Gopal Kashyap

Vellore Institute of Technology University

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Pankaj Jain

Indian Institute of Technology Kanpur

Naveen K. Singh

Jawaharlal Nehru University

Naveen Singh

Indian Agricultural Research Institute

Prasenjit Sanyal

Konkuk University

K. S. Phukon

University of Birmingham

Shamik Ghosh

Lawrence Berkeley National Laboratory

A. Nayak

National Institute of Technology Meghalaya

Pankaj Jain

Indian Institute of Technology Kanpur

Rahul Kothari

University of the Western Cape

Subhadip Mitra

International Institute of Information Technology, Hyderabad

Cooperative Institutions

Indian Institute of Technology Kanpur

Chinese Academy of Sciences

National Institute for Subatomic Physics

Utrecht University

University of Massachusetts Dartmouth

University of Wisconsin–Milwaukee

Louisiana State University

Centre National de la Recherche Scientifique

Vellore Institute of Technology University

Jawaharlal Nehru University

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Imprint of inhomogeneous and anisotropic primordial power spectrum on CMB polarization

Monthly Notices of the Royal Astronomical Society (2016)

Rahul Kothari Shamik Ghosh Pranati K. Rath Gopal Kashyap Pankaj Jain

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 (CMBR). 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 ± 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.

Cosmic background radiation

10.1093/mnras/stw1039

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Citations (16)

Explaining galactic gamma ray excess in a conformal model

arXiv (Cornell University) (2017)

A. Nayak Pankaj Jain Gopal Kashyap Prasenjit Sanyal

We study the excess 1-3 GeV gamma rays signal observed from the galactic center within a conformal model. The model has a dark sector with strong self interactions. This sector couples weakly with the Standard Model particles via a scalar messenger. The lightest dark sector particle is a pion-like fermion anti-fermion bound state. We find that the observed gamma ray excess at Fermi-LAT is nicely explained in terms of decay of the dark pions. The dark matter relic density is also explained by the freeze-in mechanism in which the dark pions are produced in the early Universe by annihilation and decay of standard model particles. We have used Higgs invisible decay and dark matter direct detection experiments in order to put bounds on the parameters of the model.

Light dark matter

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A Note on Stability of Neutron Star and Cosmological Constant

Gopal Kashyap

Constant (computer programming)

Cosmological constant

Star (game theory)

10.2139/ssrn.5110163

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Relating the cosmological constant and slow-roll to conformal symmetry breaking

Modern Physics Letters A (2014)

Pankaj Jain Gopal Kashyap

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.

Cosmological constant

Perfect Cosmological Principle

Constant (computer programming)

Cosmological constant problem

10.1142/s0217732314501958

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Stability of Neutron Star and Cosmological Constant

arXiv (Cornell University) (2024)

Naveen K. Singh Gopal Kashyap

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.

Cosmological constant

Constant (computer programming)

Star (game theory)

10.48550/arxiv.2411.04455

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A simple solution to the fine tuning problem of the cosmological constant

arXiv (Cornell University) (2014)

Pankaj Jain Gopal Kashyap

Department of Physics, IIT Kanpur, Kanpur 208 016, IndiaAugust 13, 2014Abstract: We show that the ﬁne tuning problem of the cosmological constant can be solved ina conformal model, with explicit conformal symmetry breaking. We argue that the model hastwodiﬀerent termswhich essentiallycontribute tothe cosmologicalconstant. A localconformaltransformation leads to a cancellation of these two terms among one another. Hence theseproduce no physical consequence. The model generates a small cosmological constant withoutrequiring any ﬁne tuning.

Cosmological constant

Constant (computer programming)

Cosmological constant problem

Fine-tuning

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Cosmological implications of unimodular gravity

Journal of Cosmology and Astroparticle Physics (2012)

Pankaj Jain Atul Jaiswal Purnendu Karmakar Gopal Kashyap Naveen K. Singh

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.

Unimodular matrix

Cosmological constant

Conformal gravity

10.1088/1475-7516/2012/11/003

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Unimodular Theory of Gravity in Light of the Latest Cosmological Data

arXiv (Cornell University) (2023)

Naveen Singh Gopal Kashyap

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 $70.57 \pm 0.56 \ \mbox{Km s}^{-1} \mbox{Mpc} ^{-1}$.

Unimodular matrix

10.48550/arxiv.2306.17754

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Dipole Anisotropy in Gravitational Wave Source Distribution

arXiv (Cornell University) (2022)

Gopal Kashyap Naveen Singh K. S. Phukon Sarah Caudill Pankaj Jain

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.

Mass distribution

Rest frame

10.48550/arxiv.2204.07472

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Unimodular Theory of Gravity in Light of the Latest Cosmological Data

Universe (2023)

Naveen K. Singh Gopal Kashyap

The unimodular theory of gravity is an alternative perspective to the traditional general relativity of Einstein and opens new possibilities for exploring its implications in cosmology. In this paper, we investigated Unimodular Gravity (UG) with the cosmological data from the Pantheon sample of Type Ia Supernovae (SNs) (2018), Baryon Acoustic Oscillations (BAOs), and the observational H(z) data from the Differential Age method (DA). We also used the Cosmic Microwave Background (CMB) distance priors from the Planck 2018 results. We considered a model consisting of a generalized cosmological constant, radiation, and a dark matter component along with normal matter. The considered theory respects only unimodular coordinate transformations. We first fit our model with low-redshift data from SNs and DA and determined the value of the model parameters (ξ,H0). We found the best-fit value of parameter ξ=6.03±0.40, which deviates slightly from 6, for which the theory becomes the standard general theory of relativity. We observed a small deviation in the value of the Hubble constant (H0=72.6±3.5 km s−1 Mpc−1) in the UG model compared with the standard ΛCDM model (H0=72.2±1.2 km s−1 Mpc−1). Using the BAO + CMB constraint in the UG model, we obtained H0=68.45±0.66kms−1Mpc−1, and ξ is ∼6.029. For the combined datasets (SN + DA + BAO + CMB), the estimated H0=69.01±0.60kms−1Mpc−1 with ξ∼6.037, and in standard gravity, H0=68.25±0.40kms−1Mpc−1.

Baryon Acoustic Oscillations

Cosmic background radiation

Cosmological constant

Unimodular matrix

10.3390/universe9110469

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