes next. I think we’re opening a window on the universe — a window of gravitational wave astronomy.” -Dave Reitze

    On September 14, 2015, a tiny effect lasting 200 milliseconds passed through the Earth at the speed of light. The entire planet compressed and expanded in two mutually perpendicular directions by less than the width of a proton, oscillating back and forth roughly seven times in that span. And in two detectors separated by 2,000 miles, an interference pattern formed by two isolated lasers, reflected back-and-forth in a vacuum and then brought together again, gave us the telltale explanation for this effect.

    Caltech/MIT   Advanced Ligo Hanford, WA, USA installation
    Caltech/MIT Advanced aLigo Hanford, WA, USA installation

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

    From 1.3 billion light years away, two black holes some 30 times the mass of the Sun had spiraled into one another, merging together and sending energetic ripples through the fabric of space itself. For the first time, a gravitational wave — one of the oldest unverified predictions of Einstein’s General Relativity — had been directly detected.

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

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    Image credit: ESA–C.Carreau, of the “ripple” effect on spacetime that a passing gravitational wave imparts.

    Optical telescopes didn’t see anything, as expected. Merging black holes weren’t anticipated to emit any light, unlike merging stars (which create a larger star), white dwarfs (which create a supernova), or neutron stars (which are thought to create a gamma ray burst); they should only be detectable by their gravitational wave signal. Yet there was a curious possible exception, as a team from NASA’s Fermi satellite claimed to detect gamma rays coincident with this event, offset by a meagre 0.4 seconds. An array of 14 crystal detectors on board — the Gamma-ray Burst detection Monitor (GBM) instrument — detected an unexpected burst of X-rays, and claimed there was only a 0.2% chance of a false positive.

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    This image, taken in May 2008 as the Fermi Gamma-ray Space Telescope was being readied for launch, highlights the detectors of its Gamma-ray Burst Monitor (GBM). The GBM is an array of 14 crystal detectors. Image credit: NASA/Jim Grossmann.

    While NASA was celebrating, however, cautious scientists all over the world were skeptical. Not only would this overthrow the leading theoretical models for black hole mergers, and not only does a 99.8% chance of success correspond only to a 3-σ significance (rather than the 5-σ significance typically required for a discovery in physics), but a complimentary satellite in orbit — the ESA’s INTEGRAL satellite — failed to see the corroborating evidence it should have if this signal were real.

    ESA/Integral
    ESA/Integral

    On the contrary, INTEGRAL searched through all the data and failed to find any interesting signal coincident with LIGO’s gravitational wave at all. Far from a definitive detection, this conflicting data raised more questions than it answered.

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    A marginal detection only is available for the gravitational wave event associated with LIGO’s detection on September 14, 2015. Image credit: D. Bagoly et al., 2016 (submitted to A&A), via http://arxiv.org/abs/1603.06611.

    Thanks to a new paper now available from J. Greiner, J.M. Burgess, V. Savchenko and H.-F. Yu, however, the apparent conflict may at last be resolved. The secret lies in understanding how the GBM instrument aboard NASA’s Fermi satellite actually works. Rather than measuring an absolute signal, it measures a steady, continuous background of photons over a large energy range. The spikes above that background, when they appear, can show us either a real, physical event (like a burst or merger), or they can simply be evidence of a random fluctuation that has no physical origin at all. If you use an imperfect algorithm for discriminating which fluctuations are physical vs. non-physical, you could wind up drawing invalid conclusions about what’s real and what’s phantasmal. The huge advance of the new paper, submitted to the Astrophysical Journal as a Letter, isn’t observational or theoretical, but rather statistical; it more robustly and successfully discriminates between normal noise and a burst of high-energy light from an astrophysical source.

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    Various statistical techniques analyzing the Fermi data. The original analysis (purple) shows a signal, but the improved analysis (orange) shows only something consistent with pure noise. Image credit: Figure 5 from J. Greiner, J.M. Burgess, V. Savchenko and H.-F. Yu, retrieved from the preprint at http://arxiv.org/abs/1606.00314.

    Above, you can see a number of different ways of reconstructing the apparent signal coincident with LIGO’s gravitational wave. The original Fermi team’s analysis is shown in purple: a clear detection. However, the superior reconstruction of this new paper is shown in orange, and lines up with both the raw data (blue) and also — more importantly — is consistent with a non-detection, meaning that there is no electromagnetic signal here. According to one of the paper’s authors, J. Michael Burgess, the original paper (claiming a detection) had some statistical flaws his team was able to spot, relating the following:

    When I saw the announcement and the paper, the spectrum looked like what I always see as background.

    After pulling his team together and developing some new analysis tools, they confirmed their suspicions:

    We instantly saw that we got a much different answer. The spectrum of the event was basically zero: nothing there.

    The new statistical technique developed by Burgess and his collaborators has proven to be incredibly powerful, successfully pulling out even faint gamma ray signals from noisy data and drastically reducing the number of false positives. By combining this new technique with the existing Fermi data, it should be possible to make huge strides forward in identifying true astrophysical events.

    Gamma ray burst artist depiction Credit NASA Swift Mary Pat Hrybyk-Keith and John Jones
    Gamma ray burst artist depiction Credit NASA Swift Mary Pat Hrybyk-Keith and John Jones

    It’s important to remember that there can and will be correlations in the future not only between gravitational waves and gamma rays, but between LIGO and Fermi’s GBM instrument. When asked for comment, Burgess said the following:

    “GBM is an amazing instrument and its synergy with LIGO provides an amazing way for us to view the Universe. The GBM team has made a huge effort for this, and when a neutron star merger happens nearby, it is very likely GBM and LIGO (and others) will see something… and this will be amazing!”

    But in order to make sure we aren’t fooling ourselves, we have to do it right. Collaboration between the teams — the Fermi team, the INTEGRAL team, and the gravitational wave teams — are incredibly important. But the necessity of calibrating the signals that multiple observatories will see is essential to getting the right results. Merging black holes may, in fact, sometimes lead to electromagnetic radiation, a possibility which future events will hopefully test. But the golden rule in situations like these is the null hypothesis: in the absence of extraordinary evidence, as is the case here, bet on exactly what the leading physics ideas predict.

    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 6:52 am on June 3, 2016 Permalink | Reply
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    From Ethan Siegel: “Could black holes be the dark matter?” 

    Ethan Siegel

    6.2.16

    It’s an old idea made new again, but it just might fall apart.

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    A massive black hole accreting matter off of a nearby star. Image credit: NASA/JPL-Caltech.

    “[The black hole] teaches us that space can be crumpled like a piece of paper into an infinitesimal dot, that time can be extinguished like a blown-out flame, and that the laws of physics that we regard as ‘sacred,’ as immutable, are anything but.” -John Wheeler

    Sometimes, when you look at the Universe in a new way, it surprises you. When the LIGO collaboration announced the first detection of gravitational waves, that was serendipity and the confirmation of one of the longest-enduring unconfirmed predictions of science, but it wasn’t exactly a surprise.

    Caltech/MIT Advanced Ligo Hanford, WA, USA installation
    Caltech/MIT Advanced Ligo Hanford, WA, USA installation

    The surprising part was the source of those gravitational waves: two black holes of 36 and 29 solar masses apiece, far more massive than the black holes we expect from supernova and far less massive than the ones at the centers of galaxies.

    Cornell SXS team. Two merging black holes simulation
    Illustration of two black holes merging, of comparable mass to what LIGO saw. Image credit: SXS, the Simulating eXtreme Spacetimes (SXS) project (http://www.black-holes.org).

    Perhaps this would revitalize a previously disfavored idea: that black holes had been around since very early on in the Universe, shortly after the Big Bang. Moreover, if this were the case, perhaps they made up the missing mass of the Universe: the dark matter.

