Research and Collaborations
Primary Research Directions
Dark Energy
Several lines of evidence have led to the surprising conclusion that the expansion rate of the universe is increasing with time; on large scales, this means that galaxies are moving away from each other at an accelerated rate. In the present era, this implies that two-thirds of all energy in the universe is made of something we call "dark energy." Dark energy drives the accelerated expansion of the universe, influences the growth of structure, and determines the ultimate fate of our universe. However, its fundamental nature remains a mystery. Our group investigates dark energy by modeling and measuring its effects across time. We use cosmological simulations to understand how dark energy impacts various observables in detail. We then work with large cosmic surveys to directly measure these effects: e.g. measurements of the clustering of galaxies, weak gravitational lensing, and the abundance of the most massive clusters in our universe are highly sensitive to this physics.
Dark Matter
Dark matter makes up most of the matter in the universe. It plays a foundational role in the formation and growth of galaxies and has a strong influence on how the universe expands and forms structure on cosmic scales. However, the detailed physics of dark matter is still a mystery. The exact properties of dark matter can change how cosmic structure forms—including the nature of small dark matter structures and the even smaller galaxies inside them. Our group looks for these telltale changes to try to falsify or confirm different dark matter theories. Our approaches range from vast cosmological scales to the smallest accessible scales. At large scales, we study the statistical impact of dark matter physics on massive galaxy surveys like DESI, Rubin, and Roman. At small scales, we use techniques such as searching for gravitational lenses made by invisible dark matter structures, studying the tiniest galaxies in the universe, or looking for features in disrupting star clusters within the Milky Way.
Galaxy Formation
Galaxies are some of the most foundational and interesting systems in our Universe—their formation and evolution bring together a myriad of astrophysical processes, from star formation to the dynamics of dark matter. Even though we have observed and studied countless galaxies across cosmic time, there remain many open questions about how exactly they form and on the interplay between these astrophysical processes. Our group’s focus spans from detailed studies of individual objects to analysis of large surveys of galaxies where internal galaxy physics is a chief systematic uncertainty. The wide range of expertise in our group allows us to approach these problems from both observational and theoretical perspectives. Our group pursues analyses that require collecting and understanding new data from a range of ground- and space-based instruments and pushes the limits of models and simulations for galaxy formation from detailed simulations to large data-driven models.
Milky Way
The Milky Way galaxy is our home, and the best measured galaxy in the universe. These detailed measurements allow us to extract more information about the physics and history of the Milky Way than are possible for any other galaxy and allow for unique tests of fundamental physics. Our group leverages observational and numerical approaches to understanding the Milky Way and the dark matter halo that surrounds it. We participate in large scientific surveys such as DES, DESI, Rubin LSST, and Via that are mapping out the Milky Way, its satellite galaxies, and the structure of the stellar halo that surrounds it. These imaging and spectroscopic surveys can be used together to inform the nature of dark matter and the Milky Way’s formation history. In addition, we are actively developing tailored cosmological simulations and models that are needed to match the detailed picture provided by current and future surveys of the Milky Way.
Simulations
Simulations are a primary technique our group uses to answer complex questions about galaxies, dark matter, and cosmology, on scales ranging from star clusters to universes. Many astrophysical objects are so complex that simulations are the only sources of reliable theoretical predictions. Our group leverages a broad spectrum of modeling techniques, from data-driven approaches designed to produce mock universes for large surveys to detailed hydrodynamical simulations designed to test models of galaxy formation, and from the scales of our own galaxy to the scale of the entire universe. Our group also develops field-leading simulation analysis tools. These tools range from clustering algorithms to post-processing physics models to emulators that can quickly generate results to match full simulations at high accuracy. Our group has a particularly strong tradition in simulations and techniques that model and constrain the connection between dark matter halos and the galaxies within them.
AI and machine learning
AI and machine learning are transforming galaxy formation and cosmology—enabling discovery in large datasets, uncovering hidden patterns, and accelerating modeling of diverse cosmic phenomena. Our group applies AI/ML to three key areas: detection, modeling, and inference. We use computer vision to extract information from survey data, revealing faint structures like dwarf galaxies and stellar streams missed by traditional methods. Clustering and deep learning classify subtle features across surveys to improve detection. To accelerate modeling, we build emulators that reduce computation from months to minutes, allowing us to test many models and connect theory with data from large surveys. We use techniques such as Simulation-Based Inference to construct and analyze complex probability distributions describing our data and models. In collaboration with the Center for Decoding the Universe, we are exploring new uses of AI to accelerate our work and advance discovery.
