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Joshua Frieman has been awarded an Honorary Fellowship of the Royal Astronomical Society
January 10, 2014
Royal Astronomical Society
Professor Joshua Frieman, Professor of Astronomy and Astrophysics at the University of Chicago, and member of the Theoretical Astrophysics group at Fermilab. Freiman's research centres on theoretical and observational cosmology, including studies of the nature of dark energy, the early Universe, gravitational lensing, the large-scale structure of the Universe, and supernovae as cosmological distance indicators. He is a founder of and currently serves as Director of the Dark Energy Survey, a collaboration of over 120 scientists from 20 institutions on 3 continents. Honorary Fellowship is conferred to mark his singular contributions to the study of dark energy.Learn more >>
Faculty members recognized for research, teaching and professional service with new professorships
February 5, 2014
University of Chicago News Office
Angela Olinto, who has made important contributions to the physics of quark stars, inflationary theory, cosmic magnetic fields and particle astrophysics, has been named a Homer J. Livingston Professor in Astronomy & Astrophysics and the College.
Olinto's research interests span theoretical astrophysics, particle and nuclear astrophysics, and cosmology. She has focused much of her work on understanding the origins of the highest energy cosmic rays and the ultra-compressed core of matter in neutron stars. Ultra-high-energy cosmic rays enter the atmosphere with so much energy that they produce a giant cascade of many tens of billions of secondary particles, which can be observed by large detectors such as the Auger Observatory.
Last year, Olinto was elected a fellow of the American Association for the Advancement of Science for her distinguished contributions to the field of astrophysics, particularly exotic states of matter and extremely high-energy cosmic ray studies at the Pierre Auger Observatory in Argentina.
She also is a fellow of the American Physical Society and has received the Chaire d'Excellence Award of the French Agence Nationale de Recherche. Olinto also is a recipient of the University's highest teaching honor, the Llewellyn John and Harriet Manchester Quantrell Award for Excellence in Undergraduate Teaching.
A faculty member at UChicago since 1996, Olinto now leads the U.S. collaboration of the Japanese Experiment Module-Extreme Universe Space Observatory mission to observe these ultra-energy particles from the International Space Station.Learn more >>
Communicating Science with Alan Alda
February 25, 2014
The University of Chicago News Office
Kavli Institute workshop helps scientists engage more clearly with reporters, philanthropists, policymakers and the public.
YouTube | iTunes ULearn more >>
Case for Dark Matter Signal Strengthens
March 6, 2014
by Natalie Wolchover, Quanta Magazine, Simons Foundation
Dan Hooper, Tracy Slatyer, Tim Linden and Stephen Portillo (shown clockwise from top left), and collaborators claim that the annihilation of dark matter particles called WIMPs is the only plausible source for the gamma-ray excess coming from the center of the galaxy.Learn more >>
A big-bang theory gets a big boost: Evidence that vast cosmos was created in split second
March 18, 2014
by Joel Achenbach, The Washington Post
In the beginning, the universe got very big very fast, transforming itself in a fraction of an instant from something almost infinitesimally small to something imponderably vast, a cosmos so huge that no one will ever be able to see it all.
This is the premise of an idea called cosmic inflation - a powerful twist on the big-bang theory - and Monday it received a major boost from an experiment at the South Pole called BICEP2. A team of astronomers led by John Kovac of the Harvard-Smithsonian Center for Astrophysics announced that it had detected ripples from gravitational waves created in a violent inflationary event at the dawn of time.
"We're very excited to present our results because they seem to match the prediction of the theory so closely," Kovac said in an interview. "But it's the case that science can never actually prove a theory to be true. There could always be an alternative explanation that we haven't been clever enough to think of."
The reaction in the scientific community was cautiously exultant. The new result was hailed as potentially one of the biggest discoveries of the past two decades.
Cosmology, the study of the universe on the largest scales, has already been roiled by the 1998 discovery that the cosmos is not merely expanding but doing so at an accelerating rate, because of what has been called "dark energy." Just as that discovery has implications for the ultimate fate of the universe, this new one provides a stunning look back at the moment the universe was born.
"If real, it's magnificent," said Harvard astrophysicist Lisa Randall.
Lawrence Krauss, an Arizona State University theoretical physicist, said of the new result, "It gives us a new window on the universe that takes us back to almost the very beginning of time, allowing us to turn previously metaphysical questions about our origins into scientific ones."
The measurement, however, is a difficult one. The astronomers chose the South Pole for BICEP2 and earlier experiments because the air is exceedingly dry, almost devoid of water vapor and ideal for observing subtle quirks in the ancient light pouring in from the night sky. They spent four years building the telescope, and then three years observing and analyzing the data. Kovac, 43, who has been to the South Pole 23 times, said of the conditions there, "It's almost like being in space."
The BICEP2 instrument sorts through the cosmic microwave background (CMB), looking for polarization of the light in a pattern that reveals the ripples of gravitational waves. The gravitational waves distort space itself, squishing and tugging the fabric of the universe. This is the first time that anyone has announced the detection of gravitational waves from the early universe.
There are other experiments by rival groups trying to detect these waves, and those efforts will continue in an attempt to confirm the results announced Monday.
"I would say it's very likely to be correct that we are seeing a signal from inflation," said Adrian Lee, a University of California at Berkeley cosmologist who is a leader of PolarBear, an experiment based on a mountaintop in Chile that is also searching for evidence of inflation. "But it's such a hard measurement that we really would like to see it measured with different experiments, with different techniques, looking at different parts of the sky, to have confidence that this is really a signal from the beginning of the universe."
