Gravitational waves could soon accurately measure universe’s expansion

Cosmos by John Hussey


New LIGO readings could improve disputed measurement quickly

Scientists estimate that given how quickly LIGO researchers saw the first neutron star collision, they could have a very accurate measurement of the rate of the expansion of the universe within five to 10 years.

The Dark Energy Camera, mounted on the Blanco Telescope in Chile, picked up images of the bright spot in the sky from the neutron star collision. UChicago, Argonne and Fermilab scientists are members of the international Dark Energy Survey collaboration.

Credit: Photo by Reidar Hahn/Fermilab

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For almost 10 years, an international team of astrophys

Twenty years ago, scientists were shocked to realize that our universe is not only expanding, but that it’s expanding faster over time.

Pinning down the exact rate of expansion, called the Hubble constant after famed astronomer and UChicago alumnus Edwin Hubble, has been surprisingly difficult. Since then scientists have used two methods to calculate the value, and they spit out distressingly different results. But last year’s surprising capture of gravitational waves radiating from a neutron star collision offered a third way to calculate the Hubble constant.

That was only a single data point from one collision, but in a new paper published Oct. 17 in Nature, three University of Chicago scientists estimate that given how quickly researchers saw the first neutron star collision, they could have a very accurate measurement of the Hubble constant within five to ten years.

“The Hubble constant tells you the size and the age of the universe; it’s been a holy grail since the birth of cosmology. Calculating this with gravitational waves could give us an entirely new perspective on the universe,” said study author Daniel Holz, a UChicago professor in physics who co-authored the first such calculation from the 2017 discovery. “The question is: When does it become game-changing for cosmology?”

In 1929, Edwin Hubble announced that based on his observations of galaxies beyond the Milky Way, they seemed to be moving away from us — and the farther away the galaxy, the faster it was receding. This is a cornerstone of the Big Bang theory, and it kicked off a nearly century-long search for the exact rate at which this is occurring.

To calculate the rate at which the universe is expanding, scientists need two numbers. One is the distance to a faraway object; the other is how fast the object is moving away from us because of the expansion of the universe. If you can see it with a telescope, the second quantity is relatively easy to determine, because the light you see when you look at a distant star gets shifted into the red as it recedes. Astronomers have been using that trick to see how fast an object is moving for more than a century — it’s like the Doppler effect, in which a siren changes pitch as an ambulance passes.

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‘Major questions in calculations’

But getting an exact measure of the distance is much harder. Traditionally, astrophysicists have used a technique called the cosmic distance ladder, in which the brightness of certain variable stars and supernovae can be used to build a series of comparisons that reach out to the object in question. “The problem is, if you scratch beneath the surface, there are a lot of steps with a lot of assumptions along the way,” Holz said.

Perhaps the supernovae used as markers aren’t as consistent as thought. Maybe we’re mistaking some kinds of supernovae for others, or there’s some unknown error in our measurement of distances to nearby stars. “There’s a lot of complicated astrophysics there that could throw off readings in a number of ways,” he said.

The other major way to calculate the Hubble constant is to look at the cosmic microwave background — the pulse of light created at the very beginning of the universe, which is still faintly detectable. While also useful, this method also relies on assumptions about how the universe works.

The surprising thing is that even though scientists doing each calculation are confident about their results, they don’t match. One says the universe is expanding almost 10 percent faster than the other. “This is a major question in cosmology right now,” said the study’s first author, Hsin-Yu Chen, then a graduate student at UChicago and now a fellow with Harvard University’s Black Hole Initiative.

Then the LIGO detectors picked up their first ripple in the fabric of space-time from the collision of two stars last year. This not only shook the observatory, but the field of astronomy itself: Being able to both feel the gravitational wave and see the light of the collision’s aftermath with a telescope gave scientists a powerful new tool. “It was kind of an embarrassment of riches,” Holz said.

Gravitational waves offer a completely different way to calculate the Hubble constant. When two massive stars crash into each other, they send out ripples in the fabric of space-time that can be detected on Earth. By measuring that signal, scientists can get a signature of the mass and energy of the colliding stars. When they compare this reading with the strength of the gravitational waves, they can infer how far away it is.

This measurement is cleaner and holds fewer assumptions about the universe, which should make it more precise, Holz said. Along with Scott Hughes at MIT, he suggested the idea of making this measurement with gravitational waves paired with telescope readings in 2005. The only question is how often scientists could catch these events, and how good the data from them would be.

 

‘It’s only going to get more interesting’

The paper predicts that once scientists have detected 25 readings from neutron star collisions, they’ll measure the expansion of the universe within an accuracy of 3 percent. With 200 readings, that number narrows to 1 percent.

“It was quite a surprise for me when we got into the simulations,” Chen said. “It was clear we could reach precision, and we could reach it fast.”

A precise new number for the Hubble constant would be fascinating no matter the answer, the scientists said. For example, one possible reason for the mismatch in the other two methods is that the nature of gravity itself might have changed over time. The reading also might shed light on dark energy, a mysterious force responsible for the expansion of the universe.

“With the collision we saw last year, we got lucky — it was close to us, so it was relatively easy to find and analyze,” said Maya Fishbach, a UChicago graduate student and the other author on the paper. “Future detections will be much farther away, but once we get the next generation of telescopes, we should be able to find counterparts for these distant detections as well.”

