Research Papers (TP)

Permanent URI for this collectionhttp://localhost:4000/handle/11007/3

Browse

Now showing 1 - 20 of 258

1/3 factor in the CMB Sachs-Wolfe effect
(American Physical Society, 1992-01-23) Hwang, J.; Padmanabhan, T.; Lahav, O.; Noh, H.
We point out that a pseudo Newtonian interpretation of the 1/3 factor in the Sachs-Wolfe effect, which relates the ﬂuctuations in temperature and potential, dT/T5(1/3)dF, is not supported by the general relativ-istic analysis. Dividing the full gravitational effect into separate parts depends on the choice of time slicing ~gauge! and there exist inﬁnitely many different choices. More importantly, interpreting the parts as being due to the gravitational redshift and the time dilation is not justiﬁed in the rigorous relativistic perturbation theory. We suggest regarding the 1/3 factor as a general relativistic result that applies in a restricted situation of adiabatic perturbation in the K505L model with the last scattering occurring in the matter dominated era. For an isocurvature initial condition the corresponding result dT/T52dF has a different numerical coefﬁcient.
A 3D Automated Classification Scheme for the TAUVEX data pipeline
(Mon. Not. R. Astron. Soc, 2007-01-28) Bora, Archana; Gupta, Ranjan; Singh, Harinder P; et
In order to develop a pipeline for automated classification of stars to be observed by the TAUVEX ultraviolet space Telescope, we employ an artificial neural network (ANN) technique for classifying stars by using synthetic spectra in the UV region from 1250°A to 3220°A as the training set and International Ultraviolet Explorer (IUE) low resolution spectra as the test set. Both the data sets have been pre-processed to mimic the observations of the TAUVEX ultraviolet imager. We have successfully classified 229 stars from the IUE low resolution catalog to within 3-4 spectral sub-class using two different simulated training spectra, the TAUVEX spectra of 286 spectral types and UVBLUE spectra of 277 spectral types. Further, we have also been able to obtain the colour excess (i.e. E(B-V) in magnitude units) or the interstellar reddening for those IUE spectra which have known reddening to an accuracy of better than 0.1 magnitudes. It has been shown that even with the limitation of data from just photometric bands, ANNs have not only classified the stars, but also provided satisfactory estimates for interstellar extinction. The ANN based classification scheme has been successfully tested on the simulated TAUVEX data pipeline. It is expected that the same technique can be employed for data validation in the ultraviolet from the virtual observatories. Finally, the interstellar extinction estimated by applying the ANNs on the TAUVEX data base would provide an extensive extinction map for our galaxy and which could in turn be modeled for the dust distribution in the galaxy.
Accelerated expansion of the universe driven by tachyonic matter
(American Physical Society, 2002-06-28) Padmanabhan, T.
Acceptable density perturbations from inflation due to quantum gravitational damping
(American Physical Society, 1988-05-30) Padmanabhan, T.
Nonperturbative quantum gravitational effects can act as a damping mechanism in primordial inflation. It is shown that this effect can reduce the value of δρ/ρ to an acceptable value of ∼10-4 in primordial inflation. The same effect reduces the gravitational-wave background by a factor 10-6, making the primordial inflation viable.
Action principle for the fluid-gravity correspondence and emergent gravity
(Astronomical Society of India, 2012-01-04) Kolekar, Sanved; Padmanabhan, T.
It has been known for a long time that Einstein’s field equations when projected onto a black hole horizon look very similar to a Navier-Stokes equation in suitable variables. More recently, it was shown that the projection of Einstein’s equation onto any null surface in any spacetime reduces exactly to the Navier-Stokes form when viewed in the freely falling frame. We develop an action principle, the extremization of which leads to the above result, in an arbitrary spacetime. The degrees of freedom varied in the action principle are the null vectors in the spacetime and not the metric tensor. The same action principle was introduced earlier in the context of the emergent gravity paradigm wherein it was shown that the corresponding Lagrangian can be interpreted as the entropy density of spacetime. The current analysis strengthens this interpretation and reinforces the idea that field equations in gravity can be thought of as emergent. We also find that the degrees of freedom on the null surface are equivalent to a fluid with equation of state PA = TS. We demonstrate that the same relation arises in the context of a spherical shell collapsing to form a horizon
Ambiguities in Fluid Flow
(Indian Academy of Sciences, 2008-09-28) Padmanabhan, T.
