arXiv subjects

Shashank Shekhar Pandey

Publications and source records attributed to Shashank Shekhar Pandey.

7 recordsLinked to original sources

arXiv2026

Modified Cosmological Expansion and the JWST CMB Optical Depth Tension in Self Interaction Gravity

Recent James Webb Space Telescope (JWST) observations favor an earlier and more efficient reionization history, leading to Thomson-scattering optical depths larger than those inferred from the cosmic microwave background (CMB). We investigate whether this tension can be alleviated within the framework of self-interaction (SI) gravity by modifying the cosmological expansion history while retaining the standard astrophysical description of reionization. The SI gravity parameters are constrained through a Bayesian MCMC analysis of the Union3 Type Ia supernova and DESI DR2 Baryon Acoustic Oscillations (BAO) data. The resulting expansion history is then used to predict the ionization history and Thomson optical depth. We find that the predicted optical depth decreases from $τ_{\rm{CMB}}\simeq0.076$ in $Λ$CDM to $τ_{\rm{CMB}}\simeq0.061$, consistent with the Planck PR4 measurement within $1σ$, thereby substantially reducing the optical-depth tension.

astro-ph.CO↗

arXiv2026

Optical depth to reionization in a Universe with multiple inhomogeneous domains

We study the optical depth to reionization in a cosmological setting that includes backreaction from matter inhomogeneities, using the Buchert averaging formalism. We construct a spacetime model consisting of multiple inhomogeneous domains, hereafter referred to as the backreaction model, characterized by a set of parameters. We first examine how these parameters influence the computation of the optical depth to reionization, $τ_{reion}$. Next, we carry out a Markov Chain Monte Carlo (MCMC) analysis based on the PantheonPlus+SH0ES Type Ia supernova sample to infer the best-fit values of the model parameters, and then use these to evaluate $τ_{reion}$. We obtain $τ_{reion} = 0.0581^{+0.0105}_{-0.0096}$ (68$\%$ confidence limits). This result indicates that, when PantheonPlus+SH0ES data are used to constrain the model parameters, our backreaction model yields a value of $τ_{reion}$ that aligns more closely with observational estimates than the value predicted by the standard cosmological model. We further demonstrate that the backreaction model leads to a modest reduction of the Hubble tension, while avoiding the need for exotic or non-standard physics.

astro-ph.CO↗

arXiv2025

Constraining the Hubble parameter with the 21 cm brightness temperature signal in a universe with inhomogeneities

We consider the 21\,cm brightness temperature as a probe of the Hubble tension in the framework of an inhomogeneous cosmological model. Employing Buchert's averaging formalism to study the effect of inhomogeneities on the background evolution, we consider scaling laws for the backreaction and curvature consistent with structure formation simulations. We calibrate the effective matter density using MCMC analysis using Union 2.1 Supernova Ia data. Our results show that a higher Hubble constant ($\sim73$\,km/s/Mpc) leads to a shallower absorption feature in the brightness temperature versus redshift curve. On the other hand, a lower value ($\sim67$\,km/s/Mpc) produces a remarkable dip in the brightness temperature $T_{21}$. Such a substantial difference is absent in the standard $Λ$CDM model. Our findings indicate that inhomogeneities could significantly affect the 21\,cm signal, and may shed further light on the different measurements of the Hubble constant.

astro-ph.CO↗

arXiv2024

Analyzing the 21-cm signal brightness temperature in the Universe with inhomogeneities

We explore the 21-cm signal in our Universe containing inhomogeneous matter distribution at considerably large scales. Employing Buchert's averaging procedure in the context of a model of spacetime with multiple inhomogeneous domains, we evaluate the effect of our model parameters on the observable 21-cm signal brightness temperature. Our model parameters are constrained through the Markov Chain Monte Carlo method using the Union 2.1 supernova Ia observational data. We find that a significant dip in the brightness temperature compared to the $Λ$CDM prediction could arise as an effect of the inhomogeneities present in the Universe.

astro-ph.CO↗

arXiv2023

Future deceleration due to backreaction in a Universe with multiple inhomogeneous domains

We formulate a model of spacetime with inhomogeneous matter distribution in multiple domains. In the context of the backreaction framework using Buchert's averaging procedure, we evaluate the effect of backreaction due to the inhomogeneities on the late time global evolution of the Universe. Examining the future evolution of this universe, we find that it can transit from the presently accelerating phase to undergo future deceleration. The future deceleration is governed by our model parameters. We constrain the model parameters using observational analysis of the Union 2.1 supernova Ia data employing the Markov Chain Monte Carlo method.

gr-qc↗

arXiv2023

Viscous attenuation of gravitational waves propagating through an inhomogeneous background

We consider the propagation of gravitational waves in the late-time Universe in the presence of matter distribution inhomogeneities, and we also consider the cosmic fluid to be viscous. In this work, we investigate the cumulative effect of inhomogeneities and viscosity of the cosmic-fluid on the observables associated with the sources of the gravitational waves. Employing Buchert's averaging procedure in the backreaction framework, we consider a model of spacetime in which matter is distributed in-homogeneously across space. Using the modified redshift versus distance relation, through the averaging process in the context of the model, we study the variation of the redshift-dependent part of the observed gravitational wave amplitude for different combinations of our model parameters while simultaneously considering damping of the gravitational wave amplitude due to viscosity of the cosmic-fluid. Then, we investigate the differences occurring in the variation of the redshift-dependent part of the observed gravitational wave amplitude due to consideration of viscous attenuation. We show that there are significant deviations after the inclusion of viscous attenuation in our analysis, depending on the chosen value of the coefficient of viscosity. Our result signifies the importance of the effect of viscosity, within the model of an inhomogeneous Universe, on precision measurements of parameters of compact-binary sources of gravitational waves.

gr-qc↗

arXiv2022

Global 21-cm brightness temperature in viscous dark energy models

We investigate the global 21-cm brightness temperature in the context of viscous dark energy (VDE) models. The bulk viscosity of dark energy perturbs the Hubble evolution of the Universe which could cool baryons faster, and hence, alter the 21-cm brightness temperature. An additional amount of entropy is also produced as an outcome of the viscous flow. We study the combined contribution of Hawking radiation from primordial black holes, decay and annihilation of particle dark matter and baryon-dark matter scattering in the backdrop of VDE models towards modification of the 21-cm temperature. We obtain bounds on the VDE model parameters which can account for the observational excess of the EDGES experiment ($-500^{+200}_{-500}$ mK at redshift $14<z<20$) due to the interplay of the above effects. Moreover, our analysis yields modified constraints on the dark matter mass and scattering cross-section compared to the case of the $Λ$CDM model.

astro-ph.CO↗