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Sanjay Jhingan

Publications and source records attributed to Sanjay Jhingan.

At least 19 recordsLinked to original sources
arXiv2019

Component separation map-making for stochastic gravitational wave background

Superposition of unresolved sources of gravitational waves (GW) is expected to create a persistent background of stochastic gravitational wave background (SGWB). Different types of astrophysical and cosmological sources are however likely to be present in the background. We present an algorithm for separation of the components with distinct frequency spectral indices into individual sky-maps. To demonstrate the method, we simulate GW signals for different spectral indices and corresponding sky-maps, e.g., point sources like the Virgo cluster and extended ones like the Milky Way Galaxy, and apply our method to recover the injected maps.

gr-qc↗
arXiv2018

Energy emission from high curvature region and its backreation

A strong gravity naked singular region can give important clues towards understanding classical as well as spontaneous nature of General Relativity. We propose here a model for energy emission from a naked singular region in a self-similar dust spacetime by gluing two self-similar dust solutions at the Cauchy horizon. The energy is defined and evaluated as a surface energy of a null hypersurface, the null shell. Also included are scenarios of spontaneous creation or disappearance of a singularity, end of inflation, black hole formation and bubble nucleation. Our examples investigated here explicitly show that one can model unlimitedly luminous and energetic objects in the framework of General Relativity.

gr-qc↗
arXiv2016

Primordial black hole formation in the matter-dominated phase of the Universe

We investigate primordial black hole formation in the matter-dominated phase of the Universe, where nonspherical effects in gravitational collapse play a crucial role. This is in contrast to the black hole formation in a radiation-dominated era. We apply the Zel'dovich approximation, Thorne's hoop conjecture, and Doroshkevich's probability distribution and subsequently derive the production probability $β_{0}$ of primordial black holes. The numerical result obtained is applicable even if the density fluctuation $σ$ at horizon entry is of the order of unity. For $σ\ll 1$, we find a semi-analytic formula $β_{0}\simeq 0.05556 σ^{5}$, which is comparable with the Khlopov-Polnarev formula. We find that the production probability in the matter-dominated era is much larger than that in the radiation-dominated era for $σ\lesssim 0.05$, while they are comparable with each other for $σ\gtrsim 0.05$. We also discuss how $σ$ can be written in terms of primordial curvature perturbations.

astro-ph.CO↗
arXiv2016

Spherical and nonspherical models of primordial black hole formation: exact solutions

We construct spacetimes which provide spherical and nonspherical models of black hole formation in the flat Friedmann--Lemaitre--Robertson--Walker (FLRW) universe with the Lemaitre--Tolman--Bondi solution and the Szekeres quasispherical solution, respectively. These dust solutions may contain both shell-crossing and shell-focusing naked singularities. These singularities can be physically regarded as the breakdown of dust description, where strong pressure gradient force plays a role. We adopt the simultaneous big bang condition to extract a growing mode of adiabatic perturbation in the flat FLRW universe. If the density perturbation has a sufficiently homogeneous central region and a sufficiently sharp transition to the background FLRW universe, its central shell-focusing singularity is globally covered. If the density concentration is sufficiently large, no shell-crossing singularity appears and a black hole is formed. If the density concentration is not sufficiently large, a shell-crossing singularity appears. In this case, a large dipole moment significantly advances shell-crossing singularities and they tend to appear before the black hole formation. In contrast, a shell-crossing singularity unavoidably appears in the spherical and nonspherical evolution of cosmological voids. The present analysis is general and applicable to cosmological nonlinear structure formation described by these dust solutions.

gr-qc↗
arXiv2015

Component Separation of a Isotropic Gravitational Wave Background

A Gravitational Wave Background (GWB) is expected in the universe from the superposition of a large number of unresolved astrophysical sources and phenomena in the early universe. Each component of the background (e.g., from primordial metric perturbations, binary neutron stars, milli-second pulsars etc.) has its own spectral shape. Many ongoing experiments aim to probe GWB at a variety of frequency bands. In the last two decades, using data from ground-based laser interferometric gravitational wave (GW) observatories, upper limits on GWB were placed in the frequency range of ~50-1000 Hz, considering one spectral shape at a time. However, one strong component can significantly enhance the estimated strength of another component. Hence, estimation of the amplitudes of the components with different spectral shapes should be done jointly. Here we propose a method for "component separation" of a statistically isotropic background, that can, for the first time, jointly estimate the amplitudes of many components and place upper limits. The method is rather straightforward and needs negligible amount of computation. It utilises the linear relationship between the measurements and the amplitudes of the actual components, alleviating the need for a sampling based method, e.g., Markov Chain Monte Carlo (MCMC) or matched filtering, which are computationally intensive and cumbersome in a multi-dimensional parameter space. Using this formalism we could also study how many independent components can be separated using a given dataset from a network of current and upcoming ground based interferometric detectors.

