arXiv subjects

B. Modak

Publications and source records attributed to B. Modak.

12 recordsLinked to original sources

arXiv2014

Modified theory of gravity and the history of cosmic evolution

A continuous transition from early Friedmann-like radiation era through to late time cosmic acceleration passing through a long Friedmann-like matter dominated era followed by a second phase of radiation era has been realized in modified theory of gravity containing a combination of curvature squared term, a linear term, a three-half term and an ideal fluid. Thus the history of cosmic evolution is explained by modified theory of gravity singlehandedly. The second phase of radiation-like era might provide an explanation to the hydrogen and helium reionization at low redshift.

astro-ph.CO↗

arXiv2003

Noether symmetry in the higher order gravity theory

Noether symmetry for higher order gravity theory has been explored, with the introduction of an auxiliary variable which gives the only correct quantum desccription of the theory, as shown in a series of earlier papers. The application of Noether theorem in higher order theory of gravity turned out to be a powerful tool to find the solution of the field equations. A few such physically reasonable solutions like power law inflation are presented.

astro-ph↗

arXiv2001

Quantum Cosmology With R+R^2 Gravity

Canonical quantization of an action containing curvature squared term requires introduction of an auxiliary variable. Boulware etal[1] prescribed a technique to choose such a variable, by taking derivative of an action with respect to the highest derivative of the field variable, present in the action.It has been shown that[2] this technique can even be applied in situations where introduction of auxiliary variables are not at all required, leading to wrong Wheeler-deWitt equation. It has also been pointed out that[2] Boulware etal's[1] prescription should be taken up only after removing all removable total derivative terms from the action. Once this is done only a unique description of quantum dynamics would emerge. For curvature squared term this technique yields, for the first time, a quantum mechanical probability interpretation of quantum cosmology, and an effective potential, whose extremization leads to Einstein's equation. We conclude that Einstein-Hilbert action should essentially be modified by at least a curvature squared term to get a quantum mechanical formulation of quantuum cosmology and hence extend our previous work[2] for such an action along with a scalar field.

gr-qc↗

arXiv2001

Is Noether Symmetric Approach Consistent With Dynamical Equation In Non-minimal Scalar-Tensor Theories?

The form of the coupling of the scalar field with gravity and the potential have been found by applying Noether theorem to two dimensional minisuperspaces in induced gravity model. It has been observed that though the forms thus obtained are consistent with all the equations $£_{X}L=0$, yet they do not satisfy the field equations for $k=\pm 1$, in Robertson-Walker model. It has been pointed out that this is not due to the degeneracy of the Lagrangian, since this problem does not appear in $k=0$ case.It has also been shown that though Noether theorem fails to extract any symmetry from the Lagrangian of such model for $k=\pm 1$, symmetry exists, which can easily be found by studying the continuity equation.

gr-qc↗

arXiv2001

Quantum cosmology with a curvature squared action

The correct quantum description for a curvature squared term in the action can be obtained by casting the action in the canonical form with the introduction of a variable which is the negative of the first derivative of the field variable appearing in the action, only after removing the total derivative terms from the action. We present the Wheeler-DeWitt equation and obtain the expression for the probability density and current density from the equation of continuity. Furthermore, in the weak energy limit we obtain the classical Einstein equation. Finally we present a solution of the wave equation.

gr-qc↗

arXiv1999

Evolution of Dynamical Coupling in Scalar Tensor Theory from Noether Symmetry

We present the gravitational coupling function $ω(ϕ)$ in the vacuum scalar-tensor theory as allowed by the Noether symmetry. We also obtain some exact cosmological solutions in the spatially homogeneous and isotropic background thereby showing that the attractor mechanism is not effective enough to reduce the theory to Einstein theory. It is observed that, asymptotically, the scalar tensor theory goes over to Einstein theory with finite value of $ω$. This work thus supports earlier works in this direction.

