Massive

In a new study published in the journal Nature, a team of astronomers finally found this missing mass with distant radio signals known as fast radio bursts, or FRBs. The amount of matter they detected was exactly consistent with what cosmologists predicted we’d find more than two decades ago. 

Soon after FRBs were discovered in 2007, however, astronomers realized their potential as probes of this faint region of the Universe. While we still don’t understand exactly what FRBs are or how they’re emitted, we do know that most of them originate outside of the Milky Way and travel through vast reaches of interstellar space — including the WHIM — to reach telescopes here on Earth.

Radio signals slow down as they pass through matter, with longer radio waves being slowed down more than shorter ones — a phenomenon known as “dispersion.” By measuring the amount of dispersion in a sample of FRBs, the team was able to determine just how much matter there really was hidden away within the WHIM.

It only took 6 FRBs to weigh the Universe, but telescopes around the world are detecting more of these signals every day. Future observations will allow astronomers to map out how subatomic particles are distributed throughout the WHIM, shedding further light on one of the most mysterious regions of the cosmos.

A census of baryons in the Universe from localized fast radio bursts | Nature
▻https://www.nature.com/articles/s41586-020-2300-2

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°Abstract°
More than three-quarters of the baryonic content of the Universe resides in a highly diffuse state that is difficult to detect, with only a small fraction directly observed in galaxies and galaxy clusters. Censuses of the nearby Universe have used absorption line spectroscopy to observe the ‘invisible’ baryons, but these measurements rely on large and uncertain corrections and are insensitive to most of the Universe’s volume and probably most of its mass. In particular, quasar spectroscopy is sensitive either to the very small amounts of hydrogen that exist in the atomic state, or to highly ionized and enriched gas in denser regions near galaxies. Other techniques to observe these invisible baryons also have limitations; Sunyaev–Zel’dovich analyses can provide evidence from gas within filamentary structures, and studies of X-ray emission are most sensitive to gas near galaxy clusters.

Here we report a measurement of the baryon content of the Universe using the dispersion of a sample of localized fast radio bursts; this technique determines the electron column density along each line of sight and accounts for every ionized baryon. We augment the sample of reported arcsecond-localized fast radio bursts with four new localizations in host galaxies that have measured redshifts of 0.291, 0.118, 0.378 and 0.522. This completes a sample sufficiently large to account for dispersion variations along the lines of sight and in the host-galaxy environments11, and we derive a cosmic baryon density of 𝛺b=0.051 [+0.021,−0.025] ℎ^−1 / 70 (95 per cent confidence; h70 = H0/(70 km s^−1 Mpc^−1) and H0 is Hubble’s constant).

This independent measurement is consistent with values derived from the cosmic microwave background and from Big Bang nucleosynthesis.

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The probability distribution of DMcosmic due to the cosmic baryons, p(DM), in semi-analytic models and simulations, as encoded in black, blue, red and green in order of increasing redshift (z; see key), is compared to the analytic form used in our analysis (DM; equation (4))..