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T. R. Seshadri

Publications and source records attributed to T. R. Seshadri.

At least 37 records · Page 2Linked to original sources
arXiv2013

Inflation in Higher Dimensional Gauss-Bonnet Cosmology

A Gauss-Bonnet term naturally appears in the action for gravity when one considers the existence of space time with dimensions more than 1+3. A variety of inflationary models can be obtained within such a framework, once the scale factor for the hidden dimension(s) is not constrained to be the same as that of the visible ones. In particular, the need for an adhoc inflaton field is eliminated. The phase space has a rich structure with different types of solutions, both stable and unstable. For a large class of solutions, the scale factors rapidly approach an asymptotic exponential form. Furthermore, sufficient inflation can be obtained for only a modest compression of the hidden world, if the latter is of a sufficiently large dimension.

gr-qc↗
arXiv2012

Cosmic Microwave Background Trispectrum and Primordial Magnetic Field Limits

Primordial magnetic fields will generate non-Gaussian signals in the cosmic microwave background (CMB) as magnetic stresses and the temperature anisotropy they induce depend quadratically on the magnetic field. We compute a new measure of magnetic non-Gaussianity, the CMB trispectrum, on large angular scales, sourced via the Sachs-Wolfe effect. The trispectra induced by magnetic energy density and by magnetic scalar anisotropic stress are found to have typical magnitudes of approximately a few times 10^{-29} and 10^{-19}, respectively. Observational limits on CMB non-Gaussianity from WMAP data allow us to conservatively set upper limits of a nG, and plausibly sub-nG, on the present value of the primordial cosmic magnetic field. This represents the tightest limit so far on the strength of primordial magnetic fields, on Mpc scales, and is better than limits from the CMB bispectrum and all modes in the CMB power spectrum. Thus, the CMB trispectrum is a new and more sensitive probe of primordial magnetic fields on large scales.

astro-ph.CO↗
arXiv2011

Late-time acceleration in Higher Dimensional Cosmology

We investigate late time acceleration of the universe in higher dimensional cosmology. The content in the universe is assumed to exert pressure which is different in the normal and extra dimensions. Cosmologically viable solutions are found to exist for simple forms of the equation of state. The parameters of the model are fixed by comparing the predictions with supernovae data. While observations stipulate that the matter exerts almost vanishing pressure in the normal dimensions, we assume that, in the extra dimensions, the equation of state is of the form $P \propto \, ρ^{1 - γ}$. For appropriate choice of parameters, a late time acceleration in the universe occurs with $q_0$ and $z_{tr}$ being approximately -0.46 and 0.76 respectively.

gr-qc↗
arXiv2010

Primordial Magnetic Field Limits from Cosmic Microwave Background Bispectrum of Magnetic Passive Scalar Modes

Primordial magnetic fields lead to non-Gaussian signals in the cosmic microwave background (CMB) even at the lowest order, as magnetic stresses and the temperature anisotropy they induce depend quadratically on the magnetic field. In contrast, CMB non-Gaussianity due to inflationary scalar perturbations arises only as a higher order effect. Apart from a compensated scalar mode, stochastic primordial magnetic fields also produce scalar anisotropic stress that remains uncompensated till neutrino decoupling. This gives rise to an adiabatic-like scalar perturbation mode that evolves passively thereafter (called the passive mode). We compute the CMB reduced bispectrum ($b_{l_{_1}l_{_2}l_{_3}}$) induced by this passive mode, sourced via the Sachs-Wolfe effect, on large angular scales. For any configuration of bispectrum, taking a partial sum over mode-coupling terms, we find a typical value of $l_1(l_1+1)l_3(l_3+1) b_{l_{_1}l_{_2}l_{_3}} \sim 6-9 \times 10^{-16}$, for a magnetic field of $B_0 \sim 3$ nG, assuming a nearly scale-invariant magnetic spectrum . We also evaluate, in full, the bispectrum for the squeezed collinear configuration over all angular mode-coupling terms and find $l_1(l_1+1)l_3(l_3+1) b_{l_{_1}l_{_2}l_{_3}} \approx -1.4 \times 10^{-16}$. These values are more than $\sim 10^6$ times larger than the previously calculated magnetic compensated scalar mode CMB bispectrum. Observational limits on the bispectrum from WMAP7 data allow us to set upper limits of $B_0 \sim 2$ nG on the present value of the cosmic magnetic field of primordial origin. This is over 10 times more stringent than earlier limits on $B_0$ based on the compensated mode bispectrum.

