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  • 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

    1
    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.

    2
    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.

    3
    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.

    4
    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.

    5
    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.

    6
    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.

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    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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  • richardmitnick 10:54 am on May 14, 2016 Permalink | Reply
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    From Science Alert: “The existence of massive particles of light could finally explain dark energy” 

    ScienceAlert

    Science Alert

    14 MAY 2016
    BRENDAN COLE

    1
    asharkyu/Shutterstock.com

    But we’ll have to give up our fundamental understanding of light first.

    In the late 1990s, astronomers discovered something mysterious pushing galaxies apart faster than gravity pulls them together. It seemed like every little bit of space had some amount of energy that spread it away from every other little bit of space, and that strange pushing came to be known as ‘dark energy’ – dark, because no one knows what it is.

    And now a group of physicists have shown that dark energy could probably be explained – as long as we’re willing to give up a fundamental piece of our understanding of light…

    Most scientists think that dark energy exists because of what’s known as a cosmological constant – something acting throughout the Universe that tells different bits of space to repel each other. It’s sort of like an anti-gravity force, but it acts everywhere instead of just being between two things with mass and it always acts with the same strength.

    The cosmological constant explanation works, but it’s a hollow victory. Physicists don’t like having numbers they can’t explain – things like the mass of the electron, for example.

    As far as we can tell, there isn’t a way to really derive or predict an electron’s mass using other physics. It’s not because of something else; it’s just that when the Universe formed some 14 billion years ago, it was able to generate these little bits of mass that, 14 billion years later, we started calling electrons. We just have to put this number (the mass) into our equations and begrudgingly accept that we can’t explain it.

    (The appeal of string theory is that it might be able to explain some of these numbers.)

    Originally, it seemed like the cosmological constant wasn’t like this. Using all of the physics we know right now, we can predict the strength of the cosmological constant. But instead of matching what we see, the result is known as the most embarrassing number in physics: if you multiply dark energy’s measured strength by 10^120 – that’s a one with 120 zeros – you get the value we predict.

    Hypotheses that have passed every single other test we’ve ever thrown at them predict a cosmological constant 100 million trillion googols stronger than dark energy. Dark energy is a tiny effect that doesn’t match any of these giant predictions.

    This prompted physicists to look for alternatives. If dark energy isn’t caused by a cosmological constant, then the cosmological constant is actually equal to zero, and we don’t have to worry as much about predicting it. So the hunt was on for something that might produce such a tiny repulsion.

    One way to come up with something really small is to find something else that’s really small and ask if the two might be related – like trying to explain a child’s hair colour by asking what colour their parents’ hair is.

    This is what a team of physists led by Seyen Kouwn from the Korea Astronomy and Space Science Institute did – except they didn’t just take something we already know to be small, like some previous groups have. Instead, they asked what would happen if light – which we’ve been assuming for a century and a half is massless – has some very tiny mass. And what they found was surprising.

    Experiments have already proven that light can’t be more massive than about 10^62 kilograms. That’s a zero, a decimal point, 61 more zeros, and then a one, which is a really small number. But it’s not zero, as Kouwn and his team point out*.

    They showed that if the photon’s mass is 10 million times smaller than that limit, the way that photons interact with the different fields and forces in the Universe leads to a repulsive effect that looks an awful lot like what we’ve been calling dark energy. In other words, massive photons could cause dark energy.

    Physicists probably won’t be flocking to rewrite the textbooks, though. The proposed mass of the photon is, for all intents and purposes, immeasurably small, so using it instead of a cosmological constant to explain dark energy is trading something we can’t verify for something we can’t explain. And physicists dislike mechanisms they can’t measure just as much as they dislike numbers they can’t explain.

    Plus, we’re also trading one number we can’t explain for another: why should photons have this exact mass?

    It’ll be interesting to see if groups working on other problems find that massive photons explain more than just dark energy. If light having a non-zero mass turns out to solve a bunch of other unsolved problems, physicists might warm up to this very strange idea.

    *Science paper:
    Massive photon and dark energy, Physical Review D

    See the full article here .

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

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    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!”)

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    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!

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    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.

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    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?

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    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 12:34 pm on April 11, 2016 Permalink | Reply
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    From SA: “Cosmic Speed Measurement Suggests Dark Energy Mystery” 

    Scientific American

    Scientific American

    April 11, 2016
    Clara Moskowitz

    1
    ESA/NASA/Hubble

    NASA/ESA Hubble Telescope
    NASA/ESA Hubble Telescope

    Our universe is flying apart, with galaxies moving away from each other faster each moment than they were the moment before. Scientists have known about this acceleration since the late 1990s, but whatever is causing it—dubbed dark energy—remains a mystery. Now the latest measurement of how fast the cosmos is growing thickens the plot further: The universe appears to be ballooning more quickly than it should be, even after accounting for the accelerating expansion caused by dark energy.

    Scientists came to this conclusion after comparing their new measurement of the cosmic expansion rate, called the Hubble constant, to predictions of what the Hubble constant should be based on evidence from the early universe. The puzzling conflict—which was hinted at in earlier data and confirmed in the new calculation—means that either one or both of the measurements are flawed, or that dark energy or some other aspect of nature acts differently than we think.

    “The bottom line is that the universe looks like it’s expanding about eight percent faster than you would have expected based on how it looked in its youth and how we expect it to evolve,” says study leader Adam Riess of the Space Telescope Science Institute in Baltimore, Md. “We have to take this pretty darn seriously.” He and his colleagues described their findings, based on observations from the Hubble Space Telescope, in a paper* submitted last week to the Astrophysical Journal and posted on the preprint server arXiv.