gr-qc↗

arXiv1999

Decoherence in the Starobinsky Model

Starobinsky described an inflationary scenario in which quantum corrections to vacuum Einstein equations drive the inflation. The quantum cosmology of the model is studied by solving the Wheeler-DeWitt equation. A connection between uncertainty requirement, randomness in initial states and curvature fluctuation is studied with a Schrödinger-type equation through a time parameter prescription. The result obtained is applied to understand the decoherence mechanism in quantum gravity in the Starobinsky description.

astro-ph↗

arXiv1999

Schwinger's Result On Particle Production From Complex Paths WKB Approximation

This paper presents the derivation of Schwinger's gauge invariant result of $Im \cal{L}_{eff}$ upto one loop approximation, for particle production in an uniform electric field through the method of complex trajectory WKB approximation (CWKB). The CWKB proposed by one of the author's \cite{bis:ijtp} looks upon particle production as due to the motion of a particle in complex space-time plane, thereby requiring tunneling paths both in space and time. Recently \cite{srini:iucaa,srini1:iucaa} there have been some efforts to calculate the reflection and transmission co-efficients for particle production in uniform electric field that differ from our expressions for the same. In this paper we clarify the confusion in this regard and establish the correctness of CWKB.

gr-qc↗

arXiv1999

Quantum Gravity Equation In Schroedinger Form In Minisuperspace Description

We start from classical Hamiltonian constraint of general relativity to obtain the Einstein-Hamiltonian-Jacobi equation. We obtain a time parameter prescription demanding that geometry itself determines the time, not the matter field, such that the time so defined being equivalent to the time that enters into the Schroedinger equation. Without any reference to the Wheeler-DeWitt equation and without invoking the expansion of exponent in WKB wavefunction in powers of Planck mass, we obtain an equation for quantum gravity in Schroedinger form containing time. We restrict ourselves to a minisuperspace description. Unlike matter field equation our equation is equivalent to the Wheeler-DeWitt equation in the sense that our solutions reproduce also the wavefunction of the Wheeler-DeWitt equation provided one evaluates the normalization constant according to the wormhole dominance proposal recently proposed by us.

gr-qc↗

arXiv1999

Time in Quantum Gravity

The Wheeler-DeWitt equation in quantum gravity is timeless in character. In order to discuss quantum to classical transition of the universe, one uses a time prescription in quantum gravity to obtain a time contained description starting from Wheeler-DeWitt equation and WKB ansatz for the WD wavefunction. The approach has some drawbacks. In this work, we obtain the time-contained Schroedinger-Wheeler-DeWitt equation without using the WD equation and the WKB ansatz for the wavefunction. We further show that a Gaussian ansatz for SWD wavefunction is consistent with the Hartle-Hawking or wormhole dominance proposal boundary condition. We thus find an answer to the small scale boundary conditions.

gr-qc↗

arXiv1999

Wormhole Dominance Proposal and Wave Function Discord

Using the wormhole dominance proposal, it is shown that quantum corrections to the usual WKB ansatz for the wave function of the universe ably circumvent many of the drawbacks present in the current proposals. We also find that the recent criticism by Hawking and Turok does not apply to the tunneling proposal.

astro-ph↗

arXiv1999

The Complex Time WKB Approximation And Particle Production

The complex time WKB (CWKB) approximation has been an effective technique to understand particle production in curved as well as in flat spacetime. Earlier we obtained the standard results on particle production in time dependent gauge in various curved spacetime. In the present work we generalize the technique of CWKB to the equivalent problems in space dependent gauge. Using CWKB, we first obtain the gauge invariant result for particle production in Minkowski spacetime in strong electric field. We then carry out particle production in de-Sitter spacetime in space dependent gauge and obtain the same result that we obtained earlier in time dependent gauge. The results obtained for de-Sitter spacetime has a obvious extension to particle production in black hole spacetime. It is found that the origin of Planckian spectrum is due to repeated reflections between the turning points. As mentioned earlier, it is now explicitly shown that particle production is accompanied by rotation of currents.

astro-ph↗