Lecture 8: Understanding the Power
Spectrum of the Temperature Anisotropy and
Introduction to CMB Polarisation
1
The lecture slides are available at
https://wwwmpa.mpa-garching.mpg.de/~komatsu/
lectures--reviews.html
Part I: Cosmological Parameter
Dependence of the Temperature Power Spectrum
2
Before starting: Let’s recap the Big Picture
In words!
•
How does the power spectrum constrain the baryon density?•
Via the speed of sound, the increased inertia of a photon-baryon fluid, and Silk damping.•
They all depend on R (the baryon-photon energy density ratio), which is proportional to ΩBh2. Note h2! It is not just ΩB.3
q q
q
q
EQ= a
EQH
EQ~ 0.01 (Ω
Mh
2/0.14) Mpc
–1Before starting: Let’s recap the Big Picture
In words!
•
How does the power spectrum constrain the total matter density?•
Via the boost of the amplitude of sound waves and the ISW due to a decaying gravitational after the horizon re-entry.•
They all depend on qEQ (the wavenumber of the matter-radiation equality), which is proportional to ΩMh2. Note h2! It is not just ΩM.q q q
Before starting: Things you haven’t learned yet
•
How does the power spectrum constrain the Hubble constant?•
Via the (comoving) distance to the last scattering surface, rL. Particularly interesting now because of the “Hubble constant tension”.•
How does the power spectrum constrain the dark energy?•
The same: via rL.•
How does the power spectrum constrain the epoch of reionization?•
The temperature power spectrum cannot constrain this.5
How does r L depend on H 0 and dark energy?
•
We must know ΩMh2 in advance. The CMB peak heights tell us ΩMh2, which then enables us to determine Η0 if we assume a flat Universe. Otherwise, we cannot really determine H0 or ΩΛ! (Unless we use gravitational lensing of the CMB.)6
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a 0 r L = ca 0
Z t
0t
Ldt
a(t) = ca 0
Z a
0a
Lda
a a ˙ = ca 0
Z a
0a
Lda
a 2 H (a)
Friedmann’s equation
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H
2(a) = H
02⌦
Ma
3+ ⌦
ka
2+ ⌦
⇤! H
02⌦
Ma
3+ 1 ⌦
MFlat Universe Ωk=0
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/ ⌦
Mh
2a
3+ h
2⌦
Mh
2ΩM + Ωk + ΩΛ = 1
7
The sound horizon, rs, changes when the baryon density changes, resulting in a shift in the peak positions.
Adjusting it makes the physical effect at the last scattering manifest
8
Zero-point shift of the oscillations
9
Zero-point shift effect
compensated by (1+R)–1/4 and Silk damping
10
Less tight coupling:
Enhanced Silk damping for low baryon density
11
Total Matter Density
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Total Matter Density
13
Total Matter Density
First Peak:More ISW and boost due to the decay of Φ
14
Total Matter Density
2nd, 3rd, 4th Peaks:Boosts due to the decay of Φ
Less and less effects at larger multipoles
15
Two Other Effects
• Spatial curvature
•
We have been assuming a spatially-flat Universe with zero curvature (i.e., Euclidean space). What if it is curved?• Optical depth to Thomson
scattering in a low-redshift Universe
•
We have been assuming that the Universe is transparent to photons since the last scattering at z=1090. What if there is an extra scattering in a low-redshift Universe?16
Spatial Curvature
•
It changes theangular diameter distance, d
A, to the last scattering surface; namely,•
rL -> dA = R sin(rL/R) = rL(1–
rL2/6R2) + … for positively- curved space•
rL -> dA = R sinh(rL/R) = rL(1+
rL2/6R2) + … for negatively- curved spaceSmaller angles (larger multipoles) for a negatively curved Universe
17
18
19
late-time ISW
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Optical Depth
•
Extra scattering by electrons in a low-redshift Universe damps temperature anisotropy• C
l-> C
lexp(–2τ)
at l >~ 10•
where τ is the optical depthre-ionisation
21
22
23
•
Since the power spectrum is uniformly suppressed by exp(–2τ) at l>~10, we cannot determine the amplitude of the power spectrum of the gravitational potential, Pφ(q), independently of τ.•
Namely, what we constrain is the combination: exp(–2τ)Pφ(q)Important consequence of the optical depth
•
Breaking this degeneracy requires an independent determination of the optical depth. This requiresPOLARISATION
of the CMB./ exp( 2⌧ )A s
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+CMB Lensing Planck
[100 Myr]
Cosmological Parameters Derived from the Power Spectrum
The Hubble Constant Tension
The role of the CMB
•
Can we explain, in words, how this measurement was obtained?Wong et al. (2020)
•
The CMB peak positions are controlled by cos(qrs).•
We measure q in the angular wavenumber, l ~ qrL.•
Thus, the CMB power spectrum gives a direct measurement of the distance ratio: rs/rL.•
You already know how to obtain a0rs = 145 Mpc (see Lecture 5).•
Today we saw how rL depends on cosmology (in a flat Universe):27
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a
0r
L= ca
0Z
a0aL
da
a
2H (a) /
Z
a0aL
da a
2p
⌦
Mh
2a
3+ h
2⌦
Mh
2CMB -> Distance Ratio -> (Physics) -> H 0
Physical Assumption: the sound horizon r
sCMB -> Distance Ratio -> (Physics) -> H 0
Physical Assumption: the sound horizon r
s•
The CMB peak positions are controlled by cos(qrs).•
We measure q in the angular wavenumber, l ~ qrL.•
Thus, the CMB power spectrum gives a direct measurement of the distance ratio: rs/rL.•
You already know how to obtain a0rs = 145 Mpc (see Lecture 5).•
Today we saw how rL depends on cosmology (in a flat Universe):28
<latexit sha1_base64="tvvcBXmPIDrpOwDLma0bwz/eZtw=">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</latexit>
a
0r
L= ca
0Z
a0aL
da
a
2H (a) /
Z
a0aL
da a
2p
⌦
Mh
2a
3+ h
2⌦
Mh
2If we used an incorrect value of r
s, we would infer r
Lincorrectly, hence
an incorrect value of H
0!
