Browsing by Author "Dadhich, Naresh"

Now showing 1 - 20 of 54

Brane corresponding to the nariai bulk
(2011-07-05) Dadhich, Naresh; Shtanov, Yuri
We consider the ﬁve-dimensional bulk spacetime with negative Λ described by the Nariai metric (which is not conformally ﬂat) and match it with a vacuum brane satisfying the proper boundary conditions. It is shown that the brane metric corresponds to a cloud of string dust of constant energy density.
A class of cylindrically symmetric models in string cosmology
(2015-02-09) Tikekar, R.; Patel, L.K.; Dadhich, Naresh
A new class of physically relevant explicit solutions for string cosmological models endowed with cylindrical symmetry on the background of singularity free cosmological space times have been obtained and their physical and the kinematical features are discussed. The matter free limits of this class of solutions are observed to be the singularity free vacuum solutions of Patel and Dadhich.
Cosmological Dynamics of Phantom Field
(2011-07-05) Singh, Parampreet; Sami, M.; Dadhich, Naresh
We study the general features of the dynamics of the phantom ﬁeld in the cosmological context. In the case of inverse coshyperbolic potential, we demonstrate that the phantom ﬁeld can successfully drive the observed current accelerated expansion of the universe with the equation of state parameter wφ < −1. The de-Sitter universe turns out to be the late time attractor of the model. The main features of the dynamics are independent of the initial conditions and the parameters of the model. The model ﬁts the supernova data very well, allowing for −2.4 < wφ < −1 at 95 % conﬁdence level
Cylindrical universes with heat and null radiation flow
(2014-11-25) Patel, L.K.; Dadhich, Naresh
In paper-I(Patel and Dadhich ,1992) we have discussed cylindrically symmetric viscous fluid models with the Kasnerian time evolution. In this paper we incorporate heat flow and null radiation flow with the perfect fluid. Here again, in the case of heat flow the Kasner spacetime is the matter-free limit of the model. We establish a general result that a static perfect fluid distribution will on Kasnerization (introduce t4/3, t4/3, and t−2/3 in the coefficients of dr2, dΦ2, and dz2) yield a time-dependent distribution with the same equation of state and with or without heat flow.
Cylindrically symmetric cosmological models in the Kaluza-Klein space time
(2015-01-27) Patel, L.K.; Dadhich, Naresh
We consider a non-diagonal cylindrically symmetric metric in the Kaluza-Klein spacetime. We obtain a number of homogeneous and inhomogeneous perfect fluid cosmological models, which include the 5-dimensional analogue of the recently found 4-dimensional non-singular stiff fluid model. Amongst the homogeneous models, which are all as expected big-bang singular, there is the 5-dimensional version of the Friedman-Robertson-Walker flat model.
Cylindrically symmetric viscous universes
(2014-11-26) Patel, L.K.; Dadhich, Naresh
We have considered here cylindrically symmetric cosmological models with viscous fluid having the Kasnerian time evolution. It turns out that the ‘‘no‐matter’’ limit of our geodetic viscous universes, which can as well be looked upon as string dust universes, is the non-flat empty Kasner spacetime. We thus propose that the universe may be born with the Kasner geometry at t = 0 with matter in the form of viscous fluid, then it may follow the path: geodetic to non-geodetic and finally to the radiation phase when viscosity is fully worn out. We have solutions to describe each phase separately.
Derivation of the Maxwell equations and the relation between electric and magnetic charge
(2015-03-01) Singh, Parampreet; Dadhich, Naresh
Domain walls in kaluza-klein spacetime
(2015-03-01) Patel, L.K.; Dadhich, Naresh; Tikekar, R.
Three families of exact solutions of Einstein field equations are found. Each family contains three parameters. Two of these families represent thick domain walls in a five dimensional Kaluza-Klein spacetime.The dynamical behaviour of our models is briefly discussed. The spacetime in all the cases is found to be reflection symmetric with respect to the wall.
Dual spacetimes, mach’s principle and topological defects
(2015-03-01) Dadhich, Naresh
By resolving the Riemann curvature relative to a unit timelike vector into electric and magnetic parts, we define a duality transformation which interchanges active and passive electric. parts. It implies interchange of roles of Ricci and Einstein curvatures. 'Further by modifying the vacuum/flat equation we construct spacetimes dual to the Schwarzschild solution and flat spacetime. The dual spacetimes describe the original spacetimes with global monopole charge and global texture. The duality so defined is thus intimately related to the topologiccJ, defects ~nd also renders the Schwarzschild field asymptotically non-flat which augurs well with Mach's Principle.
A duality relation for fluid spacetime
(2015-03-11) Dadhich, Naresh; Patel, L.K.; Tikekar, R.
