Graduate Students

Dark Energy Survey scientists release new analysis of how the universe expands

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The latest results combined weak lensing and galaxy clustering and incorporated four dark energy probes from a single experiment for the first time.

This story is amended from one published by Fermilab, which includes information about the full results published by the DES collaboration

The Dark Energy Survey (DES) collaboration — including scientists at the University of Wisconsin–Madison — is releasing results that, for the first time, combine all six years of data from weak lensing and galaxy clustering probes. In the paper, which represents a summary of 18 supporting papers, they also present their first results found by combining all four probes — baryon acoustic oscillations (BAO), type-Ia supernovae, galaxy clusters, and weak gravitational lensing — as proposed at the inception of DES 25 years ago.

diagram showing how the different approaches listed in the story were applied to determine the distribution of dark matter in the universe
The correlations used by DES scientists to map the distribution of matter in the universe. The DES analysis uses shape measurements of source galaxies, shown in yellow, and the positions of lens galaxies, shown in red. Credit: Jessie Muir, DES

“We combined multiple approaches to measure dark energy from a single dataset into a summative result,” says Keith Bechtol, physics professor at UW–Madison and DES collaboration scientist. “More than one hundred people have been working on these results for over a decade, and our group is one of many who contributed.”

The analysis yielded new, tighter constraints that narrow down the possible models for how the universe behaves. These constraints are more than twice as strong as those from past DES analyses, while remaining consistent with previous DES results.

“The constraints have gotten tighter and, so far, are consistent with the cosmological model that has withstood ever more stringent tests during past two decades,” Bechtol says. “The results sharpen the mysteries surrounding the detailed physics that would explain dark energy and dark matter.”

UW–Madison contributions

Bechtol, former physics graduate student Megan Tabbutt, and current graduate student Julián Beas-González all contributed to the current results, developing methods to ensure the data products were scientifically validated. Current Bechtol group postdoc Jason Lee worked on the type Ia supernovae analysis as a graduate student at the University of Pennsylvania.

Bechtol was involved with data collection and curation for DES — a dataset that amounted to over 75,000 individual images with almost 700 million individual stars and galaxies.

“I helped coordinate the effort to assemble, scientifically validate, and document the data products that served as the foundation of the cosmology results presented today,” Bechtol says.

a mostly black background (deep space) dotted with (relatively) small stars and galaxies
A field in the southern constellation Lepus showing stars in the Milky Way (small colored dots) and a group of galaxies (the larger fuzzy objects) about 300 million light-years away. The image shows just a tiny part of one image captured by the Dark Energy Camera. DES analysis measures the faintest galaxies visible in this picture. Credit: Erin Sheldon and the DES collaboration

Tabbutt developed a software pipeline to provide detailed characterization for the detection and measurement of stars and galaxies, and Beas-González further refined that pipeline and conducted the final analysis. The analysis used a method known as synthetic source injection, where synthetic stars and galaxies with known properties are inserted into actual night sky images, and the augmented images are consistently re-processed through the regular measurement pipeline.

“From these measurements, we can compare what we measured versus what we injected. It’s a way to translate between the things we measure and the things that are supposed to be out there in the night sky,” Beas-González says. “It can be used as both a diagnostic tool to see how well we’re detecting and measuring things, but it also has other downstream applications.”

These downstream applications, including calibrating photometric redshifts and obtaining magnification estimates of gravitational weak lensing, are also important to DES collaboration work. Redshifts help explain matter distribution and weak lensing affects galaxy counts or sizes of galaxies, and including both into data analyses is crucial to mapping matter density in the cosmos.

While the DES work is wrapping up, it is also a launching point for more detailed surveys that will help scientists better understand the makeup, origins, and evolution of the universe.

“It’s a very exciting time to be a grad student in the field,” Beas-González says. “I get to see the final stages of DES, and I feel like there’s this whole generation of young scientists like myself that are excited to collaborate on newer projects, see them to their final stages, and get even better and more constrained results.”

Read the full story

More information on the DES collaboration and the funding for this project can be found here.

