arXiv subjects

Prabhakar Tiwari

Publications and source records attributed to Prabhakar Tiwari.

31 records · Page 2Linked to original sources

arXiv2019

Weak Lensing Effect on CMB in the Presence of a Dipole Anisotropy

We investigate weak lensing effect on cosmic microwave background (CMB) in the presence of dipole anisotropy. The approach of flat-sky approximation is considered. We determine the functions $σ_0^2$ and $σ_2^2$ that appear in expressions of the lensed CMB power spectrum in the presence of a dipole anisotropy. We determine the correction to B-mode power spectrum which is found to be appreciable at low multipoles ($l$). However, the temperature and E-mode power spectrum are not altered significantly.

astro-ph.CO↗

arXiv2018

Evidence of isotropy at large-scale from polarizations of radio sources

We test the isotropy of radio polarization angle orientations with a robust and reliable dual frequency polarimetric survey of active galactic nuclei (AGN). We find that the polarization orientations are consistent with the assumption of isotropy for scales larger than or equal to $\sim 800$ Mpc. This provides further evidence of isotropy at large distance scales and is likely to impose strong constraints on some of the physical mechanisms that may be invoked to explain past observations of alignment of radio and optical polarizations at large distance scales.

astro-ph.CO↗

arXiv2018

Large angular scale multipoles at redshift ~0.8

We prepare the full sky radio galaxy map ($|b|>10^{\circ}$) using the north NVSS and south SUMSS galaxy catalogs and study the large scale multipoles anomalies. These galaxies are roughly at redshift $z \sim 0.8$ and therefore tracing the matter distribution at very large scales. The quadruple and octopole from radio galaxy catalog are consistent with $Λ$CDM for a reasonable value of galaxy bias and we do not find dipole--quadruple--octopole alignment as seen in CMB temperature maps. The quadrupole direction is roughly $46^{\circ}$ away from dipole, and octopole direction is approximately $33^{\circ}$ from dipole. The angle between quadrupole and octopole is around $70^\circ$ degree. We have large errors in multipole directions due to shot noise, even so with this data we are able to rule out dipole--quadruple and quadruple--octopole alignment. The magnitude of all multipoles, except dipole, are roughly consistent with $Λ$CDM for reasonable galaxy bias. The dipole magnitude remains inconsistent with CMB as reported in previous studies. The results may impose stringent constraints on cosmological models with large scale anisotropy features.

astro-ph.CO↗

arXiv2016

Constraining axionlike particles using the distance-duality relation

One of the fundamental results used in observational cosmology is the distance duality relation (DDR), which relates the luminosity distance, $D_L$, with angular diameter distance, $D_A$, at a given redshift $z$. We employ the observed limits of this relation to constrain the coupling of axion like particles (ALPs) with photons. With our detailed $3D$ ALP-photon mixing simulation in standard $Λ$CDM universe and latest DDR limits observed in Holand \& Barros (2016) we limit the coupling constant $g_ϕ\le 6 \times 10^{-13} GeV^{-1}\left(\frac{nG}{\langle B \rangle_{\rm Mpc}}\right)$ for ALPs of mass $\le 10^{-15}$ eV. The DDR observations can provide very stringent constraint on ALPs mixing in future. Also any deviation in DDR can be conventionally explained as photons decaying to axions or vice-versa.

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↗

arXiv2016

Revisiting the NVSS number count dipole

We present a realistic modeling of the dipole component of the projected sky distribution of NVSS radio galaxies. The modeling relies on mock catalogs generated within the context of $Λ$CDM cosmology, in the linear regime of structure formation. After removing the contribution from the solar motion, the mocks show that the remaining observed signal is mostly (70\%) due to structures within $z\lesssim0.1$. The amplitude of the model signal depends on the bias factor $b$ of the NVSS mock galaxies. For sources with flux density, $ S> 15 \; \rm mJy$, the bias recipe inferred from higher order moments is consistent with the observed dipole signal at $2.12σ$. Flux thresholds above $20 \; \rm mJy$ yield a disagreement close to the $3σ$ level. A constant high bias, $b=3$ is needed to mitigate the tension to the $\sim 2.3σ$ level.

astro-ph.CO↗

arXiv2016

Extracting Spectral Index of Intergalactic Magnetic Field from Radio Polarizations

We explain the large scale correlations in radio polarization in terms of the correlations of galaxy cluster/supercluster magnetic field. Assuming that the polarization correlations closely follow the spatial correlations of the background magnetic field we recover the magnetic field spectral index as -2.74$\pm$0.04.This remarkably agrees with cluster magnetic field spectral index obtained in cosmological magneto-hydrodynamic simulations. We discuss possible physical scenarios in which the observed polarization alignment is plausible.

astro-ph.CO↗

arXiv2015

The clustering of radio galaxies: biasing and evolution versus stellar mass

We study the angular clustering of $\sim 6\times 10^5$ NVSS sources on scales $\gtrsim 50 h^{-1}$ Mpc in the context of the $Λ$CDM scenario. The analysis partially relies on the redshift distribution of 131 radio galaxies, inferred from the Hercules and CENSORS survey, and an empirical fit to the stellar to halo mass (SHM) relation. For redshifts $z\lesssim 0.7$, the fraction of radio activity versus stellar mass evolves as $f_{_{\rm RL}}\sim M_*^{α_0+α_1 z}$ where $α_0=2.529{\pm0.184}$ and $α_1=1.854^{+0.708}_{-0.761}$. The estimate on $α_0$ is largely driven by the results of Best et al. (2005), while the constraint on $α_1$ is new. We derive a biasing factor $b(z=0.5)=2.093^{+0.164}_{-0.109}$ between radio galaxies and the underlying mass.The function $b(z)=0.33z^2+0.85 z +1.6$ fits well the redshift dependence. We also provide convenient parametric forms for the redshift dependent radio luminosity function, which are consistent with the redshift distribution and the NVSS source count versus flux.

astro-ph.CO↗

arXiv2014

Dipole anisotropy in sky brightness and source count distribution in radio NVSS data

We study the dipole anisotropy in number counts and flux density weighted number counts {or sky brightness} in the NRAO VLA Sky Survey (NVSS) data. The dipole anisotropy is expected due to our local motion with respect to the CMBR rest frame. We analyse data with an improved fit to the number density, n(S), as a function of the flux density S, which allows deviation from a pure power law behaviour. We also impose more stringent cuts to remove the contribution due to clustering dipole. In agreement with earlier results, we find that the amplitude of anisotropy is significantly larger in comparison to the prediction based on CMBR measurements. The extracted speed is found to be roughly 3 times the speed corresponding to CMBR. The significance of deviation is smaller, roughly 2 sigma, in comparison to earlier estimates. For the cut, S>30 mJy, the speed is found to be $1110\pm370$ Km/s using the source count analysis. The direction of the dipole anisotropy is found to be approximately in agreement with CMBR. We find that the results are relatively insensitive to the lower as well as upper limit imposed on the flux density. Our results suggest that the Universe is intrinsically anisotropic with the axis of anisotropy axis pointing roughly towards the CMBR dipole direction. Finally we present a method which may allow an independent extraction of the local speed and an intrinsic dipole anisotropy, provided a larger data set becomes available in future.

astro-ph.CO↗

arXiv2014

Dipole Anisotropy in Integrated Linearly Polarized Flux Density in NVSS Data

We study the dipole anisotropy in integrated linearly polarization flux density in NRAO VLA Sky Survey (NVSS). We extract the anisotropy parameters in the number counts, number counts weighted by polarization observables, i.e. degree of polarization (p) and polarization flux (P). We also determine the anisotropy in the degree of polarization per source and polarization flux per source. We consider data with several different cuts on the flux density, S>10,20,30,40,50 mJy. For studies with degree of polarization we impose the additional cut 0.01 20 mJy to S>50 mJy. The corresponding direction parameters are found to be stable with the cut on flux density. The significance is even higher for the case of p weighted number counts. The observed anisotropy is found to be much larger in comparison to the CMBR expectations. We find that polarization observables show a much higher level of anisotropy in comparison to pure number counts or flux weighted number counts.

astro-ph.CO↗

arXiv2013

Polarization Alignment in JVAS/CLASS flat spectrum radio surveys

We present a detailed statistical analysis of the alignment of polarizations of radio sources at high redshift. We use the JVAS/CLASS 8.4-GHz surveys for our study. This study is motivated by the puzzling signal of alignment of polarizations from distant quasars at optical frequencies. We explore several different cuts on the polarization flux for our analysis. We find that the entire data shows a very significant signal of alignment on very large distance scales of order 500 Mpc. The alignment starts to decay only at much larger distances of order Gpc. If we only consider data with polarization flux greater than 1 mJy, we find alignment at distance scales less than 150 Mpc. We also find that data with polarization flux less than 0.5 mJy does not show significant alignment. Similar results are seen for data with degree of polarization less than 0.01, although here a mild signal of alignment is observed for a narrow range of angular separations. We argue that the signal cannot be explained in terms of bias due to systematic errors in removal of instrumental polarization. We also find that the degree of polarization shows a strong negative correlation with the total flux. The data appears to fall into two classes, one of which shows such a correlation. The remaining set, which has total flux greater than 100 mJy and degree of polarization lying between 0.01 and 0.1, shows a more random behaviour. The latter set is also found to show no alignment whereas the first set shows a very strong polarization alignment.

astro-ph.CO↗

arXiv2012

New limit on pseudoscalar-photon mixing from WMAP Observations

The pseudoscalar-photon mixing in presence of large scale magnetic field induces polarization in light from distant cosmological sources. We study the effect of these pseudoscalars or axion like particles (ALPs) on Cosmic Microwave Background Radiation (CMBR) and constrain the product of mixing strength $g_ϕ$ times background magnetic field $B$. The background magnetic field has been assumed to be primordial and we assume large scale correlations with the correlation length of 1Mpc. We use WMAP seven year foreground reduced polarization and temperature data to constrain pseudoscalar-photon mixing parameter. We look for different mass limits of the pseudoscalars and find $g_ϕB\le 1.6\times10^{-13} GeV^{-1} nG$ with ALPs of mass $10^{-10} eV$ and $g_ϕB\le3.4\times10^{-15} GeV^{-1} nG$ for ultra light ALPs of mass $10^{-15} eV$.

hep-ph↗

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↗