5. The Hot Early Universe Flashcards
Why is gas in the early universe at a uniform temperature?
Scattering between photons and electrons
Why is the early universe opaque?
Scattering between photons and electrons
Why is the later universe transparent?
The universe cools enough so that atomic hydrogen forms (recombination)
There are few electrons left so no more scattering and BB radiation can propagate unimpeded
Before recombination, what can we ignore?
K and lambda until inflation
Why does the early universe emit BB radiation?
It is a purely thermal gas in equilibrium
Why do we need to have a radiometer in the sky to measure CMB?
Water vapour in the atmosphere can interfere with measurement of microwaves
If DM is non-baryonic, what happens at recombination? Why is it in equilibrium earlier on?
It is not in equilibrium with baryons by the time recombination happens
Earlier on, before aeq it is driven by radiation
When does CDM stop caring about radiation?
At a_eq
For kBT»_space; mc^2, how do particles behave?
The X are relativistic – they behave like radiation
Why ispresent day
radiation density a close match to the redshifted
radiation density from the early Universe?
The fraction of e, p etc left are insignificant compared
to the fraction that annihilated
What are the last massive particle to become relativistic?
Electrons (and positrons)
mc^2 ~ 0.5eV
How is the density of radiation now found?
It is composed of photons and neutrinos
Photon component is fixed observationally by looking at CMB
Neutrino component is assumed to be 3 species of light neutrino
What is the neutrino component in the radiation density now made of?
Electron, mu and tau neutrinos
How does the current CMB+neutrino energy density compare to the energy density of all the relativistic particles at high z?
(1+z)-4 times
energy density of all the relativistic particles at high z
What do we measure the energy of the CMB now as?
4.19 x 10^-14 J/m^3