The fact that the universe is dynamic at the grandest scale, and not static as it appears to be when we gaze at the "fixed stars" in the night sky, has been known since the late 1920s, when astronomer Edwin Hubble revealed that the light from galaxies showed that they were moving away from one another.
This led to the theory that the universe, once compact, is expanding. Scientists in recent years have been able to narrow down the age of the universe to about 13.8 billion years. Multiple lines of evidence, including the detection of the CMB exactly 50 years ago, have bolstered the consensus model of modern cosmology, which shows that the universe was initially infinitely hot and dense, literally dimensionless. There was no space, no time.
Then something happened. The universe began to expand and cool. This was the big bang.
Cosmic inflation throws gasoline on that fire. It makes the big bang even bangier right at the start. Instead of a linear expansion, the universe would have undergone an exponential growth.
In 1979, theorist Alan Guth, then at Stanford, seized on a potential explanation for some of the lingering mysteries of the universe, such as the remarkable homogeneity of the whole place - the way distantly removed parts of the universe had the same temperature and texture even though they had never been in contact with each other. Perhaps the universe did not merely expand in a stately manner but went through a much more dramatic, exponential expansion, essentially going from microscopic in scale to cosmically huge in a tiny fraction of a second.
It is unclear how long this inflationary epoch lasted. Kovac calculated that in that first fraction of a second the volume of the universe increased by a factor of 10 to the 26th power, going from subatomic to cosmic.
This is obviously difficult terrain for theorists, and the question of why there is something rather than nothing creeps into realms traditionally governed by theologians. But theoretical physicists say that empty space is not empty, that the vacuum crackles with energy and that quantum physics permits such mind-boggling events as a universe popping up seemingly out of nowhere.
"Inflation - the idea of a very big burst of inflation very early on - is the most important idea in cosmology since the big bang itself," said Michael Turner, a University of Chicago cosmologist. "If correct, this burst is the dynamite behind our big bang."
Princeton University astrophysicist David Spergel said after Monday's announcement, "If true, this has revolutionary impacts for our understanding of the physics of the early universe and gives us insight into physics on really small scales."
Spergel added, "We will soon know if this result is revolutionary or due to some poorly understood systematics."
The inflationary model implies that our universe is exceedingly larger than what we currently observe, which is humbling already in its scale. Moreover, the vacuum energy that drove the inflationary process would presumably imply the existence of a larger cosmos, or "multiverse," of which our universe is but a granular element.
"These ideas about the multiverse become interesting to me only when theories come up with testable predictions based on them," Kovac said Monday. "The powerful thing about the basic inflationary paradigm is that it did offer us this clear, testable prediction: the existence of gravitational waves which are directly linked to the exponential expansion that's intrinsic to the theory."
The cosmological models favored by scientists do not permit us to have contact with other potential universes. The multiverse is, for now, conjectural, because it is not easily subject to experimental verification and is unobservable - from the South Pole or from anywhere else.Learn more >>
Space Ripples Reveal Big Bang's Smoking Gun
March 18, 2014
by Dennis Overbye, The New York Times
CAMBRIDGE, Mass. - One night late in 1979, an itinerant young physicist named Alan Guth, with a new son and a year's appointment at Stanford, stayed up late with his notebook and equations, venturing far beyond the world of known physics.
He was trying to understand why there was no trace of some exotic particles that should have been created in the Big Bang. Instead he discovered what might have made the universe bang to begin with. A potential hitch in the presumed course of cosmic evolution could have infused space itself with a special energy that exerted a repulsive force, causing the universe to swell faster than the speed of light for a prodigiously violent instant.
If true, the rapid engorgement would solve paradoxes like why the heavens look uniform from pole to pole and not like a jagged, warped mess. The enormous ballooning would iron out all the wrinkles and irregularities. Those particles were not missing, but would be diluted beyond detection, like spit in the ocean.
"SPECTACULAR REALIZATION," Dr. Guth wrote across the top of the page and drew a double box around it.
On Monday, Dr. Guth's starship came in. Radio astronomers reported that they had seen the beginning of the Big Bang, and that his hypothesis, known undramatically as inflation, looked right.
Reaching back across 13.8 billion years to the first sliver of cosmic time with telescopes at the South Pole, a team of astronomers led by John M. Kovac of the Harvard-Smithsonian Center for Astrophysics detected ripples in the fabric of space-time - so-called gravitational waves - the signature of a universe being wrenched violently apart when it was roughly a trillionth of a trillionth of a trillionth of a second old. They are the long-sought smoking-gun evidence of inflation, proof, Dr. Kovac and his colleagues say, that Dr. Guth was correct.
Inflation has been the workhorse of cosmology for 35 years, though many, including Dr. Guth, wondered whether it could ever be proved.
If corroborated, Dr. Kovac's work will stand as a landmark in science comparable to the recent discovery of dark energy pushing the universe apart, or of the Big Bang itself. It would open vast realms of time and space and energy to science and speculation.
Confirming inflation would mean that the universe we see, extending 14 billion light-years in space with its hundreds of billions of galaxies, is only an infinitesimal patch in a larger cosmos whose extent, architecture and fate are unknowable. Moreover, beyond our own universe there might be an endless number of other universes bubbling into frothy eternity, like a pot of pasta water boiling over.
'As Big as It Gets'
In our own universe, it would serve as a window into the forces operating at energies forever beyond the reach of particle accelerators on Earth and yield new insights into gravity itself. Dr. Kovac's ripples would be the first direct observation of gravitational waves, which, according to Einstein's theory of general relativity, should ruffle space-time.
Marc Kamionkowski of Johns Hopkins University, an early-universe expert who was not part of the team, said, "This is huge, as big as it gets."
He continued, "This is a signal from the very earliest universe, sending a telegram encoded in gravitational waves."
