Baryons in the Darkest Sites of the Universe

Journal article

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 constraint on baryon underdensity in voids, exploiting the dispersion measures (DMs) of fast radio bursts (FRBs) as tracers of the electron column. By stacking 3455 sight lines from CHIME/FRB with ∼15′ localizations on 1228 Sloan Digital Sky Survey (SDSS) BOSS voids over redshifts 0.2 < z < 0.7, we measure a DM deficit toward void centers at 3.2σ significance, indicating 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 voids, and a baryonic model calibrated to the FRB DM–redshift relation (αv = 1.80 ± 0.87). A uniform-density void model yields an electron density contrast of δe,v = −0.58 ± 0.30, implying a tentative ∼60% ± 30% underdensity of baryons in void interiors relative to the cosmic mean. Jointly interpreting our FRB measurement with existing stacks of the thermal Sunyaev–Zel’dovich effect on voids further constrains the mean gas temperature to Te ≲ (1.1 ± 0.7) × 10⁶ K, pointing to a warm-hot diffuse phase, consistent with hydrodynamical simulations. With forthcoming FRB and galaxy surveys, this approach opens a new window onto baryon mapping, with direct implications for feedback models governing gas expulsion into low-density environments, and for the use of cosmic voids to extract cosmological constraints.

Authors: Sharma, K; Ravi, V; Anbajagane, D; Coulton, WR; Krause, E

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: American Astronomical Society
• Journal: The Astrophysical Journal Letters
• Volume: 1006
• Issue: 1
• Pages: L3-L3
• Publication date: 2026-07-10
• DOI: 10.3847/2041-8213/ae81ac
• EISSN: 2041-8213
• ISSN: 2041-8205
• Language: English
• Keywords: Cosmic microwave background; COSMIC cancer database; Baryon; Redshift; Universe; Galaxy; Cosmic background radiation; Void (composites); Local Void