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Science and Information / Autolens analysis steps up for Euclid’s 100,000 strong gravitational lens challe

« on: January 15, 2017, 08:21:54 PM »

The European Space Agency's Euclid satellite, due for launch in 2020, will set astronomers a huge challenge: to analyse one hundred thousand strong gravitational lenses. The gravitational deflection of light from distant astronomical sources by massive galaxies (strong lenses) along the light path can create multiple images of the source that are not just visually stunning, but are also valuable tools for probing our Universe.

Now, in preparation for Euclid's challenge, researchers from the University of Nottingham have developed 'AutoLens', the first fully-automated analysis software for strong gravitational lenses. James Nightingale will present the first results from AutoLens at the National Astronomy Meeting 2016 in Nottingham on Friday, 1st July.

"AutoLens has demonstrated its capabilities with this stunning image of a strong gravitational lens system captured by the Hubble Space Telescope," said Nightingale, who developed AutoLens together with his colleague, Dr Simon Dye. "The software's reconstruction of the lensed source reveals in detail a distant pair of star-forming galaxies that are possibly in the early stages of merging. Within the lensed image of the source are small-scale distortions, which encode an imprint of how the lens galaxy's mass is distributed. AutoLens has a novel new approach to exploit this imprinted information and can accurately measure the distribution of dark matter in the lensing galaxy."

Historically, the analysis of strongly lensed images has been a very time consuming process, requiring a large amount of manual input to study just one system. To date, only around two hundred strong lens systems have been analysed. AutoLens can be run on 'massively parallel' computing architecture that uses multiple processors and requires no user input, so will be able to manage the huge amount of data delivered by the Euclid mission.

"Some of astronomy's most important results in the past five years have come from studying a handful of strong lenses. This small sample has allowed us to start to unravel the dark matter content of galaxies and the complex physics that drives their formation and evolution," said Nightingale. "It will be breathtaking to embark on a study of up to one hundred thousand such systems. We can only speculate as to what it will reveal about the nature of dark matter and its role in galaxy evolution."

32

Science and Information / Accelerating research into dark energy

« on: January 15, 2017, 08:21:35 PM »

A quick method for making accurate, virtual universes to help understand the effects of dark matter and dark energy has been developed by UCL and CEFCA scientists. Making up 95% of our universe, these substances have profound effects on the birth and lives of galaxies and stars and yet almost nothing is known about their physical nature.

The new approach, published today in Monthly Notices of the Royal Astronomical Society and funded by the Royal Society, is twenty-five times faster than current methods but is just as accurate, allowing scientists more computer power to focus on understanding why the universe is accelerating and galaxies are positioned where they are.

"To uncover the nature of dark energy and the origin of our 14 billion year old accelerating universe, we have to compare the results from big studies to computational models of the universe," explained Dr Andrew Pontzen, UCL Physics & Astronomy.

"Exciting new ventures, including the Large Synoptic Survey Telescope and the Javalambre Physics of the Accelerating Universe survey, are on the horizon, and we want to be ready to do the best possible job of understanding them," added joint author Dr Raul Angulo, CEFCA, Spain.

Dr Pontzen continued: "But every computer simulation we run gives a slightly different answer. We end up needing to take an average over hundreds of simulations to get a 'gold standard' prediction. We've shown it's possible to achieve the same model accuracy by using only two carefully-constructed virtual universes, so a process that would take weeks on a superfast computer, can now be done in a day."

The scientists say their method will speed up research into the unseen forces in the universe by allowing many model universes to be rapidly developed to test alternate versions of dark energy and dark matter.

"Our method allows cosmologists to run more creative experiments which weren't feasible before due to the large amount of computer time needed. For example, scientists can now generate lots of different models of dark energy to find the one which best explains real-world survey data. We could also use this approach to see how individual galaxies look and fit inside the overall structure of the universe by spending the freed-up time on computing the virtual universes in much greater detail," said Dr Pontzen.

The new method removes the biggest uncertainties in the model universe by comparing its properties with an 'inverted' version. In the inverted model universe, galaxies are replaced by empty voids, and the empty voids of space with galaxies. The scientists tried this approach after noticing a mathematical symmetry linking the two seemingly different pictures.

When they compared the output of the paired universes to that of the gold standard method -- which averages 300 virtual universes to remove uncertainties -- they found the results to be very similar. The new approach showed less than 1% deviation from the gold standard, suggesting the new approach makes predictions that are accurate enough to use in forthcoming experiments.

"In addition to the reversal process, we also adjust the ripples of the early universe to carefully-chosen values, to further eliminate inaccuracies" added Dr Angulo.

The team now plan on using the new method to investigate how different forms of dark energy affect the distribution of galaxies through the universe. "Because we can get a more accurate prediction in a single shot, we don't need to spend so much computer time on existing ideas and can instead investigate a much wider range of possibilities for what this weird dark energy might really be made from," said Dr Pontzen.

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Science and Information / Dark energy measured with record-breaking map of 1.2 million galaxies

« on: January 15, 2017, 08:21:17 PM »

A team of hundreds of physicists and astronomers have announced results from the largest-ever, three-dimensional map of distant galaxies. The team constructed this map to make one of the most precise measurements yet of the dark energy currently driving the accelerated expansion of the Universe.

"We have spent five years collecting measurements of 1.2 million galaxies over one quarter of the sky to map out the structure of the Universe over a volume of 650 cubic billion light years," says Jeremy Tinker of New York University, a co-leader of the scientific team carrying out this effort. "This map has allowed us to make the best measurements yet of the effects of dark energy in the expansion of the Universe. We are making our results and map available to the world."

These new measurements were carried out by the Baryon Oscillation Spectroscopic Survey (BOSS) program of the Sloan Digital Sky Survey-III. Shaped by a continuous tug-of-war between dark matter and dark energy, the map revealed by BOSS allows scientists to measure the expansion rate of the Universe and thus determine the amount of matter and dark energy that make up the present-day Universe. A collection of papers describing these results was submitted this week to the Monthly Notices of the Royal Astronomical Society.

BOSS measures the expansion rate of the Universe by determining the size of the baryonic acoustic oscillations (BAO) in the three-dimensional distribution of galaxies. The original BAO size is determined by pressure waves that travelled through the young Universe up to when it was only 400,000 years old (the Universe is presently 13.8 billion years old), at which point they became frozen in the matter distribution of the Universe. The end result is that galaxies have a slight preference to be separated by a characteristic distance that astronomers call the acoustic scale. The size of the acoustic scale at 13.7996 billion years ago has been exquisitely determined from observations of the cosmic microwave background from the light emitted when the pressure waves became frozen. Measuring the distribution of galaxies since that time allows astronomers to measure how dark matter and dark energy have competed to govern the rate of expansion of the Universe.