astro-ph.CO↗
arXiv2010

Non-minimally coupled f(R) Cosmology

We investigate the consequences of non-minimal gravitational coupling to matter and study how it differs from the case of minimal coupling by choosing certain simple forms for the nature of coupling, The values of the parameters are specified at $z=0$ (present epoch) and the equations are evolved backwards to calculate the evolution of cosmological parameters. We find that the Hubble parameter evolves more slowly in non-minimal coupling case as compared to the minimal coupling case. In both the cases, the universe accelerates around present time, and enters the decelerating regime in the past. Using the latest Union2 dataset for supernova Type Ia observations as well as the data for baryon acoustic oscillation (BAO) from SDSS observations, we constraint the parameters of Linder exponential model in the two different approaches. We find that there is a upper bound on model parameter in minimal coupling. But for non-minimal coupling case, there is range of allowed values for the model parameter.

astro-ph.CO↗
arXiv2010

A CCD photometric study of the newly discovered contact binary ASAS 134738+0410.1

We present a CCD photometric study of the star with ASAS ID 134738 + 0410.1 using V band observations obtained from the $IUCAA$ Girawali Observatory (IGO) 2-metre telescope, India. The star was selected from the $δ$ Scuti database of All Sky Automated Survey (ASAS) (Pojmanski 2002). Our analysis reveals that the star is not a $δ$ Scuti variable but is in fact a W UMa type contact binary with an orbital period of 0.2853067 day. Two new times of primary and secondary minima were determined from the observed data. A preliminary solution obtained using the Wilson-Devinney light curve modelling technique indicates that the star is more likely a partially-eclipsing W UMa type contact binary. However, the determination of actual subtype of this binary is quite impossible from the photometry alone, as the observed light curve can fitted for both A- and W-type solutions. The exact classification of this binary needs to be determined from high resolution spectroscopy.

astro-ph.SR↗
arXiv2010

A CCD photometric study of the late type contact binary EK Comae Berenices

We present CCD photometric observations of the W UMa type contact binary EK Comae Berenices using the 2 metre telescope of $IUCAA$ Girawali Observatory, India. The star was classified as a W UMa type binary of subtype-W by \citet{sam1996}. The new V band photometric observations of the star reveal that shape of the light curve has changed significantly from the one observed by \citet{sam1996}. A detailed analysis of the light curve obtained from the high-precision CCD photometric observations of the star indicates that EK Comae Berenices is not a W-type but an A-type totally eclipsing W UMa contact binary. The photometric mass ratio is determined to be 0.349 $\pm$ 0.005. A temperature difference of $ΔT = 141 \pm 10 $ K between the components and an orbital inclination of $i [^{o}] = 89.800 \pm 0.075$ were obtained for the binary system. Absolute values of masses, radii and luminosities are estimated by means of the standard mass-luminosity relation for zero age main-sequence stars. The star shows O'Connell effect, asymmetries in the light curve shape around the primary and secondary maximum. The observed O'Connell effect is explained by the presence of a hot spot on the primary component.

astro-ph.SR↗
arXiv2010

Photometry of the $δ$ Scuti star HD 40372

We present B band photometry of the $δ$ Scuti star HD 40372 using the ARIES three channel fast photometer attached to the 104-cm Sampurnanand telescope in high-speed photometric mode. The star was observed for $\sim$ 5 hours on December 13, 2008. Based on the high quality photometric data, we have done period analysis by various periodogram analysis techniques. The best estimate of the period is found to be $\sim 0.067$ days. With this period and the other stellar parameters determined from $uvbyβ$ photometry available in the literature, we have calculated the $Q$ value for the star. Comparison of this $Q$ value with that determined from the model calculations shows that the star is pulsating in p$_{3}$ mode with $l = 2$.

astro-ph.SR↗
arXiv2009

CMB bispectrum from primordial magnetic fields on large angular scales

Primordial magnetic fields lead to non-Gaussian signals in the Cosmic Microwave Background (CMB) even at the lowest order, as magnetic stresses, and the temperature anisotropy they induce, depend quadratically on the magnetic field. In contrast, CMB non-Gaussianity due to inflationary scalar perturbations arise only as a higher order effect. We propose here a novel probe of stochastic primordial magnetic fields that exploits the characteristic CMB non-Gaussianity that they induce. In particular, we compute the CMB bispectrum ($b_{l_{_1}l_{_2}l_{_3}}$) induced by stochastic primordial fields on large angular scales. We find a typical value of $l_1(l_1+1)l_3(l_3+1) b_{l_{_1}l_{_2}l_{_3}} \sim 10^{-22}$, for magnetic fields of strength $B_0 \sim 3$ nano Gauss and with a nearly scale invariant magnetic spectrum. Current observational limits on the bispectrum allow us to set upper limits on $B_0 \sim 35$ nano Gauss, which can be improved by including other magnetically induced contributions to the bispectrum.