The ripples manifested themselves as faint spiral patterns in a bath of microwave radiation that permeates space and preserves a picture of the universe when it was 380,000 years old and as hot as the surface of the sun.
Dr. Kovac and his collaborators, working in an experiment known as Bicep, for Background Imaging of Cosmic Extragalactic Polarization, reported their results in a scientific briefing at the Center for Astrophysics here on Monday and in a set of papers submitted to The Astrophysical Journal.
Dr. Kovac said the chance that the results were a fluke was only one in 10 million.
Dr. Guth, now 67, pronounced himself "bowled over," saying he had not expected such a definite confirmation in his lifetime.
"With nature, you have to be lucky," he said. "Apparently we have been lucky."
The results are the closely guarded distillation of three years' worth of observations and analysis. Eschewing email for fear of a leak, Dr. Kovac personally delivered drafts of his work to a select few, meeting with Dr. Guth, who is now a professor at Massachusetts Institute of Technology (as is his son, Larry, who was sleeping that night in 1979), in his office last week.
"It was a very special moment, and one we took very seriously as scientists," said Dr. Kovac, who chose his words as carefully as he tended his radio telescopes.
Andrei Linde of Stanford, a prolific theorist who first described the most popular variant of inflation, known as chaotic inflation, in 1983, was about to go on vacation in the Caribbean last week when Chao-Lin Kuo, a Stanford colleague and a member of Dr. Kovac's team, knocked on his door with a bottle of Champagne to tell him the news.
Confused, Dr. Linde called out to his wife, asking if she had ordered anything.
"And then I told him that in the beginning we thought that this was a delivery but we did not think that we ordered anything, but I simply forgot that actually I did order it, 30 years ago," Dr. Linde wrote in an email.
Calling from Bonaire, the Dutch Caribbean island, Dr. Linde said he was still hyperventilating. "Having news like this is the best way of spoiling a vacation," he said.
By last weekend, as social media was buzzing with rumors that inflation had been seen and news spread, astrophysicists responded with a mixture of jubilation and caution.
Max Tegmark, a cosmologist at M.I.T., wrote in an email, "I think that if this stays true, it will go down as one of the greatest discoveries in the history of science."
John E. Carlstrom of the University of Chicago, Dr. Kovac's mentor and head of a competing project called the South Pole Telescope, pronounced himself deeply impressed. "I think the results are beautiful and very convincing," he said.
Paul J. Steinhardt of Princeton, author of a competitor to inflation that posits the clash of a pair of universes as the cause of genesis, said that if true, the Bicep result would eliminate his model, but he expressed reservations about inflation.
Lawrence M. Krauss of Arizona State and others also emphasized the need for confirmation, noting that the new results exceeded earlier estimates based on temperature maps of the cosmic background by the European Space Agency's Planck satellite and other assumptions about the universe.
"So we will need to wait and see before we jump up and down," Dr. Krauss said.
Corroboration might not be long in coming. The Planck spacecraft will report its own findings this year. At least a dozen other teams are trying similar measurements from balloons, mountaintops and space.
Spirals in the Sky
Gravity waves are the latest and deepest secret yet pried out of the cosmic microwaves, which were discovered accidentally by Arno Penzias and Robert Wilson at Bell Labs 50 years ago. They won the Nobel Prize.
Dr. Kovac has spent his career trying to read the secrets of these waves. He is one of four leaders of Bicep, which has operated a series of increasingly sensitive radio telescopes at the South Pole, where the thin, dry air creates ideal observing conditions. The others are Clement Pryke of the University of Minnesota, Jamie Bock of the California Institute of Technology and Dr. Kuo of Stanford.
"The South Pole is the closest you can get to space and still be on the ground," Dr. Kovac said. He has been there 23 times, he said, wintering over in 1994. "I've been hooked ever since," he said.
In 2002, he was part of a team that discovered that the microwave radiation was polarized, meaning the light waves had a slight preference to vibrate in one direction rather than another.
This was a step toward the ultimate goal of detecting the gravitational waves from inflation. Such waves, squeezing space in one direction and stretching it in another as they go by, would twist the direction of polarization of the microwaves, theorists said. As a result, maps of the polarization in the sky should have little arrows going in spirals.
Detecting those spirals required measuring infinitesimally small differences in the temperature of the microwaves. The group's telescope, Bicep2, is basically a giant superconducting thermometer.
"We had no expectations what we would see," Dr. Kovac said.
The strength of the signal surprised the researchers, and they spent a year burning up time on a Harvard supercomputer, making sure they had things right and worrying that competitors might beat them to the breakthrough.
A Special Time
The data traced the onset of inflation to a time that physicists like Dr. Guth, staying up late in his Palo Alto house 35 years ago, suspected was a special break point in the evolution of the universe.
Physicists recognize four forces at work in the world today: gravity, electromagnetism, and strong and weak nuclear forces. But they have long suspected that those are simply different manifestations of a single unified force that ruled the universe in its earliest, hottest moments.
As the universe cooled, according to this theory, there was a fall from grace, like some old folk mythology of gods or brothers falling out with each other. The laws of physics evolved, with one force after another splitting away.
That was where Dr. Guth came in.
Under some circumstances, a glass of water can stay liquid as the temperature falls below 32 degrees, until it is disturbed, at which point it will rapidly freeze, releasing latent heat.
Similarly, the universe could "supercool" and stay in a unified state too long. In that case, space itself would become imbued with a mysterious latent energy.
Inserted into Einstein's equations, the latent energy would act as a kind of antigravity, and the universe would blow itself up. Since it was space itself supplying the repulsive force, the more space was created, the harder it pushed apart.
