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  • richardmitnick 2:34 pm on April 2, 2019 Permalink | Reply
    Tags: , , , , , , , , Hubble Constant, ,   

    From University of Chicago: “How to use gravitational waves to measure the expansion of the universe” 

    U Chicago bloc

    From University of Chicago

    Mar 28, 2019
    Louise Lerner


    Prof. Daniel Holz discusses a new way to calculate the Hubble constant, a crucial number that measures the expansion rate of the universe and holds answers to questions about the universe’s size, age and history. Video by UChicago Creative

    Ripples in spacetime lead to new way to determine size and age of universe.

    On the morning of Aug. 17, 2017, after traveling for more than a hundred million years, the aftershocks from a massive collision in a galaxy far, far away finally reached Earth.

    These ripples in the fabric of spacetime, called gravitational waves, tripped alarms at two ultra-sensitive detectors called LIGO, sending texts flying and scientists scrambling.


    VIRGO Gravitational Wave interferometer, near Pisa, Italy


    Caltech/MIT Advanced aLigo Hanford, WA, USA installation


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

    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

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

    Gravity is talking. Lisa will listen. Dialogos of Eide

    ESA/eLISA the future of gravitational wave research

    Localizations of gravitational-wave signals detected by LIGO in 2015 (GW150914, LVT151012, GW151226, GW170104), more recently, by the LIGO-Virgo network (GW170814, GW170817). After Virgo came online in August 2018


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

    One of the scientists was Prof. Daniel Holz at the University of Chicago. The discovery had provided him the information he needed to make a groundbreaking new measurement of one of the most important numbers in astrophysics: the Hubble constant, which is the rate at which the universe is expanding.

    The Hubble constant holds the answers to big questions about the universe, like its size, age and history, but the two main ways to determine its value have produced significantly different results. Now there was a third way, which could resolve one of the most pressing questions in astronomy—or it could solidify the creeping suspicion, held by many in the field, that there is something substantial missing from our model of the universe.

    “In a flash, we had a brand-new, completely independent way to make a measurement of one of the most profound quantities in physics,” said Holz. “That day I’ll remember all my life.”

    As LIGO and its European counterpart VIRGO turn back on on April 1, Holz and other scientists are preparing for more data that could shed light on some of the universe’s biggest questions.

    Universal questions

    We’ve known the universe is expanding for a long time (ever since eminent astronomer and UChicago alum Edwin Hubble made the first measurement of the expansion in 1929, in fact),

    Edwin Hubble looking through a 100-inch Hooker telescope at Mount Wilson in Southern California, 1929 discovers the Universe is Expanding

    but in 1998, scientists were stunned to discover that the rate of expansion is not slowing as the universe ages, but actually accelerating over time. In the following decades, as they tried to precisely determine the rate, it has become apparent that different methods for measuring the rate produce different answers.

    One of the two methods measures the brightness of supernovae–exploding stars– in distant galaxies;

    Standard Candles to measure age and distance of the universe from supernovae NASA

    the other looks at tiny fluctuations in the cosmic microwave background [CMB], the faint light left over from the Big Bang.

    CMB per ESA/Planck

    ESA/Planck 2009 to 2013

    Scientists have been working for two decades to boost the accuracy and precision for each measurement, and to rule out any effects which might be compromising the results; but the two values still stubbornly disagree by almost 10 percent.

    2
    A neutron star collision causes detectable ripples in the fabric of spacetime, which are called gravitational waves. Photo courtesy of Aurore Simonnet

    Because the supernova method looks at relatively nearby objects, and the cosmic microwave background is much more ancient, it’s possible that both methods are right—and that something profound about the universe has changed since the beginning of time.

    “We don’t know if one or both of the other methods have some kind of systematic error, or if they actually reflect a fundamental truth about the universe that is missing from our current models,” said Holz. “Either is possible.”

    Holz saw the possibility for a third, completely independent way to measure the Hubble constant—but it would depend on a combination of luck and extreme feats of engineering.

    The ‘standard siren’

    In 2005, Holz wrote a paper [NJP] with Scott Hughes of Massachusetts Institute of Technology suggesting that it would be possible to calculate the Hubble constant through a combination of gravitational waves and light. They called these sources “standard sirens,” a nod to “standard candles”, which refers to the supernovae used to make the Hubble constant measurement.

    But first it would take years to develop technology that could pick up something as ephemeral as ripples in the fabric of spacetime. That’s LIGO: a set of enormous, extremely sensitive detectors that are tuned to pick up the gravitational waves that are emitted when something big happens somewhere in the universe.

