Mapping the Universe's missing matter

Roughly half of the Universe's ordinary matter has never been directly detected. It isn't missing so much as hidden: violent feedback from supernovae and accreting black holes blows gas out of galaxies and spreads it thin across intergalactic space, while carrying magnetic fields with it. Where that gas ends up is the dominant theoretical uncertainty limiting the next generation of cosmological experiments - surveys like Euclid, Rubin, and Roman aim to measure the clustering of matter with exquisite precision to constrain neutrino mass, dark matter, and dark energy, but feedback redistributes baryons in ways that suppress that same clustering and can mimic the very physics these surveys are built to detect. Calibrating feedback is therefore not a side problem for cosmology — it sits directly in the way of it.

Simulations predict a wide range of gas configurations because the physics governing feedback remains poorly constrained, underscoring the need for observational probes that can directly constrain these models of galaxy formation.

Five independent techniques to extract the fingerprints of diffuse gas have recently matured into precision tools - (1) X-ray emission traces the hot, dense gas; the (2) thermal and (3) kinematic Sunyaev-Zel'dovich effects (tSZ and kSZ, distortions in the Universe's oldest light) trace its temperature and bulk motion; (4) the delay in arrival time (dispersion, DM) of Fast Radio Bursts (FRBs) - the newest and fastest-growing of these five techniques, brief radio flashes slowed by intervening gas - quantifies that gas; and (5) the polarized radio light from galaxies twisted while passing through magnetized gas (Faraday rotation, RM) reveals its magnetization. Each probe is sensitive to a different piece of the puzzle, but combined, they span the full range of cosmic environments (masses and redshifts) needed to extract precise cosmological measurements from Rubin datasets - a dynamic range that no single technique can capture alone.

My primary tools are the dispersion measures of FRBs and the Faraday rotation measures of polarized radio sources. An FRB is a millisecond-duration flash of radio light from a distant galaxy; as it travels to Earth, free electrons along the way delay lower frequencies more than higher ones. That delay — the dispersion measure — is a direct, redshift-dependent readout of the column density of ionized gas the signal passed through, including gas far too diffuse to see in any other way. Faraday rotation, the twisting of a radio wave's polarization as it crosses magnetized gas, adds a second, complementary handle: the magnetization of that same gas.

The primary instrument underlying my work is the Deep Synoptic Array (DSA), based at Owens Valley Radio Observatory. DSA-110 has already more than doubled the number of FRBs localized to host galaxies worldwide, and the array is currently scaling up - DSA-2000 is designed to discover more FRBs in a single day than have been found worldwide over the past decade. That leap in sample size is what turns FRB DMs from a promising probe into a precision cosmological tool.

I have developed several components of this program, working toward establishing FRBs as precision probes of baryons - from theoretical predictions and simulations, through the first observational measurements, to forecasts for next-generation FRB surveys.

What do these measurements enable next?

Taken together, this progression - from a theoretical model linking FRB dispersion to feedback, to the first measurements of feedback's imprint on matter clustering and the cosmic web, to a quantified understanding of survey selection effects and a forecasting framework for cross-correlation statistics - is the observational foundation for a much larger goal: turning FRBs, alongside kSZ, tSZ, X-ray, and Faraday rotation measurements, into an empirically calibrated feedback model. As DSA-2000 and other next-generation radio surveys deliver orders-of-magnitude larger FRB samples, on a timescale tracking Rubin's first year of observations, that model can remove one of the largest obstacles standing between next-generation cosmological surveys and the physics they're built to reveal!