Dark Energy Survey releases most precise look at the universe’s evolution

a black background with lots of small bright white stars

This news piece has been slightly modified from this news story, first published by Fermilab. 

The Dark Energy Survey collaboration has created the largest ever maps of the distribution and shapes of galaxies, tracing both ordinary and dark matter in the universe out to a distance of more than 7 billion light years. The analysis, which includes the first three years of data from the survey, is consistent with predictions from the current best model of the universe, the standard cosmological model. Nevertheless, there remain hints from DES and other experiments that matter in the current universe is a few percent less clumpy than predicted.

New results from the Dark Energy Survey — a large international team that includes researchers from the University of Wisconsin–Madison — use the largest ever sample of galaxies over an enormous piece of the sky to produce the most precise measurements of the universe’s composition and growth to date. Scientists measured that the way matter is distributed throughout the universe is consistent with predictions in the standard cosmological model, the best current model of the universe.

Over the course of six years, DES surveyed 5,000 square degrees — almost one-eighth of the entire sky — in 758 nights of observation, cataloguing hundreds of millions of objects. The results, announced May 27, draw on data from the first three years — 226 million galaxies observed over 345 nights — to create the largest and most precise maps yet of the distribution of galaxies in the universe at relatively recent epochs.

Since DES studied nearby galaxies as well as those billions of light-years away, its maps provide both a snapshot of the current large-scale structure of the universe and a movie of how that structure has evolved over the course of the past 7 billion years.

profile photo of keith bechtol
Keith Bechtol

“This a stringent test of the current standard cosmological paradigm, a model proposing that 95% of the universe is dark matter and dark energy that we do not yet understand,” explains UW–Madison physics professor Keith Bechtol. “By measuring the apparent positions and shapes of hundreds of millions of galaxies in our survey, we test whether the cosmic structures that have formed in the universe today match the predictions based on structures observed in the early universe.”

To test cosmologists’ current model of the universe, DES scientists compared their results with measurements from the European Space Agency’s orbiting Planck observatory. Planck used light signals known as the cosmic microwave background to peer back to the early universe, just 400,000 years after the Big Bang. The Planck data give a precise view of the universe 13 billion years ago, and the standard cosmological model predicts how the dark matter should evolve to the present. If DES’s observations don’t match this prediction, there is possibly an undiscovered aspect to the universe. While there have been persistent hints from DES and several previous galaxy surveys that the current universe is a few percent less clumpy than predicted—an intriguing find worthy of further investigation—the recently released results are consistent with the prediction.

“In the area of constraining what we know about the distribution and structure of matter on large scales as driven by dark matter and dark energy, DES has obtained limits that rival and complement those from the cosmic microwave background,” said Brian Yanny, a Fermilab scientist who coordinated DES data processing and management. “It’s exciting to have precise measurements of what’s out there and a better understanding of how the universe has changed from its infancy through to today.”

a black background with lots of small bright white stars
Ten areas in the sky were selected as “deep fields” that the Dark Energy Camera imaged multiple times during the survey, providing a glimpse of distant galaxies and helping determine their 3-D distribution in the cosmos. Photo: Dark Energy

Ordinary matter makes up only about 5% of the universe. Dark energy, which cosmologists hypothesize drives the accelerating expansion of the universe by counteracting the force of gravity, accounts for about 70%. The last 25% is dark matter, whose gravitational influence binds galaxies together. Both dark matter and dark energy remain invisible and mysterious, but DES seeks to illuminate their natures by studying how the competition between them shapes the large-scale structure of the universe over cosmic time.

DES photographed the night sky using the 570-megapixel Dark Energy Camera on the Victor M. Blanco 4-meter Telescope at the Cerro Tololo Inter-American Observatory in Chile, a Program of the National Science Foundation’s NOIRLab. One of the most powerful digital cameras in the world, the Dark Energy Camera was designed specifically for DES and built and tested at Fermilab. The DES data were processed at the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign.

“These analyses are truly state-of-the-art, requiring artificial intelligence and high-performance computing super-charged by the smartest young scientists around,” said Scott Dodelson, a physicist at Carnegie Mellon University who co-leads the DES Science Committee with Elisabeth Krause of the University of Arizona. “What an honor to be part of this team.”

