Cosmic Microwave Background Flashcards
When were the dark ages?
0.5Gyr before first stars (z ~ 15)
When was the Recombination period and what happened within it?
400000 years
Atoms form from ions and electrons (microwave background)
When was the nucleosynthesis period and what happens within it?
1 - 1000 seconds
Elements were created
Origin of the photon/neutrino backgrounds
When was the inflation and extreme particle physics?
<10^-30 seconds
Planck time
What does an expanding universe indicate?
Much denser past and hotter in the past
What was the early universe like?
No neutral atoms present (everything is ionised)
Electron and photons scatter continuously which keeps everything in thermal equilibrium
What is microwave background radiation?
The radiation in the early universe that is in thermal equilibrium which results in black body radiation
It fills the universe
Why can CMB be seen now?
The universe is now transparent
CMB propagates approximately freely
It has a characteristic temperature as it emits a blackbody spectrum
What is the characteristic temperature of CMB?
T_radn = 1+ z
What is the hydrogen-ionisation potential energy?
13.6 eV
What is used to detect CMB locally?
Dicke radiometer
rotating satellite observes the sky in one beam and observes calibrated cold source in another and compares both
The temperature at a given frequency is proportional to the difference in the signal
What is used to detect CMB not locally?
Absorption line method
A bright object is observed with cool gas along the line of sight
Some light is absorbed in cloud of cyanogen and CMB excites the gas in cloud from its ground state up to only the first 2 energy level
The ratio of absorption line from ground state is compared to that from the excited state
CMB energy is given at wavelength of ground state (excited state gap)
What are the spectral properties of CMB?
Perfect black body spectrum (so it is cosmological and a property of entire universe)
T_0 = 2.7 K wherever you look in sky
What is the energy density relationship of the CMB?
E ∝ T^4
T ∝ a ^ -1
so E ∝ a^-4
The energy density of CMB is greater than all other radiation in universe
Does CMB change with redshift?
No it remains a black body from the start
It shows that all regions started together
Early universe is in thermal equilibrium
What is the number density of CMB photons and the mean energy of a black body photon?
n_total = 4.1 x 10^8 m^-3
E_mean = 2/7 k_B T
What is the number density of baryons?
n_baryon ~ ρ_baryon,0 / m_proton
What has happened to the ratio of photon/baryon number density since the early universe?
It has remained fixed
(there are many photons per baryon and this is fixef since recombination)
What is the big bang model?
Early universe was hot and dense and in thermodynamic equilibrium
What causes thermodynamic equilibrium?
Frequent interactions
Why do electrons and protons interact?
To keep baryons in equilibrium
What is Thompson scattering?
When an electron interacts with a photon (low energy version of Compton scattering)
Are there more photons than baryons in the universe?
Yes
Where is the best place for direct detection of CMB?
Space as water vapour in the atmosphere absorbs microwaves
