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A. R. Prasanna

Publications and source records attributed to A. R. Prasanna.

At least 19 recordsLinked to original sources
arXiv2017

Constraints on cosmological viscosity and self interacting dark matter from gravitational wave observations

It has been shown that gravitational waves propagate through ideal fluids without experiencing any dispersion or dissipation. However, if the medium has a non-zero shear viscosity $η$ , gravitational waves will be dissipated at a rate proportional to $G \,η$. We constrain dark matter and dark energy models with non-zero shear viscosity by calculating the dissipation of gravitational waves from GW150914 which propagate over a distance of $410\, $ Mpc through the dissipative fluid and comparing the data with the theoretical prediction. This provides a proof-of-principle demonstration of the fact that future observations gravitational waves at LIGO have the potential of better constraining the viscosity of dark matter and dark energy.

hep-ph↗
arXiv2016

Gravitational Waves- a new window to Cosmos

With the detection of Gravitational waves just about an year ago Einstein`s general theory of relativity- a space-time theory of gravity, got established on a firmer footing than any other theory in physics. Gravitational waves are just propagating disturbances in the gravitational field of extremely strong sources caused by some catastrophic event associated with cosmic bodies, like binary black hole coalescence, or neutron star mergers. As these events happen very far away in cosmos, and the signal strength would be extremely weak, it requires extraordinary detection and analysis technology to observe an event on earth. Luckily the joint collaboration LIGO-VIRGO, have so far detected two events in September and December of 2015 during their analysis of observations made with the laser interferometers over the last few observing sessions. The talk will give a brief theoretical sketch of the analysis required for describing the waves resulting from mass motion in the realm of general relativity, and point out, the serious and sincere efforts of the past fifty years that went into the final success. An attempt will be made to point out the enormous scope that is available for the new generation of students and researchers in pursuing the topic as a new window for possibly viewing the Universe with the implications for the studies of Dark matter, Dark energy and Cosmology as a whole.

gr-qc↗
arXiv2013

Neutrino coupling to cosmological background: A review on gravitational Baryo/Leptogenesis

In this work we review the theories of origin of matter-antimatter asymmetry in the Universe. The general conditions for achieving baryogenesis and leptogenesis in a CPT conserving field theory have been laid down by Sakharov. In this review we discuss scenarios where a background scalar or gravitational field spontaneously breaks the CPT symmetry and splits the energy levels between particles and anti-particles. Baryon or Lepton number violating processes in proceeding at thermal equilibrium in such backgrounds gives rise to Baryon or Lepton number asymmetry.

hep-ph↗
arXiv2009

Constraints on background torsion from birefringence of CMB polarization

We show that a non-minimal coupling of electromagnetism with background torsion can produce birefringence of the electromagnetic waves. This birefringence gives rise to a B-mode polarization of the CMB. From the bounds on B-mode from WMAP and BOOMERanG data, one can put limits on the background torsion at $ξ_{1}T_{1}=(-3.35 \pm 2.65) \times 10^{-22} GeV^{-1}$.

astro-ph.CO↗
arXiv2006

Leptogenesis from Spin-Gravity Coupling Following Inflation

The energy levels of the left and the right handed neutrinos is split in the background of gravitational waves generated during inflation which in presence of lepton number violating interactions gives rise to a net lepton asymmetry at equilibrium. Lepton number violation is achieved by the same dimension five operator which gives rise to neutrino masses after electro-weak symmetry breaking. A net baryon asymmetry of the same magnitude can be generated from this lepton asymmetry by electroweak sphaleron processes.

gr-qc↗
arXiv2004

Gauge Invariant Wave Equations in Curved Space-Times and Primordial Magnetic Fields

The inflationary production of magnetic field seeds for galaxies is discussed. The analysis is carried out by writing the wave equation of the electromagnetic field in curved spacetimes. The conformal invariance is broken by taking into account of the interaction of the electromagnetic field with the curvature tensor of the form $λR_{αβγδ} F^{αβ} F^{γδ}$. Such a term induces an amplification of the magnetic field during the reheating phase of the universe, but no growth of the magnetic field occurs in the de Sitter epoch. The resulting primordial magnetic field turns out to have strengths of astrophysical interest.

