Primordial information content of Rayleigh anisotropies

Abstract

Anisotropies in the cosmic microwave background (CMB) are primarily generated by Thomson scattering of photons by free electrons. Around the time of recombination, the Thomson scattering probability quickly diminishes as the free electrons combine with protons to form neutral hydrogen. This production of neutral hydrogen enables a new type of scattering to occur: Rayleigh scattering of photons by hydrogen atoms. Unlike Thomson scattering, Rayleigh scattering is frequency dependent resulting in the generation of anisotropies with a different spectral dependence. Unfortunately the Rayleigh scattering efficiency rapidly decreases with the expansion of the neutral universe, with the result that only a small percentage of photons are scattered off of neutral hydrogen after recombination. Although the effect is very small, future CMB missions with higher sensitivity and improved frequency coverage are poised to measure the effect of Rayleigh scattering. The uncorrelated component of the Rayleigh anisotropies contains unique information on the primordial perturbations that could potentially be leveraged to expand our knowledge of the early universe. In this paper we explore whether measurements of Rayleigh scattering anisotropies can be used to constrain primordial non-Gaussianity and examine the hints of anomalies found by WMAP and Planck satellites. We show that the additional Rayleigh information has the potential to improve primordial non-Gaussianity constraints over pure Thompson constraints by 30%, or more. Primordial bispectra that are not of the local type benefit the most from these additional scatterings, which we attribute to the different scale dependence of the Rayleigh anisotropies. Unfortunately this different scaling means that Rayleigh measurements cannot be used to constrain anomalies or features on large scales. On the other hand, anomalies that may persist to smaller scales, such as the potential power asymmetry seen in WMAP and Planck, could be improved by the addition of Rayleigh scattering measurements.

Cosmic microwave background spectral distortions from Rayleigh scattering at second order

“…Towards future precise CMB intensity spectral measurements [5][6][7][8][9], we considered a new spectral distortion from Rayleigh scattering during recombination. There are a few works on the Rayleigh anisotropies [28][29][30][31][32], but all of them are about the linear anisotropies, and thus distortions to the angular averaged spectrum have not been considered. In this work, we expanded the photon Boltzmann equation to second-order cosmological perturbations in the presence of Thomson scattering and Rayleigh scattering, and we show that the second-order effect introduces a new monopole spectral distortion.…”

“…Recently, the Rayleigh anisotropies are also considered for constraining primordial non-Gaussianity and neutrino mass in Refs. [31,32]. Thus, previous works on Rayleigh scattering are about the linear anisotropies.…”

CCAT-prime Collaboration: Science Goals and Forecasts with Prime-Cam on the Fred Young Submillimeter Telescope

“…Similarly, the constraint on the number of relativistic species N eff would be improved by ∼10% (Beringue et al 2021). The fact that the Rayleigh signal is effectively a second last-scattering surface could potentially also benefit the search for primordial non-Gaussianities (Coulton et al 2021), which could lead to an improvement greater than a factor of 2 compared to an analysis using only primary CMB modes.…”

Fundamental limits on constraining primordial non-Gaussianity

“…A similar effect is found in ref. [22], where Rayleigh scattering is included as a tracer of JCAP04(2021)050 primordial fluctuations. Adding data to the primary CMB temperature modes only improves the max scaling of shapes for which the scaling is not already optimal (e.g.…”