"We've made the largest map for studying the 95% of the universe that is dark," noted David Schlegel, an astrophysicist at Lawrence Berkeley National Laboratory (Berkeley Lab) and principal investigator for BOSS. "In this map, we can see galaxies being gravitationally pulled towards other galaxies by dark matter. And on much larger scales, we see the effect of dark energy ripping the universe apart."

Shirley Ho, an astrophysicist at Berkeley Lab and Carnegie Mellon University (CMU), co-led two of the companion papers and adds, "We can now measure how much the galaxies and stars cluster together as a function of time to such an accuracy we can test General Relativity at cosmological scales."

Ariel Sanchez of the Max-Planck Institute of Extraterrestrial Physics led the effort to estimate the exact amount of dark matter and dark energy based on the BOSS data and explains: "Measuring the acoustic scale across cosmic history gives a direct ruler with which to measure the Universe's expansion rate. With BOSS, we have traced the BAO's subtle imprint on the distribution of galaxies spanning a range of time from 2 to 7 billion years ago."

To measure the size of these ancient giant waves to such sharp precision, BOSS had to make an unprecedented and ambitious galaxy map, many times larger than previous surveys. At the time the BOSS program was planned, dark energy had been previously determined to significantly influence the expansion of the Universe starting about 5 billion years ago. BOSS was thus designed to measure the BAO feature from before this point (7 billion years ago) out to near the present day (2 billion years ago).

Jose Vazquez of Brookhaven National Laboratory combined the BOSS results with other surveys and searched for any evidence of unexplained physical phenomena in the results. "Our latest results tie into a clean cosmological picture, giving strength to the standard cosmological model that has emerged over the last eighteen years."

Rita Tojeiro of the University of St. Andrews is the other co-leader of the BOSS galaxy clustering working group along with Tinker. "We see a dramatic connection between the sound wave imprints seen in the cosmic microwave background 400,000 years after the Big Bang to the clustering of galaxies 7-12 billion years later. The ability to observe a single well-modeled physical effect from recombination until today is a great boon for cosmology."

The map also reveals the distinctive signature of the coherent movement of galaxies toward regions of the Universe with more matter, due to the attractive force of gravity. Crucially, the observed amount of infall is explained well by the predictions of general relativity.

"The results from BOSS provide a solid foundation for even more precise future BAO measurements, such as those we expect from the Dark Energy Spectroscopic Instrument (DESI)," says Natalie Roe, Physics Division director at Berkeley Lab. "DESI will construct a more detailed 3-dimensional map in a volume of space ten times larger to precisely characterize dark energy -- and ultimately the future of our universe."

Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the U.S. Department of Energy Office of Science.

For more on SDSS-III, visit http://www.sdss3.org/.

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Science and Information / World's most sensitive dark matter detector completes search

« on: January 15, 2017, 08:20:58 PM »

The Large Underground Xenon (LUX) dark matter experiment, which operates beneath a mile of rock at the Sanford Underground Research Facility in the Black Hills of South Dakota, has completed its search for the missing matter of the universe.

Today at an international dark matter conference (IDM 2016) in Sheffield, UK, LUX scientific collaborators presented the results from the detector's final 20-month run from October 2014 to May 2016. The new research result is also described with further details on the LUX Collaboration's website. http://luxdarkmatter.org

LUX's sensitivity far exceeded the original expectations of the experiment, collaboration scientists said, but yielded no trace of a dark matter particle. LUX's extreme sensitivity makes the team confident that if dark matter particles had interacted with the LUX's xenon target, the detector would almost certainly have seen them. These new limits on dark matter detection will allow scientists to eliminate many potential models for dark matter particles, offering critical guidance for the next generation of dark matter experiments.

"LUX has delivered the world's best search sensitivity since its first run in 2013," said Rick Gaitskell, professor of physics at Brown University and co-spokesperson for the LUX experiment. "With this final result from the 2014-2016 run, the scientists of the LUX Collaboration have pushed the sensitivity of the instrument to a final performance level that is 4 times better than originally expected. It would have been marvelous if the improved sensitivity had also delivered a clear dark matter signal. However, what we have observed is consistent with background alone."

Dark matter is thought to account for more than four-fifths of the mass in the universe. Scientists are confident of its existence because the effects of its gravity can be seen in the rotation of galaxies and in the way light bends as it travels through the universe, but experiments have yet to make direct contact with a dark matter particle. The LUX experiment was designed to look for weakly interacting massive particles, or WIMPs, the leading theoretical candidate for a dark matter particle. If the WIMP idea is correct, billions of these particles pass through your hand every second, and also through the Earth and everything on it. But because WIMPs interact so weakly with ordinary matter, this ghostly traverse goes entirely unnoticed.

The LUX detector consists of a third-of-a-ton of cooled liquid xenon surrounded by powerful sensors designed to detect the tiny flash of light and electrical charge emitted if a WIMP collides with a xenon atom within the tank. The detector's location at Sanford Lab beneath a mile of rock, and inside a 72,000-gallon, high-purity water tank, helps shield it from cosmic rays and other radiation that would interfere with a dark matter signal.

The 20-month run of LUX represents one of the largest exposures ever collected by a dark matter experiment, the researchers said. The rapid analysis of nearly a half-million gigabytes of data was made possible with the use Brown University's Center for Computation and Visualization (CCV) and the advanced computer simulations at Lawrence Berkeley National Laboratory's (Berkeley Lab) National Energy Research Scientific Computing Center (NERSC), a U.S. Department of Energy (DOE) Office of Science User Facility. Berkeley Lab is also the lead DOE laboratory for LUX operations.

"I am particularly pleased with the support LUX received from NERSC in processing these data," said Kevin Lesko, group leader of Berkeley Lab's Dark Matter group. "The Berkeley students, post-docs and visitors working on this analysis made extensive use of the NERSC for event scanning, calibration, Monte Carlo simulations and the data-blinding scheme."

Careful calibration

The exquisite sensitivity achieved by the LUX experiment came thanks to a series of pioneering calibration measures aimed at helping scientists tell the difference between a dark matter signal and events created by residual background radiation that even the elaborate construction of the experiment cannot completely block out.