To quantify the distribution of dark matter and the effect of dark energy, DES relied on two main phenomena. First, on large scales, galaxies are not distributed randomly throughout space but rather form a weblike structure due to the gravity of dark matter. DES measured how this cosmic web has evolved over the history of the universe. The galaxy clustering that forms the cosmic web, in turn, revealed regions with a higher density of dark matter.

images shows a huge camera inside an observatory
The Dark Energy Survey photographed the night sky using the 570-megapixel Dark Energy Camera on the 4-meter Blanco telescope at the Cerro Tololo Inter-American Observatory in Chile, a Program of the National Science Foundation’s NOIRLab. Photo: Reidar Hahn, Fermilab

Second, DES detected the signature of dark matter through weak gravitational lensing. As light from a distant galaxy travels through space, the gravity of both ordinary and dark matter can bend it, resulting in a distorted image of the galaxy as seen from Earth. By studying how the apparent shapes of distant galaxies are aligned with each other and with the positions of nearby galaxies along the line of sight, DES scientists inferred the spatial distribution (or clumpiness) of the dark matter in the universe.

Analyzing the massive amounts of data collected by DES was a formidable undertaking. The team began by analyzing just the first year of data, which was released in 2017. That process prepared the researchers to use more sophisticated techniques for analyzing the larger data set, which includes the largest sample of galaxies ever used to study weak gravitational lensing.

For example, calculating the redshift of a galaxy — the change in light’s wavelength due to the expansion of the universe — is a key step toward measuring how both galaxy clustering and weak gravitational lensing change over cosmic history.  The redshift of a galaxy is related to its distance, which allows the clustering to be characterized in both space and time.

“Redshift calibration is one topic where we significantly improved upon our year-1 data analysis,” said Ross Cawthon, a UW-Madison physics postdoc who led the redshift calibration efforts for two of the main galaxy samples. “We developed new methods and refined old ones. It has been a huge effort by DES members from all over the world.”

Ten regions of the sky were chosen as “deep fields” that the Dark Energy Camera imaged repeatedly throughout the survey. Stacking those images together allowed the scientists to glimpse more distant galaxies. The team then used the redshift information from the deep fields to calibrate measurements of redshift in the rest of the survey region. This and other advancements in measurements and modeling, coupled with a threefold increase in data compared to the first year, enabled the team to pin down the density and clumpiness of the universe with unprecedented precision.

Along with the analysis of the weak-lensing signals, DES also precisely measures other probes that constrain the cosmological model in independent ways: galaxy clustering on larger scales (baryon acoustic oscillations), the frequency of massive clusters of galaxies, and high-precision measurements of the brightnesses and redshifts of Type Ia supernovae. These additional measurements will be combined with the current weak-lensing analysis to yield even more stringent constraints on the standard model.

“DES has delivered cost-effective, leading-edge science results directly related to Fermilab’s mission of pursuing the fundamental nature of matter, energy, space and time,” said Fermilab Director Nigel Lockyer. “A dedicated team of scientists, engineers and technicians from institutions around the world brought DES to fruition.”

The DES collaboration consists of over 400 scientists from 25 institutions in seven countries.

“The collaboration is remarkably young. It’s tilted strongly in the direction of postdocs and graduate students who are doing a huge amount of this work,” said DES Director and spokesperson Rich Kron, who is a Fermilab and University of Chicago scientist. “That’s really gratifying. A new generation of cosmologists are being trained using the Dark Energy Survey.”

UW–Madison physics graduate student Megan Tabbutt was one of the many significant contributors to this work, developing new methods that contributed to an independent validation of the galaxy clustering analysis.

DES concluded observations of the night sky in 2019. With the experience of analyzing the first half of the data, the team is now prepared to handle the complete data set. The final DES analysis is expected to paint an even more precise picture of the dark matter and dark energy in the universe. And the methods developed by the team have paved the way for future sky surveys to probe the mysteries of the cosmos.

