The baryon content in the central regions of cosmic voids is constrained using dispersion measures (DM) of fast radio bursts (FRBs). A stacked sample of 3,455 FRB sightlines from the CHIME/FRB catalog, associated with 1,288 SDSS BOSS voids at redshifts 0.2 < z < 0.7, is analyzed. A 3.2σ significant DM deficit is measured toward void centers, directly indicating a suppressed (by ~60% relative to the mean) amount of diffuse baryons in the emptiest parts of the cosmic web. The electron density contrast is δ_e,v = -0.58 ± 0.30. Joint interpretation with thermal Sunyaev–Zeldovich effect measurements for voids constrains the average gas temperature: T_e ≲ (1.1 ± 0.7) × 10^6 K, consistent with a warm-hot diffuse phase. With upcoming surveys (CHORD, DSA, SKA, DESI, LSST, Euclid, etc.), this method will enable tomographic mapping of baryons and refine feedback in models of gas expulsion into underdense environments.
The Universe is structured like a sponge: there are dense regions packed with galaxies and gas, and giant pores—cosmic voids. These voids occupy most of space, but their contents have remained a mystery because matter there barely reveals itself.
Now astronomers have 'felt' the voids using fast radio bursts—super-powerful pulses from distant galaxies. As these pulses travel through the pores, they slow down slightly when they encounter stray electrons—tiny charged particles lingering in the gas. It's like water losing speed as it flows around sponge fibers. By measuring the delay in thousands of such bursts, scientists have weighed the invisible gas for the first time, mostly hydrogen.
Paradox: this gas is heated to a million degrees, but due to its incredibly low density, it barely transfers heat. If you were inside a void, you'd freeze, not burn. Studying these voids is crucial: they let us investigate dark energy and the properties of nearly elusive neutrinos, refining models of the expansion of the Universe.
🎯 If the Milky Way were at the center of such a void, the night sky would be absolutely black—no galaxies at all, only a few faint stars.