The LIGO detectors are planned to begin a new observing run in February 2019, joined by their Italian counterparts at VIRGO. Thanks to an upgrade, the detectors’ sensitivities will be much higher — expanding the number and distance of astronomical events they can pick up.

“It’s only going to get more interesting from here,” Holz said.

The authors ran calculations at the University of Chicago Research Computing Center.

 

Story Source:

Materials provided by University of Chicago. Original written by Louise Lerner.

 

Cosmos by John Hussey

 

https://www.sciencedaily.com/releases/2018/10/181022162156.htm

 

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Simulating the Universe: The Next Generation

Cosmos by John Hussey


A new suite of cosmological simulations known as Illustris: The Next Generation sheds light on the interconnected cosmic processes that shape the universe.

Screenshot from the IllustrisTNG simulation.

IllustrisTNG

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For almost 10 years, an international team of astrophysicists has been working on a project to simulate the evolution of the universe, from just after the Big Bang to the present day. Four years ago these simulations, known as Illustris, were the first to closely replicate the diversity of galaxies we observe in the real universe, producing “red and dead” ellipticals, star-forming spirals, and more exotic systems.

But a recent announcement brings a fresh level of sophistication to the table. The first three papers from Illustris: The Next Generation (IllustrisTNG) simulate a much bigger slice of universe at higher resolution than its predecessor.

Although still not on the grand, universe-wide scale of simulations that solely follow dark matter’s interaction with gravity, IllustrisTNG is large enough to provide a cosmological context to the processes it monitors. And despite not being as granular as “zoom-in” simulations that follow the small-scale evolutions within a single galaxy, IllustrisTNG captures the physical principles behind the evolution of gas, stars, and galaxies. As such, this new suite of simulations represents a big step toward bridging small-scale physics with large-scale evolution.

For one simulation run, IllustrisTNG used 24,000 processors over the course of more than two months on the fastest supercomputer in Germany: Hazel-Hen at the Supercomputing Center in Stuttgart. These simulations produced more than 500 terabytes of data — more than three times the information that the Hubble Space Telescope has collected over its entire lifetime.

These new data offer insights into fundamental astrophysical processes, including the growth and evolution of cosmic magnetic fields and galaxies, as well as the role black holes play in influencing the distribution of dark matter and killing off galactic star formation.

For example,  Annalisa Pillepich (Max Planck Institute for Astronomy, Germany) and colleagues predict what a faint stellar glow around large galaxies should look like. When smaller galaxies violently collide with each other, they lose stars, which relocate to a halo around the newly merged galaxy. IllustrisTNG’s results show that this halo can take all sorts of different shapes, depending on how the galaxy formed. Now, astronomers can match observations of a real-world stellar halo to simulation results, yielding the assembly history of that galaxy. Such comparisons could reveal how invisible dark matter halos build cosmic structure such as galaxy clusters and the filaments that connect them.

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Key to this and other findings has been enriching the physics included in the simulations. For example, previous attempts at simulating the cosmos have omitted magnetic fields for simplicity. But Mark Vogelsberger (MIT) points out, “Magnetic fields can impact galaxy and star formation, so it is important to model them properly.”

Federico Marinacci (MIT), Vogelsberger, and colleagues study how large-scale magnetic fields in the IllustrisTNG universe form and grow over time. They have found that turbulence in hot, spread-out gases in the heart of galaxies drives small-scale dynamos that can exponentially amplify the magnetic fields. The strength of these amplified fields matches observations. With little known about where magnetic fields come from or what precise role they play in various cosmic processes, further results from IllustrisTNG are expected to illuminate these mysteries.

The astrophysicists have also improved the accuracy of IllustrisTNG by comparing their original Illustris simulations to observations. For example, they realized that the model of how supermassive black holes gobble up gas and spit it out was too violent, removing too much gas from the host galaxy. As a result, they rewrote the model based on feedback that doesn’t lead to massive gas losses. With this new model, Dylan Nelson (Max Planck Institute for Astronomy) and the Illustris team found compelling evidence that black hole jets can stop galaxies of a certain mass from giving birth to new stars.

In the simulation’s larger galaxies, “the black holes are able to supply enough energy to the surrounding gas to prevent it from cooling and fragmenting into stars,” explains IllustrisTNG’s principal investigator Volker Springel (Heidelberg University, Germany).

Astronomers and astrophysicists will continue to learn from IllustrisTNG about the interconnected cosmic processes that have shaped our universe. Yet the team behind the simulations is not one to rest on its laurels: “There are still many things we would like to do better in future calculations,” observes Springel.

For example, although IllustrisTNG’s biggest simulations ran in a box nearly 1 billion light-years on a side, this volume is relatively small for cosmological simulations. The team will be keen to push the simulations to new limits. Springel and his collaborators are also already working on adding even more physics to the mix, including highly energetic particles known as cosmic rays, which may be important in regulating star formation, and novel methods to simulate the dust content in galaxies.

 

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Cosmos by John Hussey

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Primordial Chill Hints at Dark Matter Interactions in Early Universe

Cosmos by John Hussey


A simple experiment has detected a signal from the first stars forming just 180 million years after the Big Bang. The observations have intriguing implications for the nature of dark matter.

A timeline of the universe shows when the first stars emerged, 180 million years after the Big Bang. Their energetic radiation ionized the neutral hydrogen gas that then permeated the universe.

