Department of Physics
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Item Post-reionization H i 21-cm signal: a probe of negative cosmological constant(OUP, 2023-12) Sarkar, Tapomoy GuhaIn this study, we investigate a cosmological model involving a negative cosmological constant (AdS vacua in the dark energy sector). We consider a quintessence field on top of a negative cosmological constant and study its impact on cosmological evolution and structure formation. We use the power spectrum of the redshifted H i 21-cm brightness temperature maps from the post-reionization epoch as a cosmological probe. The signature of baryon acoustic oscillations (BAO) on the multipoles of the power spectrum is used to extract measurements of the angular diameter distance DA(z) and the Hubble parameter H(z). The projected errors on these are then subsequently employed to forecast the constraints on the model parameters () using Markov chain Monte Carlo techniques. We find that a negative cosmological constant with a phantom dark energy equation of state (EoS) and a higher value of H0 is viable from BAO distance measurements data derived from galaxy samples. We also find that BAO imprints on the 21-cm power spectrum obtained from a futuristic SKA-mid like experiment yield a 1σ error on a negative cosmological constant and the quintessence dark energy EoS parameters to be and , respectively.Item Probing Quintessence using BAO imprint on the cross-correlation of weak lensing and post-reionization H i 21 cm signal(OUP, 2022-11) Sarkar, Tapomoy GuhaIn this work, we investigate the possibility of constraining a thawing Quintessence scalar field model for dark energy. We propose using the imprint of baryon acoustic oscillation on the cross-correlation of post-reionization 21-cm signal and galaxy weak lensing convergence field to tomographically measure the angular diameter distance DA(z) and the Hubble parameter H(z). The projected errors in these quantities are then used to constrain the Quintessence model parameters. We find that independent 600 h radio interferometric observation at four observing frequencies 916, 650, 520, and 430 MHz with an SKA-1-Mid like radio telescope in cross-correlation with a deep weak lensing survey covering half the sky may measure the binned DA and H at a few per cent level of sensitivity. The Monte Carlo analysis for a power-law thawing Quientessence model gives the 1 − σ marginalized bounds on the initial slope λi, dark energy density parameter Ωϕ0 and the shape of the potential Γ at 8.63, 10.08, and , respectively. The constraints improve to 7.66, 4.39, and , respectively, when a joint analysis with supernovae and other probes is performed.Item Constraining neutrino mass using the large-scale H i distribution in the post-reionization epoch(OUP, 2016-07) Sarkar, Tapomoy GuhaThe neutral intergalactic medium in the post-reionization epoch allows us to study cosmological structure formation through the observation of the redshifted 21 cm signal and the Lyman α forest. We investigate the possibility of measuring the total neutrino mass through the suppression of power in the matter power spectrum. We investigate the possibility of measuring the neutrino mass through its imprint on the cross-correlation power spectrum of the 21 cm signal and the Lyman α forest. We consider a radio-interferometric measurement of the 21 cm signal with a SKA1-mid-like radio telescope and a BOSS-like Lyman α forest survey. A Fisher matrix analysis shows that at the fiducial redshift z = 2.5, a 10 000 h 21 cm observation distributed equally over 25 radio pointings and a Lyman α forest survey with 30 quasars lines of sights in 1deg2, allows us to measure Ων at a 3.25 per cent level. A total of 25 000 h radio-interferometric observation distributed equally over 25 radio pointings and a Lyman α survey with will allow Ων to be measured at a 2.26 per cent level. This corresponds to an idealized measurement of ∑mν at the precision of (100 ± 2.26)meV and fν = Ων/Ωm at 2.49 per cent level.Item Prospects of probing quintessence with H i 21-cm intensity mapping survey(OUP, 2016-12) Sarkar, Tapomoy GuhaWe investigate the prospect of constraining scalar field dark energy models using H i 21-cm intensity mapping surveys. We consider a wide class of coupled scalar field dark energy models whose predictions about the background cosmological evolution are different from the Λ cold dark matter (ΛCDM) predictions by a few per cent. We find that these models can be statistically distinguished from ΛCDM through their imprint on the 21-cm angular power spectrum. At the fiducial z = 1.5, corresponding to a radio interferometric observation of the post-reionization H i 21-cm observation at frequency 568 MHz, these models can in fact be distinguished from the ΛCDM model at signal-to-noise ratio >3σ level using a 10 000 h radio observation distributed over 40 pointings of a SKA1-mid-like radio telescope. We also show that tracker models are more likely to be ruled out in comparison with ΛCDM than the thawer models. Future radio observations can be instrumental in obtaining tighter constraints on the parameter space of dark energy models and supplement the bounds obtained from background studiesItem Cross-correlation of the H i 21-cm signal and Lyα forest: a probe of cosmology(OUP, 2011-01) Sarkar, Tapomoy GuhaSeparating the cosmological redshifted 21-cm signal from foregrounds is a major challenge. We present the cross-correlation of the redshifted 21-cm emission from neutral hydrogen (H i) in the post-reionization era with the Lyα forest as a new probe of the large-scale matter distribution in the redshift range z= 2–3 without the problem of foreground contamination. Though the 21-cm and the Lyα forest signals originate from different astrophysical systems, they are both expected to trace the underlying dark matter distribution on large scales. The multifrequency angular cross-correlation power spectrum estimator is found to be unaffected by the discrete quasar sampling, which only affects the noise in the estimate.Item Constraining large‐scale H i bias using redshifted 21‐cm signal from the post‐reionization epoch(OUP, 2012-04) Sarkar, Tapomoy GuhaIn the absence of complex astrophysical processes that characterize the reionization era, the 21‐cm emission from neutral hydrogen (H i) in the post‐reionization epoch is believed to be an excellent tracer of the underlying dark matter distribution. Assuming a background cosmology, it is modelled through (i) a bias function b(k, z), which relates H i to the dark matter distribution and (ii) a mean neutral fraction (forumla) which sets its amplitude. In this paper, we investigate the nature of large‐scale H i bias. The post‐reionization H i is modelled using gravity only N‐body simulations and a suitable prescription for assigning gas to the dark matter haloes. Using the simulated bias as the fiducial model for H i distribution at z≤ 4, we have generated a hypothetical data set for the 21‐cm angular power spectrum (Cℓ) using a noise model based on parameters of an extended version of the Giant Metrewave Radio Telescope (GMRT). The binned Cℓ is assumed to be measured with S/N ≳ 4 in the range 400 ≤ℓ≤ 8000 at a fiducial redshift z= 2.5. We explore the possibility of constraining b(k) using the principal component analysis on these simulated data. Our analysis shows that in the range 0.2 < k < 2 Mpc−1, the simulated data set cannot distinguish between models exhibiting different k‐dependences, provided 1 ≲b(k) ≲ 2 which sets the 2σ limits. This justifies the use of linear bias model on large scales. The largely uncertain forumla is treated as a free parameter resulting in degradation of the bias reconstruction. The given simulated data are found to constrain the fiducial forumla with an accuracy of ∼4 per cent (2σ error). The method outlined here could be successfully implemented on future observational data sets to constrain b(k, z) and forumla and thereby enhance our understanding of the low‐redshift Universe.