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

Abstract

Weak gravitational lensing of the CMBR manifests as a secondary anisotropy in the temperature maps. The effect, quantified through the shear and convergence fields imprint the underlying large scale structure (LSS), geometry and evolution history of the Universe. It is hence perceived to be an important observational probe of cosmology. De-lensing the CMBR temperature maps is also crucial for detecting 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 HI signal and weak lensing convergence field. Assuming that the HI follows the dark matter distribution, the cross-correlation angular power spectrum at a multipole ℓ is found to be proportional to the cold dark matter power spectrum evaluated at ℓ/r, where r denotes the comoving distance to the redshift where the HI is located. The amplitude of the cross-correlation depends on quantities specific to the HI distribution, growth of perturbations and also the underlying cosmological model. In an ideal situation, we found that a statistically significant detection of the cross-correlation signal is possible. If detected, the cross-correlation signal holds the possibility of a joint estimation of cosmological parameters and also may be used to test various CMBR de-lensing estimators.