“This work represents a ‘big statement’ from the Dark Energy Survey. DES data combined with other observations provide world-leading constraints on the nature of dark energy,” Bechtol says. “At the same time, we are training a new generation of cosmologists, and pioneering advanced methodologies that will be essential to realize the full potential of upcoming galaxy surveys, including the Vera C. Rubin Observatory Legacy Survey of Space and Time.”

The recent DES results were presented in a scientific seminar on May 27. Twenty-nine papers are available on the arXiv online repository.

Dark Energy Survey result video Exploring 7 billion light years of space with the Dark Energy Survey

The Dark Energy Survey is a collaboration of more than 400 scientists from 25 institutions in seven countries. For more information about the survey, please visit the experiment’s website.

Yang Bai promoted to full professor

Profile photo of Yang Bai
Profile photo of Yang Bai
Yang Bai

The Department of Physics is pleased to announce that Prof. Yang Bai has been promoted to the rank of full professor.

“It is my pleasure and honor as Dean to approve Prof. Yang Bai’s promotion to Full Professor. His creativity and impressive breadth in particle physics research make him a leader not only on dark matter, but also more generally on Beyond-the-Standard-Model Physics,” says Eric Wilcots, Dean of the College of Letters & Science. “He is also a valued teacher, appreciated by students especially at the graduate level. Graduate students and junior researchers in Madison are in good hands.”

Bai joined the department in 2012, and was promoted to associate professor in 2017. In addition to his robust and well-funded research program, he has trained several successful graduate students, taught all levels of departmental courses, and served on several departmental and university committees.

“Professor Yang Bai is widely recognized as one of the leading theoretical particle physicists of his generation with a broad and vigorous research program, covering both the collider-related frontiers and the cosmic frontier. His work includes significant contributions in essentially every area related to dark matter,” says Sridhara Dasu, professor and department chair. “The Physics Department very strongly endorses the promotion of Yang Bai to Full Professor.”

Congrats, Prof. Bai on this well-earned recognition!

 

Searching for Sources of Gravitational Waves

a colorful graph showing degrees of sky on a graph's axes and plots of data indicating where gravitational waves may have come from

The entire astrophysical world was blown away by the first-ever binary neutron star collision seen in August 2017 (called ‘GW170817’). This event, identified as a kilonova, was the first to be seen in both gravitational waves, by the LIGO and Virgo detectors, as well as the electromagnetic spectrum, from gamma rays to radio waves (and covered previously in this Oct 2017 DArchive ). Since then, there have been dozens of new gravitational wave events.

A group of researchers in DES, the DESGW team, have focused on finding more electromagnetic counterparts to these gravitational wave events. Members of the Dark Energy Survey — including University of Wisconsin–Madison physics grad student Rob Morgan and postdoc Ross Cawthon, both in Prof. Keith Bechtol’s group —  look at two of the most intriguing events we have followed up with DECam since 2017.

For the full story, please visit The Dark Energy Survey post.

Dark Energy Survey makes public catalog of nearly 700 million astronomical objects

a mostly-black photo of space with white and off-white dots of stars, a small galaxy halo in the left middle, and a larger aura-like glow in the center

Note: this post is adapted from this article, originally published by Fermilab

The Dark Energy Survey, a global collaboration including researchers at the University of Wisconsin–Madison, has released DR2, the second data release in the survey’s seven-year history. DR2 was the topic of sessions at the 237th Meeting of the American Astronomical Society, which was held virtually January 10-15.

The second data release from the Dark Energy Survey, or DES, is the culmination of over a half-decade of astronomical data collection and analysis with the ultimate goal of understanding the accelerating expansion of the universe and the phenomenon of dark energy, which is thought to be responsible for this accelerated expansion. It is one of the largest astronomical catalogs released to date. Keith Bechtol, assistant professor of physics at UW–Madison, has served as the DES Science Release co-coordinator since 2017, guiding the effort to assemble, scientifically validate, and document data releases for both cosmology analysis by the DES Collaboration and exploration by the broad astronomical community.

profile photo of keith bechtol
Keith Bechtol

Including a catalog of nearly 700 million astronomical objects, DR2 builds on the 400 million objects cataloged with the survey’s prior data release, or DR1, and also improves on it by refining calibration techniques, which, with the deeper combined images of DR2, lead to improved estimates of the amount and distribution of matter in the universe.

