Baryons in the Darkest Sites of the Universe
Kritti Sharma, Vikram Ravi, Dhayaa Anbajagane, William R. Coulton, Elisabeth Krause, Nico Schuster, Alice Pisani, Samuel McCarty, Liam Connor, Simone Ferraro, Nico Hamaus, and Pranjal R. S

TL;DR
This study uses Fast Radio Burst dispersion measures to observationally quantify the baryon underdensity in cosmic voids, revealing a significant baryon deficit and providing insights into the warm-hot phase of diffuse gas.
Contribution
First observational constraint on baryon underdensity in void interiors using FRB dispersion measures, linking baryonic content to galaxy underdensity and thermal state.
Findings
Detected a 3.2σ DM deficit toward void centers.
Baryons are underdense by approximately 60% in void interiors.
Void gas temperature constrained to less than about 1.1 million Kelvin.
Abstract
The pristine underdense patches of the Universe, cosmic voids, are powerful cosmological laboratories, uniquely sensitive to dark energy, modified gravity, and neutrino masses, yet their baryonic content remains uncharacterized. We present the first observational constraint on baryon underdensity in void interiors, exploiting the dispersion measures (DMs) of Fast Radio Bursts (FRBs) as tracers of the free electron column, independent of gas phase, temperature, and metallicity. By stacking 3,455 sightlines from CHIME/FRB on 1,288 SDSS BOSS voids over redshifts , we measure a DM deficit toward void centers at significance, establishing that diffuse baryons inhabit the emptiest corners of the cosmic web at a suppressed level. The measured signal amplitude is consistent with an effective Universe model built directly from the observed galaxy underdensity in these…
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