2. CMBR Flashcards
Describe the 3 main things that happen just after the BB
- 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)
What is the photon to baryon ratio?
Total number of photons relative to the total number of protons
Describe what is meant by the photons being entrained just after the BB
- They cannot travel any great distance before they scatter off of another proton
- Mean free path is very small
Describe what happens at the recombination stage
- 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
What is the surface of last scattering?
Recombination photons are eventually allowed to travel
- The light from this surface is described as the afterglow of the BB
What is assumed about the ratios of protons, electrons and hydrogen atoms before the formation of the CMB?
The ratios obeyed a form of equilibrium
Describe the saha equation
It tracks the relative number of ions in a given state as a function of temperature
Describe the consequences of the Saha equation expressed in terms of the ionisation parameter
Recombination is depedent on the temperature of the plasma
- Recombination isn’t instantaneous, but takes place in a narrow redshift/scale factor range
What value of the ionisation paramater does recombination begin at?
X_e = 0.1
What happens as CMB photons travel to the Universe?
The Universe expands
What is the temperature of the CMB, and at what redshift is it observed?
Temp is 2.7K and observed at z=0
How do we physically observe the CMB?
Need to be above the atmosphere and also avoid foreground emission
Describe the wavelength distribution of the CMB and explain how we deal with the foreground radiation
- BB shape peaking at 3mm
- Measure it at different frequencies and wavelengths, then model and subtract foreground
What is the biggest source of the foreground emission which hampers our observation?
The dust in the MW at 20-30K
What is the temp variation of the CMB?
1 in 10^5
(2.7 +- 0.000027)K
\What are the dynamical effects for measuring the CMB?
Earth’s orbit introduces a doppler shift
- Dipole shift as we are slightly moving towards and away from some CMB photons
How are the anisotropies measured for the CMB?
On a power spectrum
Describe what the different values of l, the multipole correspond to
Small l measures large scales on the sky
Large l measures small regions of the sky
When plotting the power spectrum of the anisotropies of the CMB, what do we notice?
That the CMB has more power on some scales than others
- It peaks for a value of l=250
Briefly describe the Sachs-Wolfe effect
Regions with more matter density appear cooler than their surroundings
- Regions with matter overdensities and redshifted relative to the CMB
Explain the two mechanisms that contribute to the Sachs Wolfe effect
- Gravitational redshift
- A CMB photon loses energy if it climbs out of a gravitational well - Time dilation
- Dense regions cool more slowly and recombine later as time moves more slowly in a gravitational potential
Describe the proportionality between the scale factor and time
a is prop. to t^2/3
Explain what baryon acoustic oscillations are
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
What is the necessary condition for BAOs to occur?
The dynamical timescales must be larger than the pressure timescales
- This means the pressure responds to the dynamical changes
Which regions oscillate more slowly, large or small densities?
Large regions oscillate more slowly
How many times will the largest regions have collapsed between the big bang and recombination?
Once i.e. just compressed at the time of CMB formation
Where do BAOs spend the most time?
Spend the longest time are their extremes (compressions and rarefractions)
Describe the plot of the power of the CMB vs l
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
How far into the future do BAOs continue, and what is a consequence of this?
They continue up until z=0
- Some galaxy scale clusterings are more common than others
When observing BAOs, what two things must we have?
- Need to observe a very large area of sky
- Need good redshift information which needs a spectrum from each galaxy (expensive)
Why is taking a spectrum of a galaxy more expensive than an image?
An image involves focussing the light onto a point
A spectrum spreads this light out, so we need to stare longer at the galaxies
What is the dark energy survey (DES)
Rare and difficult measurements
- At low redshift, you can’t assume matter domination
