Observations of the MIssing Baryons in the warm-hot intergalactic medium

Understand

It has been known for decades that the observed number of baryons in the local universe falls about 30-40% short of the total number of baryons predicted by Big-Bang Nucleosynthesis, as inferred from density fluctuations of the Cosmic Microwave Background and seen during the first 2-3 billion years of the universe in the so called Lyman-alpha Forest.

• A theoretical solution to this paradox locates the missing baryons in the hot and tenuous filamentary gas between galaxies, known as the warm-hot intergalactic medium.
• However, it is difficult to detect them there because the largest by far constituent of this gas - hydrogen - is mostly ionized and therefore almost invisible in far-ultraviolet spectra with typical signal-to-noise ratios.
• Indeed, despite the large observational efforts, only a few marginal claims of detection have been made so far.