What would become our observable universe mushroomed in size at least a trillion trillionfold - from a submicroscopic speck of primordial energy to the size of a grapefruit - in less than a cosmic eye-blink.
Almost as quickly, this pulse would subside, relaxing into ordinary particles and radiation. All of normal cosmic history was still ahead, resulting in today's observable universe, a patch of sky and stars billions of light-years across. "It's often said that there is no such thing as a free lunch," Dr. Guth likes to say, "but the universe might be the ultimate free lunch."
Make that free lunches. Most of the hundred or so models resulting from Dr. Guth's original vision suggest that inflation, once started, is eternal. Even as our own universe settled down to a comfortable homey expansion, the rest of the cosmos will continue blowing up, spinning off other bubbles endlessly, a concept known as the multiverse.
So the future of the cosmos is perhaps bright and fecund, but do not bother asking about going any deeper into the past.
We might never know what happened before inflation, at the very beginning, because inflation erases everything that came before it. All the chaos and randomness of the primordial moment are swept away, forever out of our view.
"If you trace your cosmic roots," said Abraham Loeb, a Harvard-Smithsonian astronomer who was not part of the team, "you wind up at inflation."Learn more >>
Telescope captures view of gravitational waves: Images of the infant Universe reveal evidence for rapid inflation after the Big Bang
March 18, 2014
by Ron Cowen, Nature
Astronomers have peered back to nearly the dawn of time and found what seems to be the long-sought 'smoking gun' for the theory that the Universe underwent a spurt of wrenching, exponential growth called inflation during the first tiny fraction of a second of its existence.
Using a radio telescope at the South Pole, the US-led team has detected the first evidence of primordial gravitational waves, ripples in space that inflation generated 13.8 billion years ago when the Universe first started to expand.
The telescope captured a snapshot of the waves as they continued to ripple through the Universe some 380,000 years later, when stars had not yet formed and matter was still scattered across space as a broth of plasma. The image was seen in the cosmic microwave background (CMB), the glow that radiated from that white-hot plasma and that over billions of years of cosmic expansion has cooled to microwave energies.
The fact that inflation, a quantum phenomenon, produced gravitational waves demonstrates that gravity has a quantum nature just like the other known fundamental forces of nature, experts say. Moreover, it provides a window into interactions much more energetic than are accessible in any laboratory experiment. In addition, the way that the team confirmed inflation is itself of major significance: it is the most direct evidence yet that gravitational waves - a key but elusive prediction of Albert Einstein's general theory of relativity - exist.
"This is a totally new, independent piece of cosmological evidence that the inflationary picture fits together," says theoretical physicist Alan Guth of the Massachusetts Institute of Technology (MIT) in Cambridge, who proposed the idea of inflation in 1980. He adds that the study is "definitely" worthy of a Nobel prize.
Guth's idea was that the cosmos expanded at an exponential rate for a few tens of trillionths of trillionths of trillionths of seconds after the Big Bang, ballooning from subatomic to football size. Inflation solves several long-standing cosmic conundrums, such as why the observable Universe appears uniform from one end to the other. Although the theory has proved to be consistent with all cosmological data collected so far, conclusive evidence for it has been lacking.
Cosmologists knew, however, that inflation would have a distinctive signature: the brief but violent period of expansion would have generated gravitational waves, which compress space in one direction while stretching it along another (see 'Ripple effect'). Although the primordial waves would still be propagating across the Universe, they would now be too feeble to detect directly. But they would have left a distinctive mark in the CMB: they would have polarized the radiation in a curly, vortex-like pattern known as the B mode (see 'Cosmic curl').
Last year, another telescope in Antarctica - the South Pole Telescope (SPT) - became the first observatory to detect a B-mode polarization in the CMB (see Nature http://doi.org/rwt; 2013). That signal, however, was over angular scales of less than one degree (about twice the apparent size of the Moon in the sky), and was attributed to how galaxies in the foreground curve the space through which the CMB travels (D. Hanson et al. Phys. Rev. Lett. 111, 141301; 2013). But the signal from primordial gravitational waves is expected to peak at angular scales between one and five degrees.
And that is exactly what John Kovac of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts, and his colleagues now say they have detected, using an instrument dubbed BICEP2 that is located just metres away from its competitor, the SPT.
Detecting the tiny B mode required measuring the CMB with a precision of one ten-millionth of a kelvin and distinguishing the primordial effect from other possible sources, such as galactic dust.
"The key question," says Daniel Eisenstein, an astrophysicist at the CfA, "is whether there could be a foreground that masquerades like this signal". But the team has all but ruled out that possibility, he says. First, the researchers were careful to point BICEP2 - an array of 512 superconducting microwave detectors - at the Southern Hole, a patch of sky that is known to contain only tiny amounts of such emissions. They also compared their data with those taken by an earlier experiment, BICEP1, and showed that a dust-generated signal would have had a different colour and spectrum.
Furthermore, data taken with a newer, more sensitive polarization experiment, the Keck array, which the team finished installing at the South Pole in 2012 and will continue operating for two more years, showed the same characteristics. "To see this same signal emerge from two other, different telescopes was for us very convincing," says Kovac.
"The details have to be worked out, but from what I know it's highly likely this is what we've all been waiting for," says astronomer John Carlstrom of the University of Chicago, Illinois, who is the lead researcher on the SPT. "This is the discovery of inflationary gravitational waves."
Cosmologist Marc Kamionkowski adds: "To me, this looks really, really solid." He was one of the first cosmologists to calculate what the signature of primordial gravitational waves should look like in the CMB. The findings are "on a par with dark energy, or the discovery of the CMB - something that happens once every several decades", says Kamionkowski, who is at Johns Hopkins University in Baltimore, Maryland.