Astronomical researchers around the world can access these unprecedented data and mine them to make new discoveries about the universe, complementary to the studies being carried out by the Dark Energy Survey collaboration. The full data release is online and available to the public to explore and gain their own insights as well.

“Most of the nearly 700 million objects visible in DES DR2 images had never been seen by humans before the past few years,” Bechtol says. “If you take a moment to look at even a small patch of sky in the DES images, you can see asteroids of our Solar System, stars out to the edge of the Milky Way, and distant galaxies as they were billions of years ago. We look forward to see how our colleagues use this enormous new dataset for research and education.”

DES was designed to map hundreds of millions of galaxies and to discover thousands of supernovae in order to measure the history of cosmic expansion and the growth of large-scale structure in the universe, both of which reflect the nature and amount of dark energy in the universe. DES has produced the largest and most accurate dark matter map from galaxy weak lensing to date, as well as a new map, three times larger, that will be released in the near future.

One early result relates to the construction of a catalog of a type of pulsating star known as “RR Lyrae,” which tells scientists about the region of outer space beyond the edge of our Milky Way. In this area nearly devoid of stars, the motion of the RR Lyrae hints at the presence of an enormous “halo” of invisible dark matter, which may provide clues on how our galaxy was assembled over the last 12 billion years. In another result, DES scientists used the extensive DR2 galaxy catalog, along with data from the LIGO experiment, to estimate the location of a black hole merger and, independent of other techniques, infer the value of the Hubble constant, a key cosmological parameter. Combining their data with other surveys, DES scientists have also been able to generate a complete map of the Milky Way’s dwarf satellites, giving researchers insight into how our own galaxy was assembled and how it compares with cosmologists’ predictions.

Covering 5,000 square degrees of the southern sky (one-eighth of the entire sky) and spanning billions of light-years, the survey data enables many other investigations in addition to those targeting dark energy, covering a vast range of cosmic distances — from discovering new nearby solar system objects to investigating the nature of the first star-forming galaxies in the early universe.

a mostly-black photo of space with white and off-white and blue dots of stars, more concentrated in the middle of the photo and representing the irregular dwarf galaxy
This irregular dwarf galaxy, named IC 1613, contains some 100 million stars (bluish in this portrayal). It is a member of our Local Group of galaxy neighbors, a collection which also includes our Milky Way, the Andromeda spiral and the Magellanic clouds. 2.4 million light-years away, it contains several examples of Cepheid variable stars — key calibrators of the cosmic distance ladder. The bulk of its stars were formed about 7 billion years ago, and it does not appear to be undergoing star formation at the present day, unlike other very active dwarf irregulars such as the Large and Small Magellanic clouds. To the lower right of IC 1613 (oriented with North to the left and East down in this view), one may view a background galaxy cluster (several hundred times more distant than IC 1613) consisting of dozens of orange-yellow blobs, centered on a pair of giant cluster elliptical galaxies. To the left of the irregular galaxy is a bright, sixth magnitude, foreground Milky Way star in the constellation of Cetus the Whale, identified here as a star by its sharp diffraction spikes radiating at 45 degree angles. For coordinate information, visit the NOIRLab webpage for this photo | Photo: DES/NOIRLab/NSF/AURA. | Image processing: DES, Jen Miller (Gemini Observatory/NSF’s NOIRLab), Travis Rector (University of Alaska Anchorage), Mahdi Zamani & Davide de Martin

“This is a momentous milestone. For six years, the Dark Energy Survey collaboration took pictures of distant celestial objects in the night sky. Now, after carefully checking the quality and calibration of the images captured by the Dark Energy Camera, we are releasing this second batch of data to the public,” said DES Director Rich Kron of Fermilab and the University of Chicago. “We invite professional and amateur scientists alike to dig into what we consider a rich mine of gems waiting to be discovered.”

The primary tool in collecting these images, the DOE-built Dark Energy Camera, is mounted to the NSF-funded Víctor M. Blanco 4-meter Telescope, part of the Cerro Tololo Inter-American Observatory in the Chilean Andes, part of NSF’s NOIRLab. Each week, the survey collected thousands of pictures of the southern sky, unlocking a trove of potential cosmological insights.