gr-qc↗
arXiv2004

Self lensing effects for compact stars and their mass-radius relation

During the last couple of years astronomers and astrophysicists have been debating on the fact whether the so called `strange stars' - stars made up of strange quark matter, have been discovered with the candidates like SAX J1808.4-3658, 4U 1728-34, RX J1856.5-3754, etc. The main contention has been the estimation of radius of the star for an assumed mass of ~ 1.4 M_sun and to see whether the point overlaps with the graphs for the neutron star equation of state or whether it goes to the region of stars made of strange matter equation of state. Using the well established formulae from general relativity for the gravitational redshift and the `lensing effect' due to bending of photon trajectories, we, in this letter, relate the parameters M and R with the observable parameters, the redshift z and the radiation radius R_\infty, thus constraining both M and R for specific ranges, without any other arbitrariness. With the required inputs from observations, one ought to incorporate the effects of self lensing of the compact stars which has been otherwise ignored in all of the estimations done so far. Nonetheless, these effect of self lensing makes a marked difference and constraints on the M-R relation.

astro-ph↗
arXiv2004

Reissner Nordström Background Metric in Dynamical Co-ordinates: Exceptional Behaviour of Hadamard States

We cast the Reissner Nordstrom solution in a particular co-ordinate system which shows dynamical evolution from initial data. The initial data for the $E<M$ case is regular. This procedure enables us to treat the metric as a collapse to a singularity. It also implies that one may assume Wald axioms to be valid globally in the Cauchy development, especially when Hadamard states are chosen. We can thus compare the semiclassical behaviour with spherical dust case, looking upon the metric as well as state specific information as evolution from initial data. We first recover the divergence on the Cauchy horizon obtained earlier. We point out that the semiclassical domain extends right upto the Cauchy horizon. This is different from the spherical dust case where the quantum gravity domain sets in before. We also find that the backreaction is not negligible near the central singularity, unlike the dust case. Apart from these differences, the Reissner Nordstrom solution has a similarity with dust in that it is stable over a considerable period of time. The features appearing dust collapse mentioned above were suggested to be generally applicable within spherical symmetry. Reissner Nordstrom background (along with the quantum state) generated from initial data, is shown not to reproduce them.

gr-qc↗
arXiv2003

Constraints on non-minimally coupled curved space electrodynamics from astrophysical observations

We study interactions of electro-magnetic fields with the curvature tensor of the form $λR_{μναβ}F^{μν}F^{αβ}$. Such coupling terms though are invariant under general coordinate transformation and CPT, however violate the Einstein equivalence principle. These couplings do not cause any energy dependent dispersion of photons but they exhibit birefringence. We put constraints on the coupling constant $λ$ using results from solar system radar ranging experiments and millisecond-pulsar observations. We find that the most stringent constraint comes from pulsar observations and is given by $ λ< 10^{11} cm^2 $ obtained from the timing of binary pulsar PSR B1534+12.

gr-qc↗
arXiv2002

`Strange Stars' - have they been discovered ?

Recently there is a suggestion about the star RXJ1856.5$-$3754, being a possible candidate for quark star. While from the estimates of the radiation radius, given by the authors, it does not seem to fit with any of the known models for such stars, the mass radius diagram admits the possibility of this being a quark star of mass ${\rm M_\odot<M<1.4M_\odot}$, provided the radius mentioned is the actual radius of the star.

astro-ph↗
arXiv2002

Fluid flow and inertial forces in black hole space times

We discuss the nature of the radial and azimuthal components of centrifugal force associated with fluid flows in the equatorial plane of black hole space times. The equations of motion are solved for the radial and azimuthal components of the 3-velocity V^i which are then used in evaluating the nature of the various components of inertial accelerations. It is shown that the reversal of centrifugal force is governed mainly by the dominance of the azimuthal velocity and the reversal occurs for r, mostly at 2m <~ r <~ 3m, depending upon the boundary condition.