astro-ph.CO↗
arXiv2008

Cosmic acceleration in a model of scalar-tensor gravitation

In this paper we consider a model of scalar-tensor theory of gravitation in which the scalar field, $ϕ$ determines the gravitational coupling G and has a Lagrangian of the form, $\mathcal{L}_ϕ =-V(ϕ)\sqrt{1 - \partial_μϕ\partial^μϕ}$. We study the cosmological consequence of this theory in the matter dominated era and show that this leads to a transition from an initial decelerated expansion to an accelerated expansion phase at the present epoch. Using observational constraints, we see that the effective equation of state today for the scalar field turns out to be $p_ϕ=w_ϕρ_ϕ$, with $w_ϕ=-0.88$ and that the transition to an accelerated phase happened at a redshift of about 0.3.

gr-qc↗
arXiv2008

Scalar Field Dark Energy Perturbations and their Scale Dependence

We estimate the amplitude of perturbation in dark energy at different length scales for a quintessence model with an exponential potential. It is shown that on length scales much smaller than hubble radius, perturbation in dark energy is negligible in comparison to that in in dark matter. However, on scales comparable to the hubble radius ($λ_{p}>1000\mathrm{Mpc}$) the perturbation in dark energy in general cannot be neglected. As compared to the $Λ$CDM model, large scale matter power spectrum is suppressed in a generic quintessence dark energy model. We show that on scales $λ_{p} < 1000\mathrm{Mpc}$, this suppression is primarily due to different background evolution compared to $Λ$CDM model. However, on much larger scales perturbation in dark energy can effect matter power spectrum significantly. Hence this analysis can act as a discriminator between $Λ$CDM model and other generic dark energy models with $w_{de} \neq -1$.

astro-ph↗
arXiv2008

Fractal Dimensions of a Weakly Clustered Distribution and the Scale of Homogeneity

Homogeneity and isotropy of the universe at sufficiently large scales is a fundamental premise on which modern cosmology is based. Fractal dimensions of matter distribution is a parameter that can be used to test the hypothesis of homogeneity. In this method, galaxies are used as tracers of the distribution of matter and samples derived from various galaxy redshift surveys have been used to determine the scale of homogeneity in the Universe. Ideally, for homogeneity, the distribution should be a mono-fractal with the fractal dimension equal to the ambient dimension. While this ideal definition is true for infinitely large point sets, this may not be realised as in practice, we have only a finite point set. The correct benchmark for realistic data sets is a homogeneous distribution of a finite number of points and this should be used in place of the mathematically defined fractal dimension for infinite number of points (D) as a requirement for approach towards homogeneity. We derive the expected fractal dimension for a homogeneous distribution of a finite number of points. We show that for sufficiently large data sets the expected fractal dimension approaches D in absence of clustering. It is also important to take the weak, but non-zero amplitude of clustering at very large scales into account. In this paper we also compute the expected fractal dimension for a finite point set that is weakly clustered. Clustering introduces departures in the Fractal dimensions from D and in most situations the departures are small if the amplitude of clustering is small. Features in the two point correlation function, like those introduced by Baryon Acoustic Oscillations (BAO) can lead to non-trivial variations in the Fractal dimensions where the amplitude of clustering and deviations from D are no longer related in a monotonic manner.

astro-ph↗
arXiv2005

Some Consequences of Dark Energy Density varying Exponentially with Scale Factor

In this paper we have explored the consequences of a model of dark energy with its energy density varying exponentially with the scale factor. We first consider the model with $ ρ_ϕ \propto e^{κa} $, where $κ$ is a constant. This is a kind of generalisation of the cosmological constant model with $κ= 0$. We show that such an exponentially varying dark energy density with the scale factor naturally leads to an equivalent phantom field. We also consider a model with $ ρ_ϕ \propto e^{κ/a} $ and we show that this also naturally leads to an equivalent phantom field.

astro-ph↗
arXiv2005

Testing homogeneity on large scales in the Sloan Digital Sky Survey Data Release One

