The Missing Baryon Problem in Physics

There are many unsolved problems in physics, the most famous being the mysteries of quantum gravity, supersymmetry, the Planck scale, dark matter and dark energy… but there is also what has become to be known as the Missing Baryon Problem – less well known but equally important to cosmologists. The Universe contains energy from various sources, with 3 still being relevant today: Dark Energy (which acts to expand spacetime itself), Dark Matter (which only acts gravitationally and provides most of the mass within galaxies and clusters) and finally Baryons (which in the astrophysical sense means all particles which we typically interact with). Baryons make up stars, gas and everything we see in the night sky. Analysis from things such as Type 1a supernovae, Galactic rotation curves and interstellar absorption spectra of gas show that only around 5% of the Universe’s current energy density is contained within baryons. However, baryon surveys are at present only able to detect around 60-70% of these baryons, with less than 10% being in the form of stars within galaxies. Detection of baryons comes in the form of absorption spectra analysis, where clouds of baryons absorb photons of light from distant stars and galaxies. The photons which have exactly the correct energy to excite electrons to higher energy levels in the gas are absorbed leading to the spectrum. Other processes such as the 21cm hydrogen line (a result of an electron spin flip hyperfine transition) and the Sunyaev–Zeldovich effect (SZ effect) are also used in detection. Analysis of the Cosmic Microwave Background (CMB) and ideas of Big Bang Nucleosynthesis (BBN) predicts there to be many more baryons in the Universe which we cannot detect. This likely isn’t an issue with our detection methods, but a lack of understanding of the processes involved in distributing baryons. Scientists have taken to using supercomputer simulations (such as the EAGLE simulation) to create entire universes with physical properties similar to our own to try and deduce where these missing baryons could end up. Such simulations are hydrodynamical, meaning they contain not only Dark Matter (which only interacts through gravity), but also baryons (which interact through gravity and fluid dynamics). It is the feedback mechanisms from things such as stars (stellar wind) and black holes (AGN jets and winds) which appear to cause baryons to be evacuated from galactic haloes into outside regions. Supercomputer simulations have numerous issues and currently cannot solve the missing baryon problem on their own but are nonetheless a powerful tool for physicists in understanding cosmic structure formation and baryon distribution. 0:00 - Unsolved Problems in Physics 0:25 - Energy Density in the Universe; Dark Energy, Dark Matter and Baryons 1:44 - Observational Evidence for Current Universe Composition 2:21 - Missing Baryons and Detection Methods 3:43 - Cosmic Microwave Background (CMB) analysis and Big Bang Nucleosynthesis (BBN) considerations. 4:28 - Hydrodynamical Simulations and Supercomputers 5:24 - Feedback Mechanisms and Subgrid Physics 6:01 - Evacuation of Baryons from Galactic Halos 6:50 - Persisting Problems and Outlook of the Problem