Research Papers (TP)
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Item Constraints on unstable heavy neutrinos from cosmology(Indian Academy of Sciences, 1985-12-11) Vasanthi, M. M.; Padmanabhan, T.Cosmological scenario with massive unstable neutrinos are discussed. Restrictions on the mass and lifetime of the unstable neutrino are derived from (a) age and mass density of the universe and (b) the growth of primordial fluctuations. It will not be possible to accommodate unstable neutrinos with masses above ~ `1 keV in standard cosmology unless they have exceedingly small lifetimeItem Conserved quantities from piecewise Killing vectors(IOP Publishing, 1988-11-10) Dray, Tevian; Padmanabhan, T.In the presence of symmetries, conserved quantities can be obtained by contracting the stress-energy tensor with a killing vector. We generalize this result to piecewise Killing vectors by giving sufficient conditions for the construction of an associated conserved quantity. A typical example namely, namely two stationary space-times joined together in such a way that the resulting space-time is not stationary, is treated in detail.Item Conformal invariance, gravity and massive gauge theories(IOP Publishing, 1985-05-07) Padmanabhan, T.A gauge theory is constructed maintaining the invariance of the action of massless scalar fields under local conformal transformations. The theory leads to gravity in a natural fashion and also allows for the gauge field to be massive.Item Is gravitational lensing a local phenomenon caused by Pop - III objects?(Gordon and Breach Science Publishers, 1986-01-31) Padmanabhan, T.; Chitre, S. M.Item Instability of flat space and origin of conformal fluctuations(Elsevier Science Publishers, 1983-01-03) Padmanabhan, T.It is shown that conformal fluctuations in the metric can be initiated by the vacuum fluctuations of a scalar field with mass greater than the Planck mass. Flatspace is unstable against such fluctuations.Item Inflation from quantum gravity(Elsevier Science Publishers, 1984-08-27) Padmanabhan, T.A model for inflation based on a quantum gravity scenario is presented. The process allows inflation of a Planck size bubble to the observed universe.Item 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.Item 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 anyItem Limitations on the operational definition of spacetime events and quantum gravity(IOP Publishing, 1987-01-12) Padmanabhan, T.Using simple arguments from general relativity and quantum theory we show that it is not possible to devise experiments (or operational procedures) which will measure the position of a particle to an accuracy better than the Planck length (Gh/c3) = cm. It is also impossible to synchronise clocks to a precision better than Planck time. The implications of the result are discussed.Item Why does an accelerated detector click?(IOP Publishing, 1984-09-28) Padmanabhan, T.The conflict between the definition of particles based on (a) field theory formalism and (b) simple detector models is discussed. An improved model for the detector is constructed taking the effect of accelerating potential into account. Analysis of this model shows that the detector results are better interpreted as a radiation process.