The 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 can not 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 measurements.

Fundamental limits on constraining primordial non-Gaussianity

“…A similar effect is found in Ref. [22], where Rayleigh scattering is included as a tracer of Thus the 3-point correlation function is the bispectrum, the 4-point correlation function is the trispectrum, etc.…”

Probing cosmic inflation with theLiteBIRDcosmic microwave background polarization survey

“…While the expected Rayleigh scattering signal is small in the frequencies observed by LiteBIRD, the superb sensitivity of LiteBIRD allows for detection of the signal at a statistical significance of 25 σ [435], which is due primarily to the temperature data. The first detection of this signal would not only be a significant achievement in cosmology (because this is a firm prediction of the standard model of cosmology that is yet to be confirmed), but could also improve determination of the cosmological parameters such as Neff and mν [435], as well as primordial non-Gaussianity [436].…”