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Curated by RSF Research Staff

Have we found the missing baryons?

Theoretical predictions of the amount of baryonic matter that should exist in the Universe does not match the amount that is observed - in fact it is off by more than a factor of 2. However, recent observations could have just found this missing baryonic matter!

Baryonic matter is any composite particle – known as a hadron - composed of quarks, such as protons and neutrons (note: mesons are also hadrons but are made of a quark and antiquark).

The standard theories on the origin and evolution of the universe suggest a process responsible for the production of elements heavier than hydrogen – known as big bang nucleosynthesis. This mechanism, along with inferences made from observations of the density fluctuations in the cosmic microwave background, predict baryonic matter to make up 4.9% of the critical density of the universe - that is the average density required for the Universe to just halt its expansion.

However, when the density of baryonic matter is determined from observations it falls 30-40% short of that which is predicted. Since 2012, astronomers have suspected that this missing baryonic matter may be hiding in the hot and tenuous filamentary gas between galaxies known as the Warm Hot Intergalactic Medium (WHIM). One of the problems in looking for the missing matter in the WHIM is its high temperature, ranging from 100,000 – 10 million Kelvin and thus the fact that most of the gas is ionized – making it near-invisible.

To overcome this obstacle an international team of scientists realized that to achieve the high-signal-to-noise-ratio required to observe the missing baryonic matter, they needed extremely bright sources with the shortest possible exposure times – so they decided to look at quasars. Quasars, thought to be powered by super-massive black holes, are one of the brightest objects in the Universe. The X-ray light signal and subsequent X-ray spectra should therefore be strong enough to readily observe tracers of the missing baryonic matter in the space between the quasar and the telescope – and that is exactly what they found. Utilizing the X-ray Multi-Mirror Mission (XMM) telescope, observations were made of one of the brightest known quasars, 1ES 1553+113, revealing tracers in the form of ionized oxygen indicative of this missing mass.

"We found the missing baryons" said co-author Michael Shull

Future observations of more quasars should confirm this result allowing for theoretical models of the nature of our Universe to be built upon more robust observational data and further developed, such that more questions can be answered.

Amira Val Baker




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