"As the charge and light signal response of the LUX experiment varied slightly over the dark matter search period, our calibrations allowed us to consistently reject radioactive backgrounds, maintain a well-defined dark matter signature for which to search and compensate for a small static charge buildup on the Teflon inner detector walls," said Dan McKinsey, professor of physics at the University of California, Berkeley, senior faculty scientist at Berkeley Lab, and co-spokesperson for the LUX experiment.

"We worked hard and stayed vigilant over more than a year and a half to keep the detector running in optimal conditions and maximize useful data time," said Simon Fiorucci, a physicist at Berkeley Lab and Science Coordination Manager for the experiment. "The result is unambiguous data we can be proud of and a timely result in this very competitive field -- even if it is not the positive detection we were all hoping for."

The quest continues

While the LUX experiment successfully eliminated a large swath of mass ranges and interaction-coupling strengths where WIMPs might exist, the WIMP model itself, "remains alive and viable," said Gaitskell, the Brown University physicist. And the meticulous work of LUX scientists will aid future direct detection experiments.

Among those next generation experiments will be the LUX-ZEPLIN (LZ) experiment, which will replace LUX at the Sanford Underground Research Facility.

Compared to LUX's one-third-ton of liquid xenon, LZ will have a 10-ton liquid xenon target, which will fit inside the same 72,000-gallon tank of pure water used by LUX to help fend off external radiation. LZ is expected to have 70 times the sensitivity of LUX and will continue the search in 2020. "We're looking forward to hosting the LUX-ZEPLIN experiment, which will provide another major step forward in sensitivity," said Mike Headley, Executive Director of the South Dakota Science and Technology Authority (SDSTA).

LUX, the first major astrophysics experiment in the Davis Campus of the Sanford Underground Research Facility (Sanford Lab), was installed in 2012 and is located in the former Homestake Gold Mine in Lead, S.D. A South Dakota-owned facility, it is managed by the SDSTA, which reopened the mine in 2007 with $40 million in funding from the South Dakota State Legislature and a $70 million donation from philanthropist T. Denny Sanford. DOE's Office of Science supports Sanford Lab's operations; Berkeley Lab provided management and oversight of the DOE operations support of Sanford Lab for the past five years.

The LUX scientific collaboration, which is supported by the DOE and National Science Foundation (NSF), includes 20 research universities and national laboratories in the United States, the United Kingdom, and Portugal.

"The announcement of this new result from LUX raises the bar in the search for dark matter, exceeding our expectations," said Natalie Roe, Physics Division Director at Berkeley Lab. "With the successful completion of LUX, we are now focused on the success of LZ, which we hope will produce a dramatic discovery."

Major support for LUX came from the DOE Office of Science.

35

Science and Information / Space... the final frontier

« on: January 15, 2017, 08:20:39 PM »

Fifty years ago Captain Kirk and the crew of the starship Enterprise began their journey into space -- the final frontier. Now, as the newest Star Trek film hits cinemas, the NASA/ESA Hubble space telescope is also exploring new frontiers, observing distant galaxies in the galaxy cluster Abell S1063 as part of the Frontier Fields programme.

Space... the final frontier. These are the stories of the Hubble Space Telescope. Its continuing mission, to explore strange new worlds and to boldly look where no telescope has looked before.

The newest target of Hubble's mission is the distant galaxy cluster Abell S1063, potentially home to billions of strange new worlds.

This view of the cluster, which can be seen in the centre of the image, shows it as it was four billion years ago. But Abell S1063 allows us to explore a time even earlier than this, where no telescope has really looked before. The huge mass of the cluster distorts and magnifies the light from galaxies that lie behind it due to an effect called gravitational lensing. This allows Hubble to see galaxies that would otherwise be too faint to observe and makes it possible to search for, and study, the very first generation of galaxies in the Universe. "Fascinating," as a famous Vulcan might say.

The first results from the data on Abell S1063 promise some remarkable new discoveries. Already, a galaxy has been found that is observed as it was just a billion years after the Big Bang.

Astronomers have also identified sixteen background galaxies whose light has been distorted by the cluster, causing multiple images of them to appear on the sky. This will help astronomers to improve their models of the distribution of both ordinary and dark matter in the galaxy cluster, as it is the gravity from these that causes the distorting effects. These models are key to understanding the mysterious nature of dark matter.

Abell S1063 is not alone in its ability to bend light from background galaxies, nor is it the only one of these huge cosmic lenses to be studied using Hubble. Three other clusters have already been observed as part of the Frontier Fields programme, and two more will be observed over the next few years, giving astronomers a remarkable picture of how they work and what lies both within and beyond them.*

Data gathered from the previous galaxy clusters were studied by teams all over the world, enabling them to make important discoveries, among them galaxies that existed only hundreds of million years after the Big Bang heic1523 and the first predicted appearance of a gravitationally lensed supernova heic1525.

Such an extensive international collaboration would have made Gene Roddenberry, the father of Star Trek, proud. In the fictional world Roddenberry created, a diverse crew work together to peacefully explore the Universe. This dream is partially achieved by the Hubble programme in which the European Space Agency (ESA), supported by 22 member states, and NASA collaborate to operate one of the most sophisticated scientific instruments in the world. Not to mention the scores of other international science teams that cross state, country and continental borders to achieve their scientific aims.

*The Hubble Frontier Fields is a three-year, 840-orbit programme which will yield the deepest views of the Universe to date, combining the power of Hubble with the gravitational amplification of light around six different galaxy clusters to explore more distant regions of space than could otherwise be seen.

36

Science and Information / Astronomers discover dizzying spin of the Milky Way galaxy's 'halo'

« on: January 15, 2017, 08:20:19 PM »

Astronomers at the University of Michigan's College of Literature, Science, and the Arts (LSA) discovered for the first time that the hot gas in the halo of the Milky Way galaxy is spinning in the same direction and at comparable speed as the galaxy's disk, which contains our stars, planets, gas, and dust. This new knowledge sheds light on how individual atoms have assembled into stars, planets, and galaxies like our own, and what the future holds for these galaxies.

"This flies in the face of expectations," says Edmund Hodges-Kluck, assistant research scientist. "People just assumed that the disk of the Milky Way spins while this enormous reservoir of hot gas is stationary -- but that is wrong. This hot gas reservoir is rotating as well, just not quite as fast as the disk."

The new NASA-funded research using the archival data obtained by XMM-Newton, a European Space Agency telescope, was recently published in the Astrophysical Journal. The study focuses on our galaxy's hot gaseous halo, which is several times larger than the Milky Way disk and composed of ionized plasma.

