CMBR weak lensing and HI 21-cm cross-correlation angular power spectrum

Abstract

Weak gravitational lensing of the CMBR manifests as a secondaryanisotropy in the temperature maps. The effect, quantified through theshear and convergence fields imprint the underlying large scalestructure (LSS), geometry and evolution history of the Universe. It is henceperceived to be an important observational probe ofcosmology. De-lensing the CMBR temperature maps is also crucial fordetecting the gravitational wave generated B-modes. Future observations of redshifted 21-cm radiation from the cosmological neutral hydrogen (HI) distribution hold the potential of probing the LSS over a large redshift range. We have investigated the correlation between post-reionization HIsignal and weak lensing convergence field. Assuming that the HI followsthe dark matter distribution, the cross-correlation angular powerspectrum at a multipole ℓ is found to be proportional to the colddark matter power spectrum evaluated at ℓ/r, where r denotesthe comoving distance to the redshift where the HI is located. Theamplitude of the cross-correlation depends on quantities specific tothe HI distribution, growth of perturbations and also the underlyingcosmological model. In an ideal situation, we found that a statisticallysignificant detection of the cross-correlation signal is possible. If detected, the cross-correlationsignal holds the possibility of a joint estimation of cosmologicalparameters and also may be used to test various CMBR de-lensing estimators.