astro-ph.CO↗
arXiv2015

XMM-Newton view of a hard X-ray transient IGR J17497-2821

We present spectral and energy dependent timing characteristics of the hard X-ray transient IGR J17497-2821 based on XMM-Newton observations performed five and nine days after its outburst on 2006 September 17. We find that the source spectra can be well described by a hard (Gamma ~ 1.50) powerlaw and a weak multicolour disk blackbody with inner disk temperature kT_{in} ~ 0.2 KeV. A broad iron K - alpha line with FWHM ~ 27000 Km/s, consistent with that arising from an accretion disk truncated at large radius, was also detected. The power density spectra of IGR J17497 - 2821, derived from the high resolution (30 micro second) timing mode XMM-Newton observations, are characterised by broadband noise components that are well modelled by three Lorentzians. The shallow power law slope, low disk luminosity and the shape of the broadband power density spectrum indicate that the source was in the hard state. The rms variability in the softer energy bands (0.3-2 KeV) found to be ~ 1.3 times that in 2-5 and 5-10 KeV energy bands. We also present the energy dependent timing analysis of the RXTE/PCA data, where we find that at higher energies, the rms variability increases with energy.

astro-ph.HE↗
arXiv2015

Testing the consistency between cosmological measurements of distance and age

We present a model independent method to test the consistency between cosmological measurements of distance and age, assuming the distance duality relation. We use type Ia supernovae, baryon acoustic oscillations, and observational Hubble data, to reconstruct the luminosity distance D_L(z), the angle averaged distance D_V(z) and the Hubble rate H(z), using Gaussian processes regression technique. We obtain estimate of the distance duality relation in the redshift range 0.1<z<0.73 and we find no evidence for inconsistency between the data sets used.

astro-ph.CO↗
arXiv2015

Synergy between ground and space based gravitational wave detectors for estimation of binary coalescence parameters

We study the advantage of the co-existence of future ground and space based gravitational wave detectors, in estimating the parameters of a binary coalescence. Using the post-Newtonian waveform for the inspiral of non-spinning neutron star-black hole pairs in circular orbits, we study how the estimates for chirp mass, symmetric mass ratio, and time and phase at coalescence are improved by combining the data from different space-ground detector pairs. Since the gravitational waves produced by binary coalescence also provide a suitable domain where we can study strong field gravity, we also study the deviations from general relativity using the parameterized post-Einsteinian framework. As an example, focusing on the Einstein telescope and DECIGO pair, we demonstrate that there exists a sweet spot range of sensitivity in the pre-DECIGO phase where the best enhancement due to the synergy effect can be obtained for the estimates of the post-Newtonian waveform parameters as well as the modification parameters to general relativity.

gr-qc↗
arXiv2014

Spherical gravitational collapse in 5D Einstein-Gauss-Bonnet gravity

We consider a spherical gravitational collapse of inhomogeneous dust (and null dust) in Einstein gravity with the Gauss-Bonnet (GB) combination of quadratic curvature terms. It turns out that the presence of the coupling constant of the GB terms $α> 0$ completely changes the causal structure of the singularities from the analogous general relativistic case. The gravitational collapse of inhomogeneous dust in the five-dimensional GB extended Einstein equations leads to formation of a massive, but weak, timelike singularity which is forbidden in general relativity. The apparent horizons of two different collapsing solutions show interesting mathematical similarity.

gr-qc↗
arXiv2014

Exploring scalar field dynamics with Gaussian processes

The origin of the accelerated expansion of the Universe remains an unsolved mystery in Cosmology. In this work we consider a spatially flat Friedmann-Robertson-Walker (FRW) Universe with non-relativistic matter and a single scalar field contributing to the energy density of the Universe. Properties of this scalar field, like potential, kinetic energy, equation of state etc. are reconstructed from Supernovae and BAO data using Gaussian processes. We also reconstruct energy conditions and kinematic variables of expansion, such as the jerk and the slow roll parameter. We find that the reconstructed scalar field variables and the kinematic quantities are consistent with a flat Lambda-CDM Universe. Further we find that the null energy condition is satisfied for the redshift range of the Supernovae data considered in the paper but, the strong energy condition is violated.

