Cosmology with Rayleigh scattering Beyond primary CMB with the next generation of surveys Benjamin Beringue with Daan Meerburg, Nick Battaglia, Joel Meyers 11/08/2020 1 CMB-S4 2020 Cosmology with Rayleigh scattering Outline : • Distortions induced by Rayleigh scattering • Detectability with the next generation of CMB surveys • Impact on cosmological parameters • Effort in component separation 11/08/2020 CMB-S4 2020 2 Cosmology with Rayleigh scattering Around recombination, photons are kept in equilibrium with the plasma by two scattering processes : Thomson scattering: Rayleigh scattering: - Off free electrons - Off neutral species (Hydrogen, Helium, …) - Frequency independent - Frequency dependent � - Cross section �! - Cross section �� ∝ � � � e- e- p+ 11/08/2020 CMB-S4 2020 3 Cosmology with Rayleigh Scattering Redshift z 1693 1269 1088 804 666 Thomson scattering Rayleigh Scattering 750Ghz For Rayleigh scattering to Rayleigh Scattering 500Ghz become effective, enough neutral species have to be present. After recombination, neutral Visibility species will dilute ∝ �!" and photons will redshift ∝ �!#. 200.0 250.0 280.7 350.0 400.0 Conformal time ¥ [Mpc] [BB et al. 2020 : 2008.xxxx] 11/08/2020 CMB-S4 2020 4 Cosmology with Rayleigh scattering 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. 12/08/2020 CMB-S4 2020 5 Cosmology with Rayleigh scattering 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 , &1.,, , 1&,,. �#�/ 1&,,1.,, �ℓ �#, �/ = �ℓ + �# �ℓ + �/ �ℓ + / �ℓ + … �0 �0 Primary CMB �� cross spectra �� auto spectra 11/08/2020 CMB-S4 2020 6 Cosmology with Rayleigh scattering Combining PLANCK and CMB-S4 allows to benefit from both the large scales temperature mode obtained from space and the clean polarized channels from CMB- S4. Improvements in detector noise in the polarized channels will directly result in improvements in constraints on Rayleigh scattering. 13/08/2020 CMB-S4 2020 7 [BB et al. 2020 : 2008.xxxx] Cosmology with Rayleigh scattering For a more futuristic experiment like PICO, Rayleigh scattering allows us to put tighter constraints on parameters : - Helium fraction �23 is directly probed by the amplitude of Rayleigh scattering signal. - Fixed length scales at the surface of last scattering will project to different angular scales depending on the frequencies at which they are observed : - The size of the sound horizon that probes densities : Ω4, Ω5 - The scale that entered the horizon at matter-radiation equality probes : �366 - Rayleigh scattering allows to break degeneracies between parameters and measure ∑�� at �� without the need of external dataset (BAO). [BB et al. 2020 : 2008.xxxx] 14/08/2020 CMB-S4 2020 8 Cosmology with Rayleigh Scattering 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. 11/08/2020 CMB-S4 2020 9 Cosmology with Rayleigh Scattering sps4lat is a code developed for the SO collaboration (with the help of Davide Poletti and Colin Hill) to implement a semi-parametric component separation method. The idea is to model the frequency covariance matrix of the observed sky and fit it to the empirical one. - What is called a model may vary from an explicit modelling of every emission to a totally free one. - Similar to the SMICA pipeline developed for Planck and tailored to the specificities of ground-based observations. - Code is currently under development on github : sps4lat 13/08/2020 CMB-S4 2020 10 Cosmology with Rayleigh scattering 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 could use this signal to further constrain Λ��� • Component separation will play a crucial role in a first detection. 11/08/2020 CMB-S4 2020 11 Cosmology with Rayleigh scattering Thank you very much for your attention ! Feel free to contact me : @ : [email protected] beringueb on (Keep an eye on the arXiv in the next few days !) 11/08/2020 CMB-S4 2020 12.