    The Aug. 17, 2017 waves came from two neutron stars, which had spiraled around and around each other in a faraway galaxy before finally slamming together at close to the speed of light. The collision sent gravitational waves rippling across the universe and also released a burst of light, which was picked up by telescopes on and around Earth.

    Neutron star collision-Robin Dienel-The Carnegie Institution for Science

    3
    Prof. Daniel Holz writes out the formula for the Hubble constant, which measures the rate at which the universe is expanding.

    That burst of light was what sent the scientific world into a tizzy. LIGO had picked up gravitational wave readings before, but all the previous ones were from collisions of two black holes, which can’t be seen with conventional telescopes.

    But they could see the light from the colliding neutron stars, and the combination of waves and light unlocked a treasure trove of scientific riches. Among them were the two pieces of information Holz needed to make his calculation of the Hubble constant.

    How does the method work?

    To make this measurement of the Hubble constant, you need to know how fast an object—like a newly collided pair of neutron stars—is receding away from Earth, and how far away it was to begin with. The equation is surprisingly simple. It looks like this: The Hubble constant is the velocity of the object divided by the distance to the object, or H=v/d.

    Somewhat counterintuitively, the easiest part to calculate is how fast the object is moving. Thanks to the bright afterglow given off by the collision, astronomers could point telescopes at the sky and pinpoint the galaxy where the neutron stars collided. Then they can take advantage of a phenomenon called redshift: As a faraway object moves away from us, the color of the light it’s giving off shifts slightly towards the red end of the spectrum. By measuring the color of the galaxy’s light, they can use this reddening to estimate how fast the galaxy is moving away from us. This is a century-old trick for astronomers.

    The more difficult part is getting an accurate measure of the distance to the object. This is where gravitational waves come in. The signal the LIGO detectors pick up gets interpreted as a curve, like this:

    4
    The signal picked up by the LIGO detector in Louisiana, as it caught the waves from two neutron stars colliding far away in space, forms a distinctive curve. Courtesy of LIGO

    The shape of the signal tells scientists how big the two stars were and how much energy the collision gave off. By comparing that with how strong the waves were when they reached Earth, they could infer how far away the stars must have been.

    The initial value from just this one standard siren came out to be 70 kilometers per second per megaparsec. That’s right in between the other two methods, which find about 73 (from the supernova method) and 67 (from the cosmic microwave background).

    Of course, that initial standard siren measurement is only from one data point, and large uncertainties remain. But the LIGO detectors are turning back on after an upgrade to boost their sensitivity. Nobody knows how often neutron stars collide, but Holz (along with former student Hsin-Yu Chen and current student Maya Fishbach) wrote a paper estimating that the gravitational wave method may provide a revolutionary, extremely precise measurement of the Hubble constant within five years.

    “As time goes on, we’ll observe more and more of these binary neutron star mergers, and use them as standard sirens to steadily improve our estimate of the Hubble constant. Depending on where our value falls, we might confirm one method or the other. Or we might find an entirely different value,” Holz said. “No matter what we find, it’s gonna be interesting—and will be an important step in learning more about our universe.”

    See the full article here .

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    One of the world’s premier academic and research institutions, the University of Chicago has driven new ways of thinking since our 1890 founding. Today, UChicago is an intellectual destination that draws inspired scholars to our Hyde Park and international campuses, keeping UChicago at the nexus of ideas that challenge and change the world.

    The University of Chicago is an urban research university that has driven new ways of thinking since 1890. Our commitment to free and open inquiry draws inspired scholars to our global campuses, where ideas are born that challenge and change the world.

    We empower individuals to challenge conventional thinking in pursuit of original ideas. Students in the College develop critical, analytic, and writing skills in our rigorous, interdisciplinary core curriculum. Through graduate programs, students test their ideas with UChicago scholars, and become the next generation of leaders in academia, industry, nonprofits, and government.

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  • richardmitnick 11:09 am on January 23, 2019 Permalink | Reply
    Tags: , , , , H0liCOW collaboration, Hubble Constant, Quasar SDSS J1206+4332, Seeing double could help resolve dispute about how fast the universe is expanding,   

    From UCLA Newsroom: “Seeing double could help resolve dispute about how fast the universe is expanding” 


    From UCLA Newsroom

    January 22, 2019
    Christopher Crockett

    1
    A Hubble Space Telescope picture of a doubly imaged quasar. NASA Hubble Space Telescope, Tommaso Treu/UCLA, and Birrer et al.

