Galaxies and baryons Sample Clauses

Galaxies and baryons. ‌ Based on the conservation of entropy, we can retrace the expansion of the Universe to a denser and hotter infant state (Lemaˆıtre, 1931; Gamow, 1946). In this epoch, the baryonic matter was completely ionized and coupled to photons. Due to the result- ing radiation pressure, the baryons could not collapse onto the primordial dark matter overdensities and moved instead in periodic motions called baryonic acoustic oscilla- tions. As the Universe expanded, electrons and nuclei recombined, and baryons de- coupled from radiation. At this point, these two components were free to evolve inde- pendently: baryons started collapsing onto the primordial dark matter overdensities, and light started streaming across the Universe, forming a cosmic relic we can still see today, the cosmic microwave background. Although they might appear related, gravi- tational collapse for baryons is not akin to its dark matter counterpart due to cooling, i.e. the ability to transform gravitational potential energy in forms of energy other than kinetic. For baryons, collapse assumes the form of a slow accretion process, and the end product is the collection of dense gas at the center of dark matter overdensities. Eventually, this gas fragments and stars are ignited, resulting in the birth of galaxies. Because the dynamics of dark and baryonic matter are so intimately connected, the distribution of galaxies in the Universe acts as a probe of the total matter distribution. This is a powerful idea, but despite what might transpire from the initial description, the relationship between the two components is not purely one-directional. Feedback, i.e. the backreaction of baryonic dynamics on the distribution and motion of dark matter, is an important phenomenon, and nowadays, its eects are studied through hydro- dynamical simulations capable of tracking both gravitational dynamics and baryonic microphysics. To provide an example of this relationship, consider the fact that lumi- nous matter can release a large amount of energy through, e.g. supernovae explosions or the bright accretion disks of supermassive black holes. The energy deposited in this fashion can then reshape the host dark matter halos and impact the relationship between the luminosity of a galaxy and the mass of its host halo. When combined, cooling and feedback are perfect examples of how cosmology can connect micro and macro-scales: physics set by quantum mechanical interactions dictates the appearance of our Universe on th...
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