ICRAR: Cosmic Bursts Unveil Universe's Missing Matter

[bystander]

Cosmic Bursts Unveil Universe's Missing Matter
International Center for Radio Astronomy Research (ICRAR) | 2020 May 27

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Astronomers have used a network of mysterious fast radio bursts (FRBs)
to detect half of the Universe’s normal matter, missing until now.
Credit: ICRAR; CSIRO/A. Cherney, ESO/Y. Beletsky, ESO/R. Wesson

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Astronomers have used mysterious fast radio bursts to solve a decades-old mystery of ‘missing matter’, long predicted to exist in the Universe but never detected—until now.

The researchers have now found all of the missing ‘normal’ matter in the vast space between stars and galaxies ...

The researchers were able to directly detect the missing matter using the phenomenon known as fast radio bursts—brief flashes of energy that appear to come from random directions in the sky and last for just milliseconds.

Scientists don’t yet know what causes them but it must involve incredible energy, equivalent to the amount released by the Sun in 80 years. They have been difficult to detect as astronomers don’t know when and where to look for them.

Associate Professor Macquart said the team detected the missing matter by using fast radio bursts as “cosmic weigh stations”. ...

A census of baryons in the Universe from localized fast radio bursts ~ Jean-Pierre Macquart et al

• Nature 581(7809):391 (28 May 2020) DOI: 10.1038/s41586-020-2300-2

[neufer]

<<Intergalactic space is the physical space between galaxies. Studies of the large scale distribution of galaxies show that the Universe has a foam-like structure, with groups and clusters of galaxies lying along filaments that occupy about a tenth of the total space. The remainder forms huge voids that are mostly empty of galaxies. Typically, a void spans a distance of (10–40) h⁻¹ Mpc, where h is the Hubble constant in units of 100 km s⁻¹ Mpc⁻¹.

Surrounding and stretching between galaxies, there is a rarefied plasma that is organized in a galactic filamentary structure. This material is called the intergalactic medium (IGM). The density of the IGM is 5–200 times the average density of the Universe [Voids]. It consists mostly of ionized hydrogen. As gas falls into the intergalactic medium from the voids, it heats up to temperatures of 10⁵ K to 10⁷ K, which is high enough so that collisions between atoms have enough energy to cause the bound electrons to escape from the hydrogen nuclei; this is why the IGM is ionized. At these temperatures, it is called the warm–hot intergalactic medium (WHIM). Computer simulations and observations indicate that up to half of the atomic matter in the Universe might exist in this warm–hot, rarefied state. When gas falls from the filamentary structures of the WHIM into the galaxy clusters at the intersections of the cosmic filaments, it can heat up even more, reaching temperatures of 10⁸ K and above in the so-called intracluster medium (ICM).>>

Computer simulation showing the distribution of warm-hot intergalactic gas

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<<The warm–hot intergalactic medium (WHIM) refers to a sparse, warm-to-hot (10⁵ to 10⁷ K) plasma that cosmologists believe to exist in the spaces between galaxies and to contain 40–50% of the baryonic 'normal matter' in the universe at the current epoch. The WHIM can be described as a web of hot, diffuse gas stretching between galaxies, and consists of plasma or as atoms and molecules, in contrast to dark matter. The WHIM is a proposed solution to the missing baryon problem, where the observed amount of baryonic matter does not match theoretical predictions from cosmology.

Much of what is known about the warm–hot intergalactic medium comes from computer simulations of the cosmos. The WHIM is expected to form a filamentary structure of tenuous, highly ionized baryons with a density of 1−10 particles per cubic meter. Within the WHIM, gas shocks are created as a result of active galactic nuclei, along with the gravitationally-driven processes of merging and accretion. Part of the gravitational energy supplied by these effects is converted into thermal emissions of the matter by collisionless shock heating.

Because of the high temperature of the medium, the expectation is that it is most easily observed from the absorption or emission of ultraviolet and low energy X-ray radiation. To locate the WHIM, researchers examined X-ray observations of a rapidly growing supermassive black hole known as an active galactic nucleus, or AGN. Oxygen atoms in the WHIM were seen to absorb X-rays passing through the medium. In May 2010 a giant reservoir of WHIM was detected by the Chandra X-ray Observatory lying along the wall-shaped structure of galaxies (Sculptor Wall) some 400 million light-years from Earth. In 2018, observations of highly-ionized extragalactic oxygen atoms appeared to confirm simulations of the WHIM mass distribution.>>

[Ann]

ICRAR wrote:

“We’ve now been able to measure the distances to enough fast radio bursts to determine the density of the Universe,” he said. “We only needed six to find this missing matter.”

I find this so irritating. The author of the article says that the team investigating fast radio bursts in order to look for the previously "missing" baryonic matter now know what the density of the Universe is. But we are not told what the density of the Universe actually is.

Ann