arXiv subjects

Kanhaiya Lal Pandey

Publications and source records attributed to Kanhaiya Lal Pandey.

8 recordsLinked to original sources

arXiv2026

Partially Thermalized eV-Scale Sterile Neutrinos in Dynamical Dark Energy and Non-Flat Cosmologies

We investigate cosmological constraints on a $(3+1)$ neutrino framework containing an additional partially thermalized, eV-scale sterile neutrino alongside the three active species. Rather than assuming full thermalization, we allow the sterile abundance to vary independently through its physical mass $m_s$ and contribution $ΔN_{\rm eff}$ to the effective number of relativistic species. The sterile state is modeled as a thermally distributed non-cold relic with a temperature below that of the active-neutrino background. We consider flat and non-flat $Λ$CDM and $w_0w_a$CDM cosmologies, all including the same $(3+1)ν$ sector. The models are constrained using Planck 2018 CMB data, DESI DR2 BAO, Pantheon+ Type Ia supernovae, a conservative compilation of $fσ_8$ measurements, and the recent H0DN. Oscillation-informed priors are imposed on the active-neutrino mass splittings, while the eV scale sterile-mass range is motivated by short-baseline oscillation searches and global $(3+1)$ analyses. We find that allowing additional freedom in the late-time cosmological background substantially changes the inferred sterile-neutrino sector. The effective sterile mass increases from $m_{s,\rm eff}\sim0.11\pm0.05~{\rm eV}$ in flat $Λ$CDM$+(3+1)ν$ to $\sim0.35\pm0.12~{\rm eV}$ in the non-flat $w_0w_a$CDM$+(3+1)ν$ model. The corresponding sterile abundance shifts from $ΔN_{\rm eff}\lesssim0.1$ to $ΔN_{\rm eff}\simeq0.35\pm0.15$, while remaining well below the fully thermalized limit, $ΔN_{\rm eff}\simeq1$. The non-flat $w_0w_a$CDM models also show $\sim 2.8σ$ preference for open spatial geometry, $Ω_k\simeq0.0045\pm0.0016$, and yield $H_0\simeq70.9\pm0.7~{\rm km\,s^{-1}\,Mpc^{-1}}$ and $S_8\simeq0.785\pm0.015$. These models improve the minimum $χ^2$ by approximately $21$-$25$ relative to flat $Λ$CDM$+(3+1)ν$.

physics.gen-ph↗

arXiv2024

Does the Amati Correlation Exhibit Redshift-Driven Heterogeneity in Long GRBs?

Long gamma-ray bursts (GRBs) offer significant insights into cosmology due to their high energy emissions and the potential to probe the early universe. The Amati relation, which links the intrinsic peak energy to the isotropic energy, is crucial for understanding their cosmological applications. This study investigates the redshift-driven heterogeneity of the Amati correlation in long GRBs. Analyzing 221 long GRBs with redshifts from 0.034 to 8.2 we divided the dataset based on redshift thresholds of 1.5 and 2. Using Bayesian marginalization and Reichart's likelihood approach, we found significant differences in the Amati parameters between low and high redshift subgroups. These variations, differing by approximately $2σ$ at $z = 1.5$ and more than $1σ$ at $z = 2$, suggest an evolution in the GRB population with redshift, possibly reflecting changes in host galaxy properties. However, selection effects and instrumental biases may also contribute. Our results challenge the assumption of the Amati relation's universality and underscore the need for larger datasets and more precise measurements from upcoming missions like Transient High-Energy Sky and Early Universe Surveyor (THESEUS) and enhanced X-ray Timing and Polarimetry mission (eXTP) to refine our understanding of GRB physics.

