Recombination

Question 1

What is recombination?

Answer 1

When electrons and protons combine to form atoms

Question 2

When is recombination a two way process?

Answer 2

When photon has energy greater than 13.6eV so it can ionise hydrogen

Question 3

What happens to the hydrogen as a result of photon?

Answer 3

The abundance of photons means hydrogen is formed and destroyed in the early universe

Question 4

What is recoupling?

Answer 4

When photons and electrons are no longer coupled by Compton scattering (symmetric reaction)

Question 5

Why does recoupling stop?

Answer 5

Electrons have been swallowed into atomic hydrogen (not due to lack of photons)

Question 6

Why does recombination and decoupling lead to?

Answer 6

CMB

Question 7

What is the number density and mean free path of free electrons before recombination?

Answer 7

n ~ n_baryon
mfp~ 2kpc

Question 8

What is the number density and mean free path of free electrons after recombination?

Answer 8

n ~ 10^-5 n_baryon
mfp ~ 200Mpc

Question 9

What does decoupling mark?

Answer 9

The break between short mean free path for photons and the mfp is roughly the size of universe

Question 10

How do we know CMB didn’t form locally?

Answer 10

It is isotropic so is more likely to be cosmological and it would appear lumpier if it was

Question 11

If hydrogen had a higher ionisation potential, when would recombination occur?

Answer 11

At a higher redshift

Question 12

What does re-ionisation require?

Answer 12

a photon with energy greater than 13.6eV
(the mfp doesn’t affect this)

Question 13

What does the intensity vs wavelength show about photons?

Answer 13

Many more photons compared to baryons so a lot of photons with energy greater than 13.6eV

at end of graph: 2 billion photons per H atom

Question 14

When do protons and electrons form atoms?

Answer 14

When no more photons are left to ionise hydrogen

Question 15

What happens after recombination/recoupling?

Answer 15

Photons and matter almost completely cease interacting

Question 16

What happens as the tail on the ionisation fraction vs redshift disappears?

Answer 16

The number of photons with 13.6 eV or greater is equal to number of baryons

Question 17

What is the visibility/surface of last scattering?

Answer 17

The redshift where we see CMB photons arising from which occurs due to scattering of electrons taken by H atom

(seen on visibility vs redshift graph which shows a Gaussian with centre z ~ 1070 and Δz ~ 80)

Question 18

What does the surface of scattering show?

Answer 18

CMB photons come from any redshift in this finite shell/width

cannot see beyond this width

Question 19

When does decoupling happen?

Answer 19

In matter dominated regime (so radiation, Λ and K can be ignored)

Question 20

When is decoupling?

Answer 20

t ~ 460000 yrs

Question 21

What happens to T_radn after recombination?

Answer 21

T_radn is proportional to 1/a

Question 22

What happens to T_matter after recombination?

Answer 22

It cools more quickly and starts to form structures