arXiv subjects

Pankaj Jain

Publications and source records attributed to Pankaj Jain.

At least 91 records · Page 5Linked to original sources

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↗

arXiv2012

A complete 3D numerical study of the effects of pseudoscalar-photon mixing on quasar polarizations

We present the results of three-dimensional simulations of quasar polarizations in the presence of pseudoscalar-photon mixing in the intergalactic medium. The intergalactic magnetic field is assumed to be uncorrelated in wave vector space but correlated in real space. Such a field may be obtained if its origin is primordial. Furthermore we assume that the quasars, located at cosmological distances, have negligible initial polarization. In the presence of pseudoscalar-photon mixing we show, through a direct comparison with observations, that this may explain the observed large scale alignments in quasar polarizations within the framework of big bang cosmology. We find that the simulation results give a reasonably good fit to the observed data.

astro-ph.CO↗

arXiv2012

Testing Unimodular Gravity

We consider models of gravitation that are based on unimodular general coordinate transformations (GCT). These transformations include only those which do not change the determinant of the metric. We treat the determinant as a separate field which transforms as a scalar under unimodular GCT. We consider a class of such theories. In general, these theories do not transform covariantly under the full GCT. We characterize the violation of general coordinate invariance by introducing a new parameter. We show that the theory is consistent with observations for a wide range of this parameter. This parameter may serve as a test for possible violations of general coordinate invariance. We also consider the cosmic evolution within the framework of these models. We show that in general we do not obtain consistent cosmological solutions if we assume the standard cosmological constant or the standard form of non-relativistic matter. We propose a suitable generalization which is consistent with cosmology. We fit the resulting model to the high redshift supernova data. We find that we can obtain a good fit to this data even if include only a single component, either cosmological constant or non-relativistic matter.

gr-qc↗

arXiv2012

Cosmological Perturbation Analysis in a Scale Invariant Model of Gravity

We consider a model for gravity that is invariant under global scale transformations. It includes one extra real scalar field coupled non-minimally to the gravity fields. In this model all the dimensionful parameters like the gravitational constant and the cosmological constant etc. are generated by a solution of the classical equations of motion which breaks scale invariance. In this paper we demonstrate the stability of such a solution against small perturbations in a flat FRW background by making a perturbative expansion around this solution and solving the resulting equations linear in the perturbations.

gr-qc↗

arXiv2011

Parity Asymmetry in the CMBR Temperature Power Spectrum

We study the power asymmetry between even and odd multipoles in the multipolar expansion of CMB temperature data from WMAP, recently reported in the literature. We introduce an alternate statistic which probes this effect more sensitively. We find that the data is highly anomalous and consistently outside $2σ$ significance level in the whole multipole range $l=[2,101]$. We examine the possibility that this asymmetry may be caused by the foreground cleaning procedure or by residual foregrounds. By direct simulations, using the Planck Sky Model for foregrounds we rule out this possibility. We also examine several possible sub-dominant foregrounds, which might lead to such an asymmetry. However in all cases we are unable to explain the signal seen in data. We next examine cleaned maps, using procedures other than the one followed by the WMAP Science team. Specifically we analysed the maps cleaned by the IPSE procedure, Needlets and the harmonic ILC procedure. In all these cases we do not find a statistically significant signal of power asymmetry. This is in contrast to the result obtained by the WMAP best fit power spectrum as well as the ILC map. Finally, we test for the contribution of low-$l$ multipoles to the observed power asymmetry. We find that if we eliminate the first six multipoles, $l=[2,7]$, the significance falls below $2σ$ CL. Hence we find that the signal gets dominant contribution from low-$l$ modes.

astro-ph.CO↗

arXiv2011

Quantum Treatment of the Weyl Vector Meson

The Weyl meson arises in theories with local scale invariance. It acts as a candidate for dark matter in a generalized Standard Model with local scale invariance. The Higgs particle is absent from the physical spectrum in this theory. We consider the quantization of this theory in detail, imposing suitable gauge fixing conditions. We also consider a further generalization of this model which includes an additional real scalar field. In this theory a Higgs like particle remains in the particle spectrum. Since this particle couples to the Weyl meson, it can lead to interesting phenomenology involving this vector field in particle colliders.

hep-ph↗

arXiv2011

Large scale anisotropy due to pre-inflationary phase of cosmic evolution

We show that perturbations generated during the anisotropic pre-inflationary stage of cosmic evolution may affect cosmological observations today for a certain range of parameters. Due to the anisotropic nature of the universe during such early times, it might explain some of the observed signals of large scale anisotropy. In particular we argue that the alignment of CMB quadrupole and octopole may be explained by the Sachs-Wolfe effect due to the large scale anisotropic modes from very early times of cosmological evolution. We also comment on how the observed dipole modulation of CMB power may be explained within this framework.