    A dark energy wrinkle

    One of the most exciting possibilities is that dark energy is even stranger than the leading theory suggests. Most observations support the idea that dark energy behaves like a “cosmological constant,” a term Albert Einstein inserted into his equations of general relativity and later removed. This kind of dark energy would arise from empty space, which, according to quantum mechanics, is not empty at all, but rather filled with pairs of “virtual” particles and antiparticles that constantly pop in and out of existence. These virtual particles would carry energy, which in turn might exert a kind of negative gravity that pushes everything in the universe outward.

    The Hubble constant discrepancy, though, suggests that dark energy might actually change over space and time, potentially causing an increasing acceleration of the cosmos instead of a constant outward force. One theory proposing this type of dark energy is called quintessence, which posits that dark energy results not from the vacuum of space but from a field that pervades spacetime and can take on different values at different points.

    An alternative explanation for the discrepancy, however, is that the universe contains an additional fundamental particle beyond the ones we know about. In particular, a new species of neutrino—a nearly massless particle that comes in three known varieties so far—could account for the divergence in Hubble constant measurements. If an extra type of neutrino exists, then more of the universe’s total energy would take the form of radiation rather than matter. (Neutrinos, because they have almost no mass, travel near light speed and therefore count as radiation in this calculation). Whereas matter clumps together under gravity, a greater radiation budget would have allowed the universe to expand faster than it would have otherwise.

    And these ideas are just two of the possible implications of the measurements. Another option, for example, is that the universe is not flat, as thought, but slightly curved. Theorists are excitedly pursuing all these notions and more, but the scientists working on the experiments say they must first try to find flaws in their measurements that could account for the divergence. “Basically is there something going on in cosmology that we don’t understand, or is there something going on with the data?” says Charles Bennett of Johns Hopkins University, who has worked on measurements of the Hubble constant from the early universe and was not involved in the latest study. “One of those is lot more exciting, but I think the other may be more likely.”

    Distance ladders

    Riess and his team calculated how fast the universe is growing by comparing the distances to various galaxies with their redshifts—a measure of how much the wavelength of their light has been stretched by the expansion of the universe. Calculating the distances was a tricky feat requiring a technique the researchers call “building a distance ladder.” First they used trusted methods to measure the distances to close galaxies, then used those distances to calibrate measurements of variable stars within the galaxies. These stars, called Cepheids, brighten and dim periodically, allowing them to serve as cosmic yardsticks. Finally, the researchers used Cepheids—which are only visible relatively nearby—to calibrate measurements of a special class of supernova explosions called Type 1a, which erupt with a known brightness that allows astronomers to infer their distances. Once they had reliable measurements of the nearby supernovae, they could compare them with farther supernovae of the same type to get very accurate readings of their distances.

    This is essentially the same technique Riess and colleagues used in the 1990s to discover the first evidence that the universe’s expansion was accelerating—a finding that later won him and two others the Nobel Prize in physics. In 2011 the team made an updated measurement of the Hubble constant based on eight galaxies containing both Cepheids and Type 1a supernovae, but the new paper added 10 more. “For each one of those 10 galaxies, we observed them about 12 different times over a span of about 100 days,” says Samantha L. Hoffmann of Texas A&M University, who analyzed much of the data. “It was quite an undertaking.” The newest measurement puts the universe’s expansion rate at 73.02, plus or minus 1.79, kilometers per second per megaparsec (about 3 million light-years), meaning that for each megaparsec you go out, space is receding about 73 kilometers a second faster.

    Looking back in time

    The Hubble constant measurement from the early universe, on the other hand, comes from observations of the cosmic microwave background (CMB)—light that is left over from the big bang and pervades the entire sky.

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

    ESA/Planck
    ESA/Planck

    Researchers studied patterns in the CMB and extrapolated to modern times, based on the best known cosmological laws, to arrive at the Hubble constant. The best observations to date of the CMB were made by the European Space Agency’s Planck satellite, whose data puts the universe’s expansion rate at 67.3, plus or minus 0.7, kilometers per second per megaparsec.

    “Before, there were these hints of tension in the two measurements,” says Dan Scolnic of the University of Chicago, a member of Riess’ team. “Now both our team and the Planck team have reanalyzed and those hints have become something stronger. We have this alarm bell that there really could be something more going on. This may be the biggest tension now in cosmology.”

    The latest result is also in good agreement with other measurements of the Hubble constant based on similar distance ladder measurements, such as a 2012 study led by Wendy Freedman of the University of Chicago. “I think it’s interesting that they’ve increased their sample size and the result is essentially unchanged,” Freedman says. “This is spectacular progress to be at this point, but actually making a definitive measurement at this level requires independent methods. How this will ultimately resolve is really too early to say.” Freedman is leading an effort to perform the same calculation using another type of cosmic yardstick—RR Lyrae variable stars—in place of Cepheids.

    On the CMB side as well, scientists continue to analyze the data and look for explanations of what might have gone wrong. Bennett, who led a CMB mapping mission before the Planck experiment called the Wilkinson Microwave Anisotropy Probe (WMAP), says that there are also discrepancies within the CMB data, for instance between what the satellites measure by looking at the sky on small scales versus larger ones.

    NASA/WMAP
    NASA/WMAP

    “Before I jump to conclusions about cosmology I’d like to understand these things first,” he says. Overall, he is thrilled with the progress.

    “We went through years and years where we didn’t know the value of the Hubble constant to a factor of two, and now we’re talking about getting it within two percent,” he adds. “These things we’re comparing have a fine accuracy and that’s a testament to a lot of people in the field. The message here is that it’s not over. We need to keep driving forward.”

