11 October 2017 News

Have astronomers found the missing mass in the Universe?

Simulation of the cosmic web.
Simulation of the cosmic web.

Baryonic matter - the material that planets, stars and galaxies are made out of – accounts for only 4.6 percent of the matter in the Universe; the rest is presumed out there somewhere, just missing. Now, two independent teams of researchers have made claims for finding this missing matter and they have found it hidden in the cosmic web.

Until about thirty years ago, astronomers thought that the Universe was composed almost entirely of "baryonic matter" – ordinary atoms made up of protons, neutrons and electrons. But, after calculating the density and composition of the Universe from measurements of the cosmic microwave background and from nuclear reactions following the Big Bang, it was found that a vast proportion of this material appeared to be missing. Finding the remaining 95 percent of baryons has since been one of the major challenges in modern cosmology .

Simulations to predict where the ‘missing baryons’ might be residing had revealed that they could be spread throughout warm-hot filamentary gas in the cosmic web. The cosmic web is just that - a web of matter clustered in thread-like structures (filaments) and nodes.

However, because the gas is relatively low temperature (around 105 − 107K) and is of low density, previous attempts to observe it via X-ray emission or absorption in quasar spectra have proven difficult.

Now, by looking at these warm-hot baryons through a phenomena known as the thermal Sunyaev-Zel'dovich (tSZ) effect, in which light left over from the Big Bang scatters as it passes through hot gas, both teams have independently found a substantial quantity of previously ‘unseen’ matter.

One team from the University of Edinburgh, whose lead author on the research paper is Anna de Graaff, suggest that their estimated gas density in filaments that stretch as much 15 Megaparsecs long, can account for as much as ∼30 percent of the total baryon content of the Universe.

The other international team, whose lead author is Hideki Tanimura from the University of British Columbia, Canada, studied data of the thermal Sunyaev-Zel'dovich effect between ∼260,000 pairs of Luminous Red Galaxies (LRG’s) taken from the Sloan Digital Sky Survey.

They suggest that the apparent over-density of baryons could be due to filaments in merger systems that have shock-heated and compressed the gas more than normal.

The papers released as part of these discoveries have yet to be peer-reviewed, however with both groups finding a similar conclusion to their analysis, it is likely that between them, these teams have helped solve the mystery of where all the missing mass has been hiding.

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