9 research outputs found
The null energy condition and instability
We extend previous work showing that violation of the null energy condition
implies instability in a broad class of models, including gauge theories with
scalar and fermionic matter as well as any perfect fluid. Simple examples are
given to illustrate these results. The role of causality in our results is
discussed. Finally, we extend the fluid results to more general systems in
thermal equilibrium. When applied to the dark energy, our results imply that w
is unlikely to be less than -1.Comment: 11 pages, 5 figures, Revte
Entropy: From Black Holes to Ordinary Systems
Several results of black holes thermodynamics can be considered as firmly
founded and formulated in a very general manner. From this starting point we
analyse in which way these results may give us the opportunity to gain a better
understanding in the thermodynamics of ordinary systems for which a
pre-relativistic description is sufficient. First, we investigated the
possibility to introduce an alternative definition of the entropy basically
related to a local definition of the order in a spacetime model rather than a
counting of microstates. We show that such an alternative approach exists and
leads to the traditional results provided an equilibrium condition is assumed.
This condition introduces a relation between a time interval and the reverse of
the temperature. We show that such a relation extensively used in the black
hole theory, mainly as a mathematical trick, has a very general and physical
meaning here; in particular its derivation is not related to the existence of a
canonical density matrix. Our dynamical approach of thermodynamic equilibrium
allows us to establish a relation between action and entropy and we show that
an identical relation exists in the case of black holes. The derivation of such
a relation seems impossible in the Gibbs ensemble approach of statistical
thermodynamics. From these results we suggest that the definition of entropy in
terms of order in spacetime should be more general that the Boltzmann one based
on a counting of microstates. Finally we point out that these results are
obtained by reversing the traditional route going from the Schr\"{o}dinger
equation to statistical thermodynamics
On Energy Conditions and Stability in Effective Loop Quantum Cosmology
In isotropic loop quantum cosmology, non-perturbatively modified dynamics of
a minimally coupled scalar field violates weak, strong and dominant energy
conditions when they are stated in terms of equation of state parameter. The
violation of strong energy condition helps to have non-singular evolution by
evading singularity theorems thus leading to a generic inflationary phase.
However, the violation of weak and dominant energy conditions raises concern,
as in general relativity these conditions ensure causality of the system and
stability of vacuum via Hawking-Ellis conservation theorem. It is shown here
that the non-perturbatively modified kinetic term contributes negative pressure
but positive energy density. This crucial feature leads to violation of energy
conditions but ensures positivity of energy density, as scalar matter
Hamiltonian remains bounded from below. It is also shown that the modified
dynamics restricts group velocity for inhomogeneous modes to remain sub-luminal
thus ensuring causal propagation across spatial distances.Comment: 29 pages, revtex4; few clarifications, references added, to appear in
CQ
A Single Scalar Field Model of Dark Energy with Equation of State Crossing -1
In this paper we study the possibility of building models of dark energy with
equation of state across -1 and propose explicitly a model with a single scalar
field which gives rise to an equation of state larger than -1 in the past and
less than -1 at the present time, consistent with the current observations.Comment: 4 pages, 1 figure, the version accepted by JCAP, presentation
improved and references adde
Holographic c-theorems in arbitrary dimensions
We re-examine holographic versions of the c-theorem and entanglement entropy
in the context of higher curvature gravity and the AdS/CFT correspondence. We
select the gravity theories by tuning the gravitational couplings to eliminate
non-unitary operators in the boundary theory and demonstrate that all of these
theories obey a holographic c-theorem. In cases where the dual CFT is
even-dimensional, we show that the quantity that flows is the central charge
associated with the A-type trace anomaly. Here, unlike in conventional
holographic constructions with Einstein gravity, we are able to distinguish
this quantity from other central charges or the leading coefficient in the
entropy density of a thermal bath. In general, we are also able to identify
this quantity with the coefficient of a universal contribution to the
entanglement entropy in a particular construction. Our results suggest that
these coefficients appearing in entanglement entropy play the role of central
charges in odd-dimensional CFT's. We conjecture a new c-theorem on the space of
odd-dimensional field theories, which extends Cardy's proposal for even
dimensions. Beyond holography, we were able to show that for any
even-dimensional CFT, the universal coefficient appearing the entanglement
entropy which we calculate is precisely the A-type central charge.Comment: 62 pages, 4 figures, few typo's correcte
A Comment or two on Holographic Dark Energy
It has, quite recently, become fashionable to study a certain class of
holographic-inspired models for the dark energy. These investigations have,
indeed, managed to make some significant advances towards explaining the
empirical data. Nonetheless, surprisingly little thought has been given to
conceptual issues such as the composition and the very nature of the implicated
energy source. In the current discourse, we attempt to fill this gap by the way
of some speculative yet logically self-consistent arguments. Our construction
takes us along a path that begins with an entanglement entropy and ends up at a
Hubble-sized gas of exotic particles. Moreover, our interpretation of the dark
energy turns out to be suggestive of a natural resolution to the
cosmic-coincidence problem.Comment: 18 pages; (v2) an oversight in Section 2.1 is rectified and a few
citations adde