    *Science Paper:
    A 2.4% Determination of the Local Value of the Hubble Constant

    The science team:
    Adam G. Riess,2,3; Lucas M. Macri, 4; Samantha L. Hoffmann, 4; Dan Scolnic, 2,5; Stefano Casertano, 3;
    Alexei V. Filippenko, 6; Brad E. Tucker, 6,7; Mark J. Reid, 8; David O. Jones, 2; Jeffrey M. Silverman, 9;
    Ryan Chornock, 10; Peter Challis, 8; Wenlong Yuan, 4; and Ryan J. Foley, 11,12.

    Affiliations:
    1 Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science
    Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555.
    2 Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218
    3 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218; ariess@stsci.edu
    4 George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy,
    Department of Physics & Astronomy, Texas A&M University, 4242 TAMU,
    College Station, TX 77843
    5 Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL 60637
    6 Department of Astronomy, University of California, Berkeley, CA 94720-3411
    7 The Research School of Astronomy and Astrophysics, Australian National University, Mount Stromlo Observa-
    tory, via Cotter Road, Weston Creek, ACT 2611, Australia
    8 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138
    9 Department of Astronomy, University of Texas, Austin, TX 78712
    10 Astrophysical Institute, Department of Physics and Astronomy, 251B Clippinger Lab, Ohio University, Athens,
    OH 45701
    11 Department of Physics, University of Illinois at Urbana-Champaign, 1110 W. Green Street, Urbana, IL 61801,
    USA
    12 Department of Astronomy, University of Illinois at Urbana-Champaign, 1002 W. Green Street, Urbana, IL 61801,
    USA

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  • richardmitnick 9:52 pm on April 5, 2016 Permalink | Reply
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    From SA: “Controversial Dark Matter Claim Faces Ultimate Test” 

    Scientific American

    Scientific American

    April 5, 2016
    Davide Castelvecchi, Nature magazine

    Dark matter cosmic web and the large-scale structure it forms The Millenium Simulation, V. Springel et al
    Dark matter cosmic web and the large-scale structure it forms The Millenium Simulation, V. Springel et al.

    It is the elephant in the room for dark-matter research: a claimed detection that is hard to believe, impossible to confirm and surprisingly difficult to explain away. Now, four instruments that will use the same type of detector as the collaboration behind the claim are in the works or poised to go online. Within three years, the experiments will be able to either confirm the existence of dark matter—or rule the claim out once and for all, say the physicists who work on them.

    “This will get resolved,” says Frank Calaprice of Princeton University in New Jersey, who leads one of the efforts.

    The original claim comes from the DAMA collaboration, whose detector sits in a laboratory deep under the Gran Sasso Massif, east of Rome.

    DAMA II at Gran Sasso
    DAMA II at Gran Sasso

    Gran Sasso LABORATORI NAZIONALI del GRAN SASSO
    Gran Sasso LABORATORI NAZIONALI del GRAN SASSO

    For more than a decade, it has reported overwhelming evidence for dark matter, an invisible substance thought to bind galaxies together through its gravitational attraction. The first of the new detectors to go online, in South Korea, is due to start taking data in a few weeks. The others will follow over the next few years in Spain, Australia and, again, Gran Sasso. All will use sodium iodide crystals to detect dark matter, which no full-scale experiment apart from DAMA’s has done previously.

    Scientists have substantial evidence that dark matter exists and is at least five times as abundant as ordinary matter. But its nature remains a mystery. The leading hypothesis is that at least some of its mass is composed of weakly interacting massive particles (WIMPs), which on Earth should occasionally bump into an atomic nucleus.

    DAMA’s sodium iodide crystals should produce a flash of light if this happens in the detector. And although natural radioactivity also produces such flashes, DAMA’s claim to have detected WIMPs, first made in 1998, rests on the fact that the number of flashes produced per day has varied with the seasons.

    This, they say, is exactly what is expected if the signal is produced by WIMPs that rain down on Earth as the Solar System moves through the Milky Way’s dark-matter halo.

    Dark matter halo  Image credit: Virgo consortium / A. Amblard / ESA
    Dark matter halo Image credit: Virgo consortium / A. Amblard / ESA

    In this scenario, the number of particles crossing Earth should peak when the planet’s orbital motion lines up with that of the Sun, in early June, and should hit a low when its motion works against the Sun’s, in early December.

    There is one big problem. “If it’s really dark matter, many other experiments should have seen it already,” says Thomas Schwetz-Mangold, a theoretical physicist at the Karlsruhe Institute of Technology in Germany—and none has. But at the same time, all attempts to find weaknesses in the DAMA experiment, such as environmental effects that the researchers had not taken into account, have failed. “The modulation signal is there,” says Kaixuan Ni at the University of California, San Diego, who works on a dark-matter experiment called XENON1T.

    XENON1T
    XENON1T

    “But how to interpret that signal—whether it’s from dark matter or something else—is not clear.”

    No other full-scale experiment has used sodium iodide in its detector, although the Korea Invisible Mass Search (KIMS), in South Korea, used caesium iodide. So the possibility remains that dark matter interacts with sodium in a different way to other elements. “Not until someone has turned on a detector made of the same material will you grow convinced that nothing is there,” says Juan Collar at the University of Chicago, Illinois, who has worked on several dark-matter experiments.

    Many have found it challenging to grow sodium iodide crystals with the required purity. Contamination by potassium, which has a naturally occurring radioactive isotope, is a particular problem.

    But now three dark-matter-hunting teams—KIMS; DM-Ice, run from Yale University in New Haven, Connecticut; and ANAIS, at the University of Zaragoza, Spain—have each obtained crystals with about twice the level of background radioactivity of DAMA’s. That is pure enough to test its results, they say.