The strength of the signal measured by BICEP2, although entirely consistent with inflation, initially surprised the researchers because it is nearly twice as large as estimated from previous experiments. According to theory, the intensity of a B-mode signal reveals how fast the Universe expanded during inflation, and therefore suggests the energy scale of the cosmos during that epoch. The data pinpoint the time when inflation occurred - about 10-37 seconds into the Universe's life - and its temperature at the time, corresponding to energies of about 1016 gigaelectronvolts, says cosmologist Michael Turner of the University of Chicago. That is the same energy at which three of the four fundamental forces of nature - the weak, strong and electromagnetic force - are expected to become indistinguishable from one another in a model known as the grand unified theory.
Because inflation took place in the realm of quantum physics, seeing gravitational waves arise from that epoch provides "the first-ever experimental evidence for quantum gravity", says MIT cosmologist Max Tegmark - in other words, it shows that gravity is at heart a quantum phenomenon, just like the other three fundamental forces. Physicists, however, have yet to fully understand how to reconcile general relativity with quantum physics from a theory standpoint.
The researchers reported the findings on 17 March at a press briefing at the CfA, held just after they described their results to scientists in a technical talk. The team also released several papers describing the results. In so doing, it seems to have beaten the SPT and also several other groups racing to find the fingerprint of inflation using an assortment of balloon-borne and ground-based experiments and one satellite, the European Space Agency's Planck spacecraft.
More-extensive maps of the B-mode polarization, and especially a full-sky survey, which the Planck telescope may be able to obtain later this year, should provide more clues about how inflation unfolded and what drove it. In addition to looking farther back in time than ever before, the discovery "is opening a window a trillion times higher in energy than we can access with the Large Hadron Collider", the world's premiere atom smasher, notes cosmologist Avi Loeb of the CfA, who is not part of the BICEP2 team.Learn more >>
Fermi Telescope data tantalize with new clues to dark matter
April 3, 2014
The University of Chicago News Office
A new study of gamma-ray light from the center of the galaxy makes the strongest case to date that some of this emission may arise from dark matter, an unknown substance making up most of the material universe.
Using publicly available data from NASA's Fermi Gamma-ray Space Telescope, independent scientists at the Fermi National Accelerator Laboratory, the Harvard-Smithsonian Center for Astrophysics (CfA), the Massachusetts Institute of Technology and the University of Chicago have developed new maps showing that the galactic center produces more high-energy gamma rays than can be explained by known sources and that this excess emission is consistent with some forms of dark matter.
"The new maps allow us to analyze the excess and test whether more conventional explanations, such as the presence of undiscovered pulsars or cosmic-ray collisions on gas clouds, can account for it," said Dan Hooper, University of Chicago associate professor in astronomy and astrophysics. A lead author of the study, Hooper also is an astrophysicist at Fermilab. "The signal we find cannot be explained by currently proposed alternatives and is in close agreement with the predictions of very simple dark matter models," he said.
The galactic center teems with gamma-ray sources, from interacting binary systems and isolated pulsars to supernova remnants and particles colliding with interstellar gas. It's also where astronomers expect to find the galaxy's highest density of dark matter, which only affects normal matter and radiation through its gravity. Large amounts of dark matter attract normal matter, forming a foundation upon which visible structures, like galaxies, are built.
No one knows the true nature of dark matter, but WIMPs, or Weakly Interacting Massive Particles, represent a leading class of candidates. Theorists have envisioned a wide range of WIMP types, some of which may either mutually annihilate or produce an intermediate, quickly decaying particle when they collide. Both of these pathways end with the production of gamma rays - the most energetic form of light - at energies within the detection range of Fermi's Large Area Telescope (LAT).
When astronomers carefully subtract all known gamma-ray sources from LAT observations of the galactic center, a patch of leftover emission remains. This excess appears most prominent at energies between 1 and 3 billion electron volts (GeV) - roughly a billion times greater than that of visible light - and extends outward at least 5,000 light-years from the galactic center.
Hooper and his colleagues conclude that annihilations of dark matter particles with a mass between 31 and 40 GeV provide a remarkable fit for the excess based on its gamma-ray spectrum, its symmetry around the galactic center and its overall brightness. Writing in a paper submitted to the journal Physical Review D, the researchers say that these features are difficult to reconcile with other explanations proposed so far, although they note that plausible alternatives not requiring dark matter may yet materialize.
"Dark matter in this mass range can be probed by direct detection and by the Large Hadron Collider (LHC), so if this is dark matter, we're already learning about its interactions from the lack of detection so far," said co-author Tracy Slatyer, theoretical physicist at MIT. "This is a very exciting signal, and while the case is not yet closed, in the future we might well look back and say this was where we saw dark matter annihilation for the first time."
The researchers caution that it will take multiple sightings - in other astronomical objects, the LHC or in some of the direct-detection experiments now being conducted around the world - to validate their dark matter interpretation.
"Our case is very much a process-of-elimination argument. We made a list, scratched off things that didn't work, and ended up with dark matter," said co-author Douglas Finkbeiner, professor of astronomy and physics at the CfA.
"This study is an example of innovative techniques applied to Fermi data by the science community," said Peter Michelson, professor of physics at Stanford University and the LAT principal investigator. "The Fermi LAT Collaboration continues to examine the extraordinarily complex central region of the galaxy, but until this study is complete we can neither confirm nor refute this interesting analysis."
While the great amount of dark matter expected at the galactic center should produce a strong signal, competition from many other gamma-ray sources complicates any case for detection. But turning the problem on its head provides another way to attack it. Instead of looking at the largest nearby collection of dark matter, look where the signal has fewer challenges.