Once captured, these images (and the large amount of data surrounding them) are transferred to the National Center for Supercomputing Applications for processing via the DES Data Management project. Using the Blue Waters supercomputer at NCSA, the Illinois Campus Cluster and computing systems at Fermilab, NCSA prepares calibrated data products for public and research consumption. It takes approximately four months to process one year’s worth of data into a searchable, usable catalog.

The detailed precision cosmology constraints based on the full six-year DES data set will come out over the next two years.

a dimly-lit domed-top observatory on the left at night, with the glow of the milky way visible in the sky above it
The Dark Energy Survey uses a 570-megapixel camera mounted on the Blanco Telescope, at the CTI Observatory in Chile, to image 5,000 square degrees of southern sky | Photo: Fermilab

The DES DR2 is hosted at the Community Science and Data Center, a program of NOIRLab. CSDC provides software systems, user services and development initiatives to connect and support the scientific missions of NOIRLab’s telescopes, including the Blanco Telescope at Cerro Tololo Inter-American Observatory.

NCSA, NOIRLab and the LIneA Science Server collectively provide the tools and interfaces that enable access to DR2.

“Because astronomical data sets today are so vast, the cost to handle them is prohibitive for individual researchers or most organizations. CSDC provides open access to big astronomical data sets like DES DR2 and the necessary tools to explore and exploit them — then all it takes is someone from the community with a clever idea to discover new and exciting science,” said Robert Nikutta, project scientist for Astro Data Lab at CSDC.

“With information on the positions, shapes, sizes, colors and brightnesses of over 690 million stars, galaxies and quasars, the release promises to be a valuable source for astronomers and scientists worldwide to continue their explorations of the universe, including studies of matter (light and dark) surrounding our home Milky Way galaxy, as well as pushing further to examine groups and clusters of distant galaxies, which hold precise evidence about how the size of the expanding universe changes over time,” said Dark Energy Survey Data Management Project Scientist Brian Yanny of Fermilab.

This work is supported in part by the U.S. Department of Energy Office of Science.

About DES

The Dark Energy Survey is a collaboration of more than 400 scientists from 26 institutions in seven countries. Funding for the DES Projects has been provided by the U.S. Department of Energy, the U.S. National Science Foundation, the Ministry of Science and Education of Spain, the Science and Technology Facilities Council of the United Kingdom, the Higher Education Funding Council for England, the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, the Kavli Institute of Cosmological Physics at the University of Chicago, Funding Authority for Studies and Projects in Brazil, Carlos Chagas Filho Foundation for Research Support of the State of Rio de Janeiro, Brazilian National Council for Scientific and Technological Development and the Ministry of Science, Technology and Innovation, the German Research Foundation and the collaborating institutions in the Dark Energy Survey, the list of which can be found at www.darkenergysurvey.org/collaboration.

Researchers awarded Department of Energy Quantum Information Science Grant

Three UW–Madison physics professors and their colleagues have been awarded a U.S. Department of Energy (DOE) High Energy Physics Quantum Information Science award for an interdisciplinary collaboration between theoretical and experimental physicists and experts on quantum algorithms.

The grant, entitled “Detection of dark matter and neutrinos enhanced through quantum information,” will bring a total of $2.3 million directly to UW-Madison. Physics faculty include principal investigator Baha Balantekin as well as Mark Saffman, and Sue Coppersmith. Collaborators on the grant include Kim Palladino at the University of Oxford, Peter Love at Tufts University, and Calvin Johnson at San Diego State University.

With the funding, the researchers plan to use a quantum simulator to calculate the detector response to dark matter particles and neutrinos. The simulator to be used is an array of 121 neutral atom qubits currently being developed by Saffman’s group. Much of the research plan is to understand and mitigate the behavior of the neutral atom array so that high accuracy and precision calculations can be performed. The primary goal of this project is to apply lessons from the quantum information theory in high energy physics, while a secondary goal is to contribute to the development of quantum information theory itself.

