348 research outputs found

### The analogue cosmological constant in Bose-Einstein condensates: a lesson for quantum gravity

For almost a century, the cosmological constant has been a mysterious object,
in relation to both its origin and its very small value. By using a
Bose-Einstein condensate analogue model for gravitational dynamics, we address
here the cosmological constant issue from an analogue gravity standpoint.
Starting from the fundamental equations describing a system of condensed
bosons, we highlight the presence of a vacuum source term for the analogue
gravitational field, playing the role of a cosmological constant. In this
simple system it is possible to compute from scratch the value of this
constant, to compare it with other characteristic energy scales and hence
address the problem of its magnitude within this framework, suggesting a
different path for the solution of this longstanding puzzle. We find that, even
though this constant term is related with quantum vacuum effects, it is not
immediately related to the ground state energy of the condensate. On the
gravity side this result suggests that the interpretation and computation of
the cosmological term as a form of renormalized vacuum energy might be
misleading, its origin being related to the mechanism that instead produces
spacetime from its pregeometric progenitor, shedding a different light on the
subject and at the same time suggesting a potentially relevant role of analogue
models in the understanding of quantum gravity.Comment: 24 pages, 1 figure, Proceedings of the II Amazonian Symposium on
Physic

### Entropy and Topology for Gravitational Instantons

In this work a relation between topology and thermodynamical features of
gravitational instantons is shown. The expression for the Euler characteristic,
through the Gauss-Bonnet integral, and the one for the entropy of gravitational
instantons are proposed in a form that makes the relation between them
self-evident. A new formulation of the Bekenstein-Hawking formula, where the
entropy and the Euler characteristic are related by $S=\chi A/8$, is obtained.
This formula provides the correct results for a wide class of gravitational
instantons described by both spherically and axially symmetric metrics.Comment: 25 pages, RevTeX, accepted for publication in Phys. Rev.

### Perturbative superluminal censorship and the null energy condition

We argue that ``effective'' superluminal travel, potentially caused by the
tipping over of light cones in Einstein gravity, is always associated with
violations of the null energy condition (NEC). This is most easily seen by
working perturbatively around Minkowski spacetime, where we use linearized
Einstein gravity to show that the NEC forces the light cones to contract
(narrow). Given the NEC, the Shapiro time delay in any weak gravitational field
is always a delay relative to the Minkowski background, and never an advance.
Furthermore, any object travelling within the lightcones of the weak
gravitational field is similarly delayed with respect to the minimum traversal
time possible in the background Minkowski geometry.Comment: 5 pages. Uses AIP proceedings style (aipproc.sty). To appear in the
Proceedings of the Eighth Canadian Conference on General Relativity and
Relativistic Astrophysics. (McGill University, Montreal, June 1999). To be
published by AIP Pres

### Towards the observation of Hawking radiation in Bose--Einstein condensates

Acoustic analogues of black holes (dumb holes) are generated when a
supersonic fluid flow entrains sound waves and forms a trapped region from
which sound cannot escape. The surface of no return, the acoustic horizon, is
qualitatively very similar to the event horizon of a general relativity black
hole. In particular Hawking radiation (a thermal bath of phonons with
temperature proportional to the ``surface gravity'') is expected to occur. In
this note we consider quasi-one-dimensional supersonic flow of a Bose--Einstein
condensate (BEC) in a Laval nozzle (converging-diverging nozzle), with a view
to finding which experimental settings could magnify this effect and provide an
observable signal. We identify an experimentally plausible configuration with a
Hawking temperature of order 70 n K; to be contrasted with a condensation
temperature of the order of 90 n K.Comment: revtex4; 5 pages in double-column forma

### Quantum Gravity Phenomenology and Lorentz Violation

If quantum gravity violates Lorentz symmetry, the prospects for observational
guidance in understanding quantum gravity improve considerably. This article
briefly reviews previous work on Lorentz violation (LV) and discusses aspects
of the effective field theory framework for parametrizing LV effects. Current
observational constraints on LV are then summarized, focusing on effects in QED
at order E/M_Planck.Comment: 16 pages, Expanded version of a lecture by T. Jacobson, to be
published in Particle Physics and the Universe, Proceedings of the 9th
Adriatic Meeting, eds. J.Trampetic and J.Wess (Springer-Verlag, 2004

### Analogue models for FRW cosmologies

It is by now well known that various condensed matter systems may be used to
mimic many of the kinematic aspects of general relativity, and in particular of
curved-spacetime quantum field theory. In this essay we will take a look at
what would be needed to mimic a cosmological spacetime -- to be precise a
spatially flat FRW cosmology -- in one of these analogue models. In order to do
this one needs to build and control suitable time dependent systems. We discuss
here two quite different ways to achieve this goal. One might rely on an
explosion, physically mimicking the big bang by an outflow of whatever medium
is being used to carry the excitations of the analogue model, but this idea
appears to encounter dynamical problems in practice. More subtly, one can avoid
the need for any actual physical motion (and avoid the dynamical problems) by
instead adjusting the propagation speed of the excitations of the analogue
model. We shall focus on this more promising route and discuss its
practicality.Comment: This essay was awarded an "honourable mention" in the 2003 essay
competition of the Gravity Research Foundation. Uses revtex4; 6 pages in
single-column forma

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