astro-ph.CO↗

arXiv2022

Investigating the Evolution of Amati Parameters with Redshift

Gamma Ray Bursts (GRB) are among the brightest objects in the Universe and hence can be observed up to a very high redshift. Properly calibrated empirical correlations between intensity and spectral correlations of GRBs can be used to estimate the cosmological parameters. However, the possibility of the evolution of GRBs with the redshift is a long-standing puzzle. In this work, we used 162 long-duration GRBs to determine whether GRBs below and above a certain redshift have different properties. The GRBs are split into two groups, and we fit the Amati relation for each group separately. Our findings demonstrate that estimations of the Amati parameters for the two groups are substantially dissimilar. We perform simulations to investigate whether the selection effects could cause the difference. Our analysis shows that the differences may be intrinsic, and the selection effects are not their true origin.

astro-ph.CO↗

arXiv2022

Implications of an extended dark energy model with massive neutrinos

Recently there has been reports of finding a lower bound on the neutrino mass parameter ($Σm_ν$) when using ACT and SPTpol data however these bounds on the $Σm_ν$ are still weaker for most case around at 1 $σ$ level. In this context, here in this work, we study the consequences of using an enlarged four parameter dynamical dark energy equation of state on neutrino mass parameter as well as on the Hubble and S8 tensions. The four parameter dark energy equation of state incorporates a generic non-linear monotonic evolution of the dark energy equation of state, where the four parameters are the early and the present value of the equation of state, the transition scale factor and the sharpness of the transition. We report that with lensing-marginalized Planck + BAO + Pantheon and prior on absolute magnitude $M_B$ and KIDS/Viking $S_8$ prior, the model favours a non-zero value for the neutrino mass parameter at the most at $\sim 1 σ$ level ($Σm_ν= 0.1847_{-0.165}^{+0.0698}$ eV). In this case this model also brings down the Hubble tension to $\sim 2.5 σ$ level and the S8 tension to $\sim$ 1.5 $σ$ level. This model also provide a tighter constraints on the value of the dark energy equation of state at present epoch $w_0$ ($w_0 = -0.9901_{-0.0766}^{+0.0561}$) in comparison to the CPL like parameterization.

astro-ph.CO↗

arXiv2022

Formation and Abundance of Late Forming Primordial Black Holes as Dark Matter

We propose a novel mechanism where Primordial Black Hole (PBH) dark matter is formed much later in the history of the universe between the epoch of Big Bang Nucleosynthesis (BBN) and Cosmic Microwave Background (CMB) photon decoupling. In our setup, one does not need to modify the scale-invariant inflationary power spectra; instead, a late phase transition in strongly interacting fermion-scalar fluid (which naturally occurs around red-shift $ 10^6 \leq \, z_{\scriptscriptstyle T} \, \leq 10^8$ ) creates an instability in the density perturbation as sound speed turns imaginary. As a result, the dark matter perturbation grows exponentially in sub-Compton scales. This follows the immediate formation of early dense dark matter halo, which finally evolves into PBH due to cooling through scalar radiation. We calculate the variance of the density perturbations and PBH fractional abundances $f(M)$ by using a non-monochromatic mass function. We find the peak of our PBH mass function lies between $10^{-16} - 10^{-14}$ solar mass for $ z_{\scriptscriptstyle T} \simeq 10^6$, and thus it can be the entire dark matter of the universe. In PBH formation, one would expect a temporary phase where an attractive scalar balances the Fermi pressure. We numerically confirm that such a state indeed exists, and we find the radius and density profile of the temporary static structure of the dark matter halo, which finally evolves to PBH due to cooling through scalar radiation.

astro-ph.CO↗

arXiv2021

Search for the stochastic gravitational-wave background induced by primordial curvature perturbations in LIGO's second observing run