The assumption that the universe is homogeneous and isotropic on large scales is one of the fundamental postulates of cosmology. We have tested the large scale homogeneity of the galaxy distribution in the Sloan Digital Sky Survey Data Release One (SDSS-DR1) using volume limited subsamples extracted from the two equatorial strips which are nearly two dimensional (2D). The galaxy distribution was projected on the equatorial plane and we carried out a 2D multi-fractal analysis by counting the number of galaxies inside circles of different radii in the range 5 Mpc/h to 150 Mpc/h centred on galaxies. Different moments of the count-in-cells were analysed to identify a range of length-scales (60-70 Mpc/h to 150Mpc/h) where the moments show a power law scaling behaviour and to determine the scaling exponent which gives the spectrum of generalised dimension D_q. If the galaxy distribution is homogeneous, D_q does not vary with q and is equal to the Euclidean dimension which in our case is 2. We find that D_q varies in the range 1.7 to 2.2. We also constructed mock data from random, homogeneous point distributions and from LCDM N-body simulations with bias b=1, 1.6 and 2, and analysed these in exactly the same way. The values of D_q in the random distribution and the unbiased simulations show much smaller variations and these are not consistent with the actual data. The biased simulations, however, show larger variations in D_q and these are consistent with both the random and the actual data. Interpreting the actual data as a realisation of a biased LCDM universe, we conclude that the galaxy distribution is homogeneous on scales larger than 60-70 Mpc/h.

astro-ph↗
arXiv2005

CMB Anisotropy Due to Tangled magnetic fields in re-ionized models

Primordial tangled cosmological Magnetic Fields source rotational velocity perturbations of the baryon fluid, even in the post-recombination universe. These vortical modes inturn leave a characteristic imprint on the temperature anisotropy of the Cosmic Microwave Background (CMB), if the CMB photons can be re-scatterred after recombination. Observations from WMAP indicate that the Universe underwent a relatively early re-ionization ($z_{ri} \sim 15$), which does indeed lead to a significant optical depth for re-scattering of CMB photons after the re-ionization epoch. We compute the resulting additional temperature anisotropies, induced by primordial magnetic fields in the post-recombination universe. We show that in models with early re-ionization, a nearly scale-invariant spectrum of tangled magnetic fields which redshift to a present value of $B_0 \sim 3 \times 10^{-9}$ Gauss, produce vector mode perturbations which in turn induce additional temperature anisotropy of about 0.3 to 0.4 $ μ$K over very small angular scales, with $ l$ upto $\sim 10000 $ or so.

astro-ph↗
arXiv2003

Small-scale CMB polarization anisotropies due to tangled primordial magnetic fields

Tangled, primordial cosmic magnetic fields create small rotational velocity perturbations on the last scattering surface (LSS) of the cosmic microwave background radiation (CMBR). Such perturbations can contribute significantly to the CMBR temperature and polarization anisotropies at large l > 1000 or so, like the excess power detected by the CBI experiment. The magnetic contribution can be distinguished from most conventional signals, as they lead to CMBR polarization dominated by the odd parity, B-type signal. Experiments like DASI and WMAP have detected evidence for CMBR polarization at small l. Many experiments will also probe the large $l$ regime. We therefore calculate the polarization signals due to primordial magnetic fields, for different spectra and different cosmological parameters. A scale-invariant spectrum of tangled fields which redshifts to a present value B_{0}=3\times 10^{-9} Gauss, produces B-type polarization anisotropies of order 0.3 - 0.4 micro Kelvin between l of 1000-5000. Larger signals result if the spectral index of magnetic tangles is steeper, n>-3. The peak of the signal shifts to larger l for a lambda-dominated universe, or if the baryon density is larger. The signal will also have non-Gaussian statistics. We also predict the much smaller E-type polarization, and T-E cross correlations for these models.

astro-ph↗
arXiv2001

Self interacting Brans Dicke cosmology and Quintessence

Recent cosmological observations reveal that we are living in a flat accelerated expanding universe. In this work we have investigated the nature of the potential compatible with the power law expansion of the universe in a self interacting Brans Dicke cosmology with a perfect fluid background and have analyzed whether this potential supports the accelerated expansion. It is found that positive power law potential is relevant in this scenario and can drive accelerated expansion for negative Brans Dicke coupling parameter $ω$. The evolution of the density perturbation is also analyzed in this scenerio and is seen that the model allows growing modes for negative $ω$.

gr-qc↗