    The idea is pretty simple: we know the Universe started off from a hot, dense, rapidly expanding and roughly uniform state. Wherever you were located, gravitation would try to pull nearby masses towards you, while the radiation pressure from photons would try to push those masses back apart. But if on small scales, you had regions of space that were just 68% (or more) denser than average, that radiation pressure wouldn’t matter. Instead, gravitational collapse all the way to a black hole would be inevitable. If this happened at one particular mass scale in the Universe — say at 1 kilogram masses, or 10¹⁰ kilogram masses, or even 30 solar masses — you’d wind up with a large number of primordial black holes of that particular mass. They’d be strewn roughly evenly throughout the Universe, they’d form large, diffuse-but-clumpy halos around galaxies, and they’d be an excellent candidate for the dark matter.

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    Illustration of a clumpy dark matter halo around the baryons in a galaxy. Image credit: NASA, ESA, and T. Brown and J. Tumlinson (STScI).

    As soon as this idea was first suggested, it was recognized that there were a number of restrictions on this possibility. Whenever a mass passes between your line-of-sight and a distant object, that mass acts like a gravitational lens, thanks to Einstein’s relativity. The effect of a transiting dense, dark object — known as microlensing — has been searched for at some length. While there is some microlensing seen due to these compact masses in our galactic halo, they’ve been more useful as far as constraining what fraction of the matter could be at the larger end of these primordial black holes. In addition, if the black holes are too small in mass, they’ll evaporate due to Hawking radiation. All told, observations of

    the lack of Hawking radiation,
    gamma-ray-burst microlensing,

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    Rolf Bühler, https://arxiv.org/pdf/1509.00012.pdf

    neutron star capture in globular clusters,
    traditional microlensing,

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    From http://planetquest.jpl.nasa.gov/images/microlensing3-400.jpg

    and the cosmic infrared and microwave backgrounds,

    Cosmic Infrared Background, Credit: Michael Hauser (Space Telescope Science Institute), the COBE/DIRBE Science Team, and NASA)
    Cosmic Infrared Background, Credit: Michael Hauser (Space Telescope Science Institute), the COBE/DIRBE Science Team, and NASA)

    Cosmic Microwave Background per ESA/Planck
    Cosmic Microwave Background per ESA/Planck

    tell us that we can’t have primordial black holes make up the majority of dark matter over a wide variety of mass ranges.

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    Constraints on dark matter from Primordial Black Holes. Image credit: Fig. 1 from Fabio Capela, Maxim Pshirkov and Peter Tinyakov (2013), via http://arxiv.org/pdf/1301.4984v3.pdf.

    If you look at the above graph, you’ll find that ~30 solar masses — or about 6 × 10³⁴ g — is thoroughly ruled out, where only approximately 0.01% of the dark matter can exist with that mass, at most. A recent paper, however, by Alexander Kashlinsky, doubts these earlier claims about the cosmic infrared background constraints, and instead claims that a number of sources exist that could, in fact, be these primordial black holes.

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    Left: An infrared view of the sky in Ursa Major. Right: an enhanced view with known sources masked, showing fluctuations of the infrared background. Credits: NASA/JPL-Caltech/A. Kashlinsky (Goddard).

    Rather than using the cosmic infrared background to constrain primordial black holes, Kashlinsky uses the assumption that they make up 100% of the dark matter to explain the cosmic infrared background:

    “we point out that if indeed the LIGO discovery is indicative of PBHs making up the DM, the extra […] fluctuations would lead to much greater rates of collapse at early times, which would naturally produce the observed levels of the [cosmic infrared background] fluctuations.”

    The problem is, unfortunately, that there are other constraints afoot.

    The fluctuations in the cosmic microwave background (above) tell us that no more than 0.1% of the total dark matter could be in primordial black holes at ~30 solar masses, where the only argument against it (by Bird et al. (2006) is that there are some uncertainties in this physics that haven’t been quantified, and perhaps those uncertainties are large enough that this bound can be evaded. It’s true: if these ill-motivated but not 100% ruled out primordial black holes exist at ~30 solar masses, and if they account for the cosmic infrared background, and if our understanding of the radiative processes of gas onto a moving black hole are wildly incorrect, then perhaps these black holes could be the dark matter after all. But another explanation is far more likely.

    Tarantula Nebula,  Hubble 2009. Credit NASA, ESA, and F. Paresce INAF-IASF, Bologna, Italy, R. O’Connell University of Virginia, Charlottesville, and the Wide Field Camera 3 [WFC3]Science Oversight Committee
    Tarantula Nebula, Hubble 2009. Credit NASA, ESA, and F. Paresce INAF-IASF, Bologna, Italy, R. O’Connell University of Virginia, Charlottesville, and the Wide Field Camera 3 [WFC3]Science Oversight Committee

    When we produce stars, we do so in bursts, with the most massive starbursts producing dozens of stars ranging from 50 to upwards of 250 times the mass of the Sun. All of these stars will end their lives in just a few million years in core-collapse supernovae, with the innermost core resulting in a black hole. While stars under 50 solar masses likely produce black holes around 10 solar masses or even smaller, the largest ones can create black holes 20, 30, 50 or even potentially over 100 times our Sun’s mass. That’s the leading theory for where these black holes came from, and given that the most massive star cluster known, R136, actually contains a single concentration (R136a) with at least 24 independent stars, including at least six members over 100 solar masses.

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    The huge star cluster R136, with R136a1 at the center. The image was obtained at high resolution with the MAD adaptive optics instrument at ESO’s Very Large Telescope. Image credit: ESO/P. Crowther/C.J. Evans.

    ESO VLTI image
    ESO VLTI image

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    ESO MAD at http://www.eso.org/sci/facilities/develop/ao/sys/mad.html

    The two most massive members, R136a1 and R136a2, are ~250 and ~195 solar masses, respectively, and could easily give rise to black holes in the mass range LIGO saw, if not even greater. In addition, they’re in a binary system themselves with one another, and so a future inspiral and merger is completely within the realm of reasonable. Sure, it’s not 100% ruled out that black holes of around 30 solar masses could be the dark matter, but it’s far from the most likely explanation. In physics, as in life, the smart money is to bet on what’s already known as the most likely explanation for the novel phenomenon we just saw. While the more fanciful possibilities might spark our imagination, they’re also most likely wrong. Now you know why.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

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

     
  • richardmitnick 2:53 pm on May 30, 2016 Permalink | Reply
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    From Ethan Siegel: “The strongest gravitational show in the Universe” 

    From Ethan Siegel
    5.30.16

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    Six examples of the strong gravitational lenses the Hubble Space Telescope discovered and imaged. Image credit: NASA, ESA, C. Faure (Zentrum für Astronomie, University of Heidelberg) and J.P. Kneib (Laboratoire d’Astrophysique de Marseille).

    When you get enough mass together, Einstein’s theory of gravity causes space to act like a lens. Here are the results.

    “The first amazing fact about gravitation is that the ratio of inertial mass to gravitational mass is constant wherever we have checked it. The second amazing thing about gravitation is how weak it is.” -Richard Feynman

    In 1919, a solar eclipse proved one of Einstein’s greatest predictions: that mass curves space, and causes starlight to bend.

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    Positive development of the photographic plate from the solar eclipse of 1919. You can see the stars marked by vertical lines. Image credit: F. W. Dyson, A. S. Eddington, and C. Davidson, 1919.

    With even more massive objects than stars — like galaxies, quasars or galaxy clusters — gravity can do more than just bend light slightly: it can act like a lens.

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    This image illustrates a gravitational lensing effect. Image credit: NASA, ESA, and Johan Richard (Caltech, USA); Acknowledgements: Davide de Martin & James Long (ESA/Hubble).

