Browsing by Author "Seshadri, T. R."

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CCD photometric study of the late type contact binary EK comae berenices
(2010-04-26) Deb, Sukanta; Singh, Harinder P.; Seshadri, T. R.; et al.
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 Samec et al. [20]. The new V band photometric observations of the star reveal that shape of the light curve has changed signiﬁcantly from the one observed by Samec et al. [20]. 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 ± 0.005. A temperature diﬀerence of ∆T = 141 ± 10 K between the components and an orbital inclination of i[°] = 89.800 ± 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.
CMB Anisotropy Due to Tangled magnetic fields in re-ionized models
(2005-04-01) Seshadri, T. R.; Subramanian, Kandaswamy
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 in- dicate that the Universe underwent a relatively early re-ionization (zri ∼ 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 postrecombination 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 B0 ∼ 3 × 10−9 Gauss, produce vector mode perturbations which in turn induce additional temperature anisotropy of about 0.3 to 0.4 µK over very small angular scales, with l upto ∼ 10000 or so.98.62.En, 98.70.Vc, 98.80.Cq, 95.30.Qd
CMB non-Gaussianity from cosmic magnetic fields
(2011-08-12) Seshadri, T. R.
Cosmic Microwave Background Bispectrum from Primordial Magnetic Fields on Large Angular Scales
(2009-09-01) Seshadri, T. R.; Subramanian, Kandaswamy
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 (bl 1 l2 l3)induced by stochastic primordial ﬁelds on large angular scales. We ﬁnd a typical value of l1(l1 + 1)l3(l3 + 1)bl1 l2 l3∼ 10−22, for magnetic ﬁelds of strength B0 ∼ 3 nano Gauss and with a nearly scale invariantmagnetic spectrum. Current observational limits on the bispectrum allow us to set upper limits on B0 ∼ 35 nano Gauss, which can be improved by including other magnetically induced contributions to the bispectrum
Cosmic Microwave Background Trispectrum and Primordial Magnetic Field Limits
(2012-06-08) Trivedi, Pranjal; Seshadri, T. R.; Subramanian, Kandaswamy
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.
Counter images in closed elliptical friedmann universes
(American Astronomical Society, 1985-01-06) Narlikar, J. V.; Seshadri, T. R.
It is shown that the different connectivity implied by the elliptical version of a closed Friedmann model allows two images of a distant astronomical object to be seen, provided the deceleration parameter q0 of the Friedmann model exceeds unity. Of the two images the direct image is along the shortest-route light track. If the redshift of the direct image exceeds (1.5q0 - 1)/(q0 - 1)sq then a second counterimage should be visible at the diametrically antipodal position. The direct image has a maximum possible redshift, and it is suggested that the apparent cutoff in the redshifts of QSOs may be due to this effect. Other observable consequences of this result are discussed in relation to galaxies and QSOs.
Does inflation solve the horizon problem?
(IOP Publishing, 1987-08-27) Padmanabhan, T.; Seshadri, T. R.
We stress the distinction between ‘observed region of the universe’ and ‘observable region of the universe’. We show that (i) the observable region of the universe could not have evolved out of a causally connected domain in the past, even in inflationary models, and (ii) the homogeneity of CMBR is due to the fact that the universe is still young.
Effects of curvature and interactions on the dynamics of the deconfinement phase transition
(2011-07-06) Subramanian, Kandaswamy; Seshadri, T. R.; Barrow, John D.
Gaussian States in de Sitter Spacetime and the Evolution of Semiclassical density perturbations. 2. Inhomogeneous Modes
(Indian Academy of Sciences, 1989-08-08) Seshadri, T. R.; Padmanabhan, T.
In the computation of density perturbation in inflation it is conventional to assume the inflation field to be in the vacuum state. There are, however, some advantages in relaxing this assumption. In an earlier paper we have estimated the density perturbations in a Gaussian coherent state using a toy-model. Here we extend this work by doing an exact analysis of this problem. The advantages of this method is discussed and the results are compared with earlier results.
Gaussian states in the De-Sitter spacetime and the evolution of semiclassical density perturbations: 1. Homogeneous mode
(Indian Academy of Sciences, 1989-08-08) Seshadri, T. R.; Padmanabhan, T.
The evolution of Gaussian quantum states in the de Sitter phase of the early universe is investigated. The potential is approximated by that of an inverted oscillator. We study the origin and magnitude of the density perturbations with special emphasis on the nature of the semiclassical limits.
Horizon problem and inflation
(Indian Academy of Sciences, 1987-06-24) Padmanabhan, T.; Seshadri, T. R.
We show that, the part of the universe that is observable today (in principle), could not have evolved out of a domain which was causally connected in the past. This and other issues related to horizon problem in inflationary models are discussed.
Making inflation work : Damping of density perturbations due to Planck energy cut-off
(American Physical Society, 1989-04-15) Padmanabhan, T.; Seshadri, T. R.; Singh, T. P.
In this paper we propose an alternative method for the computation of classical density perturbations from a quantum field in an inflationary scenario. We compute the power spectrum of density perturbations directly from vacuum Auctuations of the "time-time" component of the energy momentum tensor. We compute the inhornogeneous part of~ the correlation function (0~ T o(x, t)T 0(y, t) ~0) for a massless minimally coupled scalar field in de Sitter space. The Fourier transform of this two-point function leads to the scale-invariant spectrum of perturbations, but is ultraviolet divergent. This expression can be made finite by introducing an (ad hot.") small-distance cutoff' in the proper length. We argue that this cutoff'should be of the order of the Planck length, and show that, in such a case, the density Auctuations have the acceptable magnitude (-10 ) for the case of primordial inflation. Thus the inflationary scenario can be made to work without any
Probing the origin of large inhomogeneities in inflation using a toy quantum mechanical model
(American Physical Society, 1986-08-15) Padmanabhan, T.; Seshadri, T. R.
We study the quantum evolution of the homogeneous mode of a scalar field with simple potentials in Robertson-Walker spacetime. The simplicity of the model allows one to work out the exact quantum evolution of a wave packet. The model mimics the inflationary scenario quite well and is used to study the nature and origin of various constraints. We conclude that any model in which (i) the potential has only a single energy scale (10¹⁴ –10¹⁵ GeV) and dimensionless parameters of order unity and (ii) the inflationary phase is endowed with a minimum temperature of (H/2π) will produce density inhomogeneity which is too large.
Quantum uncertainty in the horizon size in an inflationary universe
(World Scientific Publication Company, 1987-12-16) Padmanabhan, T.; Seshadri, T. R.
It is shown that uncertainty principle prevents simultaneous, exact measurement of the expansion factor S(t) and its time derivative Ṡ(t) in a FRW universe. We compute the uncertainties in these parameters in the semiclassical limit. It is shown that (i) there is a minimum uncertainty ΔlH in the Hubble distance lH(t)=(Ṡ/S)−1 in a semiclassical universe. (ii) this uncertainty is amplified to an unacceptably large value if there was an inflationary phase in the early universe.
Uncertainty principle and the horizon size of our universe
(Springer, 1986-07-21) Padmanabhan, T.; Seshadri, T. R.
Uncertainty principle and the quantum fluctuations of the schwarzschild light cones
(World Scientiﬁc Publishing Company, 1985-10-12) Padmanabhan, T.; Seshadri, T. R.; Singh, T. P.
We consider the gravitational field of a point mass and show that the application of the uncertainty principle leads to (i) an uncertainty relation for the metric and its conjugate momentum and (ii) finite fluctuations of the light-cone at the event horizon.