The Dark Energy Survey is jointly supported by the U.S. Department of Energy’s Office of Science and the U.S. National Science Foundation.

Velocity gradients key to explaining large-scale magnetic field structure

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a whirled, wispy, spiral galaxy has white magnetic field lines overlaid on the image, showing that the magnetic field structure is organized in large, long structures over the entirety of the galaxy

All celestial bodies — planets, suns, even entire galaxies — produce magnetic fields, affecting such cosmic processes as the solar wind, high-energy particle transport, and galaxy formation. Small-scale magnetic fields are generally turbulent and chaotic, yet large-scale fields are organized, a phenomenon that plasma astrophysicists have tried explaining for decades, unsuccessfully. 

In a paper published January 21 in Nature, a team led by scientists at the University of Wisconsin–Madison have run complex numerical simulations of plasma flows that, while leading to turbulence, also develop structured flows due to the formation of large-scale jets. From their simulations, the team has identified a new mechanism to describe the generation of magnetic fields that can be broadly applied, and has implications ranging from space weather to multimessenger astrophysics.

profile photo of Bindesh Tripathi
Bindesh Tripathi

“Magnetic fields across the cosmos are large-scale and ordered, but our understanding of how these fields are generated is that they come from some kind of turbulent motion,” says the study’s lead author Bindesh Tripathi, a former UW–Madison physics graduate student and current postdoctoral researcher at Columbia University. “Given that turbulence is known to be a destructive agent, the question remains, how does it create a constructive, large-scale field?” 

Before working on three-dimensional (3D) magnetic fields, Tripathi investigated systems with hydrodynamic flows and two-dimensional (2D) magnetic fields. After staring at the movies and images of 3D magnetic turbulence, he noticed similarities in the shapes of large-scale flows and large-scale magnetic field structures. But it wasn’t as simple as applying fluid dynamic theory to magnetic field generation: the former may be solved as a 2D problem, whereas the latter must be solved in 3D, making it a much more complex, difficult-to-solve problem.

Tripathi and his colleagues decided to tackle the problem with two key changes from previous research. 

The first difference was the input: a constantly replenished velocity gradient. A cyclist hitting a curb head-on, say, experiences a velocity gradient: the wheels stop, but momentum can cause the cyclist to fly over the handlebars. Velocity gradients exist throughout the universe; for example, within different layers of the sun or when two neutron stars merge. The team reasoned that this gradient is likely important to include while studying 3D magnetic fields. 

Second, they ran perhaps the most complex simulation to date of magnetic fields in the presence of an unstable velocity gradient — 137 billion grid points in 3D space. Altogether, they ran around 90 simulations, generating 0.25 petabytes of data and using nearly 100 million CPU hours on the Anvil supercomputer at Purdue University.

Ordered magnetic fields spontaneously emerge out of chaotic, tangled fields. This finding is consistent with astrophysical observations. Streamlines of magnetic fields are 3D-rendered and are colored red–blue by the x-component of the field. Streamlines of the electric current density are shown in green; color represents magnitude. Poloidal fields are displayed on the (y,z)-plane, after averaging them over the azimuthal (x) direction. Credit: Tripathi et al.

“We start our simulations with a flow that has a velocity gradient, then we add some tiny perturbations, like moving one fluid particle infinitesimally, we let that perturbation propagate over the system and grow, and then analyze the data over time,” Tripathi says. “Initially, these perturbations lead to turbulent flows and magnetic fields in small-scale structures, then, over time, they emerge into larger, ordered structures.” 

When Tripathi ran the same simulations where the initial velocity gradient had decayed over time, the simulation only produced the chaotic, small-scale patterns. “So that’s really the main key: to have a steady, large-scale gradient in velocity,” he emphasizes. 

Adds Paul Terry, physics professor at UW–Madison and senior author of the study: “Magnetic field generation via dynamos has been extensively studied for 70 years, with the frustrating result that the generated fields almost always end up at small scales and highly disordered, unlike observations. This work, therefore, potentially resolves a long-standing issue.”

