arXiv subjects

Gopal Kashyap

Publications and source records attributed to Gopal Kashyap.

16 recordsLinked to original sources

arXiv2026

Directional Tests of the Cosmic Distance Duality Relation using Pantheon+ and BAO

We present a model-independent test of anisotropy in the cosmic distance duality relation (CDDR), $D_L=(1+z)^2 D_A$, using the Pantheon+ type Ia supernova sample and baryon acoustic oscillation (BAO) data. The angular diameter distance is reconstructed via Gaussian Processes, enabling an estimate of $\eta(z)=D_L/[D_A(1+z)^2]$ without assuming a background cosmology. We also allow for a possible isotropic evolution, parameterized as $\eta(z)=1+\eta_1 z$, and find a redshift-dependent deviation whose significance depends on the assumed supernova calibration. Anisotropy is modeled through a dipole modulation and constrained using a full covariance-based likelihood. To assess statistical significance, we construct null realizations that preserve both the redshift distribution and the survey selection function. We find that the observed dipole amplitude is consistent with isotropic expectations and lies below the levels induced by statistical fluctuations and survey geometry. We obtain a robust 95\% upper bound $A_{95}=0.025$, stable across different supernova calibration choices. We find no evidence for intrinsic anisotropy in the CDDR. Our results highlight the importance of accounting for survey selection effects in anisotropy searches and provide a viable framework for testing directional deviations in cosmological relations.

astro-ph.CO↗

arXiv2026

Unimodular Diffusion and Interacting Vacuum Cosmology

We investigate the correspondence between unimodular diffusion cosmology and interacting dark sector models at the background and linear perturbation levels. In the diffusion framework, the effective cosmological constant becomes time dependent, $\Lambda(t)$, sourced by a diffusion current. We show that at the background level this framework can be mapped onto interacting dark energy models with $w=-1$ and energy transfer $Q$. Using two common parameterizations, $Q = \xi H \rho_{\rm de}$ and $Q = \xi H \rho_{\rm dm}$, and data from supernovae, DESI BAO, cosmic chronometers, and CMB distance priors, we find $\xi = -0.0197 \pm 0.0076$ for the vacuum-coupled case, while the matter-coupled case gives a best-fit $\xi = 0.0018$ with comparable goodness of fit. At the level of linear perturbations, however, the diffusion framework is consistent only with interacting vacuum models having homogeneous energy transfer ($Q \propto \rho_{\rm de}$ with $\delta Q=0$), thereby breaking the degeneracy with more general interacting dark energy scenarios. Including redshift-space distortion data, we obtain $\xi = -0.0147 \pm 0.0075$, consistent with $\Lambda$CDM ($\xi=0$) at $2\sigma$. The inferred clustering amplitude is $S_8 = 0.782 \pm 0.026$ for the diffusion model, compared to $S_8 = 0.77 \pm 0.025$ for $\Lambda$CDM under the same dataset, indicating a modest but non-negligible impact on structure growth.

astro-ph.CO↗

arXiv2026

Constraints on Axion-Photon Mixing from Fast Radio Burst Dispersion Measures

Fast radio bursts (FRBs) offer a powerful probe of the ionized Universe through their dispersion measures (DM). While a significant fraction of the DM arises from the intergalactic medium (IGM), the contributions from the host galaxy and the immediate environment of the source remain uncertain, and the physical origin of FRBs is still under active investigation. In this work, we investigated the possibility that FRBs originate from high-magnetic-field neutron stars (NS), whose magnetospheres can facilitate axion-photon mixing. Such mixing can modify photon propagation and induce an effective contribution to the observed dispersion. Using a sample of localized FRBs with measured redshifts, we perform a Bayesian Markov Chain Monte Carlo (MCMC) analysis to constrain the axion mass $m_a$ and axion-photon coupling $g_{a\gamma\gamma}$. Within a parametric cosmological framework, we obtain $m_a = 1.16^{+4.40}_{-1.08}\,\mu{\rm eV}$ and $g_{a\gamma\gamma} = (1.76^{+6.69}_{-1.64})\times10^{-16}\,{\rm GeV}^{-1}$, together with a physically consistent intergalactic baryon fraction $f_{\rm IGM} = 0.837^{+0.053}_{-0.056}$. We further tested the robustness of our bounds against cosmological modeling assumptions by employing a non-parametric Gaussian Process reconstruction (GPR) of the DM-$z$ relation, which gives statistically consistent results.