    Just as optical lenses can focus or distort light, gravitational lenses curve space so significantly they magnify and stretch distant, background objects.

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    The lensing distortions from galaxy cluster Abell 2390. Image credit: NASA, ESA, and Johan Richard (Caltech, USA); Acknowledgements: Davide de Martin & James Long (ESA/Hubble).

    Normally, a good alignment will distort a background galaxy into two arcs: a radial one pointing away from the foreground mass and a tangential one arcing around the mass.

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    Galaxy cluster Abell 2218, with many arcs characteristic of gravitational lensing. Image credit: NASA, ESA, and Johan Richard (Caltech, USA); Acknowledgements: Davide de Martin & James Long (ESA/Hubble).

    Occasionally, an even better alignment will create multiple images of the same object.

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    The galaxy cluster Abell 68, and its many lensed and distorted background galaxies. Image credit: NASA & ESA. Acknowledgement: N. Rose.

    The curvature of space forces some light paths to take longer to arrive than others, meaning we’re seeing the same background object at different times.

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    A quadruply-imaged supernova, thanks to gravitational lensing. Image credit: NASA, ESA, and S. Rodney (JHU) and the FrontierSN team; T. Treu (UCLA), P. Kelly (UC Berkeley), and the GLASS team; J. Lotz (STScI) and the Frontier Fields team; M. Postman (STScI) and the CLASH team; and Z. Levay (STScI).

    Most spectacularly, we’ve gotten to see a distant supernova “replay” itself due to this lensing effect.

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    A horseshoe-shaped Einstein ring, just short of the perfect alignment needed for a 360-degree ring. Image credit: ESA/Hubble & NASA.

    In the most perfect alignment of all, a complete, 360º ring will appear due to gravitational lensing: an Einstein Ring.

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    The double gravitational lens system, SDSSJ0946+1006, which shows a rare near-doubly-perfect alignment. Image credit: NASA, ESA, and R. Gavazzi and T. Treu (University of California, Santa Barbara).

    Although the science predicted these lenses for decades, the first one wasn’t observed until 1979′s Twin Quasar.

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    The Twin Quasar QSO 0957+561, as gravitationally lensed by the enormous elliptical galaxy, YGKOW G1, four billion light years away. This was the first gravitational lens ever discovered, in 1979. Image credit: ESA/Hubble & NASA.

    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 12:53 pm on May 30, 2016 Permalink | Reply
    Tags: , , , Nebulae by Hubble   

    From Ethan Siegel: “The Double Jet Death Of Sun-Like Stars” 

    From Ethan Siegel

    May 30, 2016

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    Planetary Nebula M2-9, from the Hubble Space Telescope. Image credit: Bruce Balick (University of Washington), Vincent Icke (Leiden University, The Netherlands), Garrelt Mellema (Stockholm University), and NASA/ESA.

    When stars like our Sun, between 40% and ~800% of our mass, run out of hydrogen in their core, they start to die.

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    The bipolar planetary nebula PN Hb 12, the late stages of a dying Sun-like star. Image credit: NASA, ESA; Acknowledgement: Josh Barrington.

    The core contracts and heats up, causing the outer layers to expand as the star becomes a helium-burning red giant.

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    The Egg Nebula, a proto-planetary nebula in the early stages of formation. Image credit: NASA / Hubble.

    The intense stellar winds produced gently blow off the star’s outer layers.

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    The red rectangle nebula. Image credit: ESA / Hubble & NASA.

    When the core runs out of helium to burn, the central region contracts to a white dwarf, producing intense ultraviolet light.

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    The Southern Crab Nebula (He2-104) in its entirety, as observed by the Hubble Space Telescope. Image credit: ESA / Hubble and NASA, STScI.

    This light ionizes the atoms that had previously been blown off. As the electrons recombine with their ions, they emit light of various wavelengths.

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    Nitrogen, hydrogen and oxygen are highlighted in the planetary nebula above, known as the Hourglass Nebula for its distinctive shape. Image credit: NASA/HST/WFPC2 R Sahai and J Trauger (JPL).

    Hydrogen tends to glow red, while oxygen, sulphur, sodium, carbon and nitrogen cover the greens, blues and yellows when shown in true color.

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    The Ant Nebula, also known as Menzel 3. Image credit: NASA, ESA & the Hubble Heritage Team (STScI/AURA); Acknowledgment: R. Sahai (Jet Propulsion Lab), B. Balick (University of Washington).

    Some 80% of planetary nebulae are asymmetrical, with the vast majority of those showing a bipolar form.

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    The Rotten Egg Nebula. Image credit: NASA / Hubble.

    These twin jets emerge along the parent star’s rotational axis, where streams of material most likely flow outwards and collide with previously blown-off stellar layers.

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    Observations of active nebulae show that ~10 lunar masses worth of material are ejected each year, at speeds reaching 5% the speed of 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 3:52 pm on May 26, 2016 Permalink | Reply
    Tags: , , , How would our Universe be different without dark energy?   

    From Ethan Siegel: “How would our Universe be different without dark energy?” 

    From Ethan Siegel

    5.26.16

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    Image credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona, of the Hercules Galaxy Cluster, under a c.c.a.-s.a.-4.0 license.

    U Arizona Mt Lemmon Sky Center, north of Tuscon, AZ, USA
    U Arizona Mt Lemmon Sky Center, north of Tuscon, AZ, USA

    In 1998, cosmologists got the surprise of a lifetime. Here’s how our Universe would’ve looked without cosmic acceleration.

    “We’ve known for a long time that the universe is expanding. But about 15 years ago, my colleagues and I discovered that it is expanding faster and faster. That is, the universe is accelerating, and that was not expected, but it is now attributed to this mysterious stuff called dark energy which seems to make up about 70 percent of the universe.” -Adam Riess

    In 1998, two independent groups of scientists both studying the most distant supernova explosions in the Universe reported the same unexpected phenomenon: these brilliant flashes of light, whose intrinsic brightnesses and redshifts were known to great precision, all had a problem, that they appeared to be much fainter than expected. And the higher of a redshift you went to, the greater this problem got. The interpretation? They were more distant — and hence appeared less bright — than the conventional version of the expanding Universe would have predicted. Rather than being filled only with matter and radiation throughout the fabric of space, the Universe also contained this small but important amount of energy inherent to space itself: dark energy.

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    Image credit: Ned Wright, based on the latest data from Betoule et al. (2014), via http://www.astro.ucla.edu/~wright/sne_cosmology.html.

    As our measurements got better and better, and as we accumulated data from other sources as well, like the fluctuations in the Cosmic Microwave Background (CMB) and the clustering properties of large-scale structure, we found out that approximately 68% of the energy in the Universe today was this mysterious dark energy. Yes, there was dark matter, normal matter, neutrinos and radiation all present, and they were all vital to how the Universe expanded and evolved, particularly at early times. But as the Universe aged, dark energy became more and more important, and will eventually approach a full 100% of the energy present within our Universe.

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    Constraints on dark energy from three independent sources: supernovae, the CMB and BAO. Note that even without supernovae, we’d need dark energy. Image credit: Supernova Cosmology Project, Amanullah, et al., Ap.J. (2010).

    But according to General Relativity, it didn’t have to be this way at all. We could have had a Universe with no dark energy at all: where zero-point energy of empty space was actually zero, instead of some tiny, non-zero value. If that were our Universe, how would it be different from the Universe we have today? Surprisingly, there are a few significant ways that really make an impact.

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    A Universe with dark energy: our Universe. Image credit: NASA / WMAP Science Team.