Though the theory cannot be tested in the distant universe, a lab-based experiment does support the team’s findings: in 2012, colleagues at the Wisconsin Plasma Physics Laboratory were trying to better understand the nature of the magnetic field generation process in a laboratory experiment, but their data did not fit any of the previous models. Tripathi and colleagues’ new theory of magnetic field generation more closely matches the experimental data and helps to resolve the confounding findings.

“This work has the potential to explain the magnetic dynamics relevant in, for example, neutron star mergers and black hole formation, with direct applications to multimessenger astronomy,” Tripathi says. “It may also help better understand stellar magnetic fields and predict gas ejections from the sun toward the earth.”

Top image: The magnetic fields in large-scale structures are organized despite local areas of turbulence. The magnetic field in the Whirlpool Galaxy (M51), captured by NASA’s flying Stratospheric Observatory for Infrared Astronomy (SOFIA) observatory superimposed on a Hubble telescope picture of the galaxy. The image shows infrared images of grains of dust in the M51 galaxy. Their magnetic orientation largely follows the spiral shape of the galaxy, but it is also being pulled in the direction of the neighboring galaxy at the right of the frame. (Credit: NASA, SOFIA, HAWC+, Alejandro S. Borlaff; JPL-Caltech, ESA, Hubble)

This work was supported by the National Science Foundation (2409206) and U.S. Department of Energy (DE-SC0022257) through the DOE/NSF Partnership in Basic Plasma Science and Engineering. Anvil at Purdue University was used through allocation TG-PHY130027 from the Advanced Cyberinfrastructure Coordination Ecosystem: Services & Support (ACCESS) program, which is supported by National Science Foundation (2138259, 2138286, 2138307, 2137603 and 2138296).

 

Gage Erwin named DOE Computational Science Graduate Fellow

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This post is adapted from the DOE’s announcement regarding the Computational Science Fellows

Congrats to physics PhD student Gage Erwin on being named a U.S. Department of Energy Computational Science Graduate Fellow!

Photo of Gage Erwin
Gage Erwin

The 2025-2026 incoming fellows will learn to apply high-performance computing (HPC) to research in disciplines including machine learning, quantum computing, chemistry, astrophysics, computational biology, energy, engineering and applied mathematics.

The program, established in 1991 and funded by the DOE’s Office of Science and the National Nuclear Security Administration (NNSA), trains top leaders in computational science.

“We are so pleased to congratulate the 30 new fellows,” said Ceren Susut, Associate Director of Science for DOE’s Advanced Scientific Computing Research program. “Each of these incredibly talented people has demonstrated both outstanding academic achievement and tremendous research potential. Their research topics cover some of the highest priorities of the Department of Energy, including quantum computing, artificial intelligence, and science and engineering for energy and nuclear security.”

Fellows receive support that includes a stipend, tuition, and fees, and an annual academic allowance. Renewable for up to four years, the fellowship is guided by a comprehensive program of study that requires focused coursework in science and engineering, computer science, applied mathematics and HPC. It also includes a three-month practicum at one of 22 DOE-approved sites across the country, and an annual meeting where fellows present their research in poster and talk formats.

Search for boosted Higgs advances our understanding of dark matter

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This story, featuring physics graduate student Shivani Lomte, was originally published by the CMS collaboration

The CMS Collaboration hunts for Higgs bosons recoiling against dark matter particles

Shivani Lomte

Dark matter is one of the most perplexing mysteries of our universe, accounting for roughly 27% of its total energy. Dark matter does not emit, absorb, or reflect light, and is thus invisible to telescopes. However, its effects on gravitation are unmistakable. Although dark matter’s elementary nature remains unknown, scientists hypothesize that it might be made up of weakly interacting massive particles (WIMPs) that rarely interact with ordinary matter.