    DM-Ice at IceCube
    DM-Ice at IceCube

    ANAIS Dark Matter Experiment
    ANAIS Dark Matter Experiment

    The KIMS and DM-Ice teams have built a sodium iodide detector together at Yangyang Underground Laboratory, 160 kilometres east of Seoul. This instrument uses an ‘active veto’ sensor that will enable it to separate the dark-matter signal from the noise better than DAMA does, says Yeongduk Kim, the director of South Korea’s Center for Underground Physics in Daejeon, which manages KIMS.

    ANAIS is building a similar detector in the Canfranc Underground Laboratory in the Spanish Pyrenees. Together, KIMS/DM-Ice and ANAIS will have about 200 kilograms of sodium iodide, and they will pool their data. That is comparable to DAMA’s 250 kilograms, enabling them to catch a similar number of WIMPs, they say. Even though the newer detectors will have higher levels of background noise, it should still be possible to either falsify or reproduce the very large DAMA signal, says Reina Maruyama of Yale, who leads DM-Ice.

    But Calaprice argues that high purity is more important than mass. He and his collaborators have developed a technique to lower contamination, and in January announced that they were the first to obtain crystals purer than DAMA’s. He expects to reduce the background levels further, to one-tenth of DAMA’s.

    The project, SABRE (Sodium-iodide with Active Background Rejection), will put one detector at Gran Sasso and the other at the Stawell Underground Physics Laboratory, which is being built in a gold mine in Victoria, Australia. SABRE will also use a sensor to pull out the dark-matter signal from noise, and will have a total mass of 50 kilograms.

    SABRE should complete its research and development stage in about a year, and will build its detectors soon after that, says Calaprice. It will then make its technology available to other labs—something that DAMA did not do. And having twin detectors in both the Northern and Southern hemispheres could clarify whether environmental effects could have mimicked dark matter’s seasonality in DAMA’s results—if the signal is from WIMPs, then both detectors should see peaks at the same time.

    DAMA will wait at least until 2017 to release its latest results, says spokesperson Rita Bernabei of the University of Rome Tor Vergata. She is not holding her breath about the upcoming sodium iodide detectors. “Our results have already been verified in countless cross-checks in 14 annual cycles, so we have no reason to get excited about what others may do,” she says. If other experiments do not see the annual modulation, she adds, her collaboration will conclude that they do not yet have sufficient sensitivity.

    Could the teams prove DAMA right? “I was unwilling to believe the DAMA results or even take them seriously at first,” says Katherine Freese, a theoretical astroparticle physicist at the University of Michigan in Ann Arbor, who with her collaborators first proposed the seasonal modulation technique used by DAMA. But, in part because of a lack of any other explanation for their signal, she is now more hopeful. The fact that many have tried and failed to repeat DAMA’s experiment shows that it is not easy, says Elisabetta Barberio at the University of Melbourne, who leads the Australian arm of SABRE. “The more one looks into their experiment, the more one realizes that it is very well done.”

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  • richardmitnick 9:27 am on April 3, 2016 Permalink | Reply
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    From SAO: “The Signature of Dark Matter Annihilation, Detected?” 

    Smithsonian Astrophysical Observatory
    Smithsonian Astrophysical Observatory

    March 25, 2016
    No writer credit found

    We live in a dramatic epoch of astrophysics. Breakthrough discoveries like exoplanets, gravity waves from merging black holes, or cosmic acceleration seem to arrive every decade, or even more often. But perhaps no discovery was more unexpected, mysterious, and challenging to our grasp of the “known universe” than the recognition that the vast majority of matter in the universe cannot be directly seen. This matter is dubbed “dark matter”, and its nature is unknown. According to the latest results from the Planck satellite, a mere 4.9% of the universe is made of ordinary matter (that is, matter composed of atoms or their constituents).

    ESA/Planck
    ESA/Planck

    The rest is dark matter, and it has been firmly detected via its gravitational influence on stars and other normal matter. Dark energy is a separate constituent.

    Understanding this ubiquitous yet mysterious substance is a prime goal of modern astrophysics. Some astronomers have speculated that dark matter might have another property besides gravity in common with ordinary matter: It might come in two flavors, matter and anti-matter, that annihilate and emit high energy radiation when coming into contact. The leading class of particles in this category are called weakly interacting massive particles (WIMPS). If dark matter annihilation does occur, the range of options for the theoretical nature of dark matter would be considerably narrowed.

    CfA astronomer Doug Finkbeiner and a team of colleagues claim to have identified just such a signature of dark matter annihilation. They studied the spatial distribution of gamma-ray emission in the Milky Way, in particular gamma-ray emission from the Galactic Center region. This region is both relatively nearby and has a high matter density (and nominally a high dark matter density as well). If dark matter annihilation occurred, the location would be expected to be bright in gamma-rays. Indeed, a large gamma-ray signature has been seen from the area that extends over hundreds of light-years (there is also fainter emission extending outward for thousands of light-years). There are other possible explanations, however, most notably that the gamma-rays result from a large population of rapidly spinning pulsars, the nuclear ashes of some supernovae.

    The scientists revisited the set of previously published gamma-ray observations, applying careful new data reduction methods in order to constrain more precisely the location of the emission, and they did so for each of the several observed energy regimes of the gamma-ray emission. Pulsars have a distinctive spatial distribution: they are located where stars are found, predominantly in the plane of the galaxy. The team was able to show with high significance that the distribution of gamma-ray emission is in good agreement with the predictions of simple annihilating dark matter models, but less likely to be consistent with a pulsar explanation. Their result, if confirmed, would be an impressive breakthrough in the understanding of the nature of dark matter, the dominant constituent of the cosmos.

    Reference(s):

    The Characterization of the Gamma-Ray Signal from the Central Milky Way: A Compelling Case for Annihilating Dark Matter,” Tansu Daylan, Douglas P. Finkbeiner, Dan Hooper, Tim Linden, Stephen K. N. Portillo, Nicholas L. Rodd, and Tracy R. Slatyer, in Physics of the Dark Universe, Elsevier, 2016.

