Introduction (Reionisation)
From Eric's Wiki
Introduction
Shortly after the big bang (and after the quark-hadron transition) the universe was filled with plasma. This plasma expanded and cooled adiabatically until it dropped below 104K. For the densities and temperatures around this time, the ionisation fraction of hydrogen and helium is a steep function of ρ and T, hence recombination occurs rapidly. After recombination, the plasma is gone, leaving neutral hydrogen and helium to expand. With the transition from plasma to neutral gas, the mean-free-path of photons jumped tremendously, allowing photons to travel outward uninpeded to ultimatley form the cosmic microwave background. The place where the photons last scattered is the last scattering surface and is at about z=1065.
Today, the universe is predominantly ionised. At some time between z=1065 and today, the universe was reionised. The period during which reionisation occured is called the epoch of reionisation. Ionisation can occur via collisional ionisation or absorption of energetic photons. Collisional ionisation is difficult, since any gas hot and dense enough for this to be a concern will also cool very efficiently. (Is this true? How about shock heating? Can that lead to ionisation?) Photo-ionising radiation will be produced around hot stars and AGN such as quasars. Hence, the history of reionisation away from the densest structures is a probe of the history of star formation and quasars.
Reionisation is affected by:
- the spectral index
- the rate at which damped Lyman-limit systems absorb ionising photons
- the diffuse emission
- the clumping factor.
Good overview of reionisation and simulations provided by Theuns05, CF06.
Etittley 15:49, 22 June 2007 (BST)
Back to Reionisation.