In the CMS experiment, we use the fundamental law of momentum conservation to infer the possible presence of dark matter in the detector. In particular the momentum in the transverse plane should be conserved before and after the proton-proton (pp) collision – in other words, the sum of all particle momenta combined should balance out. If momentum is missing, then this suggests that an ‘invisible’ particle, for instance a dark matter particle, has carried that momentum away. Since dark matter doesn’t interact with the detectors, we can’t directly observe it. To detect its presence, we use a ‘visible’ known particle that recoils against the dark matter particle, providing a detectable signal in the experiment. An example of this type of process is shown in Fig. 1.

Figure 1: An event display from the transverse plane which illustrates a signal-like event: the orange cone corresponds to a jet that recoils against missing transverse energy, represented as a magenta arrow. | Credit: CMS collaboration

In pp collisions, a photon, ‘jet’, W or Z boson can be emitted from the initial quark within the proton, whereas radiating a Higgs boson is extremely rare given its small coupling to the quarks. Higgs bosons might be preferentially emitted through a new particle acting as a ‘mediator’ between the standard model and dark matter sector. There is a unique possibility at LHC to produce the mediator particle and study its interaction with the standard model and dark matter.

This analysis uses the “mono-Higgs” signature to search for dark matter particles, focusing on two scenarios that both involve Higgs bosons decaying to bottom quarks. If the Higgs boson is highly energetic (boosted), its decay products become collimated and can be reconstructed in a single large-radius ‘jet’. Alternatively, if the Higgs is not as energetic, we instead look for two small-radius jets, one from each bottom quark. The two scenarios are illustrated in Fig. 2.

Schematic depiction of the “mono-Higgs” → bb̄ production process. On the left, the Higgs decay products merge into a large-radius jet. On the right, the Higgs decay products are reconstructed as two small-radius jets
Figure 2: Schematic depiction of the “mono-Higgs” → bb̄ production process. On the left, the Higgs decay products merge into a large-radius jet. On the right, the Higgs decay products are reconstructed as two small-radius jets. | Credit: CMS collaboration

“A key challenge in this search is that the dark matter signal is rare (at best) and well-known processes, as described in the standard model, produce very similar signatures. To reduce the backgrounds from known particles, we use distinguishing features like the momentum and energy distribution of the particles” says Shivani Lomte, a graduate student at the University of Wisconsin-Madison, leading this search. The precise estimation of the background is critical and is achieved using so called control regions in the data. Such control regions are dominated by background processes and this allows us to quantify the amount of backgrounds in the signal region where we search for dark matter.

In this analysis, once the backgrounds were well-understood, we looked for the dark matter signal by comparing the observed data distributions to the predicted backgrounds, looking for discrepancies. Unfortunately, the observed data agrees with the standard model predictions, and so we conclude that our result has no sign of dark matter. We can thus rule out those types of dark matter particles that would have been detected if they existed.

Regardless of the outcome, the search for dark matter is a journey that pushes the boundaries of human knowledge. Each step brings us closer to answering some of the most profound questions about the nature of the universe and our place within it.

Three grad students recognized as L&S Teaching Mentors

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Physics PhD students Sam Kramer, Michelle Marrero Garcia, and Isaac Barnhill were recently named to the L&S Teaching Mentors program. The L&S Teaching Mentors are the heart of L&S’s Teaching Assistant (TA) Trainings. They are exceptionally passionate and knowledgeable teachers with proven track records for teaching excellence who work closely with the L&S TA Training and Support Team to facilitate various trainings and mentor L&S TAs.

Kramer and Marrero Garcia earned Lead Teaching Mentor designation, meaning that they have served as Teaching Mentors more than once and are taking on an additional leadership role within the program.

Learn more about the three Physics Teaching Mentors:

Profile photo of Isaac Barnhill
Isaac Barnhill

Isaac Barnhill, Teaching Mentor

Isaac began teaching as a peer mentor tutor in the UW Physics Learning Center during undergraduate studies. Now a PhD student in the Physics Department, Isaac has primarily taught electromagnetism, circuits, and optics at the introductory level. Isaac’s research is focused on increasing student agency and decision making in the laboratory component of their physics classes. By shifting the focus of lab activities from content reinforcement to engaging in authentic scientific practices, Isaac hopes to increase students’ sense of engagement and intellectual ownership in the classroom while simultaneously helping students build their data literacy and critical thinking skills. One of his favorite aspects of teaching is seeing students improve their ability to understand, describe, and predict the physical world around them. He always seeks to center the student by promoting active learning in the classroom, allowing students to work out their thoughts in an environment with both high expectations and high support.