    The question of how quickly the universe is expanding has been bugging astronomers for almost a century. Different studies keep coming up with different answers — which has some researchers wondering if they’ve overlooked a key mechanism in the machinery that drives the cosmos.

    Now, by pioneering a new way to measure how quickly the cosmos is expanding, a team led by UCLA astronomers has taken a step toward resolving the debate. The group’s research is published today in Monthly Notices of the Royal Astronomical Society.

    At the heart of the dispute is the Hubble constant, a number that relates distances to the redshifts of galaxies — the amount that light is stretched as it travels to Earth through the expanding universe. Estimates for the Hubble constant range from about 67 to 73 kilometers per second per megaparsec, meaning that two points in space 1 megaparsec apart (the equivalent of 3.26 million light-years) are racing away from each other at a speed between 67 and 73 kilometers per second.

    “The Hubble constant anchors the physical scale of the universe,” said Simon Birrer, a UCLA postdoctoral scholar and lead author of the study. Without a precise value for the Hubble constant, astronomers can’t accurately determine the sizes of remote galaxies, the age of the universe or the expansion history of the cosmos.

    Most methods for deriving the Hubble constant have two ingredients: a distance to some source of light and that light source’s redshift. Looking for a light source that had not been used in other scientists’ calculations, Birrer and colleagues turned to quasars, fountains of radiation that are powered by gargantuan black holes. And for their research, the scientists chose one specific subset of quasars — those whose light has been bent by the gravity of an intervening galaxy, which produces two side-by-side images of the quasar on the sky.

    Light from the two images takes different routes to Earth. When the quasar’s brightness fluctuates, the two images flicker one after another, rather than at the same time. The delay in time between those two flickers, along with information about the meddling galaxy’s gravitational field, can be used to trace the light’s journey and deduce the distances from Earth to both the quasar and the foreground galaxy. Knowing the redshifts of the quasar and galaxy enabled the scientists to estimate how quickly the universe is expanding.

    The UCLA team, as part of the international H0liCOW collaboration, had previously applied the technique to study quadruply imaged quasars, in which four images of a quasar appear around a foreground galaxy. But quadruple images are not nearly as common — double-image quasars are thought to be about five times as abundant as the quadruple ones.

    To demonstrate the technique, the UCLA-led team studied a doubly imaged quasar known as SDSS J1206+4332; they relied on data from the Hubble Space Telescope, the Gemini and W.M. Keck observatories, and from the Cosmological Monitoring of Gravitational Lenses, or COSMOGRAIL, network — a program managed by Switzerland’s Ecole Polytechnique Federale de Lausanne that is aimed at determining the Hubble constant.

    NASA/ESA Hubble Telescope

    Gemini/North telescope at Maunakea, Hawaii, USA,4,207 m (13,802 ft) above sea level

    Keck Observatory, Maunakea, Hawaii, USA.4,207 m (13,802 ft), above sea level,

    2

    Tommaso Treu, a UCLA professor of physics and astronomy and the paper’s senior author, said the researchers took images of the quasar every day for several years to precisely measure the time delay between the images. Then, to get the best estimate possible of the Hubble constant, they combined the data gathered on that quasar with data that had previously been gathered by their H0liCOW collaboration on three quadruply imaged quasars.

    “The beauty of this measurement is that it’s highly complementary to and independent of others,” Treu said.

    The UCLA-led team came up with an estimate for the Hubble constant of about 72.5 kilometers per second per megaparsec, a figure in line with what other scientists had determined in research that used distances to supernovas — exploding stars in remote galaxies — as the key measurement. However, both estimates are about 8 percent higher than one that relies on a faint glow from all over the sky called the cosmic microwave background, a relic from 380,000 years after the Big Bang, when light traveled freely through space for the first time.

    “If there is an actual difference between those values, it means the universe is a little more complicated,” Treu said.

    On the other hand, Treu said, it could also be that one measurement — or all three — are wrong.

    The researchers are now looking for more quasars to improve the precision of their Hubble constant measurement. Treu said one of the most important lessons of the new paper is that doubly imaged quasars give scientists many more useful light sources for their Hubble constant calculations. For now, though, the UCLA-led team is focusing its research on 40 quadruply imaged quasars, because of their potential to provide even more useful information than doubly imaged ones.

    Sixteen other researchers from 13 institutions in seven countries contributed to the paper; the research was supported in part by grants from NASA, the National Science Foundation and the Packard Foundation.

