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Bharat Ratra

Publications and source records attributed to Bharat Ratra.

At least 109 records · Page 6Linked to original sources
arXiv2009

Constraints on dark energy models from radial baryon acoustic scale measurements

We use the radial baryon acoustic oscillation (BAO) measurements of Gaztanaga et al. (2008) to constrain parameters of dark energy models. These constraints are comparable with constraints from other "non-radial" BAO data. The radial BAO data are consistent with the time-independent cosmological constant model but do not rule out time-varying dark energy. When we combine radial BAO and the Kowalski et al. (2008) Union type Ia supernova data we get very tight constraints on dark energy.

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arXiv2009

Gravitational Radiation from Primordial Helical Inverse Cascade MHD Turbulence

We consider the generation of gravitational waves by primordial helical inverse cascade magnetohydrodynamic (MHD) turbulence produced by bubble collisions at the electroweak phase transition. We extend the previous study \cite{kgr08} by considering both currently discussed models of MHD turbulence. For popular electroweak phase transition parameter values, the generated gravitational wave spectrum is only weakly dependent on the MHD turbulence model. Compared to the unmagnetized electroweak phase transition case, the spectrum of MHD-turbulence-generated gravitational waves peaks at lower frequency with larger amplitude and can be detected by the proposed Laser Interferometer Space Antenna

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arXiv2008

The Beginning and Evolution of the Universe

We review the current standard model for the evolution of the Universe from an early inflationary epoch to the complex hierarchy of structure seen today. We summarize and provide key references for the following topics: observations of the expanding Universe; the hot early Universe and nucleosynthesis; theory and observations of the cosmic microwave background; Big Bang cosmology; inflation; dark matter and dark energy; theory of structure formation; the cold dark matter model; galaxy formation; cosmological simulations; observations of galaxies, clusters, and quasars; statistical measures of large-scale structure; and measurement of cosmological parameters. We conclude with discussion of some open questions in cosmology. This review is designed to provide a graduate student or other new worker in the field an introduction to the cosmological literature.

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arXiv2008

Gravitational Radiation from Primordial Helical MHD Turbulence

We consider gravitational waves (GWs) generated by primordial inverse-cascade helical magneto-hydrodynamical (MHD) turbulence produced by bubble collisions at the electroweak phase transitions (EWPT). Compared to the unmagnetized EWPT case, the spectrum of MHD-turbulence-generated GWs peaks at lower frequency with larger amplitude and can be detected by the proposed Laser Interferometer Space Antenna (LISA).

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arXiv2008

Constraints on Dark Energy from Galaxy Cluster Gas Mass Fraction versus Redshift data

We use the Allen et al. (2008) galaxy cluster gas mass fraction versus redshift data to constrain parameters of three different dark energy models: a cosmological constant dominated one ($Λ$CDM); the XCDM parameterization of dark energy; and a slowly-rolling scalar field model with inverse-power-law potential energy density. (Instead of using the Monte Carlo Markov Chain method, when integrating over nuisance parameters we use an alternative method of introducing an auxiliary random variable.) The resulting constraints are consistent with, and typically more constraining than, those derived from other cosmological data. A time-independent cosmological constant is a good fit to the galaxy cluster data, but slowly evolving dark energy cannot yet be ruled out.

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arXiv2006

CMB anisotropies due to cosmological magnetosonic waves

We study scalar mode perturbations (magnetosonic waves) induced by a helical stochastic cosmological magnetic field and derive analytically the corresponding cosmic microwave background (CMB) temperature and polarization anisotropy angular power spectra. We show that the presence of a stochastic magnetic field, or an homogeneous magnetic field, influences the acoustic oscillation pattern of the CMB anisotropy power spectrum, effectively acting as a reduction of the baryon fraction. We find that the scalar magnetic energy density perturbation contribution to the CMB temperature anisotropy is small compared to the contribution to the CMB $E$-polarization anisotropy.

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arXiv2006

Gamma Ray Burst Constraints on Ultraviolet Lorentz Invariance Violation

We present a unified general formalism for ultraviolet Lorentz invariance violation (LV) testing through electromagnetic wave propagation, based on both dispersion and rotation measure data. This allows for a direct comparison of the efficacy of different data to constrain LV. As an example we study the signature of LV on the rotation of the polarization plane of $γ$-rays from gamma ray bursts in a LV model. Here $γ$-ray polarization data can provide a strong constraint on LV, 13 orders of magnitude more restrictive than a potential constraint from the rotation of the cosmic microwave background polarization proposed by Gamboa, López-Sarrión, and Polychronakos (2006).

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arXiv2006

Cosmological Constraints from Hubble Parameter versus Redshift Data

We use the Simon, Verde, & Jimenez (2005) determination of the redshift dependence of the Hubble parameter to constrain cosmological parameters in three dark energy cosmological models. We consider the standard $Λ$CDM model, the XCDM parameterization of the dark energy equation of state, and a slowly rolling dark energy scalar field with an inverse power-law potential. The constraints are restrictive, consistent with those derived from Type Ia supernova redshift-magnitude data, and complement those from galaxy cluster gas mass fraction versus redshift data.