Because motion produces a shift in the wavelength of light, the U-M researchers measured such shifts around the sky using lines of very hot oxygen. What they found was groundbreaking: The line shifts measured by the researchers show that the galaxy's halo spins in the same direction as the disk of the Milky Way and at a similar speed -- about 400,000 mph for the halo versus 540,000 mph for the disk.

"The rotation of the hot halo is an incredible clue to how the Milky Way formed," said Hodges Kluck. "It tells us that this hot atmosphere is the original source of a lot of the matter in the disk."

Scientists have long puzzled over why almost all galaxies, including the Milky Way, seem to lack most of the matter that they otherwise would expect to find. Astronomers believe that about 80% of the matter in the universe is the mysterious "dark matter" that, so far, can only be detected by its gravitational pull. But even most of the remaining 20% of "normal" matter is missing from galaxy disks. More recently, some of the "missing" matter has been discovered in the halo. The U-M researchers say that learning about the direction and speed of the spinning halo can help us learn both how the material got there in the first place, and the rate at which we expect the matter to settle into the galaxy.

"Now that we know about the rotation, theorists will begin to use this to learn how our Milky Way galaxy formed -- and its eventual destiny," says Joel Bregman, a U-M LSA professor of astronomy.

"We can use this discovery to learn so much more -- the rotation of this hot halo will be a big topic of future X-ray spectrographs," Bregman says.

37

Science and Information / Better defining the signals left by as-yet-undefined dark matter at the LHC

« on: January 15, 2017, 08:19:59 PM »

In the quest for dark matter, physicists rely on particle colliders such as the LHC in CERN, located near Geneva, Switzerland. The trouble is: physicists still don't exactly know what dark matter is. Indeed, they can only see its effect in the form of gravity. Until now, theoretical physicists have used models based on a simple, abstract description of the interaction between dark matter and ordinary particles, such as the Effective Field Theories (EFTs). However, until we observe dark matter, it is impossible to know whether or not these models neglect some key signals. Now, the high energy physics community has come together to develop a set of simplified models, which retain the elegance of EFT-style models yet provide a better description of the signals of dark matter, at the LHC.

These developments are described in a review published in EPJ C by Andrea De Simone and Thomas Jacques from the International School for Advanced Studies SISSA, in Trieste, Italy.

EFT models offer the advantage of helping to define a structured approach to identify exactly what they are looking for in the quest for dark matter. They also help to combine results from several dark matter search experiments in a straightforward manner.

In this paper the authors describe an evolution of the EFT approach, referred to as 'simplified models', that yield signals not found in the EFT description. They can also be used together with other search methods for dark matter, such as indirect detection and direct detection. By comparing constraints from as many experiments as possible, this new approach makes it possible to combine the strongest constraints in the search for dark matter.

The more experimental results are gained from particle collisions at the LHC, the more we learn about the nature of dark matter interactions. Theorists can then use this data to continue developing simplified models by defining the new and unique signatures on which to focus the search.

38

Science and Information / Mapping the exotic matter inside neutron stars

« on: January 15, 2017, 08:19:42 PM »

The recent detection of gravitational waves emitted by two merging black holes by the LIGO and Virgo collaborations has opened up a new observational window into the cosmos.

Future observations of similar mergers between two neutron stars or a neutron star and a black hole may revolutionize what we know today about the properties of neutron stars, the densest stellar objects in the universe. By providing detailed dynamical information about the material properties of these stars, such measurements will shed light on their internal composition.

"Ultimately, they may answer the question, whether neutron stars are composed solely of ordinary atomic nuclei, or if they contain more exotic matter in the form of dense deconfined quark matter," says physicist Aleksi Vuorinen at the University of Helsinki.

Towards accurate theoretical understanding, as well

In order to be able to properly take advantage of the future observational data, it is essential that our theoretical understanding of the possible constituents of neutron star matter -- dense nuclear and quark matter -- be as accurate as possible.

This is, however, an extremely challenging problem, as few first principle tools exist for studying such a strongly interacting medium due to the complexity of the underlying microscopic theory, Quantum Chromodynamics (QCD). The most important tools available for such studies are so-called chiral effective theories for the nuclear interactions, applicable for nuclear matter, and thermal perturbation theory, applicable for deconfined quark matter.

In their recent paper, Cool quark matter, published in Physical Review Letters on 22.7.2016, Aleksi Kurkela (CERN and University of Stavanger) and Aleksi Vuorinen were able to perform the first accurate determination of the thermodynamic properties of dense quark matter under the violent conditions that take place in neutron star mergers.

They applied thermal perturbation theory to a high order, generalizing previous work applicable only at zero temperature. This is a very important development, as neutron star mergers may witness enormously high temperatures, reaching perhaps even 100 MeV, or 1.000.000.000.000 K.

The new results enable realistic simulations with neutron stars containing quark cores, and thus represent an important step towards eventually distinguishing between neutron and quark matter cores in neutron stars.

39

Science and Information / Physicist trio amplifies research on mysterious forms of matter

« on: January 15, 2017, 08:19:21 PM »

All material things appear to be made of elementary particles that are held together by fundamental forces. But what are their exact properties? How do they affect how our universe looks and changes? And are there particles and forces that we don't know of yet?

Questions with cosmic implications like these drive many of the scientific efforts at the Department of Energy's SLAC National Accelerator Laboratory. Three distinguished particle physicists have joined the lab over the past months to pursue research on two particularly mysterious forms of matter: neutrinos and dark matter.

Neutrinos, which are abundantly produced in nuclear reactions, are among the most common types of particles in the universe. Although they were discovered 60 years ago, their basic properties puzzle scientists to this date.

Alexander Friedland, a senior staff scientist in SLAC's Elementary Particle Physics Theory Group, works on techniques that pave the way for future analyses of neutrino bursts from supernovae. Studying the details of these powerful star explosions helps scientists understand how dying stars spit out chemical elements into deep space.

Natalia Toro and Philip Schuster, associate professors of particle physics and astrophysics at SLAC, look for something even more enigmatic. They develop ideas for experiments that search for hidden particles and forces linked to dark matter, an invisible form of matter that is five times more prevalent than ordinary matter.

"Alex, Natalia and Philip are significant additions to the SLAC family, whose outstanding expertise tremendously strengthens our research in areas of national priority," says JoAnne Hewett, head of the lab's Elementary Particle Physics Division. Neutrino physics and dark matter research are among the five science drivers for U.S. particle physics identified in 2014 by the Particle Physics Project Prioritization Panel. Neutrino research also ranked high in the 2015 long-range plan for nuclear science issued by the Nuclear Science Advisory Committee.