    See the full article here .


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    For nearly 100 years, UCLA has been a pioneer, persevering through impossibility, turning the futile into the attainable.

    We doubt the critics, reject the status quo and see opportunity in dissatisfaction. Our campus, faculty and students are driven by optimism. It is not naïve; it is essential. And it has fueled every accomplishment, allowing us to redefine what’s possible, time after time.

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  • richardmitnick 8:45 am on July 12, 2018 Permalink | Reply
    Tags: , , , , Hubble Constant, , Using gravitational waves emitted by a relatively rare system: a black hole-neutron star binary to measure how fast our universe is expanding?   

    From Kavli MIT Institute For Astrophysics and Space Research: “Could gravitational waves reveal how fast our universe is expanding?” 

    KavliFoundation

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    From Kavli MIT Institute For Astrophysics and Space Research

    Signals from rare black hole-neutron star pairs could pinpoint rate at which universe is growing, researchers say.

    July 11, 2018
    Jennifer Chu

    Since it first exploded into existence 13.8 billion years ago, the universe has been expanding, dragging along with it hundreds of billions of galaxies and stars, much like raisins in a rapidly rising dough.

    Astronomers have pointed telescopes to certain stars and other cosmic sources to measure their distance from Earth and how fast they are moving away from us — two parameters that are essential to estimating the Hubble constant, a unit of measurement that describes the rate at which the universe is expanding.

    But to date, the most precise efforts have landed on very different values of the Hubble constant, offering no definitive resolution to exactly how fast the universe is growing. This information, scientists believe, could shed light on the universe’s origins, as well as its fate, and whether the cosmos will expand indefinitely or ultimately collapse.

    Now scientists from MIT and Harvard University have proposed a more accurate and independent way to measure the Hubble constant, using gravitational waves emitted by a relatively rare system: a black hole-neutron star binary, a hugely energetic pairing of a spiraling black hole and a neutron star. As these objects circle in toward each other, they should produce space-shaking gravitational waves and a flash of light when they ultimately collide.

    In a paper to be published July 12th in Physical Review Letters, the researchers report that the flash of light would give scientists an estimate of the system’s velocity, or how fast it is moving away from the Earth. The emitted gravitational waves, if detected on Earth, should provide an independent and precise measurement of the system’s distance. Even though black hole-neutron star binaries are incredibly rare, the researchers calculate that detecting even a few should yield the most accurate value yet for the Hubble constant and the rate of the expanding universe.

    “Black hole-neutron star binaries are very complicated systems, which we know very little about,” says Salvatore Vitale, assistant professor of physics at MIT and lead author of the paper. “If we detect one, the prize is that they can potentially give a dramatic contribution to our understanding of the universe.”

    Vitale’s co-author is Hsin-Yu Chen of Harvard.

    Competing constants

    Two independent measurements of the Hubble constant were made recently, one using NASA’s Hubble Space Telescope and another using the European Space Agency’s Planck satellite.

    NASA/ESA Hubble Telescope

    ESA/Planck 2009 to 2013

    The Hubble Space Telescope’s measurement is based on observations of a type of star known as a Cepheid variable, as well as on observations of supernovae. Both of these objects are considered “standard candles,” for their predictable pattern of brightness, which scientists can use to estimate the star’s distance and velocity.

    The other type of estimate is based on observations of the fluctuations in the cosmic microwave background [CMB] — the electromagnetic radiation that was left over in the immediate aftermath of the Big Bang, when the universe was still in its infancy. While the observations by both probes are extremely precise, their estimates of the Hubble constant disagree significantly.

    CMB per ESA/Planck

    “That’s where LIGO comes into the game,” Vitale says.

    LIGO, or the Laser Interferometry Gravitational-Wave Observatory, detects gravitational waves — ripples in the Jell-O of space-time, produced by cataclysmic astrophysical phenomena.


    Caltech/MIT Advanced aLigo Hanford, WA, USA installation


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

    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project


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

    ESA/eLISA the future of gravitational wave research

    “Gravitational waves provide a very direct and easy way of measuring the distances of their sources,” Vitale says. “What we detect with LIGO is a direct imprint of the distance to the source, without any extra analysis.”

    In 2017, scientists got their first chance at estimating the Hubble constant from a gravitational-wave source, when LIGO and its Italian counterpart Virgo detected a pair of colliding neutron stars for the first time. The collision released a huge amount of gravitational waves, which researchers measured to determine the distance of the system from Earth. The merger also released a flash of light, which astronomers focused on with ground and space telescopes to determine the system’s velocity.