 

profile photo of Michelle Marerro Garcia
Michelle Marrero Garcia

Michelle Marrero Garcia, Lead Teaching Mentor

Michelle started teaching in her first semester of the Physics PhD program. She has taught either kinematics or electromagnetism at the introductory level (every semester since then), but she loves teaching any subject within Physics. Her favorite part is watching the face of her students light up as they explore the world through a new lens. In Michelle’s approach to teaching, she always tries to be empathic and put herself in the student’s position. She has found that having changed her field of study from mechanical engineering (as an undergrad) to physics (as a grad) gave her the ability to understand how students that are new to the subject think and feel.

 

profile photo of Sam Kramer
Sam Kramer

Sam Kramer, Lead Teaching Mentor

Sam is a third-year Ph.D. candidate in the Department of Physics and has been teaching for Physics 202, a course for engineering major undergraduates that focuses on electricity, magnetism, and optics, since arriving in Madison. Sam also taught for a similar course as an undergraduate at Saint Louis University. In this role, he leads both discussions, which focus on problem solving, and labs, which provide hands-on experience with the concepts being taught. Physics can be an overwhelming subject, so Sam tries to distill the material into manageable chunks for the students, emphasizing the broader concepts underlying the formulas students use and drawing explicit connections between parts of the curricula. This is meant to develop the dynamic problem solving skills students need when encountering problems they have not seen before.

Physics awarded need-based graduate fellowships by U.S. Department of Education

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This fall, the U.S. Department of Education awarded the UW–Madison department of physics with Graduate Assistance in Areas of National Need (GAANN) fellowships. These fellowships will assist graduate students with strong academic records who demonstrate financial need. Fellows must also demonstrate a commitment to improving their teaching. GAANN has identified seven Areas of National Need, including physics. 

“Advances in physics research have far-reaching implications: they strengthen scientific leadership, lead to innovations, address STEM workforce needs, and ultimately benefit society as a whole,” says Tulika Bose, professor of physics and GAANN project director at UW–Madison. “The fellowship opportunities awarded through this program will enable us to provide new opportunities to deserving incoming or continuing students. We hope it will attract low-income students into our graduate program since the attractiveness of a fellowship offer could potentially tip the balance towards graduate study in physics for some of the extremely bright undergraduate physics majors who otherwise might decide to pursue careers in non-physics disciplines.” 

Nine GAANN fellowships will be available annually for three years to current or incoming physics doctoral students. Students selected for fellowships must demonstrate both financial need and an interest in improving their physics teaching, and they may pursue any area of physics research. The department is working with the Office of Financial Aid to assess need. 

Students must complete at least one academic year of supervised training in instruction at the undergraduate or graduate level at the schedule of at least one-half-time teaching assistant. They can choose from several options for enhancing their teaching portfolio by taking advantage of teaching assistant training sessions, trainings with the Physics Learning Center, or Delta Program certification or courses. They will also be provided professional development activities designed to enhance their skills and build their professional networks

The UW–Madison Graduate School will fund one of the nine fellowships as well as provide funds for professional society membership and conference attendance. The College of Letters & Science and the Department of Physics will support recruiting activities and fund a program evaluation to be conducted by the Wisconsin Center for Education Research.

Current or incoming students can learn more about the Physics GAANN program at https://www.physics.wisc.edu/graduate/phd-program/gaann/.

The GAANN Fellows program is supported by a grant from the U.S. Department of Education (PHYSGRAD-AID: PHYSics GRADuate Fellowship for Accelerating Innovation & Discovery – Award # P200A240159), the University of Wisconsin–Madison Graduate School, the College of Letters & Science, and the Department of Physics.

