The Large-Scale Structure of Baryons with Fast Radio Bursts

The distribution of matter on very large scales - the so-called large-scale structure - contains clues, hidden within its statistics, about the origin and evolution of the Universe. While the gravity-only evolution of dark matter can be modelled robustly, the baryons that make up stars and cosmic gas are shaped by poorly understood feedback processes, in which supernovae and accreting black holes heat gas and eject it from galaxies. Most of these baryons exist as diffuse ionized gas that is exceedingly difficult to observe, and uncertainty in their distribution is now the dominant systematic error in many cosmological measurements. A new probe of the baryons has emerged in fast radio bursts (FRBs): millisecond flashes of radio light originating from distant galaxies. Each burst is smeared out, or dispersed, by an amount that precisely measures the ionized gas along its line of sight. Spatially correlating dispersion across many bursts and cross-correlating it with galaxy surveys assembles these individual sightlines into the large-scale statistics of the baryons. The Canadian Hydrogen Intensity Mapping Experiment (CHIME) detects FRBs in large numbers, and its new Outrigger telescopes provide localizations precise enough to identify host galaxies, whose redshifts are then obtained through optical follow-up. With this sample, CHIME is performing precision measurements of the large-scale structure of baryons, sharpening our understanding of galaxy evolution and eliminating a dominant systematic from cosmological analyses.