History of our universe
Historically, astronomy started in the optical band — first with the naked eye and later with increasingly more powerful telescopes and detectors. This has provided us with many new insights, even though the optical wavelength range makes up only a tiny fraction of the entire electromagnetic wavelength range. Radio astronomy, just after World war-II, initiated the first expansion of the accessible wavelength spectrum for astronomers. This uncovered an entirely new universe, as many astrophysical processes reveal themselves at different frequencies. This revolution led to a number of fundamental discoveries:
- Microwave background and Big Bang,
- HI emission
- Dark Matter
- synchrotron emission and magnetic fields
- quasars, black holes and high-energy particle accelerators, pulsars and dense states of matter, organic molecules
Nowadays, astronomy has successfully expanded into other domains of the electromagnetic spectrum, such as infrared, X-rays, and γ-rays. Opening up a new frequency window has always led to unexpected discoveries.
Matter-Radiation equality
The epoch at which radiation density equals the matter density is known as the matter-radiation equality epoch, i.e.,
Since for radiation:
Therefore the value of scale factor for which Ωr / Ωm = 1 => a = 8.4 ×10–5/0.23 = 3.7 ×10–4
Hence equivalent redshift is z ~ 2740. The differential equation for structure growth shows that density perturbations should grow ∝ a, in a matter-dominated universe. Explain why, despite this, one might expect that structures would not start to grow until the epoch of recombination at z ∼ 1100. High-density regions can be prevented from collapsing by their internal pressure. At radiation-matter equality, the radiation pressure (Pr = εr/3) is great enough to prevent the collapse of structures below the horizon size, although the gas pressure is not. While the baryonic matter is still fully ionized, the photons and baryons frequently interact, forming a single photon-baryon fluid; therefore, if radiation pressure prevents the photons from collapsing, the baryons won’t either. At recombination, protons and electrons combine to form neutral hydrogen. Neutral atoms interact far less readily with photons, so the photons decouple from the baryonic matter. This means that the radiation pressure no longer acts to prevent baryonic structures from collapsing, so structures can start to grow. Non-baryonic dark matter is electrically neutral and therefore does not interact with photons. Therefore the radiation pressure has no effect on it, and collapse can start at matter-radiation equality. However, if the dark matter is relativistic at this time, it itself will have a “radiation” equation of state, and therefore its own pressure will prevent it from collapsing until its velocity has dropped below relativistic levels. We define dark matter to be “hot” if it is relativistic (v ∼ c) at z ∼ 3000 (i.e., T ∼ 104 K) and “cold” if it is non-relativistic (v << c) at this temperature [dark matter which would be mildly relativistic, with v a significant fraction of ‘c’ is commonly called “warm”]. Therefore, if the dark matter is cold, it does not have a relativistic equation of state at matter-radiation equality, and so neither photon pressure nor its own pressure will prevent collapse.
Reference: https://sites.google.com/view/aruncosmo/Mathematica-Codes?