gr-qc↗
arXiv2002

Effect of Coriolis force on Accretion Flows around Rotating Compact Object

Using the generalised set of fluid equations that include the `Coriolis force' along with the centrifugal and pressure gradient forces, we have reanalysed the class of self similar solutions, with the pseudo-Newtonian potential. We find that the class of solutions is well behaved for almost the entire parameter space except for a few selected combinations of $γ$ and $α$ for the co-rotating flow. The analysis of the Bernoulli number shows that whereas it remains positive for co-rotating flow for $f>1/3$, for the counter-rotating flow it does admit both positive and negative values, indicating the possibility of energy transfer in either direction.

astro-ph↗
arXiv2002

Baryogenesis from primordial tensor perturbations

During inflation primordial quantum fluctuations of the spacetime metric become classical and there is a spontaneous CPT violation by the spin connection coupling terms of the metric with fermions. The energy levels of the left and the right chirality neutrinos is split which gives rise to a net lepton asymmetry at equilibrium. A net baryon asymmetry of the same magnitude can be generated from this lepton asymmetry either by a GUT, $B-L$ symmetry or by electroweak sphaleron processes which preserve $B+L$ symmetry. If the amplitude of the primordial tensor perturbations is of the order of $10^{-6}$ (as is expected from inflation models) and the lepton/baryon number violating processes freeze out at the GUT era $T_d \sim 10^{16} Gev$ then a baryon number asymmetry of the correct magnitude $10^{-10}$ can be generated.

hep-ph↗
arXiv2002

Experimental tests of curvature couplings of Fermions in General Relativity

Spin 1/2 particles in geodesic trajectories experience no gravitational potential but they still have non-zero couplings to the curvature tensor. The effect of space time curvature on fermions can be parameterized by a vector and a pseudo-vector potential. These apparent CPT violating terms can be measured with satellite based spin-polarized torsion balance and clock comparison experiments. The Earth's curvature effect is of the order of 10^{-37} Gev which is not far from the present bounds of ~10^{-29} Gev on such CPT violating couplings.

hep-ph↗
arXiv2002

Inertial Forces as Viewed from the ADM Slicing and their behaviour for Particles in Non-Circular Geodesics

Considering the definition of inertial forces acting on a test particle, following non-circular geodesics, in static and stationary space times we show that the centrifugal force reversal occurs only in the case of particles following prograde orbits around black holes. We first rewrite the covariant expressions for the acceleration components in terms of the lapse function, shift vector and the 3-metric $\gm_{ij}$, using the ADM 3+1 splitting and use these, for different cases as given by pure radial motion, pure azimuthal motion and the general non-circular motion. It is found that the reversal occurs only when the azimuthal angular velocity of the particle supersedes the radial velocity, which indeed depends upon the physical parameters $E$, $l$ and the Kerr parameter $a$.

gr-qc↗
arXiv2001

Gravitational Wave Induced Rotation of the Plane of Polarization of Pulsar Signals

We derive in this an expression for the rotation of plane of polarization, of an electromagnetic wave, induced by the field of a gravitational wave propagating along the same direction $\approx \f{Gμd^2Ω^4}{3ø}$, $ø$ and $Ω$ being their respective frequencies using the geometrical optics limit of the wave equation for fields. Estimating the effect for the case of pulses from binary pulsars, one finds it too small to be observable, there could be other sources like inspiralling binary or an asymmetric neutron star where the effect could be in the observable region.

astro-ph↗
arXiv2001

Energy deposition due to neutrino pair annhilation near rotating neutron stars

General relativistic effects have been shown to increase the energy deposition rate due to the process $ν\barν \to e^{+}e^{-}$ in supernovae and neutron stars. In this paper we study the effect of inclusion of the rotation of the star in the general relativistic treatment. We show that inclusion of rotation results in a reduction in the heating rate as compared to the no rotation case.

astro-ph↗