N.R.Fuller / National Science Foundation

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The first stars began to shine just 180 million years after the Big Bang, according to new observations by a team of American radio astronomers. The evidence comes from observations of neutral hydrogen gas that pervaded the early universe. But surprisingly, the same observations show an unexpected chill in this gas — a result that could hint at non-gravitational interactions with dark matter.

“This is a really cool result,” says Michiel Brentjens (Netherlands Institute for Radio Astronomy), who was not involved in the study. “It’s an important first step in revealing how the very early universe behaved.”

For years, astronomers like Brentjens have been trying to detect the early universe’s neutral hydrogen via its telltale 21-centimeter radio emission. In particular, they want to see how the energetic radiation from the very first stars and galaxies heated and ionized surrounding gas. This occurred during the so-called Epoch of Reionization (EoR), when bubbles of ionized gas grew and spread across the universe, some 300 to 500 million years after the Big Bang.

 

Timeline of the Universe shows epoch of reionization

A timeline of the universe shows when the first stars emerged, 180 million years after the Big Bang. Their energetic radiation ionized the neutral hydrogen gas that then permeated the universe.

However, success has eluded them so far. As the radio waves travel through expanding space for more than 13.7 billion years, they are stretched to wavelengths of a couple of meters, corresponding to difficult-to-detect frequencies below 200 megahertz. Moreover, the signal is swamped by foreground sources, such as radiation from electrons spiraling around magnetic field lines in our Milky Way Galaxy, man-made radio interference, and instrumental noise.

 

Detecting the First Stars

Now, Judd Bowman (Arizona State University), Alan Rogers (MIT Haystack Observatory), and their colleagues have tuned in to the infant universe at even lower frequencies, between 50 and 100 megahertz, corresponding to a longer look-back time. They used a relatively cheap detector called EDGES (Experiment to Detect the Global Epoch of reionization Signature), funded by the National Science Foundation and situated in the radio-quiet Australian Outback. About the size of a large desk, EDGES is extremely well calibrated to these lower frequencies. The simple setup also contains as few electronics as possible to prevent low-frequency interference.

At that very early stage in cosmic evolution, corresponding to redshifts between 15 and 20, the cosmic microwave background — the Big Bang’s fading afterglow — would have been hotter than the all-pervasive neutral hydrogen gas. As a result, the gas would show up not by emitting 21-centimeter radio waves, but by absorbing them.

As the team reports in the March 1st issue of Nature, EDGES successfully detected this absorption feature by averaging measurements across the sky. The subtle dip in the radio spectrum is centered at a frequency of 78 megahertz (a wavelength of 3.84 meters). If this is indeed a redshifted 21-centimer absorption signal, it corresponds to an epoch just 180 million years after the Big Bang. According to Rogers, this is the earliest direct detection of the hydrogen gas.

Brentjens is impressed by the team’s result. “They have succeeded in circumventing most of the instrumental noise that is so worrisome at these very low frequencies,” he says. “I hope it won’t be too long before we also detect the emission signal of the hydrogen gas from a somewhat later epoch.” Seeing the neutral gas by its emission rather than absorption would provide valuable additional information on precisely when and how reionization progressed across the universe.

Instruments on the lookout for this emission include the LOFAR telescope (Low-Frequency Array) in the Netherlands, the Murchison Wide-field Array in Western Australia, and HERA (Hydrogen Epoch of Reionization Array) in South Africa. As of yet, these facilities haven’t yet detected the emissions signal, though they’re getting close.

The discovery of the absorption signal by EDGES implies that the very first stars must already have formed when the universe was just 180 million years old, explains Bowman.

“After stars form, their ultraviolet light alters the energy states of the hydrogen atoms and knocks them out of equilibrium with the microwave background,” he says. “That causes the hydrogen to absorb some of the background radiation, creating the small dip that we’re able to observe. Without the stars, the hydrogen wouldn’t be able to produce this signal.”

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Hints of Dark Matter

To the scientists’ surprise, the absorption they observed was stronger than expected. Since the temperature difference between the cosmic background radiation and the hydrogen gas determines the depth of the absorption feature, a deeper dip could mean either that the background radiation was hotter than expected or that the hydrogen gas was cooler than expected.

It’s hard to imagine how the cosmic microwave background could have been hotter than the expected 50K (50 degrees above absolute zero) at a cosmic age of 180 million years, as we can measure this background extremely precisely. Instead, the neutral hydrogen gas in the early universe must have been cooler than theory predicts: just over 3K instead of the expected 7K.

In a companion paper in the same issue of Nature, theoretical astrophysicist Rennan Barkana (Tel Aviv University, Israel) argues that the gas might have undergone additional cooling by interacting non-gravitationally with dark matter particles. Indeed, most dark matter theories do predict some very weak interactions between “normal” matter and dark particles, through collision and scattering. Based on the EDGES results, Barkana predicts a relatively low mass for dark matter particles — at most a few times the mass of a proton — and also relatively low velocities. “These results indicate that 21-centimeter cosmology can be used as a dark-matter probe,” he writes.

Rogers is open to the idea. “So far I think that dark matter interactions are the only proposed way of getting the temperature low enough to obtain the observed amount of absorption.” But he adds, “I think it’s still too early to draw firm conclusions.”

