Cosmology with Rayleigh scattering
Beyond primary CMB with the next generation of surveys
Benjamin Beringue with Daan Meerburg, Nick Battaglia, Joel Meyers
Based on arXiv:2008.xxxxx, to be submitted by the release of this recording.
16/08/2020 1 Cosmology from Home 2020 Cosmology with Rayleigh scattering
Outline :
• Motivations
• Distortions induced by Rayleigh scattering
• Detectability with the next generation of CMB surveys
• Impact on cosmological parameters estimation
16/08/2020 Cosmology from Home 2020 2 Cosmology with Rayleigh scattering
With the next generation of CMB surveys promising to be Cosmic Variance limited up to ℓ~5000, there is a strong need to look beyond the primary CMB signal to further constrain Λ��� and its extensions.
• Spectral distortions
• Secondary anisotropies : CMB lensing, tSZ, kSZ, patchy reionisation
• Rayleigh Scattering !
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Outline :
• Motivations
• Distortions induced by Rayleigh scattering
• Detectability with the next generation of CMB surveys
• Impact on cosmological parameters estimation
16/08/2020 Cosmology from Home 2020 4 Cosmology with Rayleigh scattering
What is Rayleigh scattering ?
The recombination history is usually told as follows :
- The primordial plasma consists in protons and free electrons (mainly), - Photons are kept in equilibrium by Thomson scattering, - As the universe expands, less and less and free electrons, - Finally, one last scattering event.
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What is Rayleigh scattering ?
However, photons may also scatter off neutral species formed during recombination :
- Rayleigh scattering by Hydrogen and Helium atoms, - Strong frequency dependence : � � ∝ � � (to lowest order) - As the universe expands, neutral species are diluted ∝ � and photons redshifted ∝ � ,
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Redshift z 1693 1269 1088 804 666
Thomson scattering 1.2 Rayleigh Scattering 750Ghz Rayleigh Scattering 500Ghz 1.0
0.8
0.6 Visibility
0.4
Fraction of free electrons 0.2 Fraction of neutral hydrogen Last scattering surface 0.0 2500 2000 1500 1000 500 Redshift 200.0 250.0 280.7 350.0 400.0 Conformal time ¥ [Mpc]
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Rayleigh scattering increases the comoving opacity of the plasma and makes it frequency dependent :
�̇ = ��!�" → �̇ � = ��!�" + � �# + 0.1 �#! �$(�) This leads to :
- A damping of small scales anisotropies both in temperature and E-mode polarization. - On large angular scales, the shift of the last scattering surface towards lower redshifts, where the local quadrupole is larger, boosts the large scales E-mode signal. - A shift in the location of the acoustic peaks, both in temperature and E-mode polarization spectra.
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Rayleigh scattering effects at frequencies below ∼ 700 GHz can be modelled as a linear distortion to the primary CMB :
, , �ℓ � = �ℓ + Δ�ℓ + +� , with � = �, �, �
[Lewis 2013 :1307.8148] The power spectrum reads :
, 1 , , � � , , �ℓ � , � = �ℓ + � �ℓ + � �ℓ + �ℓ + … � �
Primary CMB �� cross spectra �� auto spectra
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16/08/2020 Cosmology from Home 2020 10 Cosmology with Rayleigh scattering
What is Rayleigh scattering ?
- Scattering of CMB photons by neutral species right after recombination,
- Increases the comoving opacity in a frequency dependent way,
- Distortions can be modelled linearly at the multipole level,
- Produces a signal that is small (~3% at 500GHz) and highly correlated to the primary CMB.
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Outline :
• Motivations
• Distortions induced by Rayleigh scattering
• Detectability with the next generation of CMB surveys
• Impact on cosmological parameters estimation
16/08/2020 Cosmology from Home 2020 12 Cosmology with Rayleigh scattering
PLANCK only
Most of the constraining power comes from temperature large scales ℓ < 1000.
Almost no contribution from the polarized Rayleigh scattering signal.
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CMB-S4 white noise only
Most of the constraining power still comes from temperature large scales ℓ < 2000. Which will be impacted by atmospheric noise.
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CMB-S4
Since temperature channels are the most impacted by atmospheric noise, most of the constraining power comes from polarized channels.
Improvements in detector noise in the polarized channels will directly result in improvements in constraints on Rayleigh scattering.
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Outline :
• Motivations
• Distortions induced by Rayleigh scattering
• Detectability with the next generation of CMB surveys
• Impact on cosmological parameters estimation
16/08/2020 Cosmology from Home 2020 16 Cosmology with Rayleigh scattering
- Rayleigh scattering amplitude directly probes Helium fraction : ���
� - Rayleigh scattering produces different last
�!(�) scattering surface and fluctuation spectrum at each frequency.
- Fixed length scales appear at different angular scales for primary and Rayleigh scattered components of the CMB.
- Ratio of these angular scales helps constraining parameters.
� - ��, ��� , ���� are improved.
Courtesy of Joel Meyers. 16/08/2020 Cosmology from Home 2020 17 Cosmology with Rayleigh scattering
PICO-like experiment [arXiv:1902.10541]
ℓ"#$ = 5000, �%& fixed
More details in arXiv:2008.xxxxx 16/08/2020 Cosmology from Home 2020 18 Cosmology with Rayleigh scattering
PICO-like experiment [arXiv:1902.10541]
ℓ"#$ = 5000, �%& fixed
More details in arXiv:2008.xxxxx
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0.120 2 h c
≠ 0.119 PICO (unlensed) 67.860 PICO+BAO (unlensed) 0 H 66.860 PICO+Rayleigh (unlensed) CVL (unlensed)
s 2.109 A 9
10 2.089
0.967 s n 0.963
0.056 ø 0.052
∫ 0.092 m
P 0.027
0.02235 0.02240 0.11950 0.12050 66.86000 67.86000 2.08900 2.10900 0.96290 0.96690 0.05240 0.05640 0.02750 0.09250 2 2 9 ≠bh ≠ch H0 10 As ns ø m∫
16/08/2020 Cosmology from Home 2020 P 20 Cosmology with Rayleigh scattering
PICO-like experiment [arXiv:1902.10541]
ℓ"#$ = 5000, �%& fixed
More details in arXiv:2008.xxxxx
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4.0e-02
3.5e-02 13% A modest 13% improvements on � , would require 50 times more effort without Rayleigh scattering.
2 3 10° 3.0e-02£ ) e Æ N
( 2.6e-02 æ This holds for a 5’ beam experiment (space-based), 51.22 using lensed spectra up to ℓ = 5000 and not 2.3e-02 £ including �ℓ .
2.0e-02 PICO + rayleigh PICO
404.1 101.0 51.2 2.0 1.0 0.4 0.3 EÆort relative to PICO (# detectors or integration time)
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In all this work we have neglected foregrounds contamination. Several aspects of Rayleigh scattering signal will play in our favour when looking for it :
- It has a strong and unique scaling with frequency which make it suitable for blind methods such as Internal Linear Combination (ILC).
- Its correlation with the primary CMB means would require to be dealt with by using a constrained-ILC.
- Rayleigh scattering signal is well understood and modelled which make it usable in parametric component separation codes.
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Conclusion
• Rayleigh Scattering is a weak yet robustly predicted signal in the sky .
• Its first detection could be achieved with the next generation of CMB surveys.
• Future space missions will use this signal to further constrain Λ��� and its extension
• Component separation will play a crucial role in a first detection.
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Thank you very much for your attention !
(Have a look on the arxiv for our paper on Rayleigh scattering !)
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