Primordial density perturbations in the radiation-dominated era of the early Universe are expected to generate stochastic gravitational waves (GWs) due to nonlinear mode coupling. In this \emph{Letter}, we report on a search for such a stochastic GW background in the data of the two LIGO detectors during their second observing run (O2). We focus on the primordial perturbations in the range of comoving wavenumbers $10^{16}-10^{18}~{\rm Mpc}^{-1}$ for which the stochastic background falls within the detectors' sensitivity band. We do not find any conclusive evidence of this stochastic signal in the data, and thus place the very first GW-based constraints on the amplitude of the power spectrum at these scales. We assume a lognormal shape for the power spectrum and Gaussian statistics for the primordial perturbations, and vary the width of the power spectrum to cover both narrow and broad spectra. Derived upper limits ($95\%$) on the amplitude of the power spectrum are $0.01-0.1$. As a byproduct, we are able to infer upper limits on the fraction of the Universe's mass in ultralight primordial black holes ($M_\mathrm{PBH} \simeq 10^{-20}-10^{-19}M_{\odot}$) at their formation time to be $\lesssim 10^{-25}$.

gr-qc↗

arXiv2020

Prospects for probing ultralight primordial black holes using the stochastic gravitational-wave background induced by primordial curvature perturbations

Ultralight primordial black holes (PBHs) with masses $\lesssim 10^{15}$g and subatomic Schwarzschild radii, produced in the early Universe, are expected to have evaporated by the current cosmic age due to Hawking radiation. Based on this assumption, a number of constraints on the abundance of ultralight PBHs have been made. However, Hawking radiation has thus far not been verified experimentally. It would, therefore, be of interest if constraints on ultralight PBHs could be placed independent of the assumption of Hawking-radiation. In this paper, we explore the possibility of probing these PBHs, within a narrow mass range, using gravitational-wave (GW) data from the two LIGO detectors. The idea is that large primordial curvature perturbations that result in the formation of PBHs, would also generate GWs through non-linear mode couplings. These induced GWs would produce a stochastic background. Specifically, we focus our attention on PBHs of mass range $\sim 10^{13} - 10^{15}$g for which the induced stochastic GW background peak falls in the sensitivity band of LIGO. We find that, for both narrow and broad Gaussian PBH mass distributions, the corresponding GW background would be detectable using presently available LIGO data, provided we neglect the existing constraints on the abundance of PBHs, which are based on Hawking radiation. Furthermore, we find that these stochastic backgrounds would be detectable in LIGO's third observing run, even after considering the existing constraints on PBH abundance. A non-detection should enable us to constrain the amplitude of primordial curvature perturbations as well as the abundance of ultralight PBHs. We estimate that by the end of the third observing run, assuming non-detection, we should be able to place constraints that are orders of magnitude better than currently existing ones.

astro-ph.CO↗

arXiv2019

Are $H_0$ and $σ_8$ tensions generic to present cosmological data?

Yes, for a wide range of cosmological models ($Λ$CDM, non-interacting $w_z$CDM or models with possible interactions between dark energy and dark matter, in either phantom or non-phantom regimes). In the recent past there have been many attempts to solve the tension between direct measurements of $H_0$ and $σ_8 \sqrt{Ω_{0 {\rm m}}}$ from the respective low redshift observables and indirect measurements of these quantities from the cosmic microwave background (CMB). In this work we reconstruct a model independent approach that boils down to different classes of cosmological models under suitable parameters choices. We test this parameterization against the latest Planck CMB data combined with recent BAO, SNeIa datasets and the R16 direct $H_0$ measurements, and compare among different cosmological models. Our analysis reveals that a strong positive correlation between $H_0$ and $σ_8$ is more or less generic, irrespective of the choice of cosmological models. We also find that present data slightly prefers a phantom equation of state for dark energy and a slight negative value for effective equation of state for dark matter (which is a direct signature of interacting models) with a relatively high value for $H_0$ consistent with R16 and simultaneously, a consistent value for $Ω_{0 {\rm m}}$. Thus, even though the tensions cannot be fully resolved, interacting models with phantom equation of state get a slight edge over the others for currently available data. We also see that allowing interaction between dark energy and dark matter may resolve the tension between the high redshift CMB data and individual low redshift datasets, but the low redshift datasets have inconsistencies between them (e.g. between BAO and $H_0$, SNeIa and BAO, and cluster counts and $H_0$) that are practically independent of the cosmological model.

astro-ph.CO↗