2. CMBR

Question 1

Describe the 3 main things that happen just after the BB

Answer 1

• Matter and anti-matter can form, but most annihilates giving 10^9 photons per proton
• Surviving protons and electrons form a plasma
• Photons are frequently scattered (entrained/trapped)

Question 2

What is the photon to baryon ratio?

Answer 2

Total number of photons relative to the total number of protons

Question 3

Describe what is meant by the photons being entrained just after the BB

Answer 3

• They cannot travel any great distance before they scatter off of another proton
• Mean free path is very small

Question 4

Describe what happens at the recombination stage

Answer 4

• First atoms form as p + e -> H + photon
• 1 more photon per atom
• Photons have a much longer free path
• Photons don’t scatter again in the age of the Universe

Question 5

What is the surface of last scattering?

Answer 5

Recombination photons are eventually allowed to travel
- The light from this surface is described as the afterglow of the BB

Question 6

What is assumed about the ratios of protons, electrons and hydrogen atoms before the formation of the CMB?

Answer 6

The ratios obeyed a form of equilibrium

Question 7

Describe the saha equation

Answer 7

It tracks the relative number of ions in a given state as a function of temperature

Question 8

Describe the consequences of the Saha equation expressed in terms of the ionisation parameter

Answer 8

Recombination is depedent on the temperature of the plasma
- Recombination isn’t instantaneous, but takes place in a narrow redshift/scale factor range

Question 9

What value of the ionisation paramater does recombination begin at?

Answer 9

X_e = 0.1

Question 10

What happens as CMB photons travel to the Universe?

Answer 10

The Universe expands

Question 11

What is the temperature of the CMB, and at what redshift is it observed?

Answer 11

Temp is 2.7K and observed at z=0

Question 12

How do we physically observe the CMB?

Answer 12

Need to be above the atmosphere and also avoid foreground emission

Question 13

Describe the wavelength distribution of the CMB and explain how we deal with the foreground radiation

Answer 13

• BB shape peaking at 3mm
• Measure it at different frequencies and wavelengths, then model and subtract foreground

Question 14

What is the biggest source of the foreground emission which hampers our observation?

Answer 14

The dust in the MW at 20-30K

Question 15

What is the temp variation of the CMB?

Answer 15

1 in 10^5
(2.7 +- 0.000027)K

Question 16

\What are the dynamical effects for measuring the CMB?

Answer 16

Earth’s orbit introduces a doppler shift
- Dipole shift as we are slightly moving towards and away from some CMB photons

Question 17

How are the anisotropies measured for the CMB?

Answer 17

On a power spectrum

Question 18

Describe what the different values of l, the multipole correspond to

Answer 18

Small l measures large scales on the sky
Large l measures small regions of the sky

Question 19

When plotting the power spectrum of the anisotropies of the CMB, what do we notice?

Answer 19

That the CMB has more power on some scales than others
- It peaks for a value of l=250

Question 20

Briefly describe the Sachs-Wolfe effect

Answer 20

Regions with more matter density appear cooler than their surroundings
- Regions with matter overdensities and redshifted relative to the CMB

Question 21

Explain the two mechanisms that contribute to the Sachs Wolfe effect

Answer 21

1. Gravitational redshift
   - A CMB photon loses energy if it climbs out of a gravitational well
2. Time dilation
   - Dense regions cool more slowly and recombine later as time moves more slowly in a gravitational potential

Question 22

Describe the proportionality between the scale factor and time

Answer 22

a is prop. to t^2/3

Question 23

Explain what baryon acoustic oscillations are

Answer 23

Overdense regions collapse due to gravity, and the pressure rises at the centre
- When the pressure is greater than the force of gravity, the system stops collapsing so the system expands, overshooting equilibrium
- Pressure driven expansion as there is a net force outwards
- BAOs are the cyclic oscillations in pressure which emit sound waves

Question 24

What is the necessary condition for BAOs to occur?

Answer 24

The dynamical timescales must be larger than the pressure timescales
- This means the pressure responds to the dynamical changes

Question 25

Which regions oscillate more slowly, large or small densities?

Answer 25

Large regions oscillate more slowly

Question 26

How many times will the largest regions have collapsed between the big bang and recombination?

Answer 26

Once i.e. just compressed at the time of CMB formation

Question 27

Where do BAOs spend the most time?

Answer 27

Spend the longest time are their extremes (compressions and rarefractions)

Question 28

Describe the plot of the power of the CMB vs l

Answer 28

Expon. decay in amplitude due to the silk damping
- Odd peaks are stronger relative to the trend due to baryon interactions
k = 1: collapsed by z_rec
k=2: collapsed + expanded
k=3: collapsed + expanded + collapsed

Question 29

How far into the future do BAOs continue, and what is a consequence of this?

Answer 29

They continue up until z=0
- Some galaxy scale clusterings are more common than others

Question 30

When observing BAOs, what two things must we have?

Answer 30

1. Need to observe a very large area of sky
2. Need good redshift information which needs a spectrum from each galaxy (expensive)

Question 31

Why is taking a spectrum of a galaxy more expensive than an image?

Answer 31

An image involves focussing the light onto a point
A spectrum spreads this light out, so we need to stare longer at the galaxies

Question 32

What is the dark energy survey (DES)

Answer 32

Rare and difficult measurements
- At low redshift, you can’t assume matter domination