Neutrinos from Across the Country and from Across the Galaxy

One of the major neutrino projects with SLAC involvement is the international Deep Underground Neutrino Experiment (DUNE) at the planned Long-Baseline Neutrino Facility (LBNF) -- the world's flagship neutrino experiment for the coming decade and beyond. Researchers will send a neutrino beam produced at Fermi National Accelerator Laboratory in Illinois to the Sanford Underground Facility in South Dakota. After travelling 800 miles through the Earth, some of these neutrinos will be detected by the DUNE Far Detector, which will eventually consist of four 10,000-ton modules of liquid argon located 4,850 feet underground.

The ultrasensitive neutrino "eye" will measure how the three known types of neutrinos, called flavors, and their antiparticles morph from one into another during their underground journey. This study will provide crucial insights into the relative masses of neutrino flavors and the possibility that antineutrinos behave differently than neutrinos, which could potentially help explain why the universe is made of matter rather than antimatter. The experiment will also follow up on hints that there may be more than three neutrino flavors in nature.

"To help DUNE reach its full potential, my work addresses a number of fundamental questions," says Friedland, SLAC's first neutrino theorist, who joined the lab in the summer of 2015. "How can additional neutrinos be incorporated into our theories? Are there also additional forces? Is there a link between neutrinos and dark matter? How do neutrinos interact with atomic nuclei in the detector material?"

In addition to neutrinos from Fermilab, DUNE will also be able to detect very brief neutrino bursts from supernovae -- powerful explosions of massive stars with cores that can no longer resist gravity and collapse to form dense neutron stars.

"Such a burst should be an exquisite probe of neutrino properties," Friedland says. "Our goal is to understand how to read the signal and optimize our detector for it."

Supernova explosions are important events in the universe. They inject chemical elements, synthesized inside stars over their lifetimes, into space, including crucial elements of life. Friedland hopes that DUNE's data will reveal never-before-seen details in the related neutrino bursts that could open a window into the processes inside dying stars.

"Our calculations show that those neutrino signals have a certain time structure that is linked to what's going on in the star," he says. "Measuring these minute details could help us understand the different stages of a supernova, from the collapse of the star's core to the outward propagation of powerful shock waves."

Such detailed analysis can only be done by looking at neutrinos. Unlike other particles, which frequently interact with their surroundings on their way out of the star and therefore carry the imprint of this complicated environment, neutrinos stream out nearly undisturbed and deliver direct information about the processes in which they were set free.

"Supernovae go off without warning, and detectable ones don't occur very often," says Friedland, who co-leads the DUNE supernova working group. "Although the next supernova neutrino burst may be a decade or more away, what will be seen then is affected by crucial decisions about the detector design made now. My job is to make sure that we'll be prepared."

SLAC provides a unique environment for the pursuit of this line of research, according to Friedland. "The lab is building a strong neutrino program, with experimentalists and theorists working closely together," he says. "It also unites a number of disciplines under one roof that stimulate and complement each other, from particle physics to astrophysics to computing."

Before coming to SLAC, Friedland was at Los Alamos National Laboratory, first as a Richard P. Feynman Fellow and then as a staff scientist. He received his doctorate in physics from the University of California, Berkeley in 2000 and pursued postdoctoral research at the Institute for Advanced Study in Princeton, New Jersey from 2000 to 2002. In addition to neutrinos, Friedland's studies look into unknown ultraweak forces in nature, extra dimensions beyond space and time and the effect of postulated particles on the evolution of stars.

Searching for 'Light Dark Matter'

Another burning question researchers around the world are yearning to answer is: What is dark matter? With 85 percent of all matter in the universe being dark, this invisible substance has tremendous influence on how the cosmos evolves. Although scientists know that dark matter exists because it gravitationally pulls on ordinary matter, they have yet to find out what it is made of.

At SLAC, Natalia Toro and Philip Schuster search for entire dark sectors of hypothetical particles and forces that could be linked to dark matter.

"We work on a number of small-scale experiments that have a real shot at discovering what dark matter is or what it isn't," Schuster says. "Unlike most dark matter searches, which focus on rather massive particles, we look for much lighter ones, in a mass range that is surprisingly unexplored."

The researchers participate in two experiments that hunt for light dark matter at the Thomas Jefferson National Accelerator Facility in Virginia: the Heavy Photon Search (HPS), for which the scientists developed the theoretical framework, and the A Prime Experiment (APEX), which they co-lead. Both experiments hope to catch a glimpse of dark photons -- hypothetical carriers of a new force -- that could potentially be produced when powerful electron beams slam into a target. Toro and Schuster are also members of a collaboration that proposed a third experiment at Jefferson Lab to search for dark matter, the Beam Dump Experiment (BDX).

Similar searches could also be done at SLAC once the upgrade to the lab's Linac Coherent Light Source (LCLS) X-ray laser, a DOE Office of Science User Facility, is complete. The future LCLS-II will produce X-rays from a rapid sequence of electron bunches -- up to a million per second -- that will fly through the facility's linear particle accelerator.

"We're developing ideas for an experiment that would use the dark current of LCLS-II's electron beam," Toro says. "This is a small number of unused electrons in between the main bunches that we could extract and shoot into targets for light dark matter searches."

A proposal based on this concept is the Light Dark Matter Experiment (LDMX), whose young collaboration is led by researchers from the University of California, Santa Barbara, the University of Minnesota and SLAC.

At the moment, the parasitic use of LCLS-II is only an idea, but Toro and Schuster have already teamed up with members of SLAC's Accelerator Directorate to think about how these experiments could be designed and, most importantly, operated without interfering with X-ray laser operations. Together they are exploring the possibility for a future facility for Dark Sector Experiments at LCLS-II (DASEL).

"The lab has a unique culture of vibrant collaborations," Toro says. "It creates an ideal environment to follow through with our projects from beginning to end. Here we can establish the theoretical foundation, work on the engineering aspects and turn them into successful experiments, all in one place."

The husband-and-wife team joined SLAC's faculty on Dec. 1, 2015. In addition to their work on dark sectors, the couple shares a variety of other research interests, such as searching for new physics in data from the Large Hadron Collider at CERN, the European particle physics laboratory, and making theories that aim at better understanding the spin of massless particles.

"It's great to share your passion for the most basic aspects of nature also outside work," Schuster says. "We amplify each other's excitement and hold each other to high standards. On top of that, it's also a lot of fun to go off on wild research adventures and explore new places together."

40

Science and Information / Insight into why the universe is dominated by matter and why we exist

« on: January 15, 2017, 08:18:53 PM »

New findings that reveal why the universe is dominated by matter and why we exist will be presented by the international T2K Collaboration, a team a researchers who will demonstrate why matter and antimatter are different.

