Department of Physics
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Item The post-reionization neutral hydrogen - a cosmological probe(COSPAR Scientific Assembly, 2012-07) Sarkar, Tapomoy GuhaIn the absence of complex astrophysical processes that characterize the reionization era, bulk of the post-reionization neutral gas is believed to be housed in dense self shielded DLA clouds. The neutral hydrogen from this epoch can be seen either through the 21-cm emission or Lyman-alpha absorption. We present the cross-correlation of the redshifted 21-cm emission from neutral hydrogen (HI) in the post-reionization era with the Ly-alpha forest as a new probe of the large scale matter distribution in the redshift range z=2 to 3 with a significantly lesser problem arising from foreground contamination. Though the 21-cm and the Lyman-alpha forest signals originate from different astrophysical systems, they are both expected to trace the underlying dark matter distribution on large scales. The post-reionization 21-cm signal is studied using numerical N-body simulation to justify its use as a biased tracer. We investigate the imprint of baryon acoustic oscillation (BAO) in the cross-correlation power spectrum and the feasibility of a detection of the BAO scale with future radio interferometric observations and quasar surveys. The multi-frequency angular power spectrum, shall allow the detection of the BAO feature along both radial and transverse directions, opening new avenues for precision cosmology and enhance our understanding of Dark energy.Item Cosmology and Astrophysics using the Post-Reionization HI(Springer, 2016-11) Sarkar, Tapomoy GuhaWe discuss the prospects of using the redshifted 21-cm emission from neutral hydrogen in the post-reionization epoch to study our Universe. The main aim of the article is to highlight the efforts of Indian scientists in this area with the SKA in mind. It turns out that the intensity mapping surveys from SKA can be instrumental in obtaining tighter constraints on the dark energy models. Cross-correlation of the HI intensity maps with the Ly α forest data can also be useful in measuring the BAO scale.Item Probing primordial non-Gaussianity: the 3D Bispectrum of Ly-α forest and the redshifted 21-cm signal from the post reionization epoch(IOP, 2013) Sarkar, Tapomoy GuhaWe explore possibility of using the three dimensional bispectra of the Ly-α forest and the redshifted 21-cm signal from the post-reionization epoch to constrain primordial non-Gaussianity. Both these fields map out the large scale distribution of neutral hydrogen and maybe treated as tracers of the underlying dark matter field. We first present the general formalism for the auto and cross bispectrum of two arbitrary three dimensional biased tracers and then apply it to the specific case. We have modeled the 3D Ly-α transmitted flux field as a continuous tracer sampled along 1D skewers which corresponds to quasars sight lines. For the post reionization 21-cm signal we have used a linear bias model. We use a Fisher matrix analysis to present the first prediction for bounds on fNL and the other bias parameters using the three dimensional 21-cm bispectrum and other cross bispectra. The bounds on fNL depend on the survey volume, and the various observational noises. We have considered a BOSS like Ly-α survey where the average number density of quasars bar n = 10−3Mpc−2 and the spectra are measured at a 2-σ level. For the 21-cm signal we have considered a 4000 hrs observation with a futuristic SKA like radio array. We find that bounds on fNL obtained in our analysis (6 ⩽ ΔfNL ⩽ 65) is competitive with CMBR and galaxy surveys and may prove to be an important alternative approach towards constraining primordial physics using future data sets. Further, we have presented a hierarchy of power of the bispectrum-estimators towards detecting the fNL. Given the quality of the data sets, one may use this method to optimally choose the right estimator and thereby provide better constraints on fNL. We also find that by combining the various cross-bispectrum estimators it is possible to constrain fNL at a level ΔfNL ∼ 4.7. For the equilateral and orthogonal template we obtain ΔfNLequ ∼ 17 and ΔfNLorth ∼ 13 respectively for the combined estimator. This shall be important in the quest towards understanding the mechanism behind the generation of primordial perturbations.Item CMBR weak lensing and HI 21-cm cross-correlation angular power spectrum(IOP, 2010-02) Sarkar, Tapomoy GuhaWeak 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.