Research Publications

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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.
21 June, 2001 che suryagrahan
(Marathi Vidnyan Parishad Patrika, 2015-04-10) Narlikar, J. V.
3C 273 : A gravitationally lensed quasar?
(EDP Sciences, 1984-05-30) Chitre, S. M.; Narasimha, D.; Narlikar, J. V.; et al.
It is suggested that the abnormal brightness of the quasar 3C 273, coupled with the observations of superluminal motions in its radio core B as well as the misalignment of its VLBI jet with the optical jet, can be understood by postulating a faint spheroidal lensing galaxy located about halfway along the line of sight to the quasar. The lens model also helps to understand why the quasar is found slightly off-centre with respect to its nebulosity. The probability for such a lens system to arise for 3C 273 by chance is shown to be no less than that computed on the basis of the relativistic beaming hypothesis. Further tests of the lens model are discussed.
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.
A 3D Automated Classification Scheme for the TAUVEX data pipeline
(Mon. Not. R. Astron. Soc., 2007-02-02) Bora, Archana; Gupta, Ranjan; Singh, Harinder P; et.al
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.
400 years of astronomical discoveries
(Financial chronicle, 2009-11-02) Narlikar, J. V.
Aakashatale dole
(Saptahik sihasan, 2000-08-02) Narlikar, J. V.
Aakashvani aani me
(2015-04-10) Narlikar, J. V.
Abbe' Georges Lamaitree:Father of the Primeval Atom
(Current science, 1994-01-01) Naralikar, J.V.
Abhyaranya
(Chhatra prabodhan, 2000-02-01) Narlikar, J. V.
Absorber theory of radiation in expanding universe
(Kluwer Academic Publishers, 2002-10-28) Narlikar, J. V.
The Wheeler-Feynman absorber theory of radiation of the symmetric combination of retarded and advanced potentials, originally developed in a static universe model, is applied to asymptotic boundary conditions for an action-at-a-distance electro dynamic framework of a Quasi-Steady State Universe; which as discussed is in opposition to the broad class of Big bang cosmologies.
Absorber theory of radiation in expanding universes
(Astronomical Society of India, 1995-04-14) Narlikar, J. V.
Academy fellowship problem
(Indian Academy of Sciences, 1995-12-25) Narlikar, J. V.
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 and reaction in the theories of direct interparticle action
(Elsevier Science Publishers, 1975-08-12) Narlikar, J. V.
Newton’s third law of motion is examined in the context of the theories of direct interparticle action. In such theories, interactions between particles travel backward and forward in time at speeds not exceeding the speed of light. It is shown that while in the flat spacetime the equality of action and reaction can be clearly demonstrated, the situation is considerably more complicated in the curved spacetime. The phenomenon of gravitational scattering intervenes to destroy the equality of action and reaction. Nevertheless, when gravitation is taken into account, there is no violation of the con- servation law of energy and momentum. These results are discussed in the framework of general relativity for the case of the electromagnetic interaction.
Action at a distance and cosmology : A historical perspective
(Annual Reviews Inc., 2003-06-04) Narlikar, J. V.
The first law of theoretical physics, the Newtonian law of gravitation, relies on the concept of action at a distance. The success of this law led to the concept being applied to electricity and magnetism, which were next to be explored in depth. Here the action at a distance had a limited success and ultimately had to be abandoned in favor of the increasingly more popular field theory. Nevertheless, in the 1940s, an attempt was made to revive the concept of action at a distance in a relativistically in-variant way by Wheeler & Feynman (1945, 1949). It inspired a series of investigations in both electrodynamics and gravity in which the field concept was not used but the interaction was described as taking place directly between particles. As it impinged very intimately on cosmology, Hoyle was keenly interested in it. This review discusses the work by Hoyle, the author, and others on the development of electrodynamics and gravitation as direct particle theories. In this review, the author discusses how the work was started and went through stages of increasing sophistication, e.g., extending the Wheeler-Feynman electrodynamics to curved spacetime, its consequences in different cosmologies, and the issues arising from its quantization. The resolution of ultraviolet divergences in quantum electrodynamics is also briefly discussed. The parallel development of a Machian theory of gravitation followed the lead from electrodynamics. In both theories one sees a strong link between the large-scale structure of the universe and local physics, as might be expected from an action-at-a-distance framework.
Action at a distance electromagnetism
(Science today, 1974-01-01) Narlikar, J. V.
Action at a distance in electrodynamics and inertia
(Nova Science Publishers, 2001-03-14) Narlikar, J. V.
This paper reviews the parallel development of electrodynamics and gravitation as direct particle theories, echoing the ideals of Ernst Mach. It is shown that a synthesisj of Mach's ideas with these two interactions leads to their better conceptual understanding, Both interactions, however, show that the resulting framework is relativistically invariant and thus precludes instantaneous action at a distance. At the same time, they establish a strong link between the large scale structure of the universe and local phys.cs, as might be expected from an action at a distance framework.
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