    1.) The Universe would be a little bit different. Right now, in our 13.8 billion year old Universe, 32% of the energy density is in the form of matter, 68% is dark energy, the expansion rate is 67 km/s/Mpc and the limits of our observable reach is 46.1 billion light years. If wanted the Universe to have the same exact amount of matter in it, but with no dark energy, our Universe would have expanded faster early on, and would be expanding slower today. It would:

    be 47.7 billion light years in size, rather than 46.1 billion,
    have a current Hubble rate of 56 km/s/Mpc rather than 67 km/s/Mpc,
    the CMB temperature would be just slightly lower, at 2.62 K instead of 2.73 K,
    and have a whopping 71% less energy overall, due to the total lack of dark energy.

    But the major differences would show up far in the future, especially when we considered our eventual fates.

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    The GOODS-N field, with galaxy GN-z11 highlighted: the presently most-distant galaxy ever discovered. Image credit: NASA, ESA, P. Oesch (Yale University), G. Brammer (STScI), P. van Dokkum (Yale University), and G. Illingworth (University of California, Santa Cruz).

    2.) Every galaxy in the visible Universe would still be reachable.

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

    In our dark-energy dominated Universe, the rate a distant galaxy recedes from us increases as time goes on. Galaxies presently more than 15 billion light years away are receding faster than the speed of light [?], and so nothing leaving Earth today — not a relativistic spaceship, not a deep-space probe, not even light itself — could ever reach it. Already, 97% of the galaxies in our Universe are forever beyond our reach. But if we were to take that dark energy away, everything would be reachable eventually, even if it took tens or hundreds of billions of years. We’d get there in the end.

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    A portion of the Hubble eXtreme Deep Field in full UV-vis-IR light, the deepest image ever obtained. Image credit: NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI).

    3.) New galaxies beyond our horizon would continually become accessible. Not only that, but even galaxies whose light has never reached us yet will someday have that light catch up to us in the future! While a dark energy Universe has the currently visible galaxies “red out,” or redshift away to the point where they’ll no longer be seen in the far future, a Universe without it would’ve seen additional galaxies become visible over time, with more and more becoming apparent (and reachable) as time goes on.

    7
    Without dark energy, we’d be somewhere in between a decelerating and a coasting Universe. Image credit: NASA & ESA, of possible models of the expanding Universe.

    4.) The Hubble rate of expansion would eventually drop to zero. It would never actually reach zero, mind you, and it would never turn around and recollapse: there’s too little energy for that overall. But the Hubble rate would asymptotically approach zero as the Universe continued to expand, meaning that if an infinite amount of time were to pass, an infinite number of galaxies (though not all of them, by any means) would become accessible. With dark energy, our Universe’s Hubble rate will asymptote to a finite, significant value after an infinite amount of time: something like 46 km/s/Mpc. Without dark energy, we’d have dropped below that 46 km/s/Mpc rate after another 4.3 billion years.

    Laniakea supercluster no image credit
    Laniakea supercluster. From Nature The Laniakea supercluster of galaxies R. Brent Tully, Hélène Courtois, Yehuda Hoffman & Daniel Pomarède at http://www.nature.com/nature/journal/v513/n7516/full/nature13674.html. Milky Way is the red dot.

    5.) Superclusters would really exist. Our local supercluster, containing the local group, the Virgo Cluster (the largest supercluster member) and hundreds of other individual galaxies, groups and clusters, doesn’t really exist thanks to dark energy.

    Local Group. Andrew Z. Colvin 3 March 2011
    Local Group. Andrew Z. Colvin 3 March 2011

    Virgo Supercluster
    Virgo Supercluster. No image credit

    It looks like a large structure, but it’s not bound together and will have all of its individual components strewn apart as time goes on. But without that additional repulsion that dark energy imparts, gravitation would win in the end. On long enough timescales, all galaxies, groups and clusters that make up the Laniakea supercluster will remain bound together, and will continue to experience mergers on cosmic scales.

    6.) Which means eventually, Milkdromeda would fall into the Virgo Cluster. At 50–60 million light years distant, the Virgo Cluster contains around 1000 galaxies, and is the closest galaxy cluster to our local group. It’s currently receding from us, due to the expansion of the Universe, at over 1000 km/s, or about 100 times faster than any human-made spacecraft has ever traveled. With dark energy, Virgo will only accelerate away from us faster and faster. But if it weren’t there, the gravitational pull of Virgo would be irresistible, and even though it would take around a hundred billion years — many times the age of the Universe at present — eventually the galactic wreckage of our local group would merge with the Virgo cluster as well.

    8
    Image credit: E. Siegel, based on work by Wikimedia Commons users Andrew Colvin 429 and Frédéric MICHEL.

    With dark energy, the subtle differences of a slightly more energetic and more rapidly expanding Universe today leads to a far future where our local group is lonely and isolated, distant galaxies disappear from view and there’s no such thing as a bound, cosmic supercluster. On the largest scales, the Universe is doomed to emptiness, and it’s extra energy intrinsic to space itself that’s to blame. Part of why it was so hard to accept is because the fate of a dark energy Universe is so different — and unpalatable — from a Universe without it. Yet science doesn’t care about your personal preferences or motivations: it cares about the Universe as it actually is. The best thing we can do is listen to the story it tells us about itself, and in a way, about ourselves, too.

    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 9:36 pm on May 24, 2016 Permalink | Reply
    Tags: , ,   

    From Ethan Siegel: “Where Is New Physics Hiding, And How Can We Find It?” 

    From Forbes

    May 24, 2016
    Sabine Hossenfelder

    1
    The particle tracks emanating from a high energy collision at the LHC in 2014. Image credit: Wikimedia Commons user Pcharito, under a c.c.a.-by-s.a.-3.0 license.

    CERN CMS Higgs Event
    CERN CMS Higgs Event

    CERN/CMS Detector
    CERN/CMS Detector

    The year is 2016, and physicists are restless. Four years ago, the LHC confirmed the Higgs boson, the last outstanding prediction of the Standard Model. The chances were good, so they thought, that the LHC would also discover other new particles – naturalness seem to demand it. But, so far, given all the data they’ve collected, their greatest hopes appear to be phantasms.

    The Standard Model and General Relativity do a great job, but physicists know this can’t be it. Or at least they think they know: the theories are incomplete, not only disagreeable and staring each other in the face without talking, but inadmissibly wrong, giving rise to paradoxa with no known cure. There has to be more to find, somewhere. But where?

    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.
    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    The hiding places for novel phenomena are getting smaller. But physicists haven’t yet exhausted their options. Here are the most promising areas where they currently search:

    1.) Weak Coupling. Particle collisions at high energies, like those reached at the LHC, can produce all existing particles up to the energy that the colliding particles had.

    CERN/LHC Map
    CERN LHC Grand Tunnel
    CERN LHC particles
    LHC at CERN

    The amount of new particles you make, however, depends on the strength by which they couple to the particles that were brought to collision (for the LHC that’s protons, or their constituents quarks and gluons, respectively). A particle that couples very weakly might be produced so rarely that it could have gone unnoticed so far.

    Physicists have proposed many new particles which fall into this category because weakly interacting stuff generally looks a lot like dark matter. Most notably there are the weakly interacting massive particles (WIMPs), sterile neutrinos (that are neutrinos which don’t couple to the known leptons), and axions (proposed to solve the strong CP problem and also a dark matter candidate).

    2
    Limits on the dark matter/nucleon recoil cross-section, including the projected predicted sensitivity of XENON1T. Image credit: Ethan Brown of RPI, via http://ignatz.phys.rpi.edu/site/index.php/the-experiment/.

    These particles are being looked for both by direct detection measurements – monitoring large tanks in underground mines for rare interactions – and by looking out for unexplained astrophysical processes that could make for an indirect signal.