astro-ph.CO↗
arXiv2013

Bound orbits and gravitational theory

It can be easily shown that bound orbits around a static source can exist only in 4 dimension and in none else for any long range force. This is so not only for Maxwell's electromagnetic and Newton's gravity but also for Einstein's gravitation theory. In contrast to Maxwell's electrodynamics and Newton's gravity, GR has a natural higher dimensional generalization in Lovelock gravity which remarkably admits bound orbits around a static black hole in all even d=2N+2 dimensions where $N$ is degree of Lovelock polynomial action. This is as general a result as Bertrand's theorem of classical mechanics in which existence of closed orbits uniquely singles out the inverse square law for a long range central force.

gr-qc↗
arXiv2013

Gravitational collapse in pure Lovelock gravity in higher dimensions

We study collapse of inhomogeneous dust and null dust (Vaidya radiation) in pure Lovelock gravity in higher dimensions. Since pure Lovelock gravity is kinematic in odd d=2N+1 dimension, hence pertinent dimension for the study is even d=2N+2, where N is degree of Lovelock polynomial. It turns out that pure Lovelock collapse favors naked singularity as against black hole for the Einstein case in the same dimension while strength of singularity as measured by divergence of Kretshmann scalar is interestingly the same in the two cases; i.e. the corresponding scalars have the same fall off behavior.

gr-qc↗
arXiv2013

Renormalization group approach to Einstein-Rosen waves

We present a renormalization group analysis to Einstein-Rosen waves or vacuum spacetimes with whole-cylinder symmetry. It is found that self-similar solutions appear as fixed points in the renormalization group transformation. These solutions correspond to the explosive gravitational waves and the collapsing gravitational waves at late times and early times, respectively. Based on the linear perturbation analysis of the self-similar solutions, we conclude that the self-similar evolution is stable as explosive gravitational waves under the condition of no incoming waves, while it is weakly unstable as collapsing gravitational waves. The result implies that self-similar solutions can describe the asymptotic behavior of more general solutions for exploding gravitational waves and thus extends the similarity hypothesis in general relativity from spherical symmetry to cylindrical symmetry.

gr-qc↗
arXiv2013

Weak cosmic censorship in gravitational collapse with astrophysical parameter values

The possible violation of the weak cosmic censorship hypothesis in astrophysical phenomena can provide us with the information about trans-Planckian physics through observations. We present negative evidence, however, that one should not expect such a possibility at least when the deviation from spherical symmetry is negligible and the parameter values of collapse are astrophysically reasonable. Taking the Lemaître-Tolman-Bondi solution as the model most likely to counter the weak hypothesis, we show that the mass ($ \gtrsim 1.5 M_\odot $) and density ($ \gtrsim 1.5 \times 10^{15} \; {\rm g/cm^3}$) of the collapsing object produce a gravitational field strong enough to capture any null rays soon after emanating from the singularity.

gr-qc↗
arXiv2013

Cosmic distance duality and cosmic transparency

We compare distance measurements obtained from two distance indicators, Super- novae observations (standard candles) and Baryon acoustic oscillation data (standard rulers). The Union2 sample of supernovae with BAO data from SDSS, 6dFGS and the latest BOSS and WiggleZ surveys is used in search for deviations from the distance duality relation. We find that the supernovae are brighter than expected from BAO measurements. The luminosity distances tend to be smaller then expected from angular diameter distance estimates as also found in earlier works on distance duality, but the trend is not statistically significant. This further constrains the cosmic transparency.

astro-ph.CO↗
arXiv2013

Is dark energy evolving?

We look for evidence for the evolution in dark energy density by employing Principal Component Analysis (PCA). Distance redshift data from supernovae and baryon acoustic oscillations (BAO) along with WMAP7 distance priors are used to put constraints on curvature parameter Omega_k and dark energy parameters. The data sets are consistent with a flat Universe. The constraints on the dark energy evolution parameters obtained from supernovae (including CMB distance priors) are consistent with a flat Lambda-CDM Universe. On the other hand, in the parameter estimates obtained from the addition of BAO data the second principal component, which characterize a non-constant contribution from dark energy, is non-zero at 1-sigma. This could be a systematic effect and future BAO data holds key to making more robust claims.

astro-ph.CO↗
arXiv2012

Cosmological constraints for an Eddington-Born-Infeld field

We consider the Eddington-Born-Infeld (EBI) model here without assuming any cosmological constant. The EBI scalar field is supposed to play a role of both dark matter and dark energy. Different eras in cosmology are reconstructed for the model. A comparison is drawn with $Λ$CDM model using Supernova Ia, WMAP7 and BAO data. It seems that the EBI field in this form does not give good fit to observational data in comparison to the $Λ$CDM model.

gr-qc↗