Stony Brook University physicist Professor Chang Kee Jung, a leading member of the T2K Collaboration, will present the results of the T2K findings and explain why they are significant to theories of particle physics and Bing Bang Cosmology.

According to Prof. Jung, T2K's recent finding hints that neutrinos and antineutrinos may oscillate slightly differently, which is a break of fundamental symmetry (Charge-Parity) in physics.

"Physicists believe that this is deeply related to the question of why our current universe is dominated by matter and why we exist," he says. "At the onset of the Big Bang, the universe must have been symmetric, i.e. there existed the same amount of matter and antimatter. Charge-Parity violation could allow very heavy neutral particles to decay to neutrinos at a slightly higher rate than to antineutrinos which created the initial imbalance in the amount of matter and antimatter. Without this process, we would not exist, and our universe today would instead contain only light and energy resulting from the inevitable matter-antimatter annihilation after the Big Bang.

"T2K's finding is by no means a definitive discovery. Rather it is perhaps the first significant step toward the elucidation of a matter dominant universe, for which much more data and many more years of effort is needed."

41

Science and Information / Much ado about nothing: Astronomers use empty space to study the universe

« on: January 15, 2017, 08:18:13 PM »

Researchers who are looking for new ways to probe the nature of gravity and dark energy in the universe have adopted a new strategy: looking at what's not there.

In a paper to appear in upcoming issue of Physical Review Letters, the international team of astronomers reports that they were able to achieve four times better precision in measurements of how the universe's visible matter is clustered together by studying the empty spaces in between.

Paul Sutter, study co-author and staff researcher at The Ohio State University, said that the new measurements can help bring astronomers closer to testing Einstein's general theory of relativity, which describes how gravity works.

Sutter likened the new technique to "learning more about Swiss cheese by studying the holes," and offered another analogy to explain why astronomers would be interested in the voids of space.

"Voids are empty. They're boring, right? Galaxies are like the cities of the universe, full of bright lights and activity, and voids are like the miles and miles of quiet farmland in between," Sutter explained.

"But we're looking for bits of evidence that general relativity might be wrong, and it turns out that all the activity in galaxies makes those tiny effects harder to see. It's easier to pick up on effects in the voids, where there's less distraction -- like it's easier to spot the glimmer of a firefly in a dark cornfield than in a lit-up city bustling with nightlife."

The voids, he pointed out, are only empty in the sense that they contain no normal matter. They are, in fact, full of invisible dark energy, which is causing the expansion of the universe to accelerate.

While Einstein's 1915 general theory of relativity goes a long way toward explaining gravity in the universe, Einstein couldn't have known about dark energy. That's why, today, astronomers are working to find out whether the rules of general relativity hold up in a universe dominated by it.

Sutter, in Ohio State's Department of Astronomy, worked with colleagues in Germany, France and Italy to compare computer simulations of voids in space with a portion of data from the Sloan Digital Sky Survey. The statistical analysis revealed a four-times improvement in precision in their models of matter density and the growth of cosmological structure when they took the physics of voids into account.

They were looking for tiny deviations in void behavior that conflicted with general relativity, and they found none. So Einstein's theory of gravity holds true for now. The analysis and models are publicly available online, so the researchers hope that others will use them to do further work in the future.

"Our results demonstrate that a lot of unexplored cosmological information can be found in cosmic voids," Sutter concluded. "It's truly like getting something from nothing."

42

Science and Information / NASA's Fermi mission expands its search for dark matter

« on: January 15, 2017, 08:17:50 PM »

Dark matter, the mysterious substance that constitutes most of the material universe, remains as elusive as ever. Although experiments on the ground and in space have yet to find a trace of dark matter, the results are helping scientists rule out some of the many theoretical possibilities. Three studies published earlier this year, using six or more years of data from NASA's Fermi Gamma-ray Space Telescope, have broadened the mission's dark matter hunt using some novel approaches.

"We've looked for the usual suspects in the usual places and found no solid signals, so we've started searching in some creative new ways," said Julie McEnery, Fermi project scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "With these results, Fermi has excluded more candidates, has shown that dark matter can contribute to only a small part of the gamma-ray background beyond our galaxy, the Milky Way, and has produced strong limits for dark matter particles in the second-largest galaxy orbiting it."

Dark matter neither emits nor absorbs light, primarily interacts with the rest of the universe through gravity, yet accounts for about 80 percent of the matter in the universe. Astronomers see its effects throughout the cosmos -- in the rotation of galaxies, in the distortion of light passing through galaxy clusters, and in simulations of the early universe, which require the presence of dark matter to form galaxies at all.

The leading candidates for dark matter are different classes of hypothetical particles. Scientists think gamma rays, the highest-energy form of light, can help reveal the presence of some of types of proposed dark matter particles. Previously, Fermi has searched for tell-tale gamma-ray signals associated with dark matter in the center of our galaxy and in small dwarf galaxies orbiting our own. Although no convincing signals were found, these results eliminated candidates within a specific range of masses and interaction rates, further limiting the possible characteristics of dark matter particles.

Among the new studies, the most exotic scenario investigated was the possibility that dark matter might consist of hypothetical particles called axions or other particles with similar properties. An intriguing aspect of axion-like particles is their ability to convert into gamma rays and back again when they interact with strong magnetic fields. These conversions would leave behind characteristic traces, like gaps or steps, in the spectrum of a bright gamma-ray source.

Manuel Meyer at Stockholm University led a study to search for these effects in the gamma rays from NGC 1275, the central galaxy of the Perseus galaxy cluster, located about 240 million light-years away. High-energy emissions from NGC 1275 are thought to be associated with a supermassive black hole at its center. Like all galaxy clusters, the Perseus cluster is filled with hot gas threaded with magnetic fields, which would enable the switch between gamma rays and axion-like particles. This means some of the gamma rays coming from NGC 1275 could convert into axions -- and potentially back again -- as they make their way to us.

Meyer's team collected observations from Fermi's Large Area Telescope (LAT) and searched for predicted distortions in the gamma-ray signal. The findings, published April 20 in Physical Review Letters, exclude a small range of axion-like particles that could have comprised about 4 percent of dark matter.

"While we don't yet know what dark matter is, our results show we can probe axion-like models and provide the strongest constraints to date for certain masses," Meyer said. "Remarkably, we reached a sensitivity we thought would only be possible in a dedicated laboratory experiment, which is quite a testament to Fermi."