    The science team:
    Tansu Daylan a, Douglas P. Finkbeiner a, b, Dan Hooper c, d, Tim Lindene e , Stephen K.N. Portillo b, Nicholas L. Rodd f, Tracy R. Slatyerf g

    Affiliations:
    a Department of Physics, Harvard University, Cambridge, MA, United States
    b Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, United States
    c Fermi National Accelerator Laboratory, Theoretical Astrophysics Group, Batavia, IL, United States
    d University of Chicago, Department of Astronomy and Astrophysics, Chicago, IL, United States
    e University of Chicago, Kavli Institute for Cosmological Physics, Chicago, IL, United States
    f Center for Theoretical Physics, Massachusetts Institute of Technology, Boston, MA, United States
    g School of Natural Sciences, Institute for Advanced Study, Princeton, NJ, United States

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    About CfA

    The Center for Astrophysics combines the resources and research facilities of the Harvard College Observatory and the Smithsonian Astrophysical Observatory under a single director to pursue studies of those basic physical processes that determine the nature and evolution of the universe. The Smithsonian Astrophysical Observatory (SAO) is a bureau of the Smithsonian Institution, founded in 1890. The Harvard College Observatory (HCO), founded in 1839, is a research institution of the Faculty of Arts and Sciences, Harvard University, and provides facilities and substantial other support for teaching activities of the Department of Astronomy. The long relationship between the two organizations, which began when the SAO moved its headquarters to Cambridge in 1955, was formalized by the establishment of a joint center in 1973. The CfA’s history of accomplishments in astronomy and astrophysics is reflected in a wide range of awards and prizes received by individual CfA scientists.

    Today, some 300 Smithsonian and Harvard scientists cooperate in broad programs of astrophysical research supported by Federal appropriations and University funds as well as contracts and grants from government agencies. These scientific investigations, touching on almost all major topics in astronomy, are organized into the following divisions, scientific departments and service groups.

     
  • richardmitnick 8:23 am on April 2, 2016 Permalink | Reply
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    From Ethan Siegel: “Why do the tiniest galaxies have the most dark matter?” 

    Starts with a bang
    Starts with a Bang

    4.1.16
    Ethan Siegel

    Dark matter halo  Image credit: Virgo consortium / A. Amblard / ESA
    Dark matter halo Image credit: Virgo consortium / A. Amblard / ESA

    Dark matter cosmic web and the large-scale structure it forms The Millenium Simulation, V. Springel et al
    Dark matter cosmic web and the large-scale structure it forms. The Millenium Simulation, V. Springel et al http://wwwmpa.mpa-garching.mpg.de/millennium/

    Everything else has a 5:1 dark matter-to-normal matter ratio. But get a smaller and smaller galaxy, and dark matter skyrockets!

    “For the moment we might very well can them DUNNOS (for Dark Unknown Nonreflective Nondetectable Objects Somewhere).” –Bill Bryson

    When we look out at the Universe, in any direction and with a variety of methods, we find the same ratios of dark matter to normal matter all over the place: 5-to-1. Whether we’re looking at the fluctuations in the cosmic microwave background, the lensing-to-X-ray ratios of colliding clusters, the way large-scale structure clumps together or the rotation properties of the largest spiral and elliptical galaxies, that same ratio — of dark matter outmassing normal matter by a 5-to-1 ratio — exists everywhere.

    1
    Images credit: X-ray: NASA/ CXC/UVic./A.Mahdavi et al. Optical/Lensing: CFHT/UVic./A.Mahdavi et al. (top left); X-ray: NASA/CXC/UCDavis/W.Dawson et al.; Optical: NASA/STScI/UCDavis/ W.Dawson et al. (top right); ESA/XMM-Newton/F. Gastaldello (INAF/IASF, Milano, Italy)/CFHTLS (bottom left); X-ray: NASA, ESA, CXC, M. Bradac (University of California, Santa Barbara), and S. Allen (Stanford University) (bottom right). These four separate groups and clusters all show the separation between dark matter (blue) and normal matter (pink).

    NASA/Chandra Telescope
    NASA/Chandra Telescope

    CFHT Telescope, Mauna Kea, Hawaii, USA
    CFHT Interior
    CFHT Telescope, Mauna Kea, Hawaii, USA

    NASA/ESA Hubble Telescope
    NASA/ESA Hubble Telescope

    ESA/XMM Newton
    ESA/XMM Newton

    Everywhere, that is, until you start looking at the tiniest galaxies in the Universe. All the way down to Milky Way-sized galaxies, which represents the vast majority of galaxies we’ve discovered in the Universe, that 5-to-1 ratio remains constant. But when you go to smaller galaxies, down to dwarf galaxies in clusters or ultra-low-mass galaxies visible only in our local group (due to their tiny light output), you find that the less mass there is overall, the greater the dark matter fraction is.

    3
    Image credit: ESA/Hubble & NASA, of dwarf galaxy NGC 5477.

    In other words, the lower in mass your galaxy is, the smaller the percentage of stars and normal matter you’ll find inside, and the more dominated by dark matter it will turn out to be! This might seem paradoxical, since gravity affects both normal and dark matter equally. When you start from an overdense region, whether it’s a tiny one that grows into a miniature galaxy or a giant one that grows into a supermassive cluster, it should attract normal and dark matter equally.

    But if we think about it a little bit deeper — and consider the following two pictures — it might start to make sense why dark matter comes to dominate the tiniest galaxies. It isn’t because these little ones start out with more dark matter; initially, they have that same 5-to-1 ratio that everything does. But because their gravitational pull is so weak, they have a very difficult time holding on to their matter. Unfortunately for normal matter, it interacts with both light and with other normal matter, making it incredibly easy to strip away.