    With both measurements, scientists calculated a new value for the Hubble constant. However, the estimate came with a relatively large uncertainty of 14 percent, much more uncertain than the values calculated using the Hubble Space Telescope and the Planck satellite.

    Vitale says much of the uncertainty stems from the fact that it can be challenging to interpret a neutron star binary’s distance from Earth using the gravitational waves that this particular system gives off.

    “We measure distance by looking at how ‘loud’ the gravitational wave is, meaning how clear it is in our data,” Vitale says. “If it’s very clear, you can see how loud it is, and that gives the distance. But that’s only partially true for neutron star binaries.”

    That’s because these systems, which create a whirling disc of energy as two neutron stars spiral in toward each other, emit gravitational waves in an uneven fashion. The majority of gravitational waves shoot straight out from the center of the disc, while a much smaller fraction escapes out the edges. If scientists detect a “loud” gravitational wave signal, it could indicate one of two scenarios: the detected waves stemmed from the edge of a system that is very close to Earth, or the waves emanated from the center of a much further system.

    “With neutron star binaries, it’s very hard to distinguish between these two situations,” Vitale says.

    A new wave

    In 2014, before LIGO made the first detection of gravitational waves, Vitale and his colleagues observed that a binary system composed of a black hole and a neutron star could give a more accurate distance measurement, compared with neutron star binaries. The team was investigating how accurately one could measure a black hole’s spin, given that the objects are known to spin on their axes, similarly to Earth but much more quickly.

    The researchers simulated a variety of systems with black holes, including black hole-neutron star binaries and neutron star binaries. As a byproduct of this effort, the team noticed that they were able to more accurately determine the distance of black hole-neutron star binaries, compared to neutron star binaries. Vitale says this is due to the spin of the black hole around the neutron star, which can help scientists better pinpoint from where in the system the gravitational waves are emanating.

    “Because of this better distance measurement, I thought that black hole-neutron star binaries could be a competitive probe for measuring the Hubble constant,” Vitale says. “Since then, a lot has happened with LIGO and the discovery of gravitational waves, and all this was put on the back burner.”

    Vitale recently circled back to his original observation, and in this new paper, he set out to answer a theoretical question:

    “Is the fact that every black hole-neutron star binary will give me a better distance going to compensate for the fact that potentially, there are far fewer of them in the universe than neutron star binaries?” Vitale says.

    To answer this question, the team ran simulations to predict the occurrence of both types of binary systems in the universe, as well as the accuracy of their distance measurements. From their calculations, they concluded that, even if neutron binary systems outnumbered black hole-neutron star systems by 50-1, the latter would yield a Hubble constant similar in accuracy to the former.

    More optimistically, if black hole-neutron star binaries were slightly more common, but still rarer than neutron star binaries, the former would produce a Hubble constant that is four times as accurate.

    “So far, people have focused on binary neutron stars as a way of measuring the Hubble constant with gravitational waves,” Vitale says. “We’ve shown there is another type of gravitational wave source which so far has not been exploited as much: black holes and neutron stars spiraling together,” Vitale says. “LIGO will start taking data again in January 2019, and it will be much more sensitive, meaning we’ll be able to see objects farther away. So LIGO should see at least one black hole-neutron star binary, and as many as 25, which will help resolve the existing tension in the measurement of the Hubble constant, hopefully in the next few years.”

    This research was supported, in part, by the National Science Foundation and the LIGO Laboratory.

    See the full article here .


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    The mission of the MIT Kavli Institute (MKI) for Astrophysics and Space Research is to facilitate and carry out the research programs of faculty and research staff whose interests lie in the broadly defined area of astrophysics and space research. Specifically, the MKI will

    Provide an intellectual home for faculty, research staff, and students engaged in space- and ground-based astrophysics
    Develop and operate space- and ground-based instrumentation for astrophysics
    Engage in technology development
    Maintain an engineering and technical core capability for enabling and supporting innovative research
    Communicate to students, educators, and the public an understanding of and an appreciation for the goals, techniques and results of MKI’s research.

    The Kavli Foundation, based in Oxnard, California, is dedicated to the goals of advancing science for the benefit of humanity and promoting increased public understanding and support for scientists and their work.

