Missing baryon problem

Not to be confused with the dark matter problem or the baryon asymmetry problem.

The missing baryon problem was a problem related to the fact that the observed amount of baryonic matter did not match theoretical predictions. The density of baryons can be constrained according to big bang nucleosynthesis and the cosmic microwave background. The best current data, observed by the Planck spacecraft in 2015, yielded a density about 4.85% of the critical density. However, directly adding up all the known baryonic matter produces a baryonic density slightly less than half of this. The missing baryon problem is distinct from the dark matter problem, which is mainly non-baryonic in nature. There is also much more dark matter in the universe than there are missing baryons.

The missing baryons are believed to be located in the warm–hot intergalactic medium (WHIM) (hot intergalactic gas), with recent observations providing strong support.

Theoretical predictions

The density of baryonic matter can be obtained indirectly from two independent methods.

• The theory of Big Bang nucleosynthesis predicts the observed abundance of the chemical elements. If there are more baryons, then there should also be more helium, lithium and heavier elements synthesized during the Big Bang. Agreement with observed abundances requires that baryonic matter makes up between 4–5% of the universe's critical density.
• Detailed analysis of the small irregularities (anisotropies) in the cosmic microwave background (CMB), especially the second peak. The details are technical, but are based on the fact that baryonic matter interacts with photons and therefore leaves a visible imprint on the CMB.

The CMB constraint ($\Omega _{b}h^{2}=0.02230\pm 0.00014$) is much more precise than the BBN constraint ($\Omega _{b}h^{2}=0.023\pm 0.002$), but the two are in agreement.

Observations

The density of baryonic matter can be obtained directly by summing up all the known baryonic matter. This is highly nontrivial, since although luminous matter such as stars and galaxies are easily summed, baryonic matter can also exist in highly non-luminous form, such as black holes, planets, and highly diffuse interstellar gas. Nonetheless it can still be done, using techniques such as:

• Sufficient diffuse, baryonic gas or dust would be visible when backlit by stars. The resulting spectra can be used to infer the mass between the star and the observer (us).
• Gravitational microlensing. If a planet or other dark object moves between the observer and a faraway source, the image of the source is distorted. The mass of the dark object can be inferred based on the amount of distortion.

Prior to 2017, the result came to about 70% of the theoretical predictions.

Resolution

The missing baryon problem was proclaimed solved in 2017, when two groups of scientists working independently found the missing baryons in intergalactic matter. The missing baryons had been postulated to exist as hot strands between galaxy pairs. Because the strands are so diffuse, and because they are not hot enough to emit X-rays, they are difficult to detect. The groups used the Sunyaev-Zeldovich effect to measure the density of the strands. If there are baryons present, light from the cosmic microwave background should scatter off them, losing some energy. These show up as very dim patches in the CMB. The patches are too dim to see directly, but when overlaid with the visible galaxy distribution, become detectable. The density of the strands comes up to about 30% of the baryonic density, the exact amount needed to solve the problem.