So…?
Bernal, Verde and Riess (2016); Poulin et al. (2019)
•
The CMB-inferred value of H0 is too low, by 10 percent.•
This means that the inferred value of rL is too high, by 10 percent.•
This may mean that the value of rs we calculated using the standard understanding of physics was too high by 10 percent.•
If we managed to reduce the calculated value of rs by 10 percent, we could resolve the Hubble constant tension.•
Is that possible? Not really, but one way to achieve this would be to increase H(a) by 10 percent in the radiation era. => Early Dark Energy?29
Part II: Basics of the CMB Polarisation
30
31
Credit: ESA
32
Credit: ESA
CMB is weakly polarised!
Polarisation
No polarisation
Polarised in x-direction
33
Photo Credit: TALEX
34
horizontally polarised Photo Credit: TALEX
35
Photo Credit: TALEX
36
Generation of polarisation
The necessary and sufficient conditions
•
To generate polarisation, we must satisfy the following two conditions•
Scattering•
Anisotropic incident light•
However, the Universe does not have a preferred direction. How do we generate anisotropic incident light?37
Physics of CMB Polarisation
Necessary and sufficient condition: Scattering and Local Quadrupole Anisotropy
Credit:Wayne Hu 38
(l,m)=(2,0) (l,m)=(2,1)
(l,m)=(2,2)
39
Quadrupole
temperature anisotropy
seen by an electron
Generation of temperature quadrupole
The punch line
•
When the Thomson scattering is efficient (i.e., tight coupling between photons and baryons via electrons), the distribution of photons from the rest frame ofbaryons appears isotropic.
• Only when tight coupling weakens
, a local quadrupoletemperature anisotropy in the rest frame of a photon-baryon fluid can be generated.
•
In fact, “a local temperature anisotropy in the rest frame of a photon-baryon fluid” is equal toviscosity.
40
Part III: Stokes Parameters
41
Stokes Parameters
[Flat Sky, Cartesian coordinates]
a b
42
Stokes Parameters
change under coordinate rotation
x’
Under (x,y) -> (x’,y’): y’
Compact Expression
•
Using an imaginary number, writeThen, under the coordinate rotation we have
44
Alternative Expression
•
With the polarisation amplitude, P, and angle, , defined by• Then, under coordinate rotation we have We write
and P is invariant under rotation.
45
Help!
•
That Q and U depend on coordinates is not very convenient…•
Someone said, “I measured Q!” but then someone else may say, “No, it’s U!”. They flight to death, only to realise that their coordinates are 45degrees rotated from one another…
•
The best way to avoid this unfortunate fight is to define a coordinate-independent quantity for the distribution of polarisation patterns in the sky
46
To achieve this, we need
to go to Fourier space
Appendix: Effects of Neutrinos on the Temperature Power Spectrum
47
The Effects of Relativistic Neutrinos
•
To see the effects of relativistic neutrinos, weartificially increase the number of neutrino species from 3 to 7
•
Great energy density in neutrinos, i.e., greater energy density in radiation•
Longer radiation domination -> More ISW and boosts due to potential decay(1)
48
49
After correcting for more ISW and boosts due to
potential decay
50
(2): Viscosity Effect on the Amplitude of Sound Waves
The solution is
where
Hu & Sugiyama (1996)
Bashinsky & Seljak (2004) Phase shift!
51
After correcting for the viscosity effect on the
amplitude
52
(3): Change in the Silk Damping
•
Greater neutrino energy density implies greater Hubble expansion rate,Η
2=8πG∑ρ
α/3
•
Thisreduces
the sound horizon in proportion to H–1, as rs ~ csH–1•
This also reduces the diffusion length, but in proportional to H–1/2, as a/qsilk ~ (σTneH)–1/2•
As a result,l
silkdecreases relative to the first peak position
, enhancing the Silk dampingConsequence of the random walk!
Bashinsky & Seljak (2004)
53
After correcting for the diffusion length
54
Zoom in!
55
56
(4): Viscosity Effect on the Phase of Sound Waves
The solution is
where
Hu & Sugiyama (1996)
Bashinsky & Seljak (2004) Phase shift!
57
After correcting for the phase shift
Now we understand everything quantitatively!!
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