Given the incredible difficulty of detecting the absorption signal at all, his caution is understandable. After all, at 78 megahertz, the cosmic background radiation is some 10,000 times weaker than all the combined sources of foreground noise. As Peter Kurczynski (National Science Foundation) comments: “It’s like being in the middle of a hurricane and trying to hear the flap of a hummingbird’s wing.”

“We were one of the first groups to get started on this technique,” says Bowman. After 12 years of measurements and two years of tests to rule out instrumental errors, he next wants to see the same results repeated. “Several other groups around the world have set up similar experiments and are close to making the same measurement. The next step in the scientific process is for another group using a different instrument to confirm our detection.”

 

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Cosmos by John Hussey

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Hawking Takes on the Infinite Multiverse

Cosmos by John Hussey


Stephen Hawking’s last paper on cosmology, published posthumously, might solve the problem of eternal inflation, a theory that suggests our cosmos is but one in a sea of infinite universes.

This symbolic representation shows the evolution of the universe over 13.7 billion years. The far left depicts the Big Bang, the earliest moment we can yet probe, when an extremely brief moment of “inflation” produced a burst of exponential growth in the universe. (Size is symbolized by the vertical extent in this graphic.)

NASA / WMAP

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Cosmic history

This symbolic representation shows the evolution of the universe over 13.7 billion years. The far left depicts the Big Bang, the earliest moment we can yet probe, when an extremely brief moment of “inflation” produced a burst of exponential growth in the universe. (Size is symbolized by the vertical extent in this graphic.)

There’s a lot of evidence to suggest that, just a tiny fraction of a second after the Big Bang, the infant universe went through a fantastic growth spurt. The phase itself lasted only a fraction of a second, but it’s responsible for the universe we see now. Even the particles that make up stars and humans alike were created from its energy.

The trouble is, according to most theories of cosmic inflation, the growth spurt never really stopped. Though our pocket of space, what we call our universe, might have stopped its exponential expansion, the overall universe never stopped stretching.

Now, in a paper published by Stephen Hawking (posthumously) and his colleague Thomas Hertog (KU Leuven, Belgium) in the Journal of High Energy Physics, the scientists turn that picture on its head, arguing that we need to rethink our universe’s beginnings. Applying quantum and string theory to a simplified mathematical model of the universe, Hawking and Hertog conclude that cosmic inflation isn’t eternal after all.

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Eternal Inflation

Why do most cosmologists think that inflation is eternal? The idea behind inflation is that almost immediately after the Big Bang, our entire universe entered into a not entirely stable energy state known as a false vacuum. The term is a bit misleading, because what scientists mean by “vacuum” in this case is actually “ground state”; “false vacuum” is just another way of saying that the universe is in an elevated energy state. The negative pressure of this state causes all of space to expand exponentially quickly.

Obviously, our universe isn’t still in this growth spurt — otherwise we wouldn’t even be able to see neighboring galaxies. The expansion of space would have long since have spirited them out of sight. Somehow, at least our local universe had to come out of this phase.

Shortly after Alan Guth formulated the idea of inflation, scientists including Paul Steinhardt (then at University of Pennsylvania) and Alexander Vilenkin (Tufts University), figured out that quantum fluctuations could do the trick for ending the inflationary era. A pocket of space could spontaneously come out of inflation, settling into a relatively sedate phase of expansion. But, quantum fluctuations being random, inflation would stop here and there rather than everywhere at once. Meanwhile, far beyond what we can observe, space keeps inflating at an exponential rate. Even as quantum fluctuations cause other pockets of space to similarly come out of inflation, the universe as a whole would keep growing exponentially — forever.

If inflation is really eternal, then our cosmos is one of an infinite variety of universes (known as the multiverse), each following an infinite variety of physical laws.

Stephen Hawking gives a presentation in 2003 to a backdrop of the all-sky cosmic microwave background map from the WMAP satellite.

At least, that’s the conclusion cosmologists reach when treating quantum effects as small relative to a universe largely dominated by relativity. “But I have never been a fan of the multiverse,” Hawking said in an interview last fall. “If the scale of different universes in the multiverse is large or infinite the theory can’t be tested.”

Instead, Hawking and Hertog argue, relativity would break down in the early universe; only quantum theory applies. Treating the universe as a single, tiny particle, they compute its wave function: a single equation that describes all the possible states our universe could take.

In order to do this, they first simplify the math in some seemingly strange ways. First, they assume a simple universe, one without matter or energy, with an overall geometry that’s saddle-shaped. Of course, we don’t actually live in a universe without matter or energy, and the universe’s geometry appears to be flat as a sheet of a paper rather than saddle-shaped, but this is the starting point that many cosmologists take to work out new theories.

But that’s not all. They also assume that they can treat this universe as a hologram. Yes, really — it sounds weird, but basically what they’re doing is reducing three-dimensional equations into two-dimensional equations projected onto a surface. (It doesn’t mean our universe is a hologram, just that we can mathematically treat it like one.)

Rather than reducing a spatial dimension, though, what Hawking and Hertog remove is time. Relativity is no longer necessary in a timeless universe, and the pair could rely instead solely on quantum theory.

From there, Hawking and Hertog solve the equations and come to an astounding conclusion: the universe that emerges from inflation is finite. We still live in a multiverse, but now it’s one with limited possibilities.