Justin Edwards earns National Defense Science and Engineering Graduate Fellowship

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Physics PhD and ECE MS student Justin Edwards has been awarded the prestigious National Defense Science and Engineering Graduate Fellowship in the category of Physics (including Optics), with a proposal titled “Multispectral imaging in the near infrared for next-generation analog night vision systems”. Justin is advised by ECE Professor and physics affiliate professor Mikhail Kats and collaborates extensively with ECE PhD students Rabeeya Hamid and Demeng Feng, and the group of Dan Congreve at Stanford University.

MSPQC’s Preetham Tikkireddi wins second place at QED-C student poster presentation

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MSPQC student Preetham Tikkireddi won second place for his poster, “Understanding security side channel attacks on multi-tenancy quantum computers,” at the plenary meeting of the Quantum Economic Development Consortium (QED-C), held March 20-21 in Evanston, IL.

Students who attended the plenary first learned best practices for presenting their research to a non-science audience, a useful skill for a cutting-edge field where investors, hiring managers, and policy makers do not necessarily have a quantum background. Then, the students implemented those skills at the judged poster session.

“[The poster session attendees] are really smart people, but they’re not quantum people, so you set them up for asking questions, and based on the questions that they’re asking, you determine how deep you want to go into your research.” Tikkireddi says. “It was a very different kind of experience, rather than just a plain research presentation to a professor or people who already know the field.”

a group of people in business attire stand and pose in a line, they all have nametag lanyards around their necks
A total of 17 students presented posters at the first-ever QED-C student program and poster competition. UW–Madison MSPQC student Preetham Tikkireddi (right) was one of three graduate students to win the top honor at the competition. | Photo credit: QED-C

Tikkireddi’s research, conducted with computer sciences professor Swamit Tannu, looked at the potential for exploiting crosstalk when two users access the same quantum computer at the same time.

“Right now, quantum computers are really expensive, and the way we access them is by sending jobs to these quantum providers like IBM or IonQ,” Tikkireddi explains. “But the queues are really long. If you’re lucky, you can get the results back the next day.”

Quantum computing capacity is growing rapidly in the form of more and more qubits, and most jobs submitted to these long queues do not need to use all the qubits. Tikkireddi and Tannu thought that one way to increase throughput would be to allow users to share the same quantum computer, each using a subset of the qubits. But quantum computations rely on qubit entanglement, where physically separate qubits interact and share information. It was unclear if sharing a quantum computer opens users to security risks.

In his work, Tikkireddi asked if he could count C-NOTs — the gate that is used to create this entanglement — of another user. He entangled two qubits, then asked if two other qubits could “hear” what the first two were doing.

“We were able to use that to figure out how many C-NOTs the other guy is doing. That’s step one of an attack,” Tikkireddi says. “Your algorithm is your intellectual property, so you don’t want people to steal it. It’s a security problem.”

With this initial analysis identifying potential security risks amongst shared quantum computer use, Tikkireddi says providers should currently not let users share computing time, and that future research should focus on ways to mitigate these crosstalk attacks in an effort to balance efficiency with safeguarding intellectual property.

Tikkireddi credits Tannu for helping to guide his poster away from a traditional research poster and toward one more accessible to a non-science audience. He also appreciates the support from MSQPC associate director Katerina Moloni for encouraging and preparing students to take advantage of these training opportunities.

“It was a really good networking opportunity, especially for me, who is looking for a job right now,” Tikkireddi says. “I would highly recommend students to go to these kinds of events because we get a chance to interact with people in the industry.”

Cristian Vega awarded Callen Award for Excellence in Theoretical Plasma Physics Research

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Cristian Vega

Congrats to (now) Dr. Cristian Vega who won the Callen Award for Excellence in Theoretical Plasma Physics Research! Vega won the award on April 29, just days before defending his thesis on May 3.

The Callen Award is awarded annually to a UW–Madison plasma physics graduate student for achievements in plasma theory. Now-retired Professor Emeritus Jim Callen was a long-time faculty member in the Nuclear Engineering and Engineering Physics department. Callen was also an affiliate faculty member of the Physics department.