8. The CMBR

Question 1

How does the CMB radiate?

Answer 1

Very close to a perfect BB

Question 2

Describe the distribution of the CMB

Answer 2

• Small dipole distortions due to doppler shift from MW motion
• Once subtracted, small variations across the sky

Question 3

Very early in the formation of the universe, what were the two main reactions?

Answer 3

p + e < - > H + photon
photon + electron < - > photon + electron (scattering)
Here the photon energy is > 13.6 eV

Question 4

When did we have an opaque universe?

Answer 4

Very early on when the photon scattering rate was > expanion of the Universe
- Optically thick in THERMAL EQUILIBRIUM

Question 5

What happened to the photons and electrons as the Universe expanded?

Answer 5

Energy drops below 13.6eV

• H cannot be ionised anymore
• Electrons go into Hydrogen (recombination) between z = 1600 -> 1100

Question 6

When does the scattering of photons cease?

Answer 6

When the rate drops below the Hubble parameter

Question 7

What happens to the photons after the scattering rate drops below the Hubble parameter?

Answer 7

Electrons diluted faster than photons can interact with them due to scale factor expansion

• Photons decouple and undergo “last scattering”
• Photons are now CMB photons