865 research outputs found
Quantum gravity and minimum length
The existence of a fundamental scale, a lower bound to any output of a
position measurement, seems to be a model-independent feature of quantum
gravity. In fact, different approaches to this theory lead to this result. The
key ingredients for the appearance of this minimum length are quantum
mechanics, special relativity and general relativity. As a consequence,
classical notions such as causality or distance between events cannot be
expected to be applicable at this scale. They must be replaced by some other,
yet unknown, structure.Comment: 23 pages, RevTeX, few minor changes, published versio
Quantum wormholes and harmonic oscillators
The quantum state of a wormhole can be represented by a path integral over all asymptotically Euclidean four-geometries and all matter fields which have prescribed values, the arguments of the wave function, on a three-surface which divides the space time manifold into two disconnected parts. Minisuperspace models which consist of a homogeneous massless scalar field coupled to a Friedmann-Robertson-Walker space time are considered. Once the path integral over the lapse function is performed, the requirement that the space time be asymptotically Euclidean can be accomplished by fixing the asymptotic gravitational momentum in the remaining path integral. It is argued that there does not exist any wave function which corresponds to asymptotic field configurations such that the effective gravitational constant is negative in the asymptotic region. Then, the wormhole wave functions can be written as linear combinations of harmonic oscillator wave functions
The local degrees of freedom of higher dimensional pