Sourav Sur

Power-law corrections to entanglement entropy of horizons

We reexamine the idea that the origin of black-hole entropy may lie in the entanglement of quantum fields between the inside and outside of the horizon. Motivated by the observation that certain modes of gravitational fluctuations in a black-hole background behave as scalar fields, we compute the entanglement entropy of such a field, by tracing over its degrees of freedom inside a sphere. We show that while this entropy is proportional to the area of the sphere when the field is in its ground state, a correction term proportional to a fractional power of area results when the field is in a superposition of ground and excited states. The area law is thus recovered for large areas. Further, we identify the location of the degrees of freedom that give rise to the above entropy.

Multiple kinetic k-essence, phantom barrier crossing and stability

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The role of a higher derivative bulk scalar in stabilizing a warped spacetime

The back-reaction on the Randall?Sundrum warped spacetime is determined in the presence of a scalar field in the bulk. A general condition for the stability of such a model is derived for a bulk scalar field action with non-canonical higher derivative terms. It is further shown that the gauge hierarchy problem can be resolved in such a stabilized scenario by appropriate choice of various parameters of the theory.

Weakly dynamic dark energy via metric-scalar couplings with torsion

We study the dynamical aspects of dark energy in the context of a non-minimally coupled scalar field with curvature and torsion. Whereas the scalar field acts as the source of the trace mode of torsion, a suitable constraint on the pseudo-trace of the latter provides a mass term for the scalar field in the effective action. In the equivalent scalar-tensor framework, we find explicit cosmological solutions suitable for describing dark energy in both Einstein and Jordan frames. We demand the dynamical evolution of the dark energy to be weak enough, so that the present-day values of the cosmological parameters could be estimated keeping them within the confidence limits set for the standard

Constraining torsion in maximally symmetric (sub)spaces

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Entanglement and corrections to Bekenstein-Hawking entropy

CDM model from recent observations. For such estimates, we examine the variations of the effective matter density and the dark energy equation of state over different redshift ranges. In spite of being weakly dynamic, the dark energy component here differs significantly from the cosmological constant, both in characteristics and features, for e.g. it interacts with the cosmological (dust) fluid in the Einstein frame, and crosses the phantom barrier in the Jordan frame. We also obtain the upper bounds on the torsion mode parameters and the lower bound on the effective Brans-Dicke parameter. The latter turns out to be fairly large, and in agreement with the local gravity constraints, which therefore come in support of our analysis.

Where are the degrees of freedom responsible for black hole entropy

We look into the general aspects of space-time symmetries in presence of torsion, and how the latter is affected by such symmetries. Focusing in particular to space-times which either exhibit maximal symmetry on their own, or could be decomposed to maximally symmetric subspaces, we work out the constraints on torsion in two different theoretical schemes. We show that at least for a completely antisymmetric torsion tensor (for example the one motivated from string theory), an equivalence is set between these two schemes, as the non-vanishing independent torsion tensor components turn out to be the same.

Dynamical system analysis of dark energy models in scalar coupled metric-torsion theories

In this talk, we focus on the corrections to Bekenstein-Hawking entropy by associating it with the entanglement between degrees of freedom inside and outside the horizon. Using numerical techniques, we show that the corrections proportional to fractional power of area result when the field is in a superposition of ground and excited states. We explain this result by identifying that the degrees of freedom contributing to such corrections are different from those contributing to Bekenstein-Hawking entropy.

Black hole entropy from entanglement: A Review

Considering the entanglement between quantum field degrees of freedom inside and outside the horizon as a plausible source of black-hole entropy, we address the question: where are the degrees of freedom that give rise to this entropy located? When the field is in ground state, the black-hole area law is obeyed and the degrees of freedom near the horizon contribute most to the entropy. However, for excited state, or a superposition of ground state and excited state, power-law corrections to the area law are obtained, and more significant contributions from the degrees of freedom far from the horizon are shown.

Crossing the cosmological constant barrier with kinetically interacting double quintessence

We study the phase space dynamics of cosmological models in the theoretical formulations of non-minimal metric-torsion couplings with a scalar field, and investigate in particular the critical points which yield stable solutions exhibiting cosmic acceleration driven by the {\em dark energy}. The latter is defined in a way that it effectively has no direct interaction with the cosmological fluid, although in an equivalent scalar-tensor cosmological setup the scalar field interacts with the fluid (which we consider to be the pressureless dust). Determining the conditions for the existence of the stable critical points we check their physical viability, in both Einstein and Jordan frames. We also verify that in either of these frames, the evolution of the universe at the corresponding stable points matches with that given by the respective exact solutions we have found in an earlier work (arXiv: 1611.00654 [gr-qc]). We not only examine the regions of physical relevance for the trajectories in the phase space when the coupling parameter is varied, but also demonstrate the evolution profiles of the cosmological parameters of interest along fiducial trajectories in the effectively non-interacting scenarios, in both Einstein and Jordan frames.