Missing baryon problem
Observations of the cosmic microwave background and Big Bang nucleosynthesis studies have set constraints on the abundance of baryons in the early universe, finding that baryonic matter accounts for approximately 4.8% of the energy contents of the Universe.
Nonetheless, it can still be done, using a range of techniques: Prior to 2017, baryons were found to be distributed 10% inside galaxies, 50–60% in the circum-galactic medium, and 30–40% unaccounted, therefore accounting for about 70% of theoretical predictions.
The Lambda-CDM model of the big bang predicts that matter between galaxies in the universe is distributed in web-like formations with a low density (1–10 particles per cubic meter) known as the Warm-hot intergalactic medium (WHIM).
It is easier to detect the WHIM through highly ionized oxygen such as OVI and OVII absorption.
[17] The warm phase of the WHIM was detected by soft X-ray absorption in 2012 to establish 15% of total baryon content.
It is composed of mostly ionized hydrogen and is about 10% of a galaxy cluster's total mass; the rest being dark matter.
The thermal Sunyaev-Zel'dovich (tSZ) effect occurs when photons from the CMB inverse Compton scatter off ionized gas.
[24] The Lyman-alpha (Lyα) emission lines are detected from ionized hydrogen in cosmic filament.
A source, such as a quasar, ionizes hydrogen in the cosmic filament leaving detectable dips in the absorption lines.
[25] Highly ionized oxygen like O+6, O+7, and O+8 absorption lines in the soft X-rays at energies of 0.6–0.8 keV.
Various scientists have proposed explanations, but none have received acceptance as adequately addressing the issue.
One claim of a solution was published in 2017 when two groups of scientists said they found evidence for the location of missing baryons in intergalactic matter.
The missing baryons had been postulated to exist as hot strands between galaxy pairs in the Warm-hot intergalactic medium (WHIM).
The groups used the thermal Sunyaev–Zeldovich effect to measure the density of the strands in the local universe.
If baryons are present there, then some amount of energy should be lost when light from the cosmic microwave background scatters off them.
The patches are too dim to see directly, but when overlaid with the visible galaxy distribution, become detectable.
[13][26][23][16] Even if granted to be accurate, these works only describe the distribution of baryons between nearby galaxies and do not provide a complete picture of cosmic gas in the late universe.
Baryons have more or less been found, so groups are working to detect them to a higher level of significance.
[14] In 2019, a group led by Orsolya E. Kovács detected OVII absorption in the X-ray spectrum of 17 stacked quasars, corresponding to WHIM in filaments of overdensity around 5–9 times the average cosmological density at the epochs of the individual quasars.
[12] In 2020 astrophysicists reported the first direct X-ray emissions measurement of baryonic matter of cosmic web filaments.