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  • richardmitnick 9:28 am on January 18, 2018 Permalink | Reply
    Tags: , , , , , Hubble Constant, Λ Cosmological Constant, , Speed of universe’s expansion remains elusive, Type 1a supernovas as “standard candles”? Maybe not   

    From ScienceNews: “Speed of universe’s expansion remains elusive” 

    ScienceNews

    January 16, 2018
    Tom Siegfried

    1
    In August of 2011, researchers discovered SN 2011fe, a type 1a supernova 21 million light-years away in galaxy M101 (images show the galaxy before and after the supernova, with the supernova circled at right). Studies using type 1a supernovas as “standard candles” to measure how fast the universe expands (the Hubble constant) produce a result in conflict with other data used to infer the cosmic growth rate. NASA, Swift, Peter Brown, Univ. of Utah

    NASA Neil Gehrels Swift Observatory

    Unless you are a recent arrival from another universe, you’ve no doubt heard that this one is expanding. It’s getting bigger all the time. What’s more, its growth rate is accelerating. Every day, the universe expands a little bit faster than it did the day before.

    Those day-to-day differences are negligible, though, for astronomers trying to measure the universe’s expansion rate. They want to know how fast it is expanding “today,” meaning the current epoch of cosmic history. That rate is important for understanding how the universe works, knowing what its ultimate fate will be and even what it is made of. After all, the prime mission of the Hubble Space Telescope when it was launched in 1990 was to help determine that expansion rate (known, not coincidentally, as the Hubble constant, named for the astronomer Edwin Hubble).

    Since then evidence from Hubble (the telescope) and other research projects has established a reasonably precise answer for the Hubble constant: 73, in the units commonly used for this purpose. (It means that two independent astronomical bodies separated by 3.26 million light-years will appear to be moving away from each other at 73 kilometers per second.) Sure, there’s a margin of error, but not much. The latest analysis from one team, led by Nobel laureate Adam Riess, puts the Hubble constant in the range of 72–75, as reported in a paper posted online January 3 ApJ. Considering that as late as the 1980s astronomers argued about whether the Hubble constant was closer to 40 or 90, that’s quite an improvement in precision.

    But there’s a snag in this success. Current knowledge of the universe suggests a way to predict what the Hubble constant ought to be. And that prediction gives a probable range of only 66–68. The two methods don’t match.

    “This is very surprising, I think, and very interesting,” Riess, of the Space Telescope Science Institute in Baltimore, said in a talk January 9 at a meeting of the American Astronomical Society.

    It’s surprising because astrophysicists and cosmologists thought they had pretty much figured the universe out. It’s made up of a little bit of ordinary matter, a lot of some exotic “dark matter” of unknown identity, and even more of a mysterious energy permeating the vacuum of space, exerting gravitational repulsion. Remember that acceleration of the expansion rate? It implies the existence of such energy. Because nobody knows what it is, people call it “dark energy,” while suspecting that its real name is lambda, the Greek letter that stands for “cosmological constant.” (It’s called a constant because any part of space should possess the same amount of vacuum energy.) Dark energy contributes something like 70 percent of the total mass-energy content of the universe, various lines of evidence indicate.

    If all that’s right, then it’s not all that hard to infer how fast the universe should be expanding today. You just take the recipe of matter, dark matter and dark energy and add some ghostly subatomic particles known as neutrinos. Then you carefully measure the temperature of deep space, where the only heat is the faint glow remaining from the Big Bang. That glow, the cosmic microwave background radiation, varies slightly in temperature from point to point. From the size of those variations, you can calculate how far the radiation from the Big Bang has been traveling to reach our telescopes. Combine that with the universe’s mass-energy recipe, and you can calculate how fast the universe is expanding. (You can, in fact, do this calculation at home with the proper mathematical utensils.)

    An international team’s project using cosmic microwave background [CMB]data inferred a Hubble constant of 67, substantially less than the 73 or 74 based on actually measuring the expansion (by analyzing how the light from distant supernova explosions has dimmed over time).

    CMB per ESA/Planck

    When this discrepancy first showed up a few years ago, many experts believed it was just a mirage that would fade with more precise measurement. But it hasn’t.

    “This starts to get pretty serious,” Riess said at the astronomy meeting. “In both cases these are very mature measurements. This is not the first time around for either of these projects.”

    One commonly proposed explanation contends that the supernova studies are measuring the local value of the Hubble constant. Perhaps we live in a bubble, with much less matter than average, skewing expansion measurements. In that case, the cosmic microwave background data might provide a better picture of the “global” expansion rate for the whole universe. But supernovas observed by the Hubble telescope extend far enough out to refute that possibility, Riess said.