High Energy Physics group awarded three grants totaling over $14 million

a woman in a helmet wearing a disposable facemask stands in front of lots of metal hardware and wires
a woman in a helmet wearing a disposable facemask stands in front of lots of metal hardware and wires
HEP post-doc Dr. Camilla Galloni next to the CMS end cap supporting the GEM detectors that were installed this fall. The primary structure in this photo was engineered at the UW–Madison Physical Sciences Lab. The big CSC chambers were installed, upgraded and reinstalled and operated by UW physicists. The smaller GEM chambers, which are barely visible in the interstices, are being commissioned by UW–Madison physicists through the second grant mentioned in this post.

The High Energy Physics (HEP) group at UW–Madison, which broadly focuses on identifying and understanding the fundamental aspects of particles and forces in Nature, has been awarded three significant grants in 2020. The grants — two from the Department of Energy (DOE) and one from the National Science Foundation (NSF) — are awarded either directly to UW–Madison or indirectly through multi-institution international collaborations, bringing over $14 million to the department.

The first grant, $7.37 million from DOE, funds research that is expected to help physicists understand how our Universe works at its most fundamental level. At UW­–Madison, this research includes experimental and theoretical studies into topics such as using the Higgs boson as a tool for new discoveries and identifying principles of dark matter.

The grant will fund five areas of research: 1) studies of high energy proton-proton collisions; 2) studies of neutrino interactions; 3) studies of super-weak signals from galactic dark matter particles; 4) wide-area imaging surveys using powerful new telescopes; and 5) computational and mathematical methods of quantum field theory and string theory.

Sridhara Dasu is principal investigator on this DOE grant. Co-investigators include Yang Bai, Vernon Barger, Keith Bechtol, Kevin Black, Tulika Bose, Lisa Everett, Matthew Herndon, Kimberly Palladino, Brian Rebel, Gary Shiu, Jennifer Thomas (WIPAC), and Sau Lan Wu. The grant was awarded in June 2020 and provides funding through March 2023.

The other two grants awarded will provide funding for upgrades to the Compact Muon Solenoid (CMS) project at the Large Hadron Collider (LHC) at CERN. The first is an NSF-funded grant for which Kevin Black is leading the UW–Madison effort to upgrade the CMS End Cap muon system upgrade. The $900,000 to the department is part of a larger multi-institutional grant through Cornell University and runs through 2025.

“The GEM detectors are novel micropattern gas detectors which can handle the high background rates expected in the end-cap muon detectors. They will enhance the triggering and reconstruction of forward muons which are expected to make significant improvements and increased acceptance to search for new particles and make precision measurements of known particles and interactions,” Black explains. “UW has a long history with CMS muon system with Prof Matt Herndon, Senior Emeritus Scientist Dick Loveless, and Senior Scientist Armando Lanaro leading to the design, construction, operation, and upgrade of the other end-cap subdetector system instrumented with Cathode Strip Chambers.”

The other CMS-specific grant is a four-year, $5.3 million DOE grant through Fermilab that will fund the CMS trigger upgrade. This funding will allow the UW–Madison CMS group to perform all aspects of the work involved in design, prototyping, qualification, production and validation of the calorimeter trigger system for the upgrade. When completed, the project is expected to result in the collection of 25 times more data than is currently possible. Sridhara Dasu is the principal investigator of this grant.

NSF Physics Frontier Center for neutron star modeling to include UW–Madison

A green, egg-shaped density in the middle has two cones of dark blue representing the gravitational waves projecting perpendicularly out either side of the green density

A group of universities, including the University of Wisconsin–Madison, has been named the newest Physics Frontier Center, the National Science Foundation announced Aug. 17. The center expands the reach and depth of existing capabilities in modeling some of the most violent events known in the universe: the mergers of neutron stars and their explosive aftermath.

The Network for Neutrinos, Nuclear Astrophysics, and Symmetries (N3AS) is already an established hub of eight institutions, including UW–Madison, that uses the most extreme environments found in astrophysics — the Big Bang, supernovae, and neutron star and black hole mergers — as laboratories for testing fundamental physics under conditions beyond the reach of Earth-based labs. The upgrade to a Physics Frontier Center adds five institutions, provides $10.9 million in funding for postdoctoral fellowships and allows members to cover an expanded scope of research.

