Physics of the Universe

How did the Universe begin, and what physical processes set the initial conditions for the cosmic structures we see today? What is the nature of dark matter, and how does it shape the growth of structure across cosmic time? What is driving the accelerated expansion of the Universe—dark energy, new fields, or a modification of gravity? And how can precision measurements of cosmic structure—from the cosmic microwave background to gravitational lensing and galaxy surveys—map the Universe’s expansion and growth history to test fundamental physics?

At KIPAC, we work to understand the physics that shapes the origins, evolution, and fate of the Universe. We develop theoretical models of the early Universe, devise experiments to detect dark matter, and analyze precision measurements of the cosmos—from the oldest light in the cosmic microwave background to the large-scale structure mapped by galaxy surveys—to test gravity, measure the properties of dark matter and dark energy, and search for signatures of new physics.

We collaborate closely with colleagues in the Leinweber Institute for Theoretical Physics at Stanford (LITP@S) to connect theories of dark matter and the early Universe to data. For example, we develop techniques to search for primordial gravitational waves—potential signatures of inflation—and explore new ways to probe dark matter physics through both astrophysical observations and direct experiments.

Dark Matter

The matter we can see, which makes up every planet, star, and galaxy, accounts for less than five percent of the contents of our Universe. About a quarter of the Universe is composed of dark matter, which reveals its presence through gravitational effects on systems ranging from individual galaxies to the entire cosmic web and its evolution. At KIPAC, we aim to understand the nature of dark matter by studying its behavior in diverse cosmological settings—from the Milky Way to high-energy objects to the early Universe. Using state-of-the-art cosmic surveys, we search for imprints of dark matter’s interactions with regular matter and its particle properties in the sky. KIPAC scientists also devise novel experiments, including underground detectors, to directly detect different kinds of dark matter particles and to test whether dark matter interacts beyond gravity.

Dark Energy

Our Universe is expanding at an accelerating rate. This acceleration is driven by dark energy, which makes up 70 percent of the contents of our Universe and whose nature remains mysterious. Dark energy affects both the expansion history of the Universe and the growth of cosmic structure. At KIPAC, we play leading roles in observational programs that measure these effects using galaxy surveys, gravitational lensing, and precision measurements of the cosmic microwave background. By tracing the growth of structure and the expansion history across cosmic time, these measurements test whether dark energy evolves and whether cosmic acceleration points to new physics.

The Early Universe

The earliest moments of the Universe set the initial conditions for the seeds of cosmic structure. A leading idea is cosmic inflation: a brief period of accelerated expansion that can explain the Universe’s large-scale uniformity while generating tiny primordial fluctuations that later grew into galaxies and clusters. Inflation occurred when the Universe was a tiny fraction of a second old, at extreme energy densities far beyond those reached by terrestrial particle accelerators. Because inflation is sensitive to physics at these scales, observational clues from this era are uniquely valuable—and can be searched for in precision measurements of the cosmic microwave background, large-scale structure, and, potentially, primordial gravitational waves.

The Cosmic Microwave Background (CMB)

The cosmic microwave background (CMB) is the oldest light we can observe, released when the Universe was about 380,000 years old, after it cooled enough for atoms to form and light to travel freely. Tiny variations in the CMB encode a snapshot of the early Universe and the seeds of cosmic structure, while subtle distortions and gravitational lensing imprinted on the CMB as its photons traverse the Universe provide a powerful map of matter between us and the surface of last scattering. At KIPAC, we lead and contribute to major CMB experiments, developing instruments and analysis techniques to extract precise measurements of the CMB’s temperature, polarization, lensing, and Sunyaev–Zel’dovich signals. Together with galaxy surveys and gravitational lensing at late times, these CMB measurements let us trace the Universe’s expansion and growth across cosmic history—placing stringent consistency tests on cosmological models and searching for signatures of new physics.

Large-Scale Structure

The large-scale structure of the Universe—the cosmic web traced by galaxies, gas, and dark matter—provides one of our most powerful laboratories for precision cosmology. By measuring how clustering, baryon acoustic oscillations, redshift-space distortions, and gravitational lensing evolve over time, we can reconstruct the Universe’s expansion history and the growth of structure, test gravity on the largest scales, and constrain the properties of dark matter, dark energy, and light relics such as neutrinos. At KIPAC, we lead and help build major surveys to map the cosmos, and we develop statistical and computational techniques to extract cosmological information from their data—combining galaxy clustering, weak lensing, and cross-correlations with the CMB to measure the Universe’s expansion and the growth of structure with high precision. Because galaxies are biased tracers of the underlying matter field, we also build and test models that connect galaxies to their dark matter halos, enabling robust inference while disentangling astrophysical systematics from fundamental physics.