The idealized flow of fluid around a spherical body is a classic textbook problem in fluid mechanics. Interestingly enough, it leads to some curious twists and turns and conceptual conundrums.
Analytic approach to string-induced phase transitions
(American scientific journal, 1987-05-15) Yajnik, U. A.; Padmanabhan, T.
In an early work it was shown that gauge-theory strings present in the early Universe could have converted a potentially first-order phase transition into a second order one. The demonstration was based on numerical integration of the relevant equations. In this paper we discuss the model employed there in a self-contained fashion and present analytical arguments to show that the previous results are of a more general validity, applicable to "seeding" in any supercooled medium. It is shown that in the presence of an appropriate seed, the supercooled state does not minimize the free energy (even locally) below a critical temperature. This contrasts with the phase transition in the absence of seeds, which is accompanied by extensive supercooling and which can be completed only by quantum tunneling.
Angular momentum of electromagnetic field
(Indian Academy of Sciences, 2008-02-01) Padmanabhan, T.
Electromagnetic fields carry not only energy and momentum but also angular momentum . The angular momentum of the field can lead to some curious results like the one which is described here.
Approach to quantum gravity
(American Physical Society, 1983-08-15) Padmanabhan, T.
A model for quantum gravity is presented by treating the light-cone structure of space time as classical and the conformal factor as a quantum degree of freedom. The motivation and the details of the formalism are discussed. The approach is used to discuss the question of singularities in the cosmological models. It is shown that one can introduce the concept of stationary states for the quantum geometry, in analogy with the stationary states of simple quantum systems. The quantum stationary geometries QSG's)avoid the classical singularities. The light-cone structure is determined by a set of equations involving the expectations values in the QSG concerned. The cosmological implication of the formalism, especially to matter creation, flatness, etc. are discussed. The theory is conformally invariant in the quantum level.
Aspects of electrostatics in a weak gravitational ﬁeld
(Springer, 2009-11-13) Padmanabhan, Hamsa; Padmanabhan, T.
Several features of electrostatics of point charged particles in a weak, homogeneous, gravitational ﬁeld are discussed using the Rindler metric to model the gravitational ﬁeld. Some previously known results are obtained by simpler and more transparent procedures and are interpreted in an intuitive manner. Specifically: (a)We discuss possible definitions of the electric ﬁeld in curved spacetime (and noninertial frames), argue in favour of a speciﬁc definition for the electric ﬁeld and discuss its properties. (b)We show that the electrostatic potential of a charge at rest in the Rindler frame (which is known and is usually expressed as a complicated function of the coor- dinates) is expressible as A0 = q/λ where λ is the afﬁne parameter distance along the null geodesic fromthe charge to the ﬁeld point. (c) This relates well with the result that the electric ﬁeld lines of a charge coincide with the null geodesics; that is, both light and the electric ﬁeld lines ‘bend’ in the same manner in a weak gravitational ﬁeld. We provide a simple proof for this result as well as for the fact that the null geodesics (and ﬁeld lines) are circles in space. (d) We obtain the sum of the electrostatic forces exerted by one charge on another in the Rindler frame and discuss its interpretation. In particular, we compare the results in the Rindler frame and in the inertial frame and discuss their consistency. (e) We show how a purely electrostatic term in the Rindler frame appears as a radiation term in the inertial frame. (In part, this arises because charges at rest in a weak gravitational ﬁeld possess additional weight due to their electrostatic energy. This weight is proportional to the acceleration and falls inversely with distance—which are the usual characteristics of a radiation ﬁeld.) (f) We also interpret the origin of the radiation reaction term by extending our approach to include a slowly varying acceleration. Many of these results might have possible extensions for the case of electrostatics in an arbitrary static geometry.
