arXiv subjects

Chandrachud B. V. Dash

Publications and source records attributed to Chandrachud B. V. Dash.

8 recordsLinked to original sources

arXiv2026

Exploring Beyond ΛCDM with the Weak Lensing Power Spectrum and Bispectrum

In this work, we present Fisher matrix forecast of the tomographic weak lensing power spectrum and bispectrum for three physically distinct types of models of beyond-$Λ$CDM: the CPL parametrisation of dynamical dark energy, interacting dark energy (IDE) with a dark sector energy-momentum exchange, and Hu-Sawicki models of $f(R)$ gravity. We find that for all three models, including the bispectrum significantly tightens the Fisher constraints: the bispectrum reduces the marginalised $1σ$ error on the CPL equation of state parameter from $σ(w_0) = 0.2511$ (power spectrum only) to $σ(w_0) = 0.1557$, on the IDE coupling from $σ(α) = 2.6895$ to $σ(α) = 0.2944$, and on the scalaron amplitude from $σ(\ln|f_{R0}|) = 2.236$ to $σ(\ln|f_{R0}|) = 2.237$ after full marginalisation over nuisance parameters e.g., photo-z error $σ_z$ and intrinsic alignment amplitude $\mathcal{A}_{\rm IA}$. We find that $f(R)$ models are the most sensitive to systematics and especially in bispectrum. The results also demonstrates the importance of higher order weak lensing statistics as a practical necessity to maximise the scientific return of Stage IV surveys.

astro-ph.CO↗

arXiv2026

Dish Assembly Precision for HIRAX

The Hydrogen Intensity and Real-time Analysis eXperiment (HIRAX) is a radio interferometer array that is being deployed at the South African Radio Astronomy Observatory (SARAO) Square Kilometer Array (SKA) site in South Africa. Mapping the southern sky, its aim is to observe neutral hydrogen (HI) through intensity mapping (IM) across the redshift range of 0.78-2.55. The observation of HI makes it possible to tomographically probe large cosmological volumes, enabling constraints on, for example, the dark energy equation of state. Systematics are a significant concern in deriving cosmological constraints from HI IM due to the presence of strong foreground signals. Instrumental effects such as dish surface deviations and feed placement errors need to be carefully controlled and monitored to preserve the sensitivity of HIRAX's redundant array configuration. These instrumental systematics cause bright foreground power to leak into the faint cosmological signal. The 6 m parabolic dishes are made from fiberglass with an embedded aluminum mesh that acts as the reflector. The feed is held in place at the prime focus (f/D = 0.21) by four fiberglass legs. This paper presents the dish surface and feed placement precision of the first 28 dishes, derived from photogrammetry metrology measurements taken during the dish fabrication, as well as from measurements in the field. While surface deviations are predominantly well within requirements, feed placement satisfies all accuracy criteria but fails precision limits; however, precision is expected to improve with a larger sample size. These results establish a baseline for modeling primary beam effects, ultimately enabling the characterization and mitigation of systematic errors in the HI IM signal with the full HIRAX array.

astro-ph.IM↗

arXiv2025

A semi-cosmographic approach to study cosmological evolution in phase space

The signature of Baryon Acoustic Oscillation in the clustering of dark-matter tracers allows us to measure $(D_A(z), H(z))$ independently. Treating these as conjugate variables, we are motivated to study cosmological evolution in the phase space of dimensionless variables $x = H_0 D_A/c$ and $p = dx/dz$. The dynamical variables $(x(z),p(z))$ can be integrated for a known set of equation of state parameters for different matter/energy components. However, to avoid any preference for specific dark energy models, we adopt a cosmographic approach. We consider two scenarios where the Luminosity distance is expanded as Padé rational approximants using expansion in terms of $z$ and $(1+z)^{1/2}$ respectively. However, instead of directly using the Padé ratios to fit kinematic quantities with data, we adopt an alternative approach where the evolution of the cold dark matter sector is incorporated in our analysis through a semi-cosmographic equation of state, which is then, used to solve the dynamical problem in the phase space. The semi-cosmographic $(D_A(z), H(z))$, thus obtained, is fitted with BAO data from DESI DR1, cosmic chronometer (CC) data and SNIa data from Pantheon+ respectively. We also consider a futuristic 21-cm intensity mapping experiment for error projections. We further use the semi-cosmographic fitting to reconstruct some diagnostics of background cosmology and compare our results for the two scenarios of Padé expansions.

astro-ph.CO↗

arXiv2024

Probing dark energy using anisotropies in the clustering of post-EoR HI distribution