Dwarf galaxies orbiting the Milky Way lack other types of gamma-ray emitters and contain large amounts of dark matter for their size - in fact, they're the most dark-matter-dominated sources known. But there's a tradeoff. Because they lie much farther away and contain much less total dark matter than the center of the Milky Way, dwarf galaxies produce a much weaker signal and require many years of observations to establish secure detection.
For the past four years, the LAT team has been searching dwarf galaxies for hints of dark matter. The published results of these studies has set stringent limits on the mass ranges and interaction rates for many proposed WIMPs, even eliminating some models. In the study's most recent results, published in Physical Review D on Feb. 11, the Fermi team took note of a small but provocative gamma-ray excess.
"There's about a one-in-12 chance that what we're seeing in the dwarf galaxies is not even a signal at all, just a fluctuation in the gamma-ray background," explained Elliott Bloom, a member of the LAT Collaboration at the Kavli Institute for Particle Astrophysics and Cosmology, jointly located at the SLAC National Accelerator Laboratory and Stanford University. If it's real, the signal should grow stronger as Fermi acquires additional years of observations and as wide-field astronomical surveys discover new dwarfs. "If we ultimately see a significant signal," he said, "it could be a very strong confirmation of the dark matter signal claimed in the galactic center."Learn more >>
Physics Today' cover page: The Dark Energy Survey
April 10, 2014
The Dark Energy Survey will conduct a five-year census of galaxies and stars over a full eighth of the night sky in an effort to understand what is driving the accelerating expansion of the cosmos. The survey will be carried out by the 570-megapixel Dark Energy Camera, photographed here in its black housing on the Victor M. Blanco Telescope in Chile. Josh Frieman's article on page 28 describes the science underlying the survey and some of the camera's advanced technology.Learn more >>
Wayne Hu was elected to the American Academy of Arts and Sciences
May 1, 2014
The University of Chicago News Office
American Academy of Arts and Sciences elects 26 members with UChicago ties
New class of inductees includes eight faculty members, three trustees
Wayne Hu's research focuses on understanding structure formation in the universe as revealed in temperature differences found in the cosmic microwave background radiation (the afterglow of the Big Bang), gravitational lensing (an effect that distorts images of galaxies), and how galaxies and clusters of galaxies were seeded at the Big Bang. A professor in astronomy and astrophysics, Hu also develops and tests theories for dark energy and cosmic acceleration. Hu's honors include a Packard Fellowship, an Alfred P. Sloan Fellowship, the Warner Prize from the American Astronomical Society, and the Outstanding Young Researcher Award from the Overseas Chinese Physics Association.Learn more >>
"The Power of Curiosity", by Michael S. Turner, Science 2 May 2014: Vol. 344 no. 6183 p. 449
May 2, 2014
Michael S. Turner is the Rauner Distinguished Service Professor and director of the Kavli Institute of Cosmological Physics at the University of Chicago, Chicago, IL.
In March of 2014, 47 scientists from 15 institutions (including my own) announced that a South Pole - based microwave telescope had taken us back to a time when the universe was 10-38 seconds old - when everything that we can see today occupied a space much smaller than that occupied by a proton, and when the energy level of the universe was a trillion times greater than that produced by the world's most powerful accelerator, the Large Hadron Collider in Switzerland. Assuming that this amazing discovery by the Background Imaging of Cosmic Extragalactic Polarization (BICEP2) collaboration is confirmed, this cosmic remnant beats the previous record holder for earliest fossil of our cosmic birth (the helium and deuterium made when the universe was seconds old) by 38 orders of magnitude. Rarely has science advanced by such a giant leap, and it will take us years if not decades to fully comprehend all the implications of this incredible moment in science. Although we are used to cosmology stunning us with beautiful images and mind-stretching discoveries such as dark energy and dark matter, even this cosmologist with almost 40 years of experience was awed and shocked by this big, big find.
According to the standard cosmological model, during its earliest moments, the universe underwent a tremendous growth spurt known as inflation, which created the seeds of galaxies and all other cosmic structures from subatomic quantum fluctuations. Since 1992, measurements of the cosmic microwave background have amassed evidence for this theory, but the BICEP2 telescope may have found the smoking gun: gravitational waves that began as quantum fluctuations in spacetime and left an imprint on the cosmic microwave background in the tiny signal (about 100 nanokelvins) detected by the BICEP2 telescope.
Because of deep and remarkable connections between quarks and the cosmos, this cosmic discovery is related to another big discovery, the Higgs boson. The Higgs, the first of a new class of elementary particles (scalar bosons), accounts for why some elementary particles have mass. The potential instigator of cosmic inflation is a hypothetical scalar boson called the inflaton, and the Higgs discovery boosted its credibility and may even explain how it fits in. Conversely, the BICEP2 discovery has given us a window on the highest energies that particle theorists can imagine, and in doing so, will provide insights into how the fundamental forces and particles are unified.
There are differences between the BICEP2 and Higgs discoveries to be sure, in technique and scale of effort, but both are exemplars of the kind of curiosity-driven science that gets scientists out of bed in the morning and inspires young people to careers in science by asking some of the deepest questions about how the universe began and the events that have shaped our existence. BICEP2 and the Higgs will launch the careers of thousands of new scientists around the world, just as quarks and quasars sparked my career. But we should not forget that great discoveries can have unforeseen practical benefits as well. Some 100 years ago, the discovery of strange new phenomena began the esoteric study of quantum mechanics, with no hint of a practical benefit. The exploitation of these quantum phenomena has enabled the information age that now underpins our economy and way of life.