Aspects of Zeldovich approximation
(American Astronomical Society, 1993-04-15) Padmanabhan, T.; Subramanian, Kandaswamy
A generalized version of the Zel'dovich approximation which is applicable in both the radiation-dominated and the matter dominated epochs is presented. This approximation allows one to follow the growth of inhomogeneities from the time the mode enters the hubble radius until it turns around. Comparison of the results with the standard spherical model shows that the analytical approximation is quite good even in the nonlinear regime. Detailed application to cold dark matter models and seeded models are given.
Attempt to explain the smallness of the cosmological constant
(World Scientific Publication Company, 1987-11-09) Singh, T. P.; Padmanabhan, T.
Fields which couple directly to the cosmological constant (Λ) may provide a scenario for explaining the smallness of Λ at the present epoch. In this paper we postulate the existence of a scalar field which couples universally to the trace of energy—momentum tensor of matter. Various possibilities for the explicit form of the coupling function are considered. The field equations in such a theory are derived, and the cosmological models with such a scalar field are analyzed. The proposed coupling makes the effective cosmological constant a dynamically evolving quantity, which can be driven to zero by allowing the scalar field to grow to sufficiently large values. For the case of linear coupling, however, it does not seem to be possible to attain sufficient growth during the age of the universe (~1017 s). A quadratic coupling to the trace can evolve Λ to a value consistent with today’s observations, but the universe is dominated by the scalar field, rather than by radiation, at late times. The evolution is singular for couplings through a higher power law, in that the scalar field blows up at a finite time. The model is not very sensitive to initial conditions and the problems encountered can be avoided only by a severe fine-tuning of the parameters in the basic theory.
Automated classification of sloan digital sky survey (SDSS) stellar spectra using artificial neural networks
(Astrophys Space Sci, 2008-04-21) Bazarghan, Mahdi; Gupta, Ranjan
Automated techniques have been developed to automate the process of classification of objects or their analysis. The large datasets provided by upcoming spectroscopic surveys with dedicated telescopes urges scientists to use these automated techniques for analysis of such large datasets which are now available to the community. Sloan Digital Sky Survey (SDSS) is one of such surveys releasing massive datasets. We use Probabilistic Neural Network (PNN) for automatic classification of about 5000 SDSS spectra into 158 spectral type of a
Bounds on vacuum energy density in a general cosmological scenario
(Elsevier Science Publishers, 1987-02-09) Joshi, P. S.; Padmanabhan, T.; Chitre, S. M.
General limits on the cosmological constant (or equivalently the vacuum energy density) are derived for an inflationary universe. This is accomplished under the general assumption of global hyperbolicity and without the use of any special properties like spherical symmetry or homogeneity ofthe underlying spacetime. Aclear upper limit of 1/3 is obtained for the vacuum energy density parameterQ~,while the lower limit is found to depend on the age of the oldest object in the universe.
C-field cosmology: A possible solution to singularity, horizon and flatness problems
(American Physical Society, 1985-10-15) Narlikar, J. V.; Padmanabhan, T.
A solution of Einstein's equations which admits radiation and a negative-energy massless scalar creation field as a source is presented. It is shown that the cosmological model based on this solution satisfies all the observational tests and thus is a viable alternative to the standard big-bang model. The presentation model is from singularity and particle horizon and provides a natural explanation for the flatness problem. We argue that these features make the creation-field cosmological model theoretically superior to the big-bang model.
Can curvature effects be neglected in the early universe?
(Elsevier Science Publishers, 1982-05-31) Padmanabhan, T.; Vasanthi, M. M.
In the discussion of GUTS in the early universe it is assumed that there exists a locally inertial space-time region large enough to contain a sufficient number of particles justifying the use of fiat space statistical mechanics. We show that this assumption is false.
Can the clustered dark matter and the smooth dark energy arise from the same scalar ﬁeld?