astro-ph.CO↗

arXiv2025

Matter Dipole and Hubble Tension due to Large Wavelength Perturbations

We theoretically analyze the dipole anisotropy observed in the quasar distribution from the CatWISE2020 catalog. The catalog data shows a peak around $z\approx 1$, suggesting the presence of a large-scale dipole component. We explore the possibility that this dipole could be driven by primordial density fluctuations from modes that were superhorizon at the time of CMB decoupling but have since entered the horizon and become subhorizon. In particular, we consider the impact of adiabatic modes with wave numbers $k$ in the range $(10^{-4} - 4 \times 10^{-3})~\mathrm{Mpc}^{-1} $, corresponding to wavelength scales of several Gpc. Such modes can create large-scale density variations, likely causing anisotropies in the distribution of matter and, as a result, affecting the number density of observed quasars. We find that these can lead to a significant contribution to the dipole for sources up to redshifts of about 1, but are unable to explain the observed dipole. We also demonstrate that a superhorizon curvature perturbations mode, with a comoving wavenumber $k\lesssim0.3H_0$ can lead to a significant enhancement in the locally inferred Hubble constant. This effect offers a viable explanation for the observed discrepancy between local and CMB inferred measurements of $H_0$.

astro-ph.CO↗

arXiv2024

Stability of Neutron Star and Cosmological Constant

We derive the equation for pressure within a neutron star, taking into account a non-zero cosmological constant ($\Lambda$). We then examine the stability of the neutron star's equilibrium state in the presence of cosmological constant. Our analysis shows that the theorem used to assess the stability of stellar structures at equilibrium remains applicable to neutron stars even when a cosmological constant is considered. We further numerically solve the stellar structure equations and determine the mass of neutron star using different equations of state (EOS). Moreover, we observe that the value of the cosmological constant ($\Lambda \geq 10^{-11} \rm m^{-2}$) causes a significant change in the mass-radius relationship of neutron stars.

gr-qc↗

arXiv2023

Unimodular Theory of Gravity in Light of the Latest Cosmological Data

The unimodular theory of gravity is an alternative perspective to traditional Einstein's general relativity and opens new possibilities for exploring its implications in cosmology. In this paper, we investigate the unimodular gravity (UG) with the latest cosmological data from the Pantheon sample of Type Ia supernovae (SN), Baryon Acoustic Oscillations (BAO), and the observational H(z) data from Differential Age method (DA). We consider a model consisting of a generalized cosmological constant with radiation and dark matter. The considered theory respects only unimodular coordinate transformations. We fit our model with low-redshift data from SN and DA and determine the value of parameter $\xi$ of the theory. We find the best-fit value of parameter $\xi =6.23 \pm 0.5$; which deviates from 6, for which the theory becomes the standard general theory of relativity. We further study the Hubble constant problem by combining the SN and DA data with BAO data. We observe deviation in the value of $H_0$ from the standard $\Lambda$CDM model. We obtain $H_0$ as $70.7 \pm 4.1 \ \mbox{Km s}^{-1} \mbox{Mpc} ^{-1}$ and $69.24 \pm 0.90 \ \mbox{Km s}^{-1} \mbox{Mpc} ^{-1}$ from supernovae data and BAO data, respectively in unimodular gravity. Combining the BAO data with SN+DA data set, we obtain $H_0$ as $70.57 \pm 0.56 \ \mbox{Km s}^{-1} \mbox{Mpc} ^{-1}$.

astro-ph.CO↗

arXiv2022

Dipole Anisotropy in Gravitational Wave Source Distribution

Our local motion with respect to the cosmic frame of rest is believed to be dominantly responsible for the observed dipole anisotropy in the Cosmic Microwave Background Radiation (CMBR). We study the effect of this motion on the sky distribution of gravitational wave (GW) sources. We determine the resulting dipole anisotropy in GW source number counts, mass weighted number counts, which we refer to as mass intensity, and mean mass per source. The mass M dependence of the number density n(M) distribution of BBH is taken directly from the data. We also test the anisotropy in the observable mean mass per source along the direction of the CMB dipole. The current data sample is relatively small and consistent with isotropy. The number of sources required for this test is likely to become available in the near future.

astro-ph.CO↗

arXiv2019

Cosmological Dark Matter in a Conformal Model

We study the collider, astrophysical and cosmological constraints on the dark matter sector of a conformal model within the framework of both the freeze out as well as the freeze in mechanism. The model has a dark sector with strong self interactions. This sector couples weakly with the Standard Model (SM) particles via a scalar messenger. The lightest dark sector particle is a pion-like fermion anti-fermion bound state. We find that the model successfully satisfies the constraints coming from the Higgs decay to the visible as well as the invisible sector. We have used the results of the dark matter direct detection experiments, such as, XENON1T in order to impose bounds on the parameters of the model. The model satisfies the indirect detection constraints of gamma ray from the galactic center and Dwarf spheroidal galaxies. We also determine the parameter range for which it satisfies the astrophysical constraints on the dark matter self coupling.

hep-ph↗

arXiv2017

Imprint of Inhomogeneous and Anisotropic Primordial Power Spectrum on CMB Polarization

We consider an inhomogeneous model and independently an anisotropic model of primordial power spectrum in order to describe the observed hemispherical anisotropy in Cosmic Microwave Background Radiation. This anisotropy can be parametrized in terms of the dipole modulation model of the temperature field. Both the models lead to correlations between spherical harmonic coefficients corresponding to multipoles, l and l \pm 1. We obtain the model parameters by making a fit to TT correlations in CMBR data. Using these parameters we predict the signature of our models for correlations among different multipoles for the case of the TE and EE modes. These predictions can be used to test whether the observed hemispherical anisotropy can be correctly described in terms of a primordial power spectrum. Furthermore these may also allow us to distinguish between an inhomogeneous and an anisotropic model.