Whether the fruits of our curiosity are bettering our existence on Earth or our understanding of our place in the cosmos, it all begins with a burning desire to know. The BICEP2 and Higgs discoveries remind us never to underestimate the power of this curiosity, one of humankind's greatest assets.Learn more >>
Fermi Institute announces recipients of Nathan Sugarman research awards
June 5, 2014
by Steve Koppes, The University of Chicago News Office
Two fourth-year students in the College and two graduate students have received the 23rd annual Nathan Sugarman Awards for Excellence in Undergraduate and Graduate Research.
The undergraduate recipients are Jane Huang, a fourth-year in chemistry and 2013 Goldwater Scholar; and Samantha Dixon, a fourth-year in physics and mathematics.
The graduate recipients are Vinicius Miranda, a doctoral student in astronomy and astrophysics; and Eric Oberla, a doctoral student in physics.
Huang was cited "for her in-depth analysis of λ5797, the Rosetta Stone of diffuse interstellar bands." She was nominated by Takeshi Oka, professor emeritus of chemistry and astronomy and astrophysics; and Donald York, the Horace B. Horton Professor of Astronomy and Astrophysics.
Dixon was cited "for her outstanding work in the calibration set-up for the DAMIC dark matter experiment, and in the measurement of radioactive contamination of the CCD detectors." Dixon was nominated by Paolo Privitera, professor of astronomy and astrophysics.
Miranda was cited "for his thorough and careful work in elucidating the effect of inflationary features on cosmic microwave background anisotropy and non-Gaussianity." Miranda was nominated by Wayne Hu, professor of astronomy and astrophysics.
Nathan Sugarman, SB'37, PhD'41, was a charter member of the Enrico Fermi Institute and a longtime professor in chemistry.Learn more >>
Chuan He and Wayne Hu receive named professorships
June 12, 2014
The University of Chicago News Office
The Division of Physical Sciences is pleased to announce that Professors Chuan He and Wayne Hu have received named and distinguished service professorships in recognition of their outstanding contribution to scholarship, teaching and the intellectual community of the University of Chicago.
Wayne Hu, Professor in Astronomy and Astrophysics and the Enrico Fermi Institute will receive the Horace B. Horton Professorship on November 1, 2014.
Hu's research focuses on the theory and phenomenology of structure formation in the Universe as revealed in Cosmic Microwave Background anisotropies, gravitational lensing, galaxy clustering and galaxy clusters. His work has been published in Physics Review D, The Journal of Cosmology and Astroparticle Physics, The New Journal of Physics, and other publications.
Dr. Hu has received a number of awards, including the American Astronomical Society Warner Prize, an Alfred P. Sloan Fellowship, and Packard Fellowship. He joined the University of Chicago faculty in 2000.Learn more >>
Finding Dark Energy in the Details
September 23, 2014
The astrophysicist Joshua Frieman seeks to pinpoint the mysterious substance driving the accelerating expansion of the universe.
Like most theoretical cosmologists, Joshua Frieman was thrilled when astronomers announced in 1998 that the expansion of the universe appeared to be speeding up, driven by an invisible agent that they called "dark energy."
Frieman and his fellow theorists imagined two possible causes for the cosmic acceleration: Dark energy could be the quantum jitter of empty space, a "cosmological constant" that continues to accrue as space expands, pushing outward ever more forcefully. Alternately, a yet-undetected force field could pervade the cosmos, one akin to the field that scientists believe powered the exponential expansion of the universe during the Big Bang.
But the scientists also realized that the two options would have nearly identical observational consequences, and either theory could fit the crude measurements to date.
To distinguish between them, Frieman, a professor of astronomy and astrophysics at the University of Chicago and a senior staff scientist at the Fermi National Accelerator Laboratory (Fermilab) in nearby Batavia, Ill., co-founded the Dark Energy Survey (DES), a $50 million, 300-person experiment. The centerpiece of the project is the Dark Energy Camera, or DECam, a 570-megapixel, optical and near-infrared CCD detector built at Fermilab and installed on the four-meter Blanco Telescope in Chile two years ago. By observing 300 million galaxies spanning 10 billion light-years, DES aims to track the cosmic acceleration more precisely than ever before in hopes of favoring one hypothesis over the other. Frieman and his team are now reporting their first results.
Quanta Magazine caught up with Frieman in late August during COSMO 2014, a conference he helped organize. With his closely clipped gray beard, tortoiseshell glasses and organic cotton shirt, the scientist fit right in with the other gourmands lunching at Eataly Chicago down the street. Between bites of tagliatelle, he explained just what is and isn't known about dark energy, and how DES will help impel theorists toward one of the two disparate descriptions of its nature. An edited and condensed version of the interview follows.
QUANTA MAGAZINE: Why did you start the Dark Energy Survey?
JOSHUA FRIEMAN: As a theorist in the 1990s working on theoretical ideas for what could be causing the universe to speed up, I came to the conclusion that we could make different models and do a lot of theoretical speculation, but that we wouldn't know which of those paths to go down until we had much better data.
So a handful of us in Illinois started discussing possibilities for getting that data. And it just so happened that, around that time, the National Optical Astronomy Observatory announced an opportunity, saying, more or less, "If someone can build a really cool instrument for the telescope we operate in Chile, we'll give you a bunch of telescope time." That's when we formed the Dark Energy Survey collaboration and came up with the design for our camera.
Isn't it unusual for a theorist to lead a major astrophysics experiment?
It's somewhat unusual, but the boundaries between theory and observation in cosmology are getting blurred, which I think is a healthy development. It used to be that theorists like me would work with a pen and paper, and then observers would go out and take the data and analyze it. But we now have a new model in which teams are trained to analyze and interpret large data sets, and that isn't purely theory or purely observational work; it combines the two.
How do you picture an invisible unknown like dark energy?