    “Even if you thought we lived in a void…, you still are basically stuck with the same problem.”

    Consequently it seems most likely that something is wrong with the matter-energy recipe for the universe (technically, the cosmological standard model) used in making the expansion rate prediction. Maybe the vacuum energy driving cosmic acceleration is not a cosmological constant after all, but some other sort of field filling space. Such a field could vary in strength over time and throw off the calculations based on a constant vacuum energy. But Riess pointed out that the evidence is growing stronger and stronger that the vacuum energy is just the cosmological constant. “I would say there we have less and less wiggle room.”

    Another possibility, appealing to many theorists, is the existence of a new particle, perhaps a fourth neutrino or some other relativistic (moving very rapidly) particle zipping around in the early universe.

    “Relativistic particles — theorists have no trouble inventing new ones, ones that don’t violate anything else,” Riess said. “Many of them are quite giddy about the prospect of some evidence for that. So that would not be a long reach.”

    Other assumptions built into the current cosmological standard model might also need to be revised. Dark matter, for example, is presumed to be very aloof from other forms of matter and energy. But if it interacted with radiation in the early universe, it could have an effect similar to that of relativistic particles, changing how the energy in the early universe is divided up among its components. Such a change in energy balance would alter how much the universe expands at early times, corrupting the calibrations needed to infer the current expansion rate.

    It’s not the first time that determining the Hubble constant has provoked controversy. Edwin Hubble himself initially (in the 1930s) vastly overestimated the expansion rate. Using his rate, calculations indicated that the universe was much younger than the Earth, an obvious contradiction. Even by the 1990s, some Hubble constant estimates suggested an age for the universe of under 10 billion years, whereas many stars appeared to be several billion years older than that.

    Hubble’s original error could be traced to lack of astronomical knowledge. His early overestimates turned out to be signals of a previously unknown distinction between different generations of stars, some younger and some older, Riess pointed out. That threw off distance estimates to some stars that Hubble used to estimate the expansion rate. Similarly, in the 1990s the expansion rate implied too young a universe because dark energy was not then known to exist and therefore was not taken into account when calculating the universe’s age.

    So the current discrepancy, Riess suggested, might also be a signal of some astronomical unknown, whether a new particle, new interactions of matter and radiation, or a phenomenon even more surprising — something that would really astound a visitor from another universe.

    See the full article here .

    Science News is edited for an educated readership of professionals, scientists and other science enthusiasts. Written by a staff of experienced science journalists, it treats science as news, reporting accurately and placing findings in perspective. Science News and its writers have won many awards for their work; here’s a list of many of them.

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  • richardmitnick 9:46 am on December 14, 2016 Permalink | Reply
    Tags: , , , , Einstein’s General Relativity (GR), Emergent Gravity, , , Hubble Constant, ,   

    From astrobites: “Emergent Gravity faces its First Test in Galaxy Lensing” 

    Astrobites bloc

    Astrobites

    Dec 13, 2016
    Gourav Khullar

    Title: First test of Verlinde’s theory of Emergent Gravity using Weak Gravitational Lensing measurements
    Authors: M. M. Brower, M.R. Visser, A Dvornik, et al.
    First Author’s Institution: Leiden Observatory, Leiden, The Netherlands
    Status: Submitted to The Monthly Notices of the Royal Astronomical Society (MNRAS), December 2016 [open access]

    Despite being a near-perfect model and explaining everything ranging from galactic rotation curves to high-redshift supernovae observations, Lambda-CDM has its problems. A lack of clear candidates for a dark matter particle and dark energy are two that certainly keep physicists up at night. This leads us towards alleys unexplored – theories that are creative, innovative and crucial to the scientific process, theories that could lead us to the eventual model of the universe with a clear explanations of all observations. One such theory that garnered some attention in the last few years is Emergent Gravity.

    1
    Fig 1. Galaxy rotation curves observed over the last few years indicate a dominant matter halo on the outskirts of galaxies, something that’s explained concretely by dark matter.

    What is ‘Emergent’ in Emergent Gravity?

    The idea is pretty radical yet basic – gravity isn’t a manifestation of mass in spacetime as proposed by Einstein’s General Relativity (GR) or a fundamental force that fits perfectly in a four-force model of the universe. Instead, gravity is proposed to be ’emerging’ from interactions between even more fundamental particles. This is akin to seeing thermodynamical parameters like pressure and temperature arising from interactions between atoms and molecules – what’s crucial to our discussion is the macroscopic quantity. In the case here, that quantity would be gravity. This idea has been developing over the last few decades, with Theodore Jacobson, Thanu Padmanabhan and more recently, Erik Verlinde contributing heavily to its development.