astro-ph.CO↗

arXiv2011

Alignments in quasar polarizations: pseudoscalar-photon mixing in the presence of correlated magnetic fields

We investigate the effects of pseudoscalar-photon mixing on electromagnetic radiation in the presence of correlated extragalactic magnetic fields. We model the Universe as a collection of magnetic domains and study the propagation of radiation through them. This leads to correlations between Stokes parameters over large scales and consistently explains the observed large-scale alignment of quasar polarizations at different redshifts within the framework of the big bang model.

hep-ph↗

arXiv2011

Scale Invariance as a Solution to the Cosmological Constant Problem

We show that scale invariance provides a solution to the fine tuning problem of the cosmological constant. We construct a generalization of the standard model of particle physics which displays exact quantum scale invariance. The matter action is invariant under global scale transformations in arbitrary dimensions. However the gravitational action breaks scale invariance explicitly. The scale symmetry is broken spontaneously in the matter sector of the theory. We show that the contribution to the vacuum energy and hence the cosmological constant is identically zero from the matter sector within the full quantum theory. However the gravitational sector may give non-zero contributions to the cosmological constant at loop orders. No fine tuning may be required at loop orders since the matter sector gives zero contribution to the cosmological constant. We also show that we do not require full scale invariance in order to constrain the vacuum energy from the matter sector. We only require invariance under pseudoscale transformations. Using this idea and motivated by the concept of unimodular gravity we propose an alternative model. In this case also we show that matter sector gives exactly zero contribution to the vacuum energy.

hep-ph↗

arXiv2010

New physics, the cosmic ray spectrum knee, and $pp$ cross section measurements

We explore the possibility that a new physics interaction can provide an explanation for the knee just above $10^6$ GeV in the cosmic ray spectrum. We model the new physics modifications to the total proton-proton cross section with an incoherent term that allows for missing energy above the scale of new physics. We add the constraint that the new physics must also be consistent with published $pp$ cross section measurements, using cosmic ray observations, an order of magnitude and more above the knee. We find that the rise in cross section required at energies above the knee is radical. The increase in cross section suggests that it may be more appropriate to treat the scattering process in the black disc limit at such high energies. In this case there may be no clean separation between the standard model and new physics contributions to the total cross section. We model the missing energy in this limit and find a good fit to the Tibet III cosmic ray flux data. We comment on testing the new physics proposal for the cosmic ray knee at the Large Hadron Collider.

astro-ph.CO↗

arXiv2010

Effect of Foregrounds on the CMBR Multipole Alignment

We analyze the effect of foregrounds on the observed alignment of CMBR quadrupole and octopole. The alignment between these multipoles is studied by using a symmetry based approach which assigns a principal eigenvector (PEV) or an axis with each multipole. We determine the significance of alignment between these multipoles by using the Internal Linear Combination (ILC) 5 and 7 year map s and also the maps obtained by using the Internal Power Spectrum Estimation (IPSE) procedure. The effect of foreground cleaning is studied in detail within the framework of the IPSE method both analytically and numerically. By using simulated CMBR data, we study how the PEVs of the pure simulated CMB map differ from those of the final cleaned map. We find that, in general, the shift in the PEVs is relatively small and in random directions. Due to the random nature of the shift we conclude that it can only lead to misalignment rather than alignment of multipoles. We also directly estimate the significance of alignment by using simulated cleaned maps. We find that the results in this case are identical to those obtained by simple analytic estimate or by using simulated pure CMB maps.

astro-ph.CO↗

arXiv2009

Constraints on the Cosmological Constant due to Scale Invariance

We consider the standard model with local scale invariance. The theory shows exact scale invariance of dimensionally regulated action. We show that massless gauge fields, which may be abelian or non-abelian, lead to vanishing contribution to the cosmological constant in this theory. This result follows in the quantum theory, to all orders in the gauge couplings. However we have not considered contributions higher orders in the gravitational coupling. Similarly we also find that massless fermion fields yield null contribution to the cosmological constant. The effective cosmological constant in this theory is non-zero due to the phenomenon of cosmological symmetry breaking, which also gives masses to all the massive fields, besides generating the Planck mass. We find a simple relationship between the curvature scalar and the vacuum value of the Higgs field in the limit when we ignore all other contributions to the energy density besides the vacuum energy.