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arXiv2006

Inflation and accelerated expansion TeVeS cosmological solutions

We find exact exponentially expanding and contracting de Sitter solutions of the spatially homogeneous TeVeS cosmological equations of motion in the vacuum TeVeS model and a power law accelerated expanding solution in thepresence of an additional ideal fluid with equation of state parameter $-5/3 < ω< -1$. A preliminary stability analysis shows that the expanding vacuum solution is stable, while in the ideal fluid case stability depends on model parameter values. These solutions might provide a basis for incorporating early-time inflation or late-time accelerated expansion in TeVeS cosmology.

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arXiv2006

Supernova Ia and Galaxy Cluster Gas Mass Fraction Constraints on Dark Energy

We use the Riess et al.(2004) supernova Ia apparent magnitude versus redshift data and the Allen et al.(2004) galaxy cluster gas mass fraction versus redshift data to constrain dark energy models. These data provide complementary constraints that when combined together ignificantly restrict model parameters and favor lowly-evolving dark energy density models, close to the Einstein cosmological constant limit of dark energy.

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arXiv2005

Polarized Cosmological Gravitational Waves from Primordial Helical Turbulence

We show that helical turbulence produced during a first-order phase transition generates circularly polarized cosmological gravitational waves (GWs). The characteristic frequency of these GWs for an extreme case of the phase transition model is around $10^{-3}$ --- $10^{-2}$ Hz with an energy density parameter as high as $10^{-12}$ --- $10^{-11}$. The possibility of detection is briefly discussed.

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arXiv2005

Neutrino Mass Limit from Galaxy Cluster Number Density Evolution

Measurements of the evolution with redshift of the number density of massive galaxy clusters are used to constrain the energy density of massive neutrinos and so the sum of neutrino masses $\sum m_ν$. We consider a spatially-flat cosmological model with cosmological constant, cold dark matter, baryonic matter, and massive neutrinos. Accounting for the uncertainties in the measurements of the relevant cosmological parameters we obtain a limit of $\sum m_ν$ $<$ 2.4 eV (95 % C.L.).

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arXiv2005

Effects of Cosmological Magnetic Helicity on the Cosmic Microwave Background

Cosmological magnetic fields induce temperature and polarization fluctuations in the cosmic microwave background (CMB) radiation. A cosmological magnetic field with current amplitude of order $10^{-9}$ G is detectable via observations of CMB anisotropies. This magnetic field (with or without helicity) generates vector perturbations through vortical motions of the primordial plasma. This paper shows that magnetic field helicity induces parity-odd cross correlations between CMB temperature and $B$-polarization fluctuations and between $E$- and $B$-polarization fluctuations, correlations which are zero for fields with no helicity (or for any parity-invariant source). Helical fields also contribute to parity-even temperature and polarization anisotropies, cancelling part of the contribution from the symmetric component of the magnetic field. We give analytic approximations for all CMB temperature and polarization anisotropy vector power spectra due to helical magnetic fields. These power spectra offer a method for detecting cosmological helical magnetic fields, particularly when combined with Faraday rotation measurements which are insensitive to helicity.

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arXiv2005

Faraday Rotation of the Cosmic Microwave Background Polarization by a Stochastic Magnetic Field

A primordial cosmological magnetic field induces Faraday rotation of the cosmic microwave background polarization. This rotation produces a curl-type polarization component even when the unrotated polarization possesses only gradient-type polarization, as expected from scalar density perturbations. We compute the angular power spectrum of curl-type polarization arising from small Faraday rotation due to a weak stochastic primordial magnetic field with a power-law power spectrum. The induced polarization power spectrum peaks at arcminute angular scales. Faraday rotation is one of the few cosmological sources of curl-type polarization, along with primordial tensor perturbations, gravitational lensing, and the vector and tensor perturbations induced by magnetic fields; the Faraday rotation signal peaks on significantly smaller angular scales than any of these, with a power spectrum amplitude which can be comparable to that from gravitational lensing. Prospects for detection are briefly discussed.

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arXiv2004

Constraints on Scalar-Field Dark Energy from Galaxy Cluster Gas Mass Fraction versus Redshift

We use the Allen et al. (2004) Chandra measurements of x-ray gas mass fraction of 26 rich clusters to place constraints on the scalar-field dark energy model with inverse power law potential energy density. The constraints are consistent with, and typically more constraining than, those from other cosmological tests, and mildly favor the Einstein cosmological constant limit of the dark energy model.

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arXiv2004

Looking for Cosmological Alfven Waves in WMAP Data

A primordial cosmological magnetic field induces and supports vorticity or Alfven waves, which in turn generate cosmic microwave background (CMB) anisotropies. A homogeneous primordial magnetic field with fixed direction induces correlations between the $a_{l-1,m}$ and $a_{l+1,m}$ multipole coefficients of the CMB temperature anisotropy field. We discuss the constraints that can be placed on the strength of such a primordial magnetic field using CMB anisotropy data from the WMAP experiment. We place 3 $σ$ upper limits on the strength of the magnetic field of $B < 15$ nG for vector perturbation spectral index $n=-5$ and $B<1.7$ nG for $n=-7$.

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