    2
    Erik Verlinde

    3
    Fig 2. High speeds of particle collision against the walls of a container lead to higher temperature, since the system possesses more kinetic energy that gets converted to thermal energy.

    Diving deep into Entropy and Gravity

    One aspect of a theoretical model like emergent gravity (EG) is that we are allowed to derive macroscopic results without having to worry about the underlying fundamental particles that could lead to gravity ’emerging’ – at least for now. This ’emergence’ can be thought of as the result of the tendency of a physical system to increase its entropy. Early work in the field towards a ‘thermodynamics-like theory of gravity’ used something called ‘holographic scaling of entropy’, which essentially scales with surface area of an enclosed volume of spacetime. Verlinde’s new work insists that due to dark energy, we see deviations in GR at long distances that can be resolved if this entropy scaling scales as volume instead of area. Keeping details aside, this leads to a different ‘force-law’, that has additional dominant matter terms that could explain dark matter (called ‘apparent dark matter’ in this case). This and this piece are excellent sources for details on the model. It can be seen that in some sense, this model combines the origin of dark matter and dark energy in a novel way.

    Basics of Weak Gravitational Lensing

    Well, how do we test this theory? Perhaps, passing it through the same standards as GR would seem appropriate.

    The idea of gravitational lensing was one of the first tests of GR i.e. the idea that light’s path gets distorted when traveling through curved spacetime surrounding massive objects. This distortion can change the light ray received from background galaxies (and hence, apparent shape and size) due to a foreground massive object like a galaxy or a galaxy cluster, leading to weak gravitational lensing. This galaxy-galaxy lensing signal is a massive success story of GR, as observations of this phenomena in the Universe fit into the model very well.

    4
    Fig 3. Gravitational lensing leading to a drastic distortion in light coming from background galaxies. Credit: NASA-Hubble Space Telescope.

    Since EG still gives rise to ‘apparent dark matter’, it is safe to say that the gravitational lensing formalism stays the same, since we do apply this formalim to our universe’s dark matter-dominated objects like galaxy clusters (if we believe Lambda-CDM and its predictions). This allows us to use weak lensing as a test for emergent gravity, and match observations against the predictions of this theory.

    This work

    The regime studied in this work is the low-redshift universe, or the relatively local universe, where the Hubble Constant can be treated as a constant. This is almost true because of the dominance of dark energy after redshift ~0.7-0.9. Since Verlinde’s EG isn’t evolved enough as a theory to quantify cosmology before this epoch, this work assumes a background Lambda-CDM cosmology. For studying galaxy-galaxy lensing, Brower et al. select ~33,000 galaxies from the Galaxy And Mass Assembly (GAMA) survey as ‘lenses’ and KiDS survey galaxies as background galaxies that get lensed. They model these galaxies as having a static, spherically symmetric distribution of mass- something like a point mass or an extended source resembling a point mass- because that’s what EG can handle so far.

    This work calculates the lensing effect by measuring distortions in the background galaxies’ images, termed as a ‘shear’. In the framework of GR, this quantity is comprised in something called the Extended Surface Density (ESD) profile. Brower et al. calculated the ESD for these galaxies under the many assumptions of this model, compared them with Navarror-Frenk-White (NFW) profiles of galaxies from Lambda-CDM, and found that there was general agreement in the ESD progression between the two.

    5
    Fig 4. From the paper, a model-fit of Emergent Gravity(Point mass model), Emergent Gravity (Extended model) and Dark Matter(NFW model). The lensing signal measured in the form of an ESD is plotted for four different galactic mass bins. It can be seen that Verlinde’s Emergent Gravity model assisted by teh assumptions made by Brower et al. match NFW profile predictions very well.

    Conclusion and Summary

    So what are the assumptions? For one, EG cannot deal with evolution of the universe at the moment. Moreover, the theory isn’t developed enough to have a basic framework of what causes gravity to ’emerge’ from fundamental interactions. The paper agrees that a more ‘sophisticated implementation of both theories’ is needed to make a statement about whether apparent dark matter explains observations better than Lambda-CDM dark matter. Till then, EG shall keep on evolving and observations shall keep on being pitted against these evolving frameworks. A very exciting space to watch!

    See the full article here .

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    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
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