Expansion History
Since the dawn of time, the Universe has been expanding. However, the rate of that expansion has not always been the same. Einstein’s theory of General Relativity tells us that the content of the Universe (i.e. matter and radiation) dictates the expansion rate of the Universe. In the standard cosmological model, the energy density of the Universe was dominated by radiation but then, 54,000 years after the big bang, it transitioned to being dominated by non-relativistic matter. Our current measurements of the Cosmic Microwave Background (CMB), Large Scale Structure (LSS), and probes of big bang nucleosynthesis (BBN) seem to be in good agreement with this model. However, measurements of the present-day expansion rate (H0) inferred from local probes like Type Ia supernovae are in tension with what we infer from observations of the CMB. This begs the question: how reliable is our standard model of cosmology?
Temperature map of the CMB observed by the Planck satellite in 2018.
I am interested in non-standard cosmologies that alter the content of the Universe prior to the formation of the CMB. Adding extra energy density to the Universe during this time can increase the value of H0 inferred from observations of the CMB and reduce tensions with local measurements. However, this process can be tricky; adding new energy density to the Universe can have some unintended side effects on cosmological observables. I am interested in quantifying how much we can alter our standard model of cosmology prior to the epoch of recombination while still maintaining consistency with precise measurements of the CMB and LSS.
Small-scale Structure
If a subhalo passes through or near a stellar stream, it can leave behind a detectable imprint in the form of a gap or perturbation. Identifying and characterizing these features provides a way to indirectly detect otherwise invisible dark matter structure. With recent advances in observational data, we are now approaching the precision needed to reliably identify stream member stars—an essential step in detecting these subtle signatures.
By combining observations with theoretical models, we can predict how many subhalos should exist, what their physical properties are, and how frequently they should interact with stellar streams. Comparing these predictions to observations allows us to test models of dark matter and reconstruct the small-scale history of structure formation in our Universe.
Artist’s impression of stellar streams in and around the Milky Way. Credit: NOIRLab
My research focuses on using structure on the smallest astronomical scales to understand how our Universe evolved and what dark matter is made of. Dark matter halos—extended, invisible structures that surround galaxies—are the basic building blocks of cosmic structure. These halos have grown under the influence of gravity since their origins as tiny density fluctuations in the early Universe. The way they form, evolve, and cluster depends on both the underlying cosmology and the physics of dark matter.
Cosmological models predict the existence of subhalos—smaller clumps of dark matter that orbit within a galaxy’s larger host halo. These subhalos can interact gravitationally with visible structures such as stellar streams. Stellar streams are elongated trails of stars formed when globular clusters or dwarf galaxies are tidally disrupted by the Milky Way.