“For 20 years, we’ve expected that the growing precision of astrophysical and cosmological measurements would make this field an increasingly important part of fundamental physics. Indeed, four monumental discoveries — neutrino masses, dark matter, the accelerating universe, and gravitational waves — have confirmed this prediction,” says A. Baha Balantekin, a professor of physics at UW–Madison and one of the principal investigators for N3AS.

Read the full story 

Dark Energy Survey census of the smallest galaxies hones the search for dark matter

two circles with clusters of stars in them, showing predictions of warm dark matter (fewer stars visible) on the left and cold dark matter (far more stars) on the right

This story is adapted from one originally published by Fermilab

Today, scientists in the Dark Energy Survey — including UW–Madison assistant professor of physics Keith Bechtol and his research group — released results that have been five years in the making. Researchers used the world’s most complete census of dwarf galaxies around our Milky Way galaxy to probe the nature of dark matter, an invisible form of matter that dominates the universe. These new measurements provide information about what dark matter can and cannot be made of.

In particular, the new results constrain the minimum mass of the dark matter particles, as well as the strength of interactions between dark matter and normal matter.

profile photo of keith bechtol
Keith Bechtol

According to these new results, a dark matter particle must be heavier than a zeptoelectronvolt, which is 10-21 electronvolts. That’s one trillionth of a trillionth of the mass of an electron. This study also shows that dark matter’s interactions with normal matter must be roughly 1,000 times weaker than the weak nuclear force. Of the known forces, only gravity is weaker.

These novel measurements used data from the Dark Energy Survey, a cosmological survey designed to study dark energy, the mysterious force driving the accelerated expansion of the universe. In contrast, dark matter is gravitationally attractive, resisting the expansion of the universe and gravitationally binding astronomical systems such as galaxies. The smallest “dwarf” galaxies can have hundreds to thousands of times more dark matter than normal matter. Over the past five years, the Dark Energy Survey has combined with other surveys to more than double the known population of these tiny galaxies. The current total is now over 50.

“The large number of dwarf galaxies that we found orbiting the Milky Way is consistent with expectations from the simplest picture of dark matter — that is, comprising slow-moving particles that interact only through gravity,” Bechtol explained. “In this new paper, we rule out several alternative possibilities for the nature of dark matter.”

profile photo of Mitch McNanna
Mitch McNanna

Dark matter makes up 85% of the matter in the universe, but we have yet to detect it directly in the laboratory. The gravitational effects of dark matter are clearly visible in the motions of stars in galaxies, the clumpy distribution of galaxies in the universe, and even in the amount of lightweight elements. The robust astronomical evidence for the existence of dark matter has motivated many experimental searches here on Earth, using instruments ranging from cryogenic detectors buried deep underground to energetic particle colliders.

“The faintest galaxies are among the most valuable tools we have to learn about dark matter because they are sensitive to several of its fundamental properties all at once,” said Ethan Nadler, the study’s lead author and graduate student at Stanford University and SLAC.

In these multi-year, multi-telescope sky surveys, the raw data comes in the form of tens of thousands high-resolution digital images. But identifying these ultrafaint galaxies, as their description implies, is not as simple as looking at an image and seeing a faint smudge of light. Bechtol and his group, including physics grad student Mitch McNanna, designed the search algorithms needed to identify, with some statistical assurance, which individual stars are part of a dwarf galaxy.

“We worked closely with experts in galaxy formation and particle physics theory to compare the Dark Energy Survey observations with predictions,” Bechtol said. “Part of our job was to determine the sensitivity of our search — how far away from the Earth could we spot a galaxy with only a few hundred stars?”

By combining the observed census of dwarf galaxies with advanced cosmological simulations of the distribution of dark matter around the Milky Way, scientists were able to predict how the physical properties of dark matter would affect the number of small galaxies. Small galaxies form in regions where the dark matter density in the early universe is very slightly above average. Physical processes that smooth out these regions of higher density (if dark matter moves too quickly or gains energy due to interactions with normal matter) or prevent density variations from collapsing to form galaxies (thanks to quantum interference effects) would reduce the number of galaxies observed by the Dark Energy Survey.