Another broad class of dark matter candidates are called Weakly Interacting Massive Particles (WIMPs). In some versions, colliding WIMPs either mutually annihilate or produce an intermediate, quickly decaying particle. Both scenarios result in gamma rays that can be detected by the LAT.

Regina Caputo at the University of California, Santa Cruz, sought these signals from the Small Magellanic Cloud (SMC), which is located about 200,000 light-years away and is the second-largest of the small satellite galaxies orbiting the Milky Way. Part of the SMC's appeal for a dark matter search is that it lies comparatively close to us and its gamma-ray emission from conventional sources, like star formation and pulsars, is well understood. Most importantly, astronomers have high-precision measurements of the SMC's rotation curve, which shows how its rotational speed changes with distance from its center and indicates how much dark matter is present. In a paper published in Physical Review D on March 22, Caputo and her colleagues modeled the dark matter content of the SMC, showing it possessed enough to produce detectable signals for two WIMP types.

"The LAT definitely sees gamma rays from the SMC, but we can explain them all through conventional sources," Caputo said. "No signal from dark matter annihilation was found to be statistically significant."

In the third study, researchers led by Marco Ajello at Clemson University in South Carolina and Mattia Di Mauro at SLAC National Accelerator Laboratory in California took the search in a different direction. Instead of looking at specific astronomical targets, the team used more than 6.5 years of LAT data to analyze the background glow of gamma rays seen all over the sky.

The nature of this light, called the extragalactic gamma-ray background (EGB) has been debated since it was first measured by NASA's Small Astronomy Satellite 2 in the early 1970s. Fermi has shown that much of this light arises from unresolved gamma-ray sources, particularly galaxies called blazars, which are powered by material falling toward gigantic black holes. Blazars constitute more than half of the total gamma-ray sources seen by Fermi, and they make up an even greater share in a new LAT catalog of the highest-energy gamma rays.

Some models predict that EGB gamma rays could arise from distant interactions of dark matter particles, such as the annihilation or decay of WIMPs. In a detailed analysis of high-energy EGB gamma rays, published April 14 in Physical Review Letters, Ajello and his team show that blazars and other discrete sources can account for nearly all of this emission.

"There is very little room left for signals from exotic sources in the extragalactic gamma-ray background, which in turn means that any contribution from these sources must be quite small," Ajello said. "This information may help us place limits on how often WIMP particles collide or decay."

Although these latest studies have come up empty-handed, the quest to find dark matter continues both in space and in ground-based experiments. Fermi is joined in its search by NASA's Alpha Magnetic Spectrometer, a particle detector on the International Space Station.

43

Science and Information / Fifth force of nature? Light particle may be key to understanding dark matter in

« on: January 15, 2017, 08:17:28 PM »

Recent findings indicating the possible discovery of a previously unknown subatomic particle may be evidence of a fifth fundamental force of nature, according to a paper published in the journal Physical Review Letters by theoretical physicists at the University of California, Irvine.

"If true, it's revolutionary," said Jonathan Feng, professor of physics & astronomy. "For decades, we've known of four fundamental forces: gravitation, electromagnetism, and the strong and weak nuclear forces. If confirmed by further experiments, this discovery of a possible fifth force would completely change our understanding of the universe, with consequences for the unification of forces and dark matter."

The UCI researchers came upon a mid-2015 study by experimental nuclear physicists at the Hungarian Academy of Sciences who were searching for "dark photons," particles that would signify unseen dark matter, which physicists say makes up about 85 percent of the universe's mass. The Hungarians' work uncovered a radioactive decay anomaly that points to the existence of a light particle just 30 times heavier than an electron.

"The experimentalists weren't able to claim that it was a new force," Feng said. "They simply saw an excess of events that indicated a new particle, but it was not clear to them whether it was a matter particle or a force-carrying particle."

The UCI group studied the Hungarian researchers' data as well as all other previous experiments in this area and showed that the evidence strongly disfavors both matter particles and dark photons. They proposed a new theory, however, that synthesizes all existing data and determined that the discovery could indicate a fifth fundamental force. Their initial analysis was published in late April on the public arXiv online server, and a follow-up paper amplifying the conclusions of the first work was released Friday on the same website.

The UCI work demonstrates that instead of being a dark photon, the particle may be a "protophobic X boson." While the normal electric force acts on electrons and protons, this newfound boson interacts only with electrons and neutrons -- and at an extremely limited range. Analysis co-author Timothy Tait, professor of physics & astronomy, said, "There's no other boson that we've observed that has this same characteristic. Sometimes we also just call it the 'X boson,' where 'X' means unknown."

Feng noted that further experiments are crucial. "The particle is not very heavy, and laboratories have had the energies required to make it since the '50s and '60s," he said. "But the reason it's been hard to find is that its interactions are very feeble. That said, because the new particle is so light, there are many experimental groups working in small labs around the world that can follow up the initial claims, now that they know where to look."

Like many scientific breakthroughs, this one opens entirely new fields of inquiry.

One direction that intrigues Feng is the possibility that this potential fifth force might be joined to the electromagnetic and strong and weak nuclear forces as "manifestations of one grander, more fundamental force."

Citing physicists' understanding of the standard model, Feng speculated that there may also be a separate dark sector with its own matter and forces. "It's possible that these two sectors talk to each other and interact with one another through somewhat veiled but fundamental interactions," he said. "This dark sector force may manifest itself as this protophobic force we're seeing as a result of the Hungarian experiment. In a broader sense, it fits in with our original research to understand the nature of dark matter."

44

Science and Information / Can one cosmic enigma help solve another?

« on: January 15, 2017, 08:17:08 PM »

Astrophysicists from the Johns Hopkins University have proposed a clever new way of shedding light on the mystery of dark matter, believed to make up most of the universe.

The irony is they want to try to pin down the nature of this unexplained phenomenon by using another, an obscure cosmic emanation known as "fast radio bursts."

In a paper published online by the journal Physical Review Letters the team of astrophysicists argues that these extremely bright and brief flashes of radio-frequency radiation can provide clues about whether a particular kind of ancient black hole is what makes up dark matter.

Julian Munoz, a Johns Hopkins graduate student and the paper's lead author, said fast radio bursts, or FRBs, provide a direct and specific way of detecting black holes of a specific mass, which are the suspect dark matter.

Munoz wrote the paper along with Ely D. Kovetz a post-doctoral fellow, Marc Kamionkowski, the William R. Kenan Jr. Professor of Physics and Astronomy, and Liang Dai, who completed his doctorate in astrophysics at Johns Hopkins last year. Dai is now a NASA Einstein Postdoctoral Fellow at the Institute for Advanced Study in Princeton.