    JUNO Chinese Neutrino Experiment
    JUNO Chinese Neutrino Experiment

    Sudbury Neutrino Observatory
    Sudbury Neutrino Observatory

    Sanford Underground levels
    SURF

    2.) High Energies. If the particles are not of the weakly interacting type, we would have noticed them already, unless their mass is beyond the energy that we have reached so far with particle colliders. In this category we find all the supersymmetric partner particles, which are much heavier than the standard model particles because supersymmetry is broken. Also at high energies could hide excitations of particles that exist in models with compactified extra dimensions. These excitations are similar to higher harmonics of a string and show up at certain discrete energy levels which depend on the size of the extra dimension.

    3
    The supersymmetric particles, next to the (normal) Standard Model ones. Image credit: DESY at Hamburg.

    Strictly speaking, it isn’t the mass that is relevant to the question whether a particle can be discovered, but the energy necessary to produce the particles, which includes binding energy. An interaction like the strong nuclear force, for example, displays “confinement” which means that it takes a lot of energy to tear quarks apart even though their masses are not all that large. Hence, quarks could have constituents – often called “preons” – that have an interaction – dubbed “technicolor” – similar to the strong nuclear force. The most obvious models of technicolor however ran into conflict with data decades ago. The idea however isn’t entirely dead, and though the surviving models aren’t presently particularly popular, some variants are still viable.

    These phenomena are being looked for at the LHC and also in highly energetic cosmic ray showers.

    3.) High Precision. High precision tests of standard model processes are complementary to high energy measurements. They can be sensitive to tiniest effects stemming from virtual particles with energies too high to be produced at colliders, but still making a contribution at lower energies due to quantum effects. Examples for this are proton decay, neutron-antineutron oscillation, the muon g-2, the neutron electric dipole moment, or Kaon oscillations. There are existing experiments for all of these, searching for deviations from the standard model, and the precision for these measurements is constantly increasing.

    4
    A diagram of neutrinoless double beta decay. The decay time through this pathway is much longer than the age of the Universe. Image credit: public domain image by JabberWok2.

    A somewhat different high precision test is the search for neutrinoless double-beta decay which would demonstrate that neutrinos are Majorana-particles, an entirely new type of particle. (When it comes to fundamental particles that is. Majorana particles have recently been produced as emergent excitations in condensed matter systems.)

    Majorano Demonstrator Experiment
    Majorano Demonstrator Experiment

    4.) Long ago. In the early universe, matter was much denser and hotter than we can hope to ever achieve in our particle colliders. Hence, signatures left over from this time can deliver a bounty of new insights. The temperature fluctuations in the cosmic microwave background (B-modes and non-Gaussianities) may be able to test scenarios of inflation or its alternatives (like phase transitions from a non-geometric phase), whether our universe had a big bounce instead of a big bang, and – with some optimism – even whether gravity was quantized back them.

    Cosmic Microwave Background per ESA/Planck
    Cosmic Microwave Background per ESA/Planck

    ESA/Planck
    ESA/Planck

    5
    A Universe with dark energy: our Universe. Image credit: NASA / WMAP Science Team.

    5.) Far away. Some signatures of new physics appear on long distances rather than of short. An outstanding question is for example what’s the shape of the universe?

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

    Is it really infinitely large, or does it close back onto itself? And if it does, then how does it do this? One can study these questions by looking for repeating patterns in the temperature fluctuation of the cosmic microwave background (CMB).

    If we live in a multiverse, it might occasionally happen that two universes collide, and this too would leave a signal in the CMB.

    Multiverse. Image credit: public domain, retrieved from https://pixabay.com/
    Multiverse. Image credit: public domain, retrieved from https://pixabay.com/

    Another novel phenomenon that would become noticeable on long distances is a fifth force, which would lead to subtle deviations from general relativity. This might have all kinds of effects, from violations of the equivalence principle to a time-dependence of dark energy. Hence, there are experiments testing the equivalence principle and the constancy of dark energy to every higher precision.

    6
    A schematic to explain the polarizations in the double slit quantum eraser experiment of Kim et al. 2007. Image credit: Wikimedia Commons user Patrick Edwin Moran under a c.c.a.-by-s.a. 3.0 license.

    6.) Right here. Not all experiments are huge and expensive. While tabletop discoveries have become increasingly unlikely simply because we’ve pretty much tried all that could be done, there are still areas where small-scale lab experiments reach into unknown territory. This is the case notably in the foundations of quantum mechanics, where nanoscale devices, single photon sources and – detectors, and increasingly sophisticated noise-control techniques have enabled previously impossible experiments. Maybe one day we’ll be able to solve the dispute over the “correct” interpretation of quantum mechanics simply by measuring which one is right.

    Physics is far from over. It has become more difficult to test new fundamental theories, but we are pushing the limits in many currently running experiments. There must be new physics out there; we simply need to look at higher energies, higher precisions, or at more subtle effects. If nature is kind to us, this decade might finally be the one that sees us break through the Standard Model to the novel Universe beyond.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

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

     
  • richardmitnick 11:07 am on May 19, 2016 Permalink | Reply
    Tags: , , , The Biggest Hopes Of What A New Particle At The LHC Might Reveal   

    From Ethan Siegel: “The Biggest Hopes Of What A New Particle At The LHC Might Reveal” 

    Starts with a Bang

    May 18, 2016
    Ethan Siegel

    1
    Inside the magnet upgrades on the LHC, that have it running at nearly double the energies of the first (2010-2013) run. Image credit: Richard Juilliart/AFP/Getty Images.

    Built over an 11-year period from 1998 to 2008, the Large Hadron Collider was designed with one goal in mind: to create the greatest numbers of the highest-energy collisions ever, in the hopes of finding new fundamental particles and of revealing new secrets of nature.

    CERN/LHC Map
    CERN LHC Grand Tunnel
    CERN LHC particles
    LHC at CERN

    Over a three year period from 2010 to 2013, the LHC collided protons together at energies nearly four times the previous record, with an upgrade nearly doubling that in 2015: to a record 13 TeV, or approximately 14,000 times the energy inherent to a proton via Einstein’s E = mc^2. The largest, most advanced detectors of all — CMS and ATLAS — were built around the main two collision points, collecting as precise and accurate data about all the debris that emerges each time two protons smash together.

    CERN/CMS Detector
    CERN/CMS detector

    CERN/ATLAS detector
    CERN/ATLAS detector

    July 2012 was a watershed moment for particle physics, as enough high-energy collisions were reconstructed to definitively announce, in both detectors, the first concrete, direct evidence for the Higgs Boson: the last undiscovered particle in the Standard Model of particle physics.

    CERN ATLAS Higgs Event
    CERN ATLAS Higgs Event

    CERN CMS Higgs Event
    CERN CMS Higgs Event

    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.
    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth

    3
    Image credit: The CMS Collaboration, “Observation of the diphoton decay of the Higgs boson and measurement of its properties”, (2014). This was the first “5-sigma” detection of the Higgs.

    But that was expected. The problem is, there are a whole host of questions about the Universe that the Standard Model of particle physics doesn’t answer at a fundamental level, including:

    Why is there more matter than antimatter in the Universe?
    What is dark matter, and what particle(s) beyond the Standard Model (which cannot account for it) explains it?
    Why does our Universe have dark energy, and what is its nature?
    Why don’t the strong interactions in the Standard Model exhibit CP-violation in the strong decays?

    Why do neutrinos have such small but non-zero masses compared to all the other particles?
    And why do the Standard Model particles have the properties and masses that they do, and not any others?

    And the great hope of the LHC, the real hope, is that we’ll learn something extra, beyond the Standard Model, that helps answer one or more of these questions.

    4
    The particles of the Standard Model, all of which have been detected. Image credit: E. Siegel, from his new book, Beyond The Galaxy.