However, Vilenkin urges caution in interpreting these results, noting that the simplified mathematical treatment, and the conclusions Hawking and Hertog derive from it, might not apply to our real, more complicated universe. “I am sure Thomas Hertog will try to go beyond this model,” Vilenkin adds, “but it is hard to tell how successful this is going to be.”

 

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Cosmos by John Hussey

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Early Star Formation Presents New Cosmic Mystery

Cosmos by John Hussey


New observations suggest that stars began forming just 250 million years after the Big Bang — a record-breaker that will likely open a new line of cosmological inquiry.

The galaxy cluster MACS J1149.5+2223 features in this image taken with the Hubble Space Telescope. The inset shows ALMA’s view of oxygen (green) in the galaxy MACS1149-JD1, whose light has traveled 13.28 billion light-years to Earth.

Inset: ALMA (ESO / NAOJ / NRAO) / Hashimoto et al.; Background: NASA / ESA Hubble Space Telescope / W. Zheng (JHU) / M. Postman (STScI) / CLASH Team

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Because only hydrogen, helium, and a little lithium emerged from the Big Bang, the young universe was pristine. It wasn’t until the first generation of stars exploded, breathing carbon, oxygen and other heavy elements into the cosmos, that the universe’s inventory of elements increased. So, the detection of oxygen 550 million years after the Big Bang suggests that a generation of stars had already formed and died by this point.

Hashimoto and his colleagues estimate that the first generation of stars would have formed 250 million years after the Big Bang — even though Planck measurements of the cosmic microwave background indicate that star formation wouldn’t have been prevalent in this epoch. That said, the results are in line with a tentative result from the EDGES experiment, which found a signal from stars forming just 180 million years after the Big Bang. The EDGES result still awaits confirmation from other groups performing similar experiments.

For decades, teams of scientists have been racing to find the signatures of the first stars — and it’s more than cosmic curiosity. “There is renewed optimism we are getting closer and closer to witnessing directly the birth of starlight,” says coauthor Richard Ellis (University College London) in a press release. “Since we are all made of processed stellar material, this is really finding our own origins.”

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An Explosive Start

What’s more, the team also used infrared data taken with the Hubble Space Telescope and the Spitzer Space Telescope to glean the number of stars within the galaxy. Typically, galaxies form a small number of high-mass stars and a large number of low-mass stars. The high-mass stars die first, exploding as supernovae a few million years after they form. But the lowest-mass stars can survive for trillions of years — much longer than the age of the universe.

But Hashimoto and his colleagues saw that the galaxy contained even fewer few high-mass stars than expected — meaning that the star formation kicked-off strong, tapered off, and then started forming stars again. That’s the opposite of predictions from simulations of the early universe. The star formation rate was expected to increase with time at these early epochs, starting out slow and then growing exponentially — at least for high-mass galaxies like the one Hashimoto’s team detected.

“It may mean that we don’t really understand the first generation of galaxies sufficiently well,” says coauthor Erik Zackrisson (Uppsala University, Sweden). “There might be some ingredient that is missing from the simulations.”

Discovering that missing ingredient will be the goal of future work, but Zackrisson has a few ideas. It could be that the very first generation of stars produced far more powerful supernovae than theorists suspect. Or perhaps this particular galaxy hosts a ravenous supermassive black hole. Both would unleash powerful winds that would push gas away from the galaxy and suppress further star formation.

Rychard Bouwens (Leiden University, The Netherlands), who was not involved in the study, argues that the paper’s conclusions are reliable yet uncertain only because the team is peering so far back into the universe’s history. “It’s always this way when you’re at the cutting edge,” he says. “It might be providing us with important clues to what happened at very early times in the universe, but we can’t be sure until we observe more objects.”

Both Bouwens and Zackrisson are excited for the 2020 launch of the James Webb Space Telescope, which will be able to directly image galaxies at these early times. Not only that, but Webb is expected to image hundreds if not thousands of these young galaxies. As such, Bouwens compares the current observations to standing on a ship that is enveloped in a thick fog. Although you might be able to see the hazy outline of a lighthouse with Hubble or Spitzer, the James Webb Space Telescope will act like rays of sunlight — forcing the fog to clear and allowing us to see the lighthouse clearly, as well as the rocky coast behind it.

 

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By: Shannon Hall

 

Cosmos by John Hussey

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Too Many Massive Stars in Universe’s Youngest Galaxies

Cosmos by John Hussey


A new method of measuring star formation in the earliest galaxies finds that they’re producing more massive stars than expected — a result that could affect our understanding of how galaxies grow their stars.

The Tarantula Nebula  in the Large Magellanic Cloud is one of the largest star-forming regions in the Local Group of galaxies. Measurements have shown that it might be forming more massive stars than expected.

NASA / ESA / D. Lennon / E. Sabbi (STSCI) et al.

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But it’s difficult to reach such conclusions on the basis of a single star-forming region, no matter how well it has been studied. Now, new work appearing in Nature appears to confirm that star formation depends not on fundamental processes but on environment. Astronomers may have to do some rethinking after all.

 

Four Distant Starbursts

The focus in this study turns to four galaxies whose light took more than 10 billion years to travel to Earth. These galaxies are bursting with stars, but they’re also dusty, which makes them immune to methods requiring ultraviolet, visible, or infrared light. Instead, Zhi-Yu Zhang (University of Edinburgh, UK, and European Southern Observatory, Germany) and colleagues trained the Atacama Millimeter/submillimeter Array (ALMA) on these galaxies, searching for emission related to carbon monoxide, a signal tied to a galaxy’s history of star formation.