    Messier 82 Cigar starburst galaxy
    Image credit: NASA, ESA, The Hubble Heritage Team, (STScI / AURA); acknowledgement: M. Mountain (STScI), P. Puxley (NSF), J. Gallagher (U. Wisconsin), of the starburst galaxy Messier 82, with matter being expelled as shown by the red jets.

    When you get a large burst of star formation, you create intense, ultraviolet radiation. When the most massive stars die, they create bursts of supernovae, which ionize matter and accelerate it to near-relativistic speeds. And when you funnel matter into a black hole, it can cause jets, which eject matter into the intergalactic medium. All of these factors are at play in all galaxies, and yet these matter-ejecting effects only touch the normal matter. Because dark matter is transparent to all electromagnetic phenomena, only the normal matter gets ejected whenever you have a star-formation, stellar-death or black-hole-infalling event. On the other hand, these effects simply pass through the dark matter, and so it remains in these low-mass galaxies.

    4
    Image credit: NASA, ESA Acknowledgements: Ming Sun (UAH), and Serge Meunier, of spiral galaxy ESO 137–001 having its normal matter stripped away as it speeds through the intracluster medium.

    This discrepancy is compounded when you have a galaxy inside of a large cluster. The intergalactic medium there is dense and full of matter, and when these galaxies pass through, they do so at high speeds. Just as a strong wind can easily blow the loosely-held seeds off of a dandelion, the intra-cluster medium easily blows the normal matter off of the smaller galaxies in the Universe, leaving only the dark matter behind.

    Take all of these effects into account, and you’ll find that the smaller and lower-mass your galaxy is, the more tenuously the normal matter is held onto in the first place, making the dark matter-to-normal matter ratio that much larger. For the smallest mini-galaxies in the Universe, ratios in the thousands-to-one are common, while if you come up to Milky Way-sized galaxies, you’re back to the 5-to-1 ratio that everything else in the Universe holds to. Everything might be born with the same ratio of dark matter to normal matter, but it’s only the big winners that hang onto their normal matter for long!

    See the full article here .

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

     
  • richardmitnick 11:28 am on March 29, 2016 Permalink | Reply
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    From Symmetry: “The Milky Way’s hot spot” 

    Symmetry Mag

    Symmetry

    03/29/16
    Ali Sundermier

    Credits: NASA/JPL-Caltech/R. Hurt (SSC/Caltech)
    Milky Way map. Credits: NASA/JPL-Caltech/R. Hurt (SSC/Caltech)

    When you look up at night, the Milky Way appears as a swarm of stars arranged in a misty white band across the sky.

    But from an outside perspective, our galaxy looks more like a disk, with spiral arms of stars reaching out into the universe. At the center of this disk is a small region around which the entire pinwheel of our galaxy rotates, a region packed with exotic astronomical phenomena ranging from dark matter and newborn stars to a supermassive black hole. Astronomers call this region of the Milky Way the galactic center.

    SGR A* NASA's Chandra X-Ray Observatory
    Milky Way’s supermassive black hole SGR A* NASA’s Chandra X-Ray Observatory

    It’s a strange neighborhood, and scientists have reason to believe it’s one of the best places to hunt for dark matter.

    1
    The Spitzer Space Telescope provides an infrared view of the galactic center region.
    Courtesy of: NASA/JPL-Caltech/ESA/CXC/STScI

    NASA/Spitzer Telescope
    NASA/Spitzer Telescope

    Phenomena in our galaxy’s heart

    In the ’70s, scientists hypothesized that a supermassive black hole might be lurking in the center of the Milky Way. Black holes are points of space-time where gravity is so strong that not even light can escape.

    After decades of trying to indirectly identify the mysterious object in the galactic center by tracing the orbits of stars and gas, astronomers were finally able to calculate its mass in 2008. It weighed more than 4 million times as much as the sun, making it a prime supermassive black hole candidate.

    About 10 percent of all new star formation in the galaxy occurs in the galactic center. This is strange because local conditions produce an extreme environment in which it should be difficult for stars to form.

    Scientists believe that at least some of the new stars being formed should explode and transform into pulsars, but they aren’t seeing any. Pulsars emit a regular pulsating signal, like a lighthouse. One early explanation for the apparent lack of pulsars in the galactic center was that the magnetic fields there could be bending their radio waves on their way to us, hiding their pulsating signals. But recently scientists measured the strength of the fields and realized the bending was much less than they had anticipated. The mystery of the missing pulsars remains unsolved.

    The galactic center also has a notably high concentration of cosmic rays, high-energy charged particles that hurtle through outer space. Scientists still don’t understand where these particles come from or how they reach such intense energies.

    2
    The Hubble Space Telescope, though better known for its visible light images, also captured an infrared light picture of the galactic center (the bright patch in the lower right).
    Courtesy of: NASA/JPL-Caltech/ESA/CXC/STScI

    NASA/ESA Hubble Telescope
    NASA/ESA Hubble Telescope

    Hunting for dark matter

    We know that the Milky Way is rotating because when we look along it, we see some stars moving towards us and some stars moving away. But the speed at which our galaxy rotates is faster than it should be for the amount of matter we can see.

    This leads scientists to believe that there is matter located in the center of our galaxy that we cannot see. Despite all of the other stuff going on there, this makes the inner galaxy the perfect hunting ground for this “dark matter,” an invisible substance that makes up most of the matter in the universe.

    Scientists looking for dark matter take advantage of the fact that it likely interacts with itself. Researchers predict that when dark matter particles run into each other, they annihilate. They believe that this might produce a distinctive spectrum of gamma rays.