    With the possible exception of dark energy, all of these problems pretty much require new fundamental particles to explain them. And many of them — the dark matter problem, the matter/antimatter problem, and the mass-of-the-particles problem (a.k.a. the Hierarchy problem) — may actually be within reach at the LHC. One way to look for this new physics is to look for deviations from the expected (and well-calculated) behavior in the decays and other properties of the known, detectable Standard Model particles. So far, to the best of our abilities, everything falls within the “normal” range, where things are perfectly consistent with the Standard Model.

    6
    Image credit: The ATLAS collaboration, 2015, of the various decay channels of the Higgs. The parameter mu = 1 corresponds to a Standard Model Higgs only. Via https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2015-007/.

    But the second way is even better: to discover, directly, evidence for a new particle beyond the Standard Model. As the LHC begins collecting even higher-energy data and with even greater numbers of collisions-per-second, it’s in the best position it’s ever going to be to find new fundamental particles; particles it never expected to find. Of course, it doesn’t exactly find particles; it finds the decay products of particles! Fortunately, because of how physics works, we can reconstruct what energy (and hence, what mass) those particles were created at, and whether we’ve got a new particle after all. At the end of the LHC’s initial run, there’s an intriguing (but not certain) hint of what might be a new particle. This “750 GeV diphoton bump” might not be real, but if it is, it could mean the world to physicists everywhere.

    7
    The ATLAS and CMS diphoton bumps, displayed together, clearly correlating at ~750 GeV. Image credit: CERN, CMS/ATLAS collaborations, image generated by Matt Strassler at https://profmattstrassler.com/2015/12/16/is-this-the-beginning-of-the-end-of-the-standard-model/.

    The preliminary signal is discernible in both the CMS and the ATLAS detectors so far, and that makes the possibility extra tantalizing. Within about 6 more months, we should know whether this signal is strengthening — and hence likely real — or whether it shows itself to be spurious. If it’s real, here are some of the top possibilities:

    1. It’s a second Higgs boson! Many extensions to the Standard Model — like supersymmetry — predict additional Higgs particles that are heavier than the current (126 GeV) one we know. If so, this could be a window into a whole world of physics beyond the Standard Model, including into the matter/antimatter asymmetry and the Hierarchy problem.
    2. It’s dark matter-related. Could this new particle be a window into the dark sector? Is there some energy non-conservation happening here that means we’re making something that the detectors can’t see? This is one of the “dare-to-dream” possibilities of particle physics: that the LHC could create dark matter. There’s even a fun little correlation here with something most people haven’t put together: there’s an excess in cosmic ray energies seen in this exact same energy range from the balloon-borne Advanced Thin Ionization Calorimeter (ATIC) experiment!

    8
    Image credit: J. Chang et al. (2008), Nature, from the Advanced Thin Ionization Calorimeter (ATIC).

    3.It’s a window into extra dimensions. If there are more than the three spatial dimensions we’re used to, especially at smaller scales, new particles can arise in our three dimensions as a result. These Kaluza-Klein particles could show up at the LHC, and might decay to two photons. Studying how they decay could tell us whether this is true.

    4.It’s a new part of the neutrino sector. This would be a little unusual — since neutrinos don’t normally decay to two photons; they’ve got the wrong spin — but a scalar neutrino could create two photons, which is actually a thing in Standard Model extensions. The couplings and decay pathways, if it’s real, could show us this.

    5. It’s a composite particle. The first particle we ever saw decay into two photons was the lightest quark-antiquark combination of all: the neutral pion. Perhaps these Standard Model particles are combining in ways we don’t yet understand, and what we’ve found is nothing new.
    Or, most excitingly, none of the above. The most exciting discoveries are the ones you never anticipated, and perhaps it isn’t any of the speculative scenarios we know to look for. Perhaps nature is more surprising than even our wildest theoretical dreams.

    6. Or, most excitingly, none of the above. The most exciting discoveries are the ones you never anticipated, and perhaps it isn’t any of the speculative scenarios we know to look for. Perhaps nature is more surprising than even our wildest theoretical dreams.

    The answers, believe it or not, are locked inside of the smallest particles in nature. All we need are the highest energies we can get to in order to find out.

    Of course, this could simply turn out to be a statistically insignificant bump that goes away with more data; it may be nothing at all. This has already happened once before, at about three times the energy. There was hint of an extra “bump” at just over 2 TeV in both detectors, as you can see for yourself.

    7
    Images credit: ATLAS collaboration (L), via http://arxiv.org/abs/1506.00962; CMS collaboration (R), via http://arxiv.org/abs/1405.3447.

    A reanalysis of the data shows there’s no significance to this signal, and that might be what we have in the 750 GeV case, too. But the possibility that it’s real is too big to ignore, and the data will come in to tell us by the end of this year. The biggest unanswered, fundamental questions in theoretical physics will get a run for the money, and all it takes is for a bump in the data to hold up a little bit longer.

    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 10:34 am on May 14, 2016 Permalink | Reply
    Tags: , , ,   

    From Ethan Siegel: “Could A New Type Of Supernova Eliminate Dark Energy?” 

    Starts with a Bang

    May 13, 2016
    Ethan Siegel

    Supernova in Messier 101
    A game-changing supernova in the galaxy Messier 101, observed in 2011. Image credit: NASA / Swift.

    NASA/SWIFT Telescope
    NASA/SWIFT Telescope

    Every once in a while, some Earth-shattering discoveries come along that forever change our view of the Universe. Back in the late 1990s, observations of distant supernovae made it clear that the Universe wasn’t only expanding, but that distant galaxies were actually speeding up as they moved away from us, a Nobel Prize-worthy discovery that told us the fate of our Universe. By measuring their optical properties and comparing them to supernovae seen nearby, we were able to determine their distances, finding that they were fainter (and hence, more distant) compared to what we’d expect. The interpretation was that this was because the Universe was accelerating due to some form of dark energy, but a 2015 study* showed another possibility: that these supernovae appeared fainter because they were inherently different from the supernovae we saw nearby. Could this alternative explanation eliminate the need for dark energy?

    Triangulum Galaxy, European Southern Observatory (ESO).
    Triangulum Galaxy, VLT, European Southern Observatory (ESO)

    This is potentially a very, very big deal for our understanding of all there is, and how our Universe will end. Let’s go back nearly 100 years to a lesson we should have learned, and then come forward to today to see why. Back in 1923, Edwin Hubble was looking at a particular class of objects — the obscure, faint “spiral nebulae” in the sky — studying novae occurring in them and trying to add to our knowledge of just what these objects were. Some people contended that they were proto-stars within the Milky Way, while others believed them to be island Universes, millions of light years beyond our own galaxy, consisting of billions of stars apiece.

    While observing the great nebula in Andromeda on October 6th of that year, he saw a nova go off, then a second, and then a third. And then something unprecedented happened: a fourth nova went off in the same location as the first.

    Andromeda Galaxy NASA/ESA Hubble
    Andromeda Galaxy NASA/ESA Hubble

    2
    The star in the great Andromeda Nebula that changed our view of the Universe forever, as imaged first by Edwin Hubble in 1923 and then by the Hubble Space Telescope nearly 90 years later. Image credit: NASA, ESA and Z. Levay (STScI) (for the illustration); NASA, ESA and the Hubble Heritage Team (STScI/AURA) (for the image).

    NASA/ESA Hubble Telescope
    NASA/ESA Hubble Telescope

    Novae do sometimes repeat, but it usually takes hundreds or thousands of years for them to do so, as they occur only when enough fuel builds up on the surface of a collapsed star to ignite. Of all the novae we’ve ever discovered, even the most rapidly replenishing takes many years to go off again. The idea that one would repeat in only a few hours? Absurd.