ALMA’s incredible resolving power received a helping hand in the form of gravitational lenses: foreground galaxies aligned just so. Their gravity acted as a cosmic lens to magnify the light from these distant starbursts.

 

ALMA images distant starburst galaxies

ALMA imaged emission from 13CO (top) and C18O (bottom) molecules in four distant starburst galaxies. The ratio of these two isotopologues allowed astronomers to determine that these starburst galaxies have an excess of massive stars.

The scientists measured two isotopologs of carbon monoxide: 13CO and C18O. 13C, (which contains one more neutron than ordinary carbon atoms) is released by stars of all masses, whereas 18O (which contains two extra neutrons compared to ordinary oxygen atoms) is released by only by more massive stars. Since more massive stars live brief lives, measuring the abundance of 13CO and C18O serves as a fossil record of how many massive stars formed relative to low-mass stars.

The signature is immune to what the study authors describe as “pernicious” effects of dust. But the authors also acknowledge that the measurement is a roundabout way of getting at the problem.

That’s because they’re observing carbon monoxide molecules that are floating in the gas between the stars, rather than measuring radiation from stars themselves. So they’re in effect probing the galaxy’s entire history of star formation. Granted, galaxies in the early universe have a shorter history and thus a shorter amount of time for confounding effects to complicate the measurements, but as Kevin Covey (Western Washington University) points out, it’s still possible.

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Dusty starburst galaxy (art)

This artist’s impression shows a dusty galaxy in the distant universe. New ALMA observations have allowed scientists to lift the veil of dust and see what was previously inaccessible — that such starburst galaxies have an excess of massive stars as compared to more peaceful galaxies.

 

Redefining Cosmic Noon

If the measurements hold up, then what we dub “starburst” galaxies in the early universe aren’t actually making as many stars as we thought. Most stars have less than the Sun’s mass, but these young galaxies appear to be pouring more of their energy into making more massive stars. That means that other methods of estimating star formation rates might be wrong. In fact, our entire understanding of the cosmic star formation — which astronomers currently think peaked when the universe was roughly 4 billion years old — might be wrong.

Profound implications aside, Zhang’s team has a ways to go when it comes to convincing all of their colleagues. But they’re only getting started: Zhang says they’re already preparing more systematic surveys that will include nearby galaxies and multiple tracers of star formation.

 

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Cosmos by John Hussey

 

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Pulsar Limits “Fifth Force” Interactions with Dark Matter

Cosmos by John Hussey


A recent experiment to better understand the nature of dark matter constrains a possible “fifth force” of nature to almost zero.

An artist’s illustration shows what pulsar PSR J1713+0747 and its white dwarf companion might look like.

ESO / L. Calçada

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Scientists recently studied a pulsar binary system to constrain the existence of a hypothetical fifth fundamental force of nature.

We already know about four fundamental forces: gravity, electromagnetism, and the strong and weak nuclear forces. However, there are some effects in the universe that cannot be explained by these forces alone. For example, a 2016 experiment in Hungary showed unexpected behavior in the decay of nuclei in the isotype beryllium-8. (After shooting protons at lithium foil, observers saw more electron-positron pairs ejected at a 140-degree angle, which is difficult to explain with standard nuclear physics theories.)

One possibility is the existence of a “fifth force” of nature, which governs the behavior of elementary particles alongside the other four forces. Some scientists suggest this force could work on dark matter, the unseen substance that makes up most of the universe’s mass. We can see dark matter’s effects on ordinary matter, but direct detection has eluded scientists and what it’s made of remains unknown.

 

Testing for a Fifth Force

One research group tested for a fifth force using a pulsar and its white dwarf star companion. Pulsars, whose atoms have been compacted into neutrons, are so dense that their extreme gravitational fields could enhance any possible interactions with dark matter. The white dwarf, while still sardine-packing its atoms, isn’t nearly so compact. General relativity predicts that normal matter ought to fall freely toward dark matter, but a fifth force that has the ability to interact with both normal and dark matter could strengthen or diminish dark matter’s pull. If a fifth force does exist, the Milky Way’s dark matter halo, whose density ought to peak in the galactic center, would pull on the neutron star and the white dwarf in different ways, slightly altering their orbit.

The researchers chose binary pulsar PSR J1713+0747, which is 3,800 light-years from Earth, lying in the direction of the galactic center. Dark matter is believed to be more populous towards the heart of the galaxy, so the pulsar binary system provides an ideal test how a fifth force would act on dark matter and standard matter. The researchers wanted to see if the movements of the pulsar and white dwarf would differ as they orbited one another.

“If there is a fifth force that acts between dark matter and standard matter, it would not be universal,” says Lijing Shao (Max Planck Institute for Radio Astronomy, Germany). “It would therefore produce an apparent difference for the neutron star and the white dwarf in their free fall towards dark matter. Thus, the orbit of the neutron star would be different than what is predicted by the general relativity.”

Using 20 years of radio observations of this system, the researchers concluded that if a fifth force does exist, it must have less than 1% of gravity’s strength . (And gravity is already the weakest of the four known forces.) The results appear in Physical Review Letters.

The researchers also discovered that the limits on the density of dark matter at this pulsar system were similar to other tests closer to Earth. In other words, the team didn’t prove or disprove other observations showing that dark matter density increases towards the center of the galaxy.