We propose an anisotropy quantifier of the HI 21-cm signal traditionally used to clock the astrophysics of the reionization era as a post-reionization dark energy diagnostic. We find that the anisotropy probe can be measured at SNR $\sim 10$ in both auto-correlation and in cross-correlation with the Ly-$α$ forest over a wide $z$ and $k-$range. We propose to use the BAO signature on the anisotropy signal to measure $( H(z), D_A(z))$. Subsequently, we put constraints on a dark energy model involving a negative cosmological constant on top of a quintessence scalar field and find that such a model is consistent with futuristic observations.

astro-ph.CO↗

arXiv2023

Post-reionization HI 21-cm signal: A probe of negative cosmological constant

In 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 HI 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 $D_A(z)$ and the Hubble parameter $H(z)$. The projected errors on these are then subsequently employed to forecast the constraints on the model parameters ($Ω_Λ, w_0, w_a$) 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 $H_0$ is viable from BAO distance measurements data derived from galaxy samples. We also find that BAO imprints on the 21cm power spectrum obtained from a futuristic SKA-mid like experiment yield a $1-σ$ error on a negative cosmological constant and the quintessence dark energy EoS parameters to be $Ω_Λ=-1.030^{0.589}_{-1.712}$ and $w_0=-1.023^{0.043}_{-0.060}$, $w_a=-0.141^{0.478}_{-0.409}$ respectively.

astro-ph.CO↗

arXiv2022

Intensity mapping of post-reionization 21-cm signal and its cross-correlations as a probe of $f(R)$ gravity

We propose the intensity mapping of the redshifted HI 21-cm signal from the post-reionization epoch as a cosmological probe of $f(R)$ gravity. We consider the Hu-Sawicki class of $f(R)$ gravity models characterized by a single parameter $f_{,R0}$. The $f(R)$ modification to gravity affects the post-reionization $21$-cm power spectrum through the change in the growth rate of density fluctuations. We find that a radio interferometric observation with a SKA1-Mid like radio telescope in both auto-correlation and cross-correlation with galaxy weak-lensing and Ly-$α$ forest may distinguish $f(R)$ models from $LCDM$ cosmology at a precision which is competitive with other probes of $f(R)$ gravity.

astro-ph.CO↗

arXiv2022

Probing Quintessence using BAO imprint on the cross-correlation of weak lensing and post-reionization HI 21 cm signal

In 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 (BAO) on the cross-correlation of post-reionization 21-cm signal and galaxy weak lensing convergence field to tomographically measure the angular diameter distance $D_A(z)$ 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 $600$hrs radio interferometric observation at four observing frequencies $916 $MHz, $650$ MHz, $520$ MHz and $430 $MHz with a SKA-1-Mid like radio telescope in cross-correlation with a deep weak lensing survey covering half the sky may measure the binned $D_A$ and $H$ at a few percent level of sensitivity. The Monte Carlo analysis for a power law thawing Quintessence model gives the $1-σ$ marginalized bounds on the initial slope $λ_i$,dark energy density parameter $Ω_{ϕ0}$ and the shape of the potential $Γ$ at 8.63%, 10.08% and 9.75% respectively. The constraints improve to 7.66%, 4.39% and 5.86% respectively when a joint analysis with SN and other probes is performed.

astro-ph.CO↗

arXiv2021

Constraining dark energy using the cross correlations of weak lensing with post-reionization probes of neutral hydrogen

We investigate the prospects of detecting the cross correlation of CMBR weak-lensing convergence field with the large scale tracers of the underlying dark matter distribution in the post-reionization epoch. The cross-correlation is then used to make error projections for dark energy equation of state (EoS)for models with a time evolving dark energy. We study the cross-correlation angular power spectrum of the weak-lensing field with the Lyman-$α$ forest and the redshifted HI 21 cm signal from the post reionization epoch. The angular power spectra is expressed as a line of sight average over the tomographic slices. We find that on using multiple $400$ hrs observation with an extended uGMRT like instrument or with a BOSS like survey with quasar (QSO) density of $16 {\rm deg}^{-2}$ the cross-correlation with weak-lensing convergence field covering half the sky can be detected at a very high SNR ($>20$). The cross-correlation of weak-lensing with Lyman-$α$ forest allows the $1-σ$ errors on the dark energy EoS parameters for different parametrizations to be constrained at a level of precision comparable to combined Planck+SNIa+BAO+HST projections. The 21-cm weak-lensing cross-correlation is found to provide strong constraints on the present value of the dark energy EoS parameter at $4\%$ for the 7CPL model. The constraints on $w_a$ is comparable ($\sim 12\%$) for models other than the 7CPL model. We also find that the CPL parametrization may not be the best constrained parametrization for dark energy evolution. The cross-correlation of CMBR weak-lensing with the post-reionization probes of neutral hydrogen thus holds the potential to give us valuable understanding about the nature of evolving dark energy.

astro-ph.CO↗