Electromagnetic duality in general relativity
(2015-03-01) Dadhich, Naresh
By resolving the Riemann curvature relative to a unit timelike vector into electric and magnetic parts, we consider duality relations analogous to the electromagnetic theory. It turns out that the duality symmetry of the Einstein action implies the Einstein vacuum equation without the cosmological term. The vacuum equation is invariant under interchange of active and passive electric parts giving rise to the same vacuum solutions but the gravitational constant changes sign. Further by modifying the equation it is possible to construct interesting dual solutions to vacuum as well as to flat spacetimes.
Exact solutions for null fluid collapse in higher dimensions
(2015-03-01) Patel, L.K.; Dadhich, Naresh
A large family of inhomogeneous non-static spherically symmetric solutions of the Einstein equation for null fluid in higher dimensions has been obtained. It encompasses higher dimensional versions of many previously known solutions such as Vaidya, charged Vaidya and Husain solutions and also some new solutions representing global monopole or string dust. It turns out that physical properties of the solutions carry over to higher dimensions.
Field theories from the relativistic law of motion
(2001-06-02) Singh, Parampreet; Dadhich, Naresh
From the relativistic law of motion we attempt to deduce the ﬁeld theories corresponding to the force law being linear and quadratic in 4-velocity of the particle. The linear law leads to the vector gauge theory which could be the abelian Maxwell electrodynamics or the non-abelian Yang-Mills theory. On the other hand the quadratic law demands spacetime metric as its potential which is equivalent to de- manding the Principle of Equivalence. It leads to the tensor theory of gravitational ﬁeld - General Relativity. It is remarkable that a purely dynamical property of the force law leads uniquely to the correspond- ing ﬁeld theories
Global monopole as dual-vacuum solution in kaluza-klein spacetime
(2015-03-01) Dadhich, Naresh; Patel, L.K.; Tikekar, R.
By application of the duality transformation, which implies interchange of active and passive electric parts of the Riemann curvature (equivalent to interchange of Ricci and Einstein tensors) it is shown that the global monopole solution in the Kaluza-Klein spacetime is dual to the corresponding vacuum solution. Further we also obtain solution dual to flat space which would in general describe a massive global monopole in 4-dimensional Euclidean space and would have massless limit analogus to the 4-dimensional dual-flat solution.
Global monopoles and scalar fields as the electrogravity dual of Schwarzschild spacetime
(2000-08-15) Dadhich, Naresh; Banerjee, Narayan
We prove that both global monopole and minimally coupled static zero mass scalar ﬁeld are electrogravity dual of the Schwarzschild solution or ﬂat space and they share the same equation of state, T0 0 − Ti i = 0. This property was however known for the global monopole spacetime while it is for the ﬁrst time being established for the scalar ﬁeld. In particular, it turns out that the Xanthopoulos - Zannias scalar ﬁeld solution is dual to ﬂat space.
Gravitational collapse of null strange quark fluid and cosmic censorship
(2002-04-01) Ghosh, S. G.; Dadhich, Naresh
We study gravitational collapse of the general spherically symmetric null strange quark ﬂuid having the equation of state, p = (ρ − 4B)/n, where B is the bag constant. An interesting feature that emerges is that the initial data set giving rise to naked singularity in the Vaidya collapse of null ﬂuid gets covered due to the presence of strange quark matter component. Its implication to the Cosmic Censorship Conjecture is discussed.
Gravoelectric-dual of the kerr solution
(2015-03-01) Dadhich, Naresh; Patel, L.K.
By decomposing the Riemann curvature into electric and magnetic parts, we define the gravoelectric duality transformation by interchange of active and passive electric parts which amounts to interchange of the Ricci and Einstein tensors. It turns out that the vacuum equation is duality-invariant. We obtain solutions dual to the Kerr solution by writing an effective vacuum equation in such a way that it still admits the Kerr solution but is not duality invariant. The dual equation is then solved to obtain the dual-Kerr solution which can be interpreted as the Kerr black hole sitting in a string dust universe.
High energy radiation from white holes
(Nature Publishing Group, 1974-10-18) Narlikar, J. V.; Apparao, K. M. V.; Dadhich, Naresh
Higher dimensional analogue of McVittie solution
(2015-03-01) Patel, L.K.; Tikekar, R.; Dadhich, Naresh
How empty must empty space be ?
(2015-02-07) Dadhich, Naresh
We propose a generalised definition of empty space in general relativity which is characterised by vanishing of gravitational charge density Rikuiuk = 0 instead of Rik = 0. It can be argued that so long as there exists energy distribution outside the empty space region, the generalised definition seems to be more appropriate for its description.