Survey Collaborations
Tiny dark matter halos offer essential clues to the nature of dark matter. The cold dark matter model predicts abundant halos from one million to one billion solar masses, while alternative models predict fewer—making their abundance a critical test of dark matter microphysics. Some of these halos host faint low-mass galaxies, whose stellar velocities trace their halo mass. Smaller halos are expected to be dark, detectable only through their gravitational effects on stellar streams from disrupted globular clusters. The Via Project—a collaboration between Stanford, Harvard, Yale, and Carnegie—is building twin high-resolution fiber-fed spectrographs for the 6.5-meter MMT and Magellan telescopes. Via will measure stellar velocities to ~100 m/s across the sky, enabling velocity dispersion measurements of low-mass galaxies and the detection of subtle perturbations from dark halos on streams. Our group is developing cosmologically grounded, flexible models of Milky Way-like hosts, satellites, and streams to interpret these data and designing new techniques to place stronger constraints on dark matter physics and the formation of the lowest-mass galaxies.
The Dark Energy Spectroscopic Instrument (DESI) is a multi-object spectrograph that has operated since 2021 on the Mayall Telescope at Kitt Peak, Arizona. With the ability to take spectra for 5,000 objects simultaneously, DESI has already observed tens of millions of galaxies, stars, and quasars—more than ten times the total from all previous spectroscopic surveys combined. This three-dimensional map of cosmic structure, spanning nearly half the sky, enables precise measurements of the expansion history of the universe and stringent tests of dark energy models. Our group has been deeply involved in DESI from its inception; Wechsler was the founding co-spokesperson of the collaboration (2014–2018). Today, group members use DESI data to study the connection between galaxies and dark matter on cosmological scales. We are also pursuing what DESI reveals about the local universe, including studies of nearby faint galaxies and stellar populations within the Milky Way, to infer more about the nature of dark matter and the formation of low mass galaxies.
The Milky Way's satellite galaxies offer detailed insight into low-mass galaxy formation and the nature of dark matter, but placing this knowledge in context requires understanding how our galaxy compares to similar systems. Our group played a leading role in the SAGA Survey, which studied 101 Milky Way analogs to characterize their satellite populations. We identified 378 satellites around these hosts, measured their properties, and compared them to predictions from simulated Milky Way analogs. SAGA also obtained spectra for many background galaxies, and we continue to analyze this rich dataset. Building on SAGA, we designed a DESI secondary target program that has collected spectra for a large sample of low-redshift galaxies across the sky, dramatically expanding our ability to study the nearby galaxy population. These datasets provide crucial context for interpreting Milky Way observations and testing models of satellite galaxy formation.
The Dark Energy Survey (DES) mapped 300 million galaxies over one-eighth of the sky using the Blanco Telescope in Chile. DES measured the properties of dark energy through galaxy clustering, weak gravitational lensing, galaxy cluster abundances, and supernovae. Our group played leading roles in these measurements and their interpretation. We ran large suites of N-body simulations needed to compare theory against observation, developed galaxy models to turn these simulations into mock measurements, and pioneered techniques for extracting cosmological information—including methods for combining cluster, lensing, and clustering probes. Beyond large-scale cosmology, our group led efforts to use DES imaging of resolved stars to discover faint dwarf galaxies orbiting the Milky Way. These dim satellites are powerful probes of dark matter physics, and we continue to use them to constrain dark matter properties.
The Vera C. Rubin Observatory, perched on Cerro Pachón in Chile, will capture the largest-ever image of the sky every few nights for ten years with its 8.4-meter telescope and the world's largest digital camera. Its flagship Legacy Survey of Space and Time (LSST) will observe 20 billion galaxies and 20 billion stars, driving discoveries across cosmology, time-domain astrophysics, and Milky Way science. Our group is preparing to maximize the scientific return of this transformative dataset. For cosmology, we are building synthetic sky surveys to test analysis pipelines, developing methods for joint-survey cosmology with Rubin and other experiments, and creating tools to accelerate reproducible inference. For dark matter, we are developing techniques to detect ultra-faint dwarf galaxies, identify strong gravitational lenses produced by dark substructure, and extract dark matter constraints from small-scale structure. We are also building models of the Milky Way stellar halo and its substructure to interpret the unprecedented stellar maps LSST will provide. We are most excited about the new discoveries that LSST will make, and we are designing new techniques for automated discovery in the Rubin data.
The Nancy Grace Roman Space Telescope is NASA's next flagship observatory, designed to study dark energy, galaxy evolution, and exoplanets. Roman's wide-field infrared instrument captures 100 times more sky per image than Hubble at comparable sensitivity, enabling a sweeping survey that will measure light from over a billion galaxies. These observations will allow us to reconstruct the history of structure formation under dark matter's influence and constrain dark energy models with unprecedented precision. Our group has contributed significantly to Roman's science definition and survey planning, particularly shaping the High Latitude Wide Area Survey for precision cosmology and studies of stellar streams. We are providing essential infrastructure for the mission, including cosmological simulations that model Roman's galaxy redshift survey and models of the Milky Way and local universe relevant to Roman's wide-field imaging. Our group is well positioned to maximize the scientific impact of this transformative mission.