astro-ph.CO↗

arXiv2016

Probing statistical isotropy of cosmological radio sources using SKA

There currently exist many observations which are not consistent with the cosmological principle. We review these observations with a particular emphasis on those relevant for Square Kilometre Array (SKA). In particular, several different data sets indicate a preferred direction pointing approximately towards the Virgo cluster. We also observe a hemispherical anisotropy in the Cosmic Microwave Background Radiation (CMBR) temperature fluctuations. Although these inconsistencies may be attributed to systematic effects, there remains the possibility that they indicate new physics and various theories have been proposed to explain them. One possibility, which we discuss in this review, is the generation of perturbation modes during the early pre-inflationary epoch, when the Universe may not obey the cosmological principle. Better measurements will provide better constraints on these theories. In particular, we propose measurement of the dipole in number counts, sky brightness, polarized flux and polarization orientations of radio sources. We also suggest test of alignment of linear polarizations of sources as a function of their relative separation. Finally we propose measurement of hemispherical anisotropy or equivalently dipole modulation in radio sources.

astro-ph.CO↗

arXiv2015

Non-relativistic matter and Dark energy in a quantum conformal model

We consider a generalization of the standard model which respects quantum conformal invariance. This model leads to identically zero vacuum energy. We show how non-relativistic matter and dark energy arises in this model. Hence the model is shown to be consistent with observations.

gr-qc↗

arXiv2015

The fine tuning of the cosmological constant in a conformal model

We consider a conformal model involving two real scalar fields in which the conformal symmetry is broken by a soft mechanism and is not anomalous. One of these scalar fields is representative of the standard model Higgs. The model predicts exactly zero cosmological constant. In the simplest version of the model, some of the couplings need to be fine tuned to very small values. We formulate the problem of fine tuning of these couplings. We argue that the problem arises since we require a soft mechanism to break conformal symmetry. The symmetry breaking is possible only if the scalar fields do not evolve significantly over the time scale of the Universe. We present two solutions to this fine tuning problem. We argue that the problem is solved if the classical value of one of the scalar fields is super-Planckian, i.e. takes a value much larger than the Planck mass. The second solution involves introduction of a strongly coupled hidden sector that we call hypercolor. In this case the conformal invariance is broken dynamically and triggers the breakdown of the electroweak symmetry. We argue that our analysis applies also to the case of the standard model Higgs multiplet.

hep-ph↗

arXiv2014

Limits on the Weyl meson parameters due to Fermi-LAT gamma-ray observations

We use gamma-ray observations by the Fermi Large Area Telescope (Fermi-LAT) to impose limits on the properties of the hypothetical particle Weyl meson. Such mesons arise in theories which display local scale invariance and act as dark matter candidate. In a generalized locally scale invariant Standard Model it can decay or annihilate to the photon pair through the Higgs channel. We find that Fermi-LAT observations severely constrain the parameters of the Weyl meson.

hep-ph↗

arXiv2014

Relating the cosmological constant and slow roll to conformal symmetry breaking

We show that a theory with conformal invariance, which is explicitly broken by small terms, provides a solution to the fine tuning problem of the cosmological constant. In the absence of the symmetry breaking terms, the cosmological constant is zero. Its value in the full theory is controlled by the symmetry breaking terms. The symmetry breaking terms also provide the slow roll conditions, which may be useful in constructing a model of inflation.

astro-ph.CO↗

arXiv2013

Weyl meson and its implications in collider physics and cosmology

Local scale invariant theory leads to the existence of a new particle called the Weyl vector meson. We study a generalized Standard Model, which displays local scale invariance. The model contains a real scalar field, besides the Higgs multiplet and coupling between the Higgs and Weyl meson. For certain range of coupling parameters, the Weyl-Higgs coupling leads to interesting phenomenon in particle colliders as well as in cosmology. Here we study the signature of the Weyl meson in particle colliders and determine the range of coupling parameters for which it can solve the dark matter problem.

hep-ph↗

arXiv2012

Cosmological Implications of Unimodular Gravity

We consider a model of gravity and matter fields which is invariant only under unimodular general coordinate transformations (GCT). The determinant of the metric is treated as a separate field which transforms as a scalar under unimodular GCT. Furthermore we also demand that the theory is invariant under a new global symmetry which we call generalized conformal invariance. We study the cosmological implications of the resulting theory. We show that this theory gives a fit to the high-z supernova data which is identical to the standard Big Bang model. Hence we require some other cosmological observations to test the validity of this model. We also consider some models which do not obey the generalized conformal invariance. In these models we can fit the supernova data without introducing the standard cosmological constant term. Furthermore these models introduce only one dark component and hence solve the coincidence problem of dark matter and dark energy.

astro-ph.CO↗