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Beyond Relativity

Aurélien Hees (Observatory of Paris), who was not involved in the study, noted that this work is the first to investigate interactions between a hypothetical fifth force and dark matter in this way. The short rotational period of the pulsar – just 4.6 milliseconds – and its stable rotation made it a good candidate for constraining the effects of the fifth force, he said.

With the fifth force, he explains, “We expect to see something a little bit beyond relativity. We are trying to search for that with all the observations available from Earth.”

Shao says his team hopes to study more binary pulsars closer to the center of the galaxy to better understand the effects of dark matter. Unlike most tests of general relativity, in this case the researchers want to find pulsars moving in relatively slow orbits around their companion The challenge, of course, is finding the pulsars in the first place. He suggested a breakthrough will come when the more sensitive Square Kilometer Array is ready in the 2020s. “Bigger radio telescopes and arrays are better because they more precisely measure the time of the [pulsar signal] arrival,” Shao said.

 

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Best Test of General Relativity on Galaxy Scales

Cosmos by John Hussey


Astronomers have conducted the best, galaxy-scale test of general relativity yet, and it rules out some (but not all) theories of modified gravity.

An image of the nearby galaxy ESO 325-G004, created using data collected by the NASA/ESA Hubble Space Telescope and the MUSE instrument on the ESO’s Very Large Telescope. The inset shows the Einstein ring resulting from the distortion of light from a more distant source by intervening lens ESO 325-004, which becomes visible after subtraction of the foreground lens light.

ESO / ESA / Hubble / NASA

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Collett and colleagues first calculated the mass of the intervening galaxy by measuring the movements of stars within it. Then they measured the spatial curvature generated by each unit mass of the intervening galaxy. The mass inferred by spacetime curvature is precisely consistent with the mass measured by the stars, exactly as general relativity predicts.

Unlike previous lensing tests of relativity, Collett’s team relied less on assumptions about the nature of the intervening galaxy. So this test is relatively free of systematic uncertainties that have plagued previous studies.

Lucas Lombriser (University of Geneva), who was not involved in the study, calls the measurements, “the most robust test of gravity of this type and on these length scales to date.”

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“However,” he adds, “while this new test rules out some alternative gravity theories, there are many others that remain compatible with the measurement.” While this study limited any variations in gravity on scales less than 6,500 light-years, additional tests will still be needed to rule out modified gravity more generally.

 

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Cosmos by John Hussey

 

Best Test of General Relativity on Galaxy Scales

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HaloSat: A Small Satellite for a Big Question

Cosmos by John Hussey


HaloSat, a mini-satellite recently deployed from the International Space Station, is on the hunt for the universe’s missing matter.

An artist’s conception of HaloSat in space.

Blue Canyon Technologies

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A small CubeSat mission may provide the data that answers a key cosmological question: where is half of the ordinary matter in the universe?

When astronomers attempt to measure the universe’s total baryonic mass using modern surveys, they keep coming up short. The team behind HaloSat, built by Blue Canyon Technologies and run by the University of Iowa, looks to answer this question by looking at X-ray emissions from the gaseous halo thought to surround the Milky Way Galaxy.

The current tally of the universe’s mass and energy distribution comes from analysis of the Cosmic Microwave Background (CMB), radiation released when the infant universe was just 370,000 years old. Measurements of this early era show that the universe is a mix of 70% dark energy, 25% dark matter, and 5% baryonic matter — the “normal” stuff, including protons, neutrons, and electrons, that makes up the stars, planets, spacecraft, and humans.

Yet astronomers tallying up the observable measure of stars, planets, galaxies, gas, and dust only come up with about half of the baryonic mass they expect.

“We should have all the matter today that we had back when the Universe was 400,000 years old,” Philip Kaaret (University of Iowa) said in a recent press release. Finding the missing matter “can help us learn how we got from the CMB’s uniform state to the large-scale structures we see today.”

One proposed location for the missing mass is the hot gas in the space between galaxies or in the halos around galaxies. For example, gas in the Milky Way’s halo should run around 2 million Kelvin. HaloSat was designed, among other things, to detect the X-rays emitted by ionized oxygen associated with this gas.

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Probing the Galactic Halo

There’s one key problem to detecting this emission: The solar wind interacts with Earth’s atmosphere to produce X-rays that swamp any signal from the galactic halo.

“Every observation we make has this solar wind emission in it to some degree, but it varies with the time and solar wind conditions,” Kip Kuntz (Johns Hopkins University) said in a recent press release. “The variations are so hard to calculate that many people just mention it and then ignore it in their observations.”

HaloSat will get around this by making observations during the 45-minute passes over Earth’s nightside, recharging its solar-powered batteries during the daytime half of its 90-minute orbit.

HaloSat detects X-rays with energies between 400 and 2,000 electron-volts, the same regime as the Chandra and XMM-Newton space telescopes. But unlike these other orbiting X-ray observatories, HaloSat has a wide field of view, 100 square degrees, allowing it to carry out an efficient survey of the entire sky.

Researchers hope to use HaloSat to discern the shape of the Milky Way’s galactic gas halo, to see if it follows more of the bulging pancake shape (the fried egg model) of the Milky Way galaxy itself, or is more of a sphere. If the distribution isn’t uniform, HaloSat should see the difference, looking out perpendicular to the plane of the galaxy versus along the plane itself.

