Cosmic Microwave Background

Question 1

When were the dark ages?

Answer 1

0.5Gyr before first stars (z ~ 15)

Question 2

When was the Recombination period and what happened within it?

Answer 2

400000 years

Atoms form from ions and electrons (microwave background)

Question 3

When was the nucleosynthesis period and what happens within it?

Answer 3

1 - 1000 seconds

Elements were created
Origin of the photon/neutrino backgrounds

Question 4

When was the inflation and extreme particle physics?

Answer 4

<10^-30 seconds
Planck time

Question 5

What does an expanding universe indicate?

Answer 5

Much denser past and hotter in the past

Question 6

What was the early universe like?

Answer 6

No neutral atoms present (everything is ionised)

Electron and photons scatter continuously which keeps everything in thermal equilibrium

Question 7

What is microwave background radiation?

Answer 7

The radiation in the early universe that is in thermal equilibrium which results in black body radiation

It fills the universe

Question 8

Why can CMB be seen now?

Answer 8

The universe is now transparent

CMB propagates approximately freely

It has a characteristic temperature as it emits a blackbody spectrum

Question 9

What is the characteristic temperature of CMB?

Answer 9

T_radn = 1+ z

Question 10

What is the hydrogen-ionisation potential energy?

Answer 10

13.6 eV

Question 11

What is used to detect CMB locally?

Answer 11

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

Question 12

What is used to detect CMB not locally?

Answer 12

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)

Question 13

What are the spectral properties of CMB?

Answer 13

Perfect black body spectrum (so it is cosmological and a property of entire universe)
T_0 = 2.7 K wherever you look in sky

Question 14

What is the energy density relationship of the CMB?

Answer 14

E ∝ T^4
T ∝ a ^ -1
so E ∝ a^-4

The energy density of CMB is greater than all other radiation in universe

Question 15

Does CMB change with redshift?

Answer 15

No it remains a black body from the start
It shows that all regions started together
Early universe is in thermal equilibrium

Question 16

What is the number density of CMB photons and the mean energy of a black body photon?

Answer 16

n_total = 4.1 x 10^8 m^-3

E_mean = 2/7 k_B T

Question 17

What is the number density of baryons?

Answer 17

n_baryon ~ ρ_baryon,0 / m_proton

Question 18

What has happened to the ratio of photon/baryon number density since the early universe?

Answer 18

It has remained fixed

(there are many photons per baryon and this is fixef since recombination)

Question 19

What is the big bang model?

Answer 19

Early universe was hot and dense and in thermodynamic equilibrium

Question 20

What causes thermodynamic equilibrium?

Answer 20

Frequent interactions

Question 21

Why do electrons and protons interact?

Answer 21

To keep baryons in equilibrium

Question 22

What is Thompson scattering?

Answer 22

When an electron interacts with a photon (low energy version of Compton scattering)

Question 23

Are there more photons than baryons in the universe?

Answer 23

Yes

Question 24

Where is the best place for direct detection of CMB?

Answer 24

Space as water vapour in the atmosphere absorbs microwaves