    But there was something we knew about that could go from very bright to dim to bright again in just a few hours: a variable star! (Hence, his crossing out of “N” for nova and excitedly writing “VAR!”)

    3
    The Variable Star RS Puppis, with its light echoes shining through the interstellar clouds. Image credit: NASA, ESA, and the Hubble Heritage Team.

    The incredible work of Henrietta Leavitt taught us that some stars in the Universe — Cepheid variable stars — get brighter-and-dimmer with a certain period, and that period is related to their intrinsic brightness. This is important, because it means that if you measure the period (something easy to do), then you know the intrinsic brightness of the thing you’re measuring. And since you can easily measure the apparent brightness, then you can immediately know how far away that object is, because the brightness/distance relationship is something we’ve known for hundreds of years!

    4
    The brightness/distance relationship dates back to at least Christiaan Huygens in the 17th century. Image credit: E. Siegel, from his book Beyond The Galaxy.

    Now, Hubble used this knowledge of variable stars and the fact that we could find them in these spiral nebulae (now known to be galaxies) to measure their distances from us. He then combined their known redshift with these distances to derive Hubble’s Law and figure out the rate of expansion of the Universe.

    Remarkable, right? But unfortunately, we often gloss over something about this discovery: Hubble’s conclusions for what that expansion rate actually was were totally wrong!

    5
    The original graph from Hubble’s findings, and the first demonstration of Hubble’s Law. Image credit: E. Hubble, 1929.

    The problem, you see, was that the Cepheid variable stars that Hubble measured in these galaxies were intrinsically different than the Cepheids that Henrietta Leavitt measured. As it turned out, Cepheids come in two different classes, something Hubble didn’t know at the time. While Hubble’s Law still held, his initial estimates for distances were far too low, and so his estimates for the expansion rate of the Universe were far too high. In time, we got it right, and while the overall conclusions — that the Universe was expanding and that these spiral nebulae were galaxies far beyond our own — didn’t change, the details of how the Universe was expanding definitely did!

    6
    An extragalactic supernova, along with the galaxy that hosts it, from 1994. Image credit: NASA/ESA, The Hubble Key Project Team and The High-Z Supernova Search Team.

    And that brings us to the present day, and a very similar problem, this time with supernovae. Far brighter than Cepheids, supernovae can often shine nearly as brightly — albeit for a very short time — as the entire galaxy that hosts it! Instead of millions of light years away, they can be seen, under the right circumstances, more than ten billion light years distant, allowing us to probe farther and farther into the Universe. In addition, a special type of supernova, type Ia supernovae, arises from a runaway fusion reaction taking place inside a white dwarf.

    When these reactions occur, the entire star is destroyed, but more importantly, the light curve of the supernova, or how it brightens and then dims over time, is well-known, and has some universal properties.

    1
    Universal light-curve properties for Type Ia supernovae. Image credit: S. Blondin and Max Stritzinger.

    By the late 1990s, enough supernova data had been collected at large enough distances that two independent teams — the High-z Supernova Search Team and the Supernova Cosmology Project — both announced that based on this data, the Universe’s expansion was accelerating, and that there was some form of dark energy dominating the Universe.

    It’s important to be appropriately skeptical of a revolutionary discovery like this. If it turned out that there was something amiss with the interpretation of this supernova data, the entire set of conclusions reached — that the Universe was accelerating — would have disappeared entirely. There were some possibilities for why this data might not be trustworthy:

    For one, there were two different methods by which supernovae could occur: from accretion of matter from a companion star (L), and from a merger with another white dwarf (R). Would both of these result in the same type of supernova?

    7
    Two different ways to make a Type Ia supernova: the accretion scenario (L) and the merger scenario (R). These may be fundamentally different from one another. Images credit: NASA / CXC / M. Weiss.

    For another, these supernovae at great distances may have been occurring in very different environments from the ones we see close by today. Are we positive that the light curves we see today reflect the light curves at great distances?

    And for still another, it’s possible that something happened to this light during their incredible travels from great distances to our eyes. Are we sure there isn’t some new type of dust or some other light-dimming property (like photon-axion oscillations) at work here?

    As it turns out, these issues were all able to be resolved and ruled out; these things aren’t issues. But recently — and this is what the 2015 study concluded — we’ve discovered that these so-called “standard candles” may not be so standard after all. Just like the Cepheids come in different varieties, these type Ia supernovae come in different varieties too.

    8
    A Type Ia supernova in the nearby galaxy M82. This one is fundamentally different from the one atop this page, observed in 2011 in M101. Image credit: NASA/Swift/P. Brown, TAMU

    Imagine you had a box of candles that you thought were all identical to one another: you could light them up, put them all at different distances, and immediately, just from measuring the brightness you saw, know how far away they are. That’s the idea behind a standard candle in astronomy, and why type Ia supernovae are so powerful.

    But now, imagine that these candle flames aren’t all the same brightness! Suddenly, some are a little brighter and some are a little dimmer; you have two classes of candles, and while you might have more of the brighter ones close by, you might have more of the dimmer ones far away.

    That’s what we think we’ve just discovered with supernovae: there are actually two separate classes of them, where one’s a little brighter in the blue/UV, and one’s a little brighter in the red/IR, and the light curves they follow are slightly different. This might mean that, at high redshifts (large distances), the supernovae themselves are actually intrinsically fainter, and not that they’re farther away.

    In other words, the inference we drew — that the Universe is accelerating — might be based on a misinterpretation of the data!

    8
    Image credit: Ned Wright, based on the latest data from Betoule et al. (2014), via http://www.astro.ucla.edu/~wright/sne_cosmology.html.

    If we’ve got the distances wrong for these supernovae, maybe we’ve got dark energy wrong, too! At least, that would be the big worry. The smaller worry would be that dark energy is still real, but there might be less of it than we previously thought.

    So which of these worries are valid? As it turns out, only the small one, and not the big one! You see, in 1998, we only had supernova data pointing towards dark energy. But as time went on, we gained two other pieces of evidence that provided evidence that was just as strong.

    9
    The best map of the CMB and the best constraints on dark energy from it. Images credit: ESA & the Planck Collaboration (top); P. A. R. Ade et al., 2014, A&A (bottom).

    1.) The Cosmic Microwave Background. The fluctuations in the leftover glow from the Big Bang — as measured by WMAP and later, to higher precision, Planck — strongly indicated that the Universe was about 5% normal matter, 27% dark matter, and about 68% dark energy. While the microwave background doesn’t do a great job by itself of telling you what the properties of this dark energy are, it does tell you that you have about 2/3 of the Universe’s energy in a form that isn’t clumpy and massive.

    For a while, this was actually an even bigger problem, as supernovae alone indicated that about 3/4 of the Universe’s energy was dark energy. It’s possible that these new revelations about supernovae, that there are two types of Type Ia supernovae with different intrinsic light curves, could help the data line up better.

    10
    An illustration of clustering patterns due to Baryon Acoustic Oscillations. Image credit: Zosia Rostomian, Lawrence Berkeley National Laboratory.

    2.) The way galaxies cluster. In the early Universe, dark matter and normal matter — and how they do-and-do-not interact with radiation — govern how galaxies wind up clustered together in the Universe today. If you see a galaxy anywhere in the Universe, there’s this odd property that you’re more likely to have another galaxy about 500 million light years away from it than you are to have one either 400 or 600 million light years away. This is due to a phenomenon known as Baryon Acoustic Oscillations (BAO), and it’s because normal matter gets pushed out by radiation, while dark matter doesn’t.

    The thing is, the Universe is expanding due to everything in it at all times, including dark energy. So as the Universe expands, that preferred scale of 500 million light years changes. Instead of a “standard candle,” BAO allows us to have a “standard ruler,” which we can also use to measure dark energy.