One way to think about dark energy is as a fluid, in the sense that it can be described by its density and its pressure. Those two properties tell you its effects on the expansion of the universe. The more dark energy there is - that is, the greater its density - the stronger its effects are. But the thing that's really crucial about dark energy is that unlike anything else we know about, it has negative pressure, and that's what makes it gravitationally repulsive.
Why does negative pressure make it repulsive?
Einstein's theory says the force of gravity is proportional to the energy density plus three times the pressure, so pressure itself actually gravitates. That's something we're not familiar with, because for ordinary matter, the pressure is just a tiny fraction of the density. But if something has a pressure that's a sizable fraction of the energy density, and if that pressure is negative, then I can flip the sign of gravity. Gravity's no longer attractive - it's repulsive.
By far the leading candidate for dark energy is the "cosmological constant." What's that?
Albert Einstein introduced the cosmological constant in 1917 as an additional term in the equation of gravity. In Einstein's theory, gravity is the curvature of space-time: You have some source of energy and pressure that curves space-time, and then other matter moves within this curved space. Einstein's equations relate the curvature of space-time to the energy and pressure of whatever's in space.
Einstein originally put the cosmological constant on the curvature side of the equation because he wanted to get a certain solution, which turned out to be wrong. But soon after that, the Belgian physicist Georges Lemaitre realized that the cosmological constant naturally lives with the pressures and energy densities, and that it could be interpreted as the energy density and pressure of something. Already on the energy density and pressure side of the equation was everything in the universe: dark matter, atoms, whatever. If I remove all that stuff, then the cosmological constant must be the energy density and pressure of empty space.
How could empty space possess energy and pressure?
In classical physics, empty space would have no energy or pressure. But quantum effects can create energy and pressure even if there are no real particles there. In quantum theory, you can imagine virtual particles zipping in and out of the vacuum, and those virtual particles - which are always being produced and then annihilating - have energy. So if dark energy is the cosmological constant, then it could be the energy associated with these virtual particles.
How do you measure dark energy?
There are two things we're trying to do that can give us constraints on dark energy: One is to measure distances, which tells us the history of cosmic expansion. The second is to measure the growth of structure in the universe.
The galaxy cluster Abell 1689, imaged by the Hubble Space Telescope, bends light around its center, distorting the shapes of more distant galaxies in an effect known as gravitational lensing.
For the latter, we're using a technique called "weak gravitational lensing," which involves measuring, very precisely, the shapes of hundreds of millions of galaxies, and then inferring how those shapes have been distorted because the light rays from those galaxies get bent by gravity as they travel to us. This lensing effect is really tiny, so in 99 cases out of 100, you can't tell just by looking at a galaxy if it has been lensed. So we have to tease out the signal statistically.
If we look at the shapes of galaxies that are not so far versus ones that are farther, part of the difference in the shapes will be due to the fact that the light has passed through different amounts of clumpy structure. Measuring the lensing signal will give us a measure of how the clumpiness of the universe has evolved over cosmic time, and that clumpiness is impacted by dark energy. Gravity pulls stuff in, making the universe become more and more clumpy over time, but dark energy does the opposite. It makes things push away from each other. So if we can measure how the clumpiness of the universe has changed over cosmic time, we can infer something about dark energy: how much of it there was, and what its properties were at different points in time.
DES will try to calculate the dark energy "equation of state" parameter, w. What does w represent?
The parameter w tells us the ratio of the pressure of the dark energy to its density. If dark energy is the cosmological constant, then you can show that the only w that's consistent for empty space is the one where the pressure is exactly equal to minus the energy density. So w has a very specific value: minus one.
If dark energy isn't the cosmological constant, what else might it be?
The simplest alternatives, and the ones that I worked on in the 1990s, are inspired by "inflation." Before we knew that the expansion of the universe is currently speeding up, we had this idea that the universe was speeding up in the very earliest fraction of a second after the Big Bang. That idea of very early cosmic acceleration is called inflation. So the simplest thing to do was to borrow the theory that explains this other epoch of sped-up expansion, and that involves scalar fields.
A scalar field is an entity that has a value everywhere in space. As the field evolves, it can act like dark energy: If it evolves really slowly, it will have negative pressure, which will cause the universe to accelerate. The simplest models of primordial inflation say that, for some period, the universe was dominated by one of these scalar fields, and it eventually decayed and disappeared. And if that's our best idea for what happened when the universe was speeding up almost 14 billion years ago, we should consider that maybe we have something like that going on now.
If you look at these models, they tend to predict that w, the ratio of the pressure to energy density, would be slightly different from minus one. We would like to test that idea.Learn more >>
Follow Wired Twitter Facebook RSS Mysterious Missing Pulsars May Have Gotten Wrapped in Dark Matter and Turned Into Black Holes
November 13, 2014
The center of the galaxy should be chock-full of rapidly spinning, dense stellar corpses known as pulsars. The problem is, astronomers can't seem to find them.
The galactic center is a bustling place. Lots of gas, dust, and stars zip about, orbiting a supermassive black hole about three million times more massive than the sun. With so many stars, astronomers estimate that there should be hundreds of dead ones, says astrophysicist Joseph Bramante of Notre Dame University. Scientists have found only a single young pulsar at the galactic center, where there should be as many as 50 such youngsters.
Bramante and astrophysicist Tim Linden of the University of Chicago have a possible solution to this missing-pulsar problem, which they describe in a paper accepted for publication in the journal Physical Review Letters. Maybe those pulsars are absent because dark matter, which is plentiful in the galactic center, gloms onto the pulsars, accumulating until the pulsars become so dense they collapse into a black hole. Poof. No more pulsars.Learn more >>