 

Launch & Deployment

HaloSat was launched from NASA’s Wallops Flight Facility on the Virginia coast on May 21, 2018, aboard the Cygnus S.S. J.R. Thompson spacecraft en route to the International Space Station. It was released from the ISS on July 13th.

Halosat is built around an XB1 CubeSat bus from Blue Canyon Technologies, and is equipped with reaction wheels, star trackers, and a solar panel. Although it’s equipped for a wide-field survey, it has a pointing accuracy of better than 30 arc seconds. Its three X-ray detectors were built by Amptek Inc, and are similar to the ones on NASA’s Neutron star Interior Composition Explorer aboard the ISS.

“We expect to operate HaloSat for one year,” says Kaaret. “We hope to show some preliminary results on one or a few bright targets within the Milky Way within that year, mostly to show how the instrument is operating. The results on the halo and the missing baryon problem will hopefully come 6-12 months after operations are done.”

It will be amazing to see if a small satellite mission such as HaloSat can answer such a big cosmological mystery.

Story Source:

By: David Dickinson

 

Cosmos by John Hussey

 

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New insight into why galaxies stop forming stars

Cosmos by John Hussey


Galaxy clusters are rare regions of the universe consisting of hundreds of galaxies containing trillions of stars. It has long been known that when a galaxy falls into a cluster, star formation is fairly rapidly shut off in a process known as ‘quenching.’ A new study has made the best measurement yet of the quenching timescale, measuring how it varies across 70 percent of the history of the universe.

Hubble Space Telescope image of one of the SpARCS clusters used in the study, seen as it appeared when the universe was 4.8 billion years old.

Credit: Jeffrey Chan, UC Riverside.

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Galaxy clusters are rare regions of the universe consisting of hundreds of galaxies containing trillions of stars, as well as hot gas and dark matter.

It has long been known that when a galaxy falls into a cluster, star formation is fairly rapidly shut off in a process known as “quenching.” What actually causes the stars to quench, however, is a mystery, despite several plausible explanations having been proposed by astronomers.

A new international study led by astronomer Ryan Foltz, a former graduate student at the University of California, Riverside, has made the best measurement yet of the quenching timescale, measuring how it varies across 70 percent of the history of the universe. The study has also revealed the process likely responsible for shutting down star formation in clusters.

Each galaxy entering a cluster is known to bring some cold gas with it that has not yet formed stars. One possible explanation suggests that before the cold gas can turn into stars, it is “stripped” away from the galaxy by the hot, dense gas already in the cluster, causing star formation to cease.

Another possibility is that galaxies are instead “strangled,” meaning they stop forming stars because their reservoirs cease getting replenished with additional cold gas once they fall inside the cluster. This is predicted to be a slower process than stripping.

A third possibility is that energy from the star formation itself drives much of the cold gas fuel away from the galaxy and prevents it from forming new stars. This “outflow” scenario is predicted to occur on a faster timescale than stripping, because the gas is lost forever to the galaxy and is unavailable to form new stars.

Because these three different physical processes predict galaxies to quench on different relative timescales over the history of the universe, astronomers have postulated that if they could compare the number of quenched galaxies observed over a long time-baseline, the dominant process causing stars to quench would more readily become apparent.

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View Sample Video – Cosmology – Telescopes – Hubble – 15 Years of Discovery

However, until recently, it was very difficult to find distant clusters, and even harder to measure the properties of their galaxies. The international Spitzer Adaptation of the Red-sequence Cluster Survey, or SpARCS, survey has now made a measurement of more than 70 percent of the history of the universe, accomplished by pioneering new cluster-detection techniques, which enabled the discovery of hundreds of new clusters in the distant universe.

Using some of their own newly discovered SpARCS clusters, the new UCR-led study discovered that it takes a galaxy longer to stop forming stars as the universe gets older: only 1.1 billion years when the universe was young (4 billion years old), 1.3 billion years when the universe is middle-aged (6 billion years old), and 5 billion years in the present-day universe.

“Comparing observations of the quenching timescale in galaxies in clusters in the distant universe to those in the nearby universe revealed that a dynamical process such as gas stripping is a better fit to the predictions than strangulation or outflows,” Foltz said.

To make this state-of-the art measurement, the SpARCS team required 10 nights of observations with the W. M. Keck Observatory telescopes (10 meters in diameter) in Hawaii, and 25 nights of observations with the twin Gemini telescopes (8 meters in diameter) in Hawaii and Chile.

“Thanks to the phenomenal investment in our work by these observatories, we now believe we have a good idea of how star formation stops in the most massive galaxies in clusters,” said Gillian Wilson, professor of physics and astronomy at UCR and leader of the SpARCS survey, in whose lab Foltz worked when the study was done. “There are good reasons, however, to believe that lower-mass galaxies may quench by a different process. That is one of the questions our team is working on answering next.”

The team has been awarded 50 additional nights of Gemini time and a $1.2 million grant from the National Science Foundation to study how star formation stops in more regular-mass galaxies. Wilson was also awarded Hubble Space Telescope observations and a NASA grant to analyze high-resolution images of the quenching galaxies.

The research paper is published in the Astrophysical Journal.

 

Story Source:

Materials provided by University of California – Riverside.

 

Cosmos by John Hussey

 

https://www.sciencedaily.com/releases/2018/10/181023185443.htm

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