Sensitivity forecasts for the cosmological recombination radiation in the presence of foregrounds

ABSTRACT The cosmological recombination radiation (CRR) is one of the inevitable Lambda cold dark matter spectral distortions of the cosmic microwave background (CMB). While it shows a rich spectral structure across dm-mm wavelengths, it is also one of the smallest signals to target. Here, we carry out a detailed forecast for the expected sensitivity levels required to not only detect but also extract cosmological information from the CRR in the presence of foregrounds. We use CosmoSpec to compute the CRR including all important radiative transfer effects and modifications to the recombination dynamics. We confirm that detections of the overall CRR signal are possible with spectrometer concepts like SuperPIXIE. However, for a real exploitation of the cosmological information, an ≃ 50 times more sensitive spectrometer is required. While extremely futuristic, this could provide independent constraints on the primordial helium abundance, Yp, and probe the presence of extra relativistic degrees of freedom during BBN and recombination. Significantly improving the constraints on other cosmological parameters requires even higher sensitivity (another factor of ≃5) when considering a combination of a CMB spectrometer with existing CMB data. To a large part, this is due to astrophysical foregrounds which interestingly do not degrade the constraints on Yp and Neff as much. A future CMB spectrometer could thus open a novel way of probing non-standard BBN scenarios, dark radiation and sterile neutrinos. In addition, inflation physics could be indirectly probed using the CRR in combination with existing and forthcoming CMB anisotropy data.

Updated fundamental constant constraints from Planck 2018 data and possible relations to the Hubble tension

ABSTRACT We present updated constraints on the variation of the fine structure constant, αEM, and effective electron rest mass, me, during the cosmological recombination era. These two fundamental constants directly affect the ionization history at redshift z ≃ 1100 and, thus, modify the temperature and polarization anisotropies of the cosmic microwave background (CMB) measured precisely with Planck . The constraints on αEM tighten slightly due to improved Planck 2018 polarization data but otherwise remain similar to previous CMB analysis. However, a comparison with the 2015 constraints reveals a mildly discordant behaviour for me, which from CMB data alone is found below its local value. Adding baryon acoustic oscillation data brings me back to the fiducial value, $m_{\rm e}=(1.0078\pm 0.0067)\, m_{\rm e,0}$, and also drives the Hubble parameter to H0 = 69.1 ± 1.2(in units of ${\rm km \, s^{-1} \, Mpc^{-1} }$). Further adding supernova data yields $m_{\rm e}=(1.0190\pm 0.0055)\, m_{\rm e,0}$ with H0 = 71.24 ± 0.96. We perform several comparative analyses using the latest cosmological recombination calculations to further understand the various effects. Our results indicate that a single-parameter extension allowing a slightly increased value of me (≃3.5σ above me, 0) could play a role in the Hubble tension.