“Astrophysical observations provide unique information about the fundamental nature of dark matter, and are complementary to searches for dark matter particles in terrestrial experiments.” Bechtol said. “With the Dark Energy Survey, we continue to learn about the deep connection between particle physics and the growth of cosmic structure, ranging from the vast network of galaxies in the cosmic web, down to smallest individual galaxies.”

two circles with clusters of stars in them, showing predictions of warm dark matter (fewer stars visible) on the left and cold dark matter (far more stars) on the right
This shows the result of two numerical simulations predicting the distribution of dark matter around a galaxy similar to our Milky Way. The left panel assumes that dark matter particles were moving fast in the early universe (warm dark matter), while the right panel assumes that dark matter particles were moving slowly (cold dark matter). The warm dark matter model predicts many fewer small clumps of dark matter surrounding our galaxy and thus many fewer satellite galaxies that inhabit these small clumps of dark matter. By measuring the number of satellite galaxies, scientists can distinguish between these models of dark matter. | Image: Bullock and Boylan-Kolchin (2017); simulations by V. Robles, T. Kelley and B. Bozek, in collaboration with Bullock and Boylan-Kolchin

The Dark Energy Survey is a collaboration of more than 300 scientists from 25 institutions in six countries. For more information about the survey, please visit the experiment’s website.

Funding for the DES Projects has been provided by the U.S. Department of Energy, the U.S. National Science Foundation, the Ministry of Science and Education of Spain, the Science and Technology Facilities Council of the United Kingdom, the Higher Education Funding Council for England, the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, the Kavli Institute of Cosmological Physics at the University of Chicago Funding Authority for Funding and Projects in Brazil, Carlos Chagas Filho Foundation for Research Support of the State of Rio de Janeiro, Brazilian National Council for Scientific and Technological Development and the Ministry of Science and Technology, the German Research Foundation and the collaborating institutions in the Dark Energy Survey.

 

 

Vandenbroucke group plays instrumental role in proving viability of innovative gamma-ray telescope

Scientists in the Cherenkov Telescope Array (CTA) consortium have detected gamma rays from the Crab Nebula using the prototype Schwarzschild-Couder Telescope (pSCT), proving the viability of the novel telescope design for use in gamma-ray astrophysics. The announcement was made today by Justin Vandenbroucke, associate professor at the University of Wisconsin–Madison, on behalf of the CTA Consortium at the virtual 236th meeting of the American Astronomical Society (AAS).

“The Crab Nebula is the brightest steady source of TeV, or very high-energy, gamma rays in the sky, so detecting it is an excellent way of proving the pSCT technology,” says Vandenbroucke, who is also affiliated with the Wisconsin IceCube Particle Astrophysics Center (WIPAC) at UW–Madison. “Very high-energy gamma rays are the highest energy photons in the universe and can unveil the physics of extreme objects, including black holes and possibly dark matter.”

Vandenbroucke is coleader of a team made up of WIPAC scientists and other collaborators that developed and operate a critical part of the telescope: its high-speed camera. Vandenbroucke has worked on the design, construction, and integration of the camera since 2009.

Read the full story on the WIPAC website. The WIPAC story was adapted from a CTA press release.

Keith Bechtol, Rob Morgan win UW’s Cool Science Image contest

pieced-together photos of space with a helix nebula the most visible

pieced-together photos of space with a helix nebula the most visibleCongrats to Prof. Keith Bechtol and graduate student Rob Morgan for their winning entry in the UW–Madison Cool Science Images contest! Their winning entry — one of 12 selected out of 101 entries — earns them a large-format print which initially will be displayed in a gallery at the McPherson Eye Research Institute’s gallery in the WIMR building.

This snapshot of the sky contains thousands of distant galaxies, each containing billions of stars. Bechtol and Morgan were looking for the flash of the explosion of a single star, the potential source of a sub-atomic particle called a neutrino, spotted zipping through the Earth by the IceCube Neutrino Observatory at the South Pole. The distant galaxies, swirling billions of light years away, are all the harder to see because of nearby objects, like the pictured Helix Nebula. The image was captured with a Dark Energy Camera and Victor M. Blanco telescope.

To learn more about the Cool Science Images contest and to view the other winning images, please visit https://news.wisc.edu/the-winners-cool-science-images-2020/.