(American Physical Society, 2002-10-29) Padmanabhan, T.; Choudhury, T. Roy
Cosmological observations suggest the existence of two different kinds of energy densities dominating at small ( ≲ 500 Mpc) and large (≳ 1000 Mpc) scales. The dark matter component, which dominates at small scales, contributes Ωm ≈ 0.35 and has an equation of state p=0 while the dark energy component, which dominates at large scales, contributes ΩV ≈ 0.65 and has an equation of state p≃ -ρ. It is usual to postulate wimps for the first component and some form of scalar field or cosmological constant for the second component. We explore the possibility of a scalar field with a Lagrangian L =- V(φ) √1 - deli φ deli φ acting as ıt both clustered dark matter and smoother dark energy and having a scale dependent equation of state. This model predicts a relation between the ratio r = ρV/ρDM of the energy densities of the two dark components and expansion rate n of the universe (with a(t) ∝ tn) in the form n = (2/3) (1+r) . For r ≈ 2, we get n ≈ 2 which is consistent with observations.
Casimir effect confronts cosmological constant
(Elsevier Science Publishers, 2006-08-23) Mahajan, Gaurang; Sarkar, Sudipta; Padmanabhan, T.
It has been speculated that the zero-point energy of the vacuum, regularized due to the existence of a suitable ultraviolet cut-off scale, could be the source of the on-vanishing cosmological constant that is driving the present acceleration of the universe. We show that the presence of such a cut-off can significantly alter the results for the Casimir force between parallel conducting plates and even lead to repulsive Casimir force when the plate separation is smaller than the cut-off scale length. Using the current experimental data we rule out the possibility that the observed cosmological constant arises from the zero-point energy which is made finite by a suitable cut-off. Any such cut-off which is consistent with the observed Casimir effect will lead to an energy density which is at least about 10¹² times larger than the observed one, if gravity couples to these modes. The implications are discussed.
Challenges in nonlinear gravitational clustering
(Elsevier, 2006-04-10) Padmanabhan, T.
This article addresses some issues related to the statistical description of gravitating systems in an expanding backgrounds. In particular, I describe (a) how the nonlinear mode-mode coupling transfers power from one scale to another in the Fourier space if the initial power spectrum is sharply peaked at a given scale and (b) what are the asymptotic characteristics of gravitational clustering that are independent of the initial conditions. The analysis uses a new approach based on an integro-differential equation for the evolution of the gravitational potential in the Fourier space. I show how this equation allows one to understand several aspects of nonlinear gravitational clustering and provides insight in to the transfer of power from one scale to another through nonlinear mode coupling. Numerical simulations as well as analytic work shows that power transfer leads to a universal power spectrum at late times, somewhat reminiscent of the existence of Kolmogorov spectrum in fluid turbulence.
Classical and quantum thermodynamics of horizons in spherically symmetric spacetime
(IOP Publishing, 2002-10-14) Padmanabhan, T.
Ageneral formalism for understanding the thermodynamics of horizons in spherically symmetric spacetimes is developed. The formalism reproduces known results in the case of black-hole spacetimes and can handle more general situations such as: (i) spacetimes whichare not asymptotically ﬂat (such as the de Sitter spacetime) and (ii) spacetimes with multiple horizons having different temperatures (such as the Schwarzschild–de Sitter spacetime) and provide a consistent interpretation for temperature, entropy and energy. I show that it is possible to write Einstein’s equations for a spherically symmetric spacetime in the form T dS − dE = P dV near any horizon of radius a with S = 1 4 (4πa2), |E|= (a/2) and the temperature T determined from the surface gravity at the horizon. The pressure P is provided by the source of Einstein’s equations and dV is the change in the volume when the horizon is displaced inﬁnitesimally. The same results can be obtained by evaluating the quantum mechanical partition function without usin g Ein stein’s equations or th eWKB approximatio n for th e a ctio n .Boththe classical and quantum analyses provide a simple and consistent interpretation of entropy and energy for de Sitter spacetime as well as for (1+2) dimensional gravity. For the Rindler spacetime the entropy per unit transverse area turns out to be 1 4 while the energy is zero. The approach also shows that the de Sitter horizon—like the Schwarzschild horizon—is effectively one dimensional as far as the ﬂow of information is concerned, while the Schwarzschild–de Sitter, Reissner–Nordstrom horizons are not. The implications for spacetimes with multiple horizons are discussed.