    Over the past few years, scientists have detected an excess of gamma rays from the Milky Way’s galactic center. Many scientists believe that this could be a very strong signal for dark matter. The events look the way they would expect dark matter to look, and the energy spectrum and the way the gamma rays are concentrated resemble what scientists would expect from dark matter.

    Other scientists believe that it is pulsars, not dark matter, that create this signal. Because the excess appears clumped, instead of smooth, scientists believe that it could be coming from compact sources like an ancient population of pulsars.

    To determine whether this excess is a dark matter signal, scientists are looking for similar signatures elsewhere in the universe, in places like dwarf galaxies. These small galaxies are cleaner places to look for dark matter with a lot less going on, but the trade-off is that they do not produce as much gamma radiation.

    See the full article here .

    Please help promote STEM in your local schools.

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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 12:56 pm on March 26, 2016 Permalink | Reply
    Tags: , , , Eternal inflation?,   

    From Ethan Siegel: “Could Dark Energy Recycle The Universe?” 

    Starts with a bang
    Starts with a Bang

    3.26.16
    Ethan Siegel

    Inflationary Universe. NASA/WMAP
    Inflationary Universe. NASA/WMAP

    Our Universe is becoming colder, sparser and emptier. But is that its inevitable, ultimate fate?

    “Quintessence is a dynamic, time-evolving, and spatially dependent form of energy with negative pressure sufficient to drive the accelerating expansion […] Whereas the cosmological constant is a very specific form of energy.”
    -Robert Caldwell, inventor of the Big Rip scenario

    There’s something eerily similar about the start of our Universe, a period of cosmic inflation, and the driver of the ultimate fate, the accelerated expansion of dark energy, that leads one to speculate if they might be related. In fact, this week’s chosen question comes from Andrew Gillett, who wants to know:

    If eternal inflation is correct, could dark energy be a precursor to a return to that original state?

    Not only is it possible, it might not even require eternal inflation to be correct. Let’s start by talking about the stage that preceded the birth of the Universe as we know it and set it up: cosmic inflation.

    When the Universe as we know it — full of matter and radiation — began, it began with a few strange properties that didn’t necessarily have to be so: it was spatially flat, it was the same temperature everywhere, it didn’t have ultra-high-energy relics, and it had a very particular pattern of overdense and underdense regions. It’s possible that the Universe just began with these conditions in place, but the idea of cosmic inflation was that if the Universe started off with a period of exponential expansion, where there was a large amount of energy inherent to space itself, and then that period came to an end, it would create the hot Big Bang with all of these conditions already in place. It took a number of years for the consequences to be worked out properly, and it took even longer for the evidence from the fluctuations in the cosmic microwave background to validate it, but cosmic inflation is now understood to be the first thing we can point to with supporting evidence for it in our Universe’s history.

    Inflation to gravitational waves derived from ESAPlanck and the DoENASA NSF interagency task force on CMB research
    Image credit: Eric Siegel. Inflation to gravitational waves derived from ESA/Planck and the DOE/NASA NSF interagency task force on CMB research

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

    MIT/Caltech Advanced aLIGO Hanford Washington USA installation
    MIT/Caltech Advanced aLIGO Hanford Washington USA installation, which detected gravitational waves

    Black holes merging Swinburne Astronomy Productions
    Black holes merging, the source of gravitational waves, Swinburne Astronomy Productions

    ESA/Planck
    ESA/Planck

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

    Eternal inflation is an offshoot of inflation, based on a property you might not think about very often. Normally, when you have a transition in nature — like a pot of very hot water that’s transitioning from the liquid state to the gaseous state — it happens in different locations to start, and those locations expand and merge together. In the case of boiling water, we call this “percolating,” when the small bubbles rise and merge together, creating larger bubbles by time they reach the surface. In inflation, however, you have this problem where the regions where inflation doesn’t end at a particular point in time continue to expand exponentially, and this prevents the regions where it does end from “percolating.” Our observable Universe, therefore, must all be contained within a single bubble where inflation ended, rather than being made from many bubbles that percolated together.

    At the far other end of the spectrum, though, there’s the fact that our Universe’s expansion appears to be accelerating. The best explanation for this, to the greatest precision and accuracy we’ve measured it, is that there’s a small component of energy inherent to space itself: what we refer to as dark energy. This energy component is omnipresent — it exists at all locations equally in space — and it’s extremely small: if you converted it into mass via Einstein’s E = mc^2, it would only equate to one proton per cubic meter of the Universe. But space is not only very large, it’s also expanding! So as time goes on, this dark energy becomes more and more important, eventually, after some 8 billion years, causing the expansion of the Universe to accelerate and to later become the dominant component of energy in the Universe.

    These two periods might seem very different: inflation and the late-time accelerated expansion. Indeed, the magnitude of these energy scales are different by about a factor of 10¹²⁰, which is tremendous! But they both represent energy inherent to space itself, they both cause the fabric of space to expand exponentially, and given enough time — fractions of a second for inflation and a trillion years for dark energy — they will take everything that isn’t bound together into a single structure in the Universe and drive it apart. There are a whole class of models out there, known generically as quintessence, that seek to unify inflation and dark energy.

    So what are the possibilities for our Universe to recycle itself? There are two good ones.

    1.) If dark energy is truly a cosmological constant, it might be the leftover, relic energy from the inflationary period that started it all. And if that’s the case, there’s no reason why, given enough time, it couldn’t further decay to a much lower energy state! Perhaps that transition will give rise to a large number of extremely low-mass particles, like neutrinos, axions or something even more exotic, that may yet bind together to form their own analogues to stars, planets or even humans on long enough timescales. Just because it isn’t really accessible to us doesn’t mean it isn’t possible, and it’s one potential fate for the very long-term future of our Universe, even if it takes googols of years to occur.