hep-ph↗

arXiv2009

Axion luminosity of Active Galactic Nuclei

We compute the flux of axions from Active Galactic Nuclei (AGN). Axions can be produced in the accretion disk by the Compton, Bremsstrahlung and Primakoff processes. We find that the axion luminosity due to these processes is negligible in comparison to the photon luminosity from AGNs. We also compute the luminosity of a hypothetical pseudoscalar, with very small mass, from the AGN atmosphere due to the phenomenon of pseudoscalar-photon mixing in background magnetic field. In this case we find that for some parameter ranges, the pseudoscalar flux can exceed that of photons. We comment on the implications of this result on the observed large scale alignment of optical polarizations from AGNs.

astro-ph.CO↗

arXiv2009

Standard Model with Cosmologically Broken Quantum Scale Invariance

We argue that scale invariance is not anomalous in quantum field theory, provided it is broken cosmologically. We consider a locally scale invariant extension of the Standard Model of particle physics and argue that it fits both the particle and cosmological observations. The model is scale invariant both classically and quantum mechanically. The scale invariance is broken cosmologically producing all the dimensionful parameters. The cosmological constant or dark energy is a prediction of the theory and can be calculated systematically order by order in perturbation theory. It is expected to be finite at all orders. The model does not suffer from the hierarchy problem due to absence of scalar particles, including the Higgs, from the physical spectrum.

hep-ph↗

arXiv2009

CMB Polarization and Temperature Power Spectra Estimation using Linear Combination of WMAP 5-year Maps

We estimate CMB polarization and temperature power spectra using WMAP 5-year foreground contaminated maps. The power spectrum is estimated by using a model independent method, which does not utilize directly the diffuse foreground templates nor the detector noise model. The method essentially consists of two steps, (i) removal of diffuse foregrounds contamination by making linear combination of individual maps in harmonic space and (ii) cross-correlation of foreground cleaned maps to minimize detector noise bias. For temperature power spectrum we also estimate and subtract residual unresolved point source contamination in the cross-power spectrum using the point source model provided by the WMAP science team. Our $TT$, $TE$ and $EE$ power spectra are in good agreement with the published results of the WMAP science team. The error bars on the polarization power spectra, however, turn out to be smaller in comparison to what is obtained by the WMAP science team. We perform detailed numerical simulations to test for bias in our procedure. We find that the bias is small in all cases. A negative bias at low $l$ in $TT$ power spectrum has been pointed in an earlier publication. We find that the bias corrected quadrupole power ($l(l+1)C_l/2π)$ is 532 $μK^2$, approximately 2.5 times the estimate (213.4 $μK^2$) made by the WMAP team.

astro-ph.CO↗

arXiv2009

One Loop Calculation of Cosmological Constant in a Scale Invariant Theory

We compute the cosmological constant in a scale invariant scalar field theory. The gravitational action is also suitably modified to respect scale invariance. Due to scale invariance the theory does not admit a cosmological constant term. The scale invariance is broken by a recently introduced mechanism called cosmological symmetry breaking. This leads to a nonzero cosmological constant. We compute the one loop corrections to the cosmological constant and show that it is finite.

hep-ph↗

arXiv2009

Model Independent Foreground Power Spectrum Estimation using WMAP 5-year Data

In this paper, we propose & implement on WMAP 5-year data, a model independent approach of foreground power spectrum estimation for multifrequency observations of CMB experiments. Recently a model independent approach of CMB power spectrum estimation was proposed by Saha et al. 2006. This methodology demonstrates that CMB power spectrum can be reliably estimated solely from WMAP data without assuming any template models for the foreground components. In the current paper, we extend this work to estimate the galactic foreground power spectrum using the WMAP 5 year maps following a self contained analysis. We apply the model independent method in harmonic basis to estimate the foreground power spectrum and frequency dependence of combined foregrounds. We also study the behaviour of synchrotron spectral index variation over different regions of the sky. We compare our results with those obtained from MEM foreground maps which are formed in pixel space. We find that relative to our model independent estimates MEM maps overestimates the foreground power close to galactic plane and underestimates it at high latitudes.

astro-ph.CO↗

arXiv2008

Signals of Statistical Anisotropy in WMAP Foreground-Cleaned Maps

Recently a symmetry-based method to test for statistical isotropy of the cosmic microwave background was developed. We apply the method to template-cleaned 3-year and 5-year WMAP-$DA$ maps. We examine a wide range of angular multipoles from $2 < l < 300$. The analysis detects statistically signicant signals of anisotropy inconsistent with an isotropic CMB in some of the foreground cleaned maps. We are unable to resolve whether the anomalies have a cosmological, local astrophysical or instrumental origin. Assuming the anisotropy arises due to residual foreground contamination, we estimate the residual foreground power in the maps. For the W band maps, we also find a highly improbable degree of isotropy we cannot explain. We speculate that excess isotropy may be caused by faulty modeling of detector noise.

astro-ph↗