The paper builds on a hypothesis offered in a paper published this spring by Munoz, Kovetz and Kamionkowski along with five Johns Hopkins colleagues. Also published in Physical Review Letters, that research made a speculative case that the collision of black holes detected early in the year by the Laser Interferometer Gravitational-Wave Observatory (LIGO) had actually revealed dark matter, a substance not yet identified but believed to make up 85 percent of the mass of the universe.

The earlier paper made what Kamionkowski called a "plausibility argument" that LIGO had found dark matter. The study took as a point of departure the fact that the objects detected by LIGO fit within the predicted range of mass of so-called "primordial" black holes. Unlike black holes that formed from imploded stars, primordial black holes are believed to have formed from the collapse of large expanses of gas during the birth of the universe.

The existence of primordial black holes has not been established with certainty, but they have been suggested before as a possible solution to the riddle of dark matter. With so little evidence of them to examine, the hypothesis had not gained a large following among scientists.

The LIGO findings, however, raised the prospect anew, especially as the objects detected in that experiment conform to the mass predicted for dark matter.

The Johns Hopkins team calculated how often these primordial black holes would form binary pairs, and eventually collide. Taking into account the size and elongated shape believed to characterize primordial black hole binary orbits, the team came up with a collision rate that conforms to the LIGO findings.

Key to the argument is that the black holes that LIGO detected fall within a range of 29 to 36 solar masses, meaning that many times the mass of the sun. The new paper considers the question of how to test the hypothesis that dark matter consists of black holes of roughly 30 solar masses.

That's where the fast radio bursts come in. First observed only a few years ago, these flashes of radio frequency radiation emit intense energy, but last only fractions of a second. Their origins are unknown, but believed to lie in galaxies outside the Milky Way.

If the speculation about their origins is true, Kamionkowski said, the radio waves would travel great distances before they're observed on Earth, perhaps passing a black hole. According to Einstein's theory of general relativity, the wave would be deflected when it passes a black hole. If it passes close enough, it could be split into two rays shooting off in the same direction -- creating two images from one source.

The new study shows that if the black hole has 30 times the mass of the sun, the two images will arrive a few milliseconds apart. If roughly 30-solar-mass primordial black holes are dark matter, there is a chance that any given fast radio burst will be deflected in this way and followed in a few milliseconds by an echo.

"The echoing of FRBs is a very direct probe of dark matter," Munoz said. "While gravitational waves might 'indicate' that dark matter is made of black holes, there are other ways to produce very-massive black holes with regular astrophysics, so it would be hard to convince oneself that we are detecting dark matter. However, gravitational lensing of fast radio bursts has a very unique signature, with no other astrophysical phenomenon that could reproduce it."

Kaimonkowski said that while the probability for any such FRB echo is small, "it is expected that several of the thousands of FRBs to be detected in the next few years will have such echoes ... if black holes make up the dark matter."

So far, only about 20 fast radio bursts have been detected and recorded since 2001. The very sensitive instruments needed to detect them can look at only very small slices of the sky at a time, limiting the rate at which the bursts can be found. A new telescope expected to go into operation this year that seems particularly promising for spotting radio bursts is the Canadian Hydrogen Intensity Mapping Experiment. The joint project of the University of British Columbia, McGill University, the University of Toronto and the Dominion Radio Astrophysical Observatory stands in British Columbia.

"Once the thing is working up to their planned specifications, they should collect enough FRBs to begin the tests we propose," said Kamionkowski, estimating results could be available in three to five years.

45

Science and Information / Scientists discover a ‘dark’ Milky Way

« on: January 15, 2017, 08:16:45 PM »

Using the world's most powerful telescopes, an international team of astronomers has found a massive galaxy that consists almost entirely of dark matter.

The galaxy, Dragonfly 44, is located in the nearby Coma constellation and had been overlooked until last year because of its unusual composition: It is a diffuse "blob" about the size of the Milky Way, but with far fewer stars.

"Very soon after its discovery, we realized this galaxy had to be more than meets the eye. It has so few stars that it would quickly be ripped apart unless something was holding it together," said Yale University astronomer Pieter van Dokkum, lead author of a paper in the Astrophysical Journal Letters.

Van Dokkum's team was able to get a good look at Dragonfly 44 thanks to the W.M. Keck Observatory and the Gemini North telescope, both in Hawaii. Astronomers used observations from Keck, taken over six nights, to measure the velocities of stars in the galaxy. They used the 8-meter Gemini North telescope to reveal a halo of spherical clusters of stars around the galaxy's core, similar to the halo that surrounds our Milky Way galaxy.

Star velocities are an indication of the galaxy's mass, the researchers noted. The faster the stars move, the more mass its galaxy will have.

"Amazingly, the stars move at velocities that are far greater than expected for such a dim galaxy. It means that Dragonfly 44 has a huge amount of unseen mass," said co-author Roberto Abraham of the University of Toronto.

Scientists initially spotted Dragonfly 44 with the Dragonfly Telephoto Array, a telescope invented and built by van Dokkum and Abraham.

Dragonfly 44's mass is estimated to be 1 trillion times the mass of the Sun, or 2 tredecillion kilograms (a 2 followed by 42 zeros), which is similar to the mass of the Milky Way. However, only one-hundredth of 1% of that is in the form of stars and "normal" matter. The other 99.99% is in the form of dark matter -- a hypothesized material that remains unseen but may make up more than 90% of the universe.

The researchers note that finding a galaxy composed mainly of dark matter is not new; ultra-faint dwarf galaxies have similar compositions. But those galaxies were roughly 10,000 times less massive than Dragonfly 44.

"We have no idea how galaxies like Dragonfly 44 could have formed," said Abraham. "The Gemini data show that a relatively large fraction of the stars is in the form of very compact clusters, and that is probably an important clue. But at the moment we're just guessing."

Van Dokkum, the Sol Goldman Family Professor of Astronomy and Physics at Yale, added: "Ultimately what we really want to learn is what dark matter is. The race is on to find massive dark galaxies that are even closer to us than Dragonfly 44, so we can look for feeble signals that may reveal a dark matter particle."

Additional co-authors are Shany Danieli, Allison Merritt, and Lamiya Mowla of Yale, Jean Brodie of the University of California Observatories, Charlie Conroy of Harvard, Aaron Romanowsky of San Jose State University, and Jielai Zhang of the University of Toronto.

Story Source:

Materials provided by Yale University. Original written by Jim Shelton. Note: Content may be edited for style and length.