    2.) Dark energy may not be a cosmological constant, but may actually increase in strength over time. If it does, then it will continue to rise and rise, potentially leading to a “big rip” scenario where every bound structure in the Universe eventually tears itself apart. But under a scenario developed by Eric Gawiser, it’s possible that right at the final moment — just before space itself rips into oblivion — that energy inherent to space, which would be indistinguishible from inflationary scenarios, transitions… into a hot Big Bang! This “rejuvenated Universe” scenario may not only be in our far future, but it could make our Universe much older than it appears, possibly even infinitely old.


    Access mp4 video here .

    Right now, the best evidence we have points towards dark energy truly being a cosmological constant, meaning that scenario #2 is out. If there is no lower-energy state for it to transition to, then scenario #1 is out as well, but we don’t know enough to rule either one of them out for now. If I had to bet, I’d say the lower-energy transition is more likely, but the idea that dark energy is truly a constant that exists for an eternity is better supported by the data we have available. But until we know for sure, we have to keep our minds open to all possibilities! The EUCLID mission, NASA’s WFIRST and finally the LSST will help us measure dark energy to an even better precision, which should turn up evidence either for or against the latter of these two possibilities, while developments in theoretical high-energy physics may tell us more about the possibility of the first.

    ESA/Euclid spacecraft
    ESA/Euclid spacecraft

    NASA/WFIRST New
    NASA/WFIRST

    LSST/Camera, built at SLAC
    LSST Interior
    LSST, currently under construction in Chile
    LSST/Camera, built at SLAC, LSST telescope, currently under construction in Chile.

    No matter what, the answer to your question, Andrew, is that dark energy may herald a return to a hot Big Bang from an inflation-like state, but it isn’t dependent on inflation’s eternal nature!

    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 2:03 pm on March 25, 2016 Permalink | Reply
    Tags: , , , Dark matter halos   

    From AAS NOVA: “Simulating Halos with the Caterpillar Project” 

    AASNOVA

    American Astronomical Society

    25 March 2016
    Susanna Kohler

    Caterpillar Project A Milky-Way-size dark-matter halo and its subhalos circled, an enormous suite of simulations . Griffen et al. 2016
    Caterpillar Project A Milky-Way-size dark-matter halo and its subhalos circled, an enormous suite of simulations . Griffen et al. 2016

    The Caterpillar Project is a beautiful series of high-resolution cosmological simulations. The goal of this project is to examine the evolution of dark-matter halos like the Milky Way’s, to learn about how galaxies like ours formed. This immense computational project is still in progress, but the Caterpillar team is already providing a look at some of its first results.
    Lessons from Dark-Matter Halos

    Why simulate the dark-matter halos of galaxies? Observationally, the formation history of our galaxy is encoded in “galactic fossil record” clues, like the tidal debris from disrupted satellite galaxies in the outer reaches of our galaxy, or chemical abundance patterns throughout our galactic disk and stellar halo.

    But to interpret this information in a way that lets us learn about our galaxy’s history, we need to first test galaxy formation and evolution scenarios via cosmological simulations. Then we can compare the end result of these simulations to what we observe today.

    1
    This figure illustrates the difference that mass resolution makes. In the left panel, the mass resolution is 1.5*10^7 solar masses per particle. In the right panel, the mass resolution is 3*10^4 solar masses per particle [Griffen et al. 2016]

    A Computational Challenge

    Due to how computationally expensive such simulations are, previous N-body simulations of the growth of Milky-Way-like halos have consisted of only one or a few halos each. But in order to establish a statistical understanding of how galaxy halos form — and find out whether the Milky Way’s halo is typical or unusual! — it is necessary to simulate a larger number of halos.

    In addition, in order to accurately follow the formation and evolution of substructure within the dark-matter halos, these simulations must be able to resolve the smallest dwarf galaxies, which are around a million solar masses. This requires an extremely high mass resolution, which adds to the computational expense of the simulation.

    First Outcomes

    These are the challenges faced by the Caterpillar Project, detailed in a recently published paper led by Brendan Griffen (Massachusetts Institute of Technology). The Caterpillar Project was designed to simulate 70 Milky-Way-size halos (quadrupling the total number of halos that have been simulated in the past!) at a high mass resolution (10,000 solar masses per particle) and time resolution (5 Myr per snapshot). The project is extremely computationally intense, requiring 14 million CPU hours and 700 TB of data storage!

    2
    Mass evolution of the first 24 Caterpillar halos (selected to be Milky-Way-size at z=0). The inset panel shows the mass evolution normalized by the halo mass at z=0, demonstrating the highly varied evolution these different halos undergo. [Griffen et al. 2016]

    In this first study, the Griffen and collaborators show the end states for the first 24 halos of the project, evolved from a large redshift to today (z=0). They use these initial results to demonstrate the integrity of their data and the utility of their methods, which include new halo-finding techniques that recover more substructure within each halo.

    The first results from the Caterpillar Project are already enough to show clear general trends, such as the highly variable paths the different halos take as they merge, accrete, and evolve, as well as how different their ends states can be. Statistically examining the evolution of these halos is an important next step in providing insight into the origin and evolution of the Milky Way, and helping us to understand how our galaxy differs from other galaxies of similar mass. Keep an eye out for future results from this project!

    Bonus

    Check out this video of how the first 24 Milky-Way-like halos from the Caterpillar simulations form. Seeing these halos evolve simultaneously is an awesome way to identify the similarities and differences between them.


    Access mp4 video here .

    Citation

    Brendan F. Griffen et al 2016 ApJ 818 10. doi:10.3847/0004-637X/818/1/10
    If you access the science paper, you can see who is on the science team and the astronomical assets used in the study.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

     
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