Astronomers Find the Missing Normal Matter in the Universe, Still Looking for Dark Matter, Though

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For decades, the predominant cosmological model used by scientists has been based on the theory that in addition to baryonic matter - aka. "normal" or "luminous" matter, which we can see - the Universe also contains a substantial amount of invisible mass. This "Dark Matter" accounts for roughly 26.8% of the mass of the Universe, whereas normal matter accounts for just 4.9%.

While the search for Dark Matter is ongoing and direct evidence is yet to be found, scientists have also been aware that roughly 90% of the Universe's normal matter still remained undetected. According to

two

new studies

that were recently published, much of this normal matter - which consists of filaments of hot, diffuse gas that links galaxies together - may have finally been found.

The first study, titled "

A Search for Warm/Hot Gas Filaments Between Pairs of SDSS Luminous Red Galaxies

", appeared in the

Monthly Notices of the Royal Astronomic Society

. The study was led by Hideki Tanimura, a then-PhD candidate at the

University of British Columbia

, and included researchers from the

Canadian Institute for Advanced Research

(CIFAR), the

Liverpool John Moores University

and the

University of KwaZulu-Natal

.

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All-sky data obtained by the ESA's Planck mission, showing the different wavelenghts. Credit: ESA

[/caption]

The second study, which recently appeared online, was titled "

Missing Baryons in the Cosmic Web Revealed by the Sunyaev-Zel'dovich Effect

". This team consisted of researchers from the

University of Edinburgh

and was led Anna de Graaff, a undergraduate student from the

Institute for Astronomy

at Edinburgh's Royal Observatory. Working independently of each other, these two team tackled a problem of the Universe's missing matter.

Based on cosmological simulations, the predominant theory has been that the previously-undetected normal matter of the Universe consists of strands of baryonic matter - i.e. protons, neutrons and electrons - that is floating between galaxies. These regions are what is known as the "Cosmic Web", where low density gas exists at a temperatures of 105 to 107 K (-168 t0 -166 °C; -270 to 266 °F).

For the sake of their studies, both teams consulted data from the

Planck Collaboration

, a venture maintained by the European Space Agency that includes all those who contributed to the

Planck mission

(ESA). This was presented

in 2015

, where it was used to create a thermal map of the Universe by measuring the influence of the Sunyaev-Zeldovich (SZ) effect.

This effect refers to a spectral distortion in the Cosmic Microwave Background, where photons are scattered by ionized gas in galaxies and larger structures. During its mission to study the cosmos, the

Planck

satellite measured the spectral distortion of CMB photons with great sensitivity, and the resulting thermal map has since been used to chart the large-scale structure of the Universe.

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IR map of the whole Galaxy showing the plane and bulge of the Galaxy full of stars and dust. Credit: SDSS

[/caption]

However, the filaments between galaxies appeared too faint for scientists to examine at the time. To remedy this, the two teams consulted data from the North and South CMASS galaxy catalogues, which were produced from the 12th data release of the

Sloan Digital Sky Survey

(SDSS). From this data set, they then selected pairs of galaxies and focused on the space between them.

They then stacked the thermal data obtained by

Planck

for these areas on top of each other in order to strengthen the signals caused by SZ effect between galaxies. As Dr. Hideki told Universe Today via email:

While Tanimura and his team stacked data from 260,000 galaxy pairs, de Graaff and her team stacked data from over a million. In the end, the two teams came up with strong evidence of gas filaments, though their measurements differed somewhat. Whereas Tanimura's team found that the density of these filaments was around three times the average density in the surrounding void, de Graaf and her team found that they were six times the average density.

"We detect the low-dense gas in the cosmic web statistically by a stacking method," said Hideki. "The other team uses almost the same method. Our results are very similar. The main difference is that we are probing a nearby Universe, on the other hand, they are probing a relatively farther Universe."

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This illustration shows the evolution of the Universe, from the Big Bang on the left, to modern times on the right. Image: NASA

[/caption]

This particular aspect of particularly interesting, in that it hints that over time, baryonic matter in the Cosmic Web has become less dense. Between these two results, the studies accounted for between 15 and 30% of the total baryonic content of the Universe. While that would mean that a significant amount of the Universe's baryonic matter still remains to be found, it is nevertheless an impressive find.

As Hideki explained, their results not only support the current cosmological model of the Universe (the

Lambda CDM model

) but also goes beyond it:

It also opens up opportunities for future studies of the Comsic Web, which will no doubt benefit from the deployment of next-generation instruments like

James Webb Telescope

, the

Atacama Cosmology Telescope

and the

Q/U Imaging ExperimenT

(QUIET). With any luck, they will be able to spot the remaining missing matter. Then, perhaps we can finally zero in on all the invisible mass!

Further Reading: MNRAS, arXiv,

Matthew Williams

Matthew Williams

Matt Williams is a space journalist, science communicator, and author with several published titles and studies. His work is featured in The Ross 248 Project and Interstellar Travel edited by NASA alumni Les Johnson and Ken Roy. He also hosts the podcast series Stories from Space at ITSP Magazine. He lives in beautiful British Columbia with his wife and family. For more information, check out his website.