    While this wasn’t the case in the late 1990s, as surveys like the 2dF GRS weren’t complete and the SDSS hadn’t even started, today’s measurements from BAO are just as good at present as the measurements from supernovae.

    SDSS Telescope at Apache Point, NM, USA
    SDSS Telescope at Apache Point, NM, USA

    What’s even more compelling is the fact that they seem to give the same results: a Universe that’s about 70% dark energy, and consistent with a cosmological constant and not domain walls, cosmic strings, or many other exotic types.

    In fact, if we combine all three data sets, we find that they all point roughly towards the same picture.

    11
    Constraints on dark energy from three independent sources: supernovae, the CMB and BAO. Note that even without supernovae, we’d need dark energy. Image credit: Supernova Cosmology Project, Amanullah, et al., Ap.J. (2010).

    What we’ve learned from this is that the amount of dark energy and the type of dark energy we infer from supernovae may change slightly and in a subtle manner, and this may actually be good for bringing the three methods — supernovae, the CMB and BAO — into better alignment. This is one of those great moments in science where one incorrect assumption doesn’t cause us to throw all our results and conclusions out, but rather where it helps us more accurately understand a phenomenon that’s puzzled us since we first discovered it. Dark energy is real, and thanks to this new discovery, we just might come to understand it — and its effects on the Universe — better than ever before.

    *Science paper:
    THE CHANGING FRACTIONS OF TYPE IA SUPERNOVA NUV–OPTICAL SUBCLASSES WITH REDSHIFT

    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 8:21 pm on May 11, 2016 Permalink | Reply
    Tags: , , , , Which Elements Will Never Be Made By Our Sun?   

    From Ethan Siegel: “Which Elements Will Never Be Made By Our Sun?” 

    Starts with a Bang

    May 11, 2016
    Ethan Siegel

    1
    A high-resolution spectrum showing the elements in the Sun, by their visible-light absorption properties. Image credit: N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF.

    Our Sun is the greatest source of heat and light in the entire Solar System, fusing hydrogen into helium in a nuclear chain reaction in its core. Because an atomic nucleus of helium is 0.7% lighter than the four hydrogen nuclei that it’s created from, that act of nuclear fusion releases a tremendously efficient amount of energy. Over the course of its 4.5 billion year lifetime (so far), the Sun had lost about the mass of Saturn due to the amount of hydrogen that’s fused into helium, through Einstein’s E = mc^2, which is the root source of all the sunlight we receive here on Earth. The Sun has a lot more going on inside of it than just fusing hydrogen (the lightest element) into helium (the second lightest), though, and is capable of making so many more elements than that. But the periodic table has a whole slew of elements the Sun can never make.

    Periodic Table 2016
    Periodic Table 2016

    We’re pretty fortunate that our Sun wasn’t among the very first stars in the Universe. Shortly after the Big Bang, the Universe was made exclusively of hydrogen and helium: 99.999999% of the Universe was composed of these two elements alone. Yet the first massive stars didn’t just fuse hydrogen into helium, but eventually fused helium into carbon, carbon into oxygen, oxygen into silicon and sulfur, and then silicon and sulfur into iron, nickel and cobalt. When the inner core reached a large enough concentration of those heavy elements, a catastrophic supernova occurred, creating a rapid burst of neutrons that were scattered into the other nuclei. Very quickly, the types of elements present in the Universe climbed up and up the periodic table, creating everything we’ve ever found in nature and many elements even heavier than that. Even the very first core-collapse supernovae created elements that are beyond the limit of what we find on Earth: elements heavier than even uranium and plutonium.

    3
    The various layers of a supernova-bound star. During the supernova itself, many trans-uranic elements are created, through rapid neutron capture. Image credit: Nicolle Rager Fuller of the NSF.

    But our Sun won’t go supernova, and won’t ever make those elements. That rapid burst of neutrons that happens in supernova allows the creation of elements through the r-process, where elements rapidly absorb neutrons and climb the periodic table in great leaps and jumps. Instead, our Sun will burn through the hydrogen in its core, and then will contract and heat up until it can begin fusing helium in its core. This phase of life — where our Sun will become a red giant star — is something that happens to all stars that are at least 40% as massive as our own.

    4
    Red Giant, SSL UC Berkeley

    Reaching the right temperatures and densities, simultaneously, for helium fusion, is what separates red dwarfs (which can’t get there) from all other stars (which can). Three helium atoms fuse together into carbon, and then through another hydrogen-fusion pathway — the CNO cycle — we can create nitrogen and oxygen, while we can continue to add helium to various nuclei to climb up the periodic table. Carbon and helium make oxygen; carbon and oxygen make neon; carbon and neon make magnesium. But two very particular reactions take place that will create the vast majority of elements we know:

    carbon-13 will fuse with helium-4, creating oxygen-16 and a free neutron, and
    neon-22 will fuse with helium-4, creating magnesium-25 and a free neutron.

    5
    Image credit: screenshot from the wikipedia article on the s-process.

    Free neutrons aren’t created in great abundance, just in relatively scarce numbers, since such a small percentage of these atoms actually are carbon-13 or neon-22 at any given time. But these free neutrons can only stick around for about 15 minutes, on average, until they decay away.

    6
    The two types (radiative and non-radiative) of neutron beta decay. Image credit: Zina Deretsky, National Science Foundation.

    Fortunately, the interior of the Sun is dense enough that 15 minutes is more than enough time for this free neutron to run into another atomic nucleus, and when it does, it inevitably gets absorbed, creating a nucleus that’s one atomic mass unit heavier than before the neutron was absorbed. There are a few nuclei this won’t work for: you can’t create a mass-5 nucleus (out of helium-4, for instance) or a mass-8 nucleus (out of lithium-7, for examples), since they’re all inherently too unstable. But everything else will either be stable on timescales of at least tens of thousands of years, or it will decay by emitting an electron (through β-decay), which causes it to move one element up the periodic table.

    8
    Image credit: E. Siegel, based on the original from the University of Oregon’s physics department, via http://zebu.uoregon.edu/2004/a321/lec10.html.

    During any star’s red giant, helium-burning phase, this enabled you to build all the elements between carbon and iron through this process of slow neutron capture, and heavy elements from iron all the way up through lead through that very same process. This process, known as the s-process (because neutrons are produced-and-captured slowly), runs into a problem when it tries to build elements heavier than lead. The most common isotope of lead is Pb-208, with 82 protons and 126 neutrons. If you add a neutron to it, it beta decays to become bismuth-209, which can then capture a neutron and β-decay again to become polonium-210. But unlike the other isotopes, which live for years, Po-210 only lives for days before emitting an alpha particle — or a helium-4 nucleus — and returning back to lead in the form of Pb-206.

    9
    The chain reaction that’s at the end of the line for the s-process. Image credit: E. Siegel and the English Language Wikipedia.

    This leads to a cycle: lead captures 3 neutrons, becomes bismuth, which captures one more and becomes polonium, which then decays back to lead. In our Sun and in all stars that won’t go supernova, that’s the end of the line. Combine that with the fact that there’s no good pathway to get the elements between helium and carbon (lithium, beryllium and boron are produced from cosmic rays, not inside of stars), and you’ll find that the Sun can make a total of 80 different elements: helium and then everything from carbon through polonium, but nothing heavier. For that, you need a supernova or a neutron star collision.

    9
    Two neutron stars colliding, which is the primary source of many of the heaviest periodic table elements in the Universe. Image credit: Dana Berry, SkyWorks Digital, Inc.

    But think about that: of all the naturally occurring elements here on Earth, the Sun makes about 90% of them, all from a tiny, non-descript star of no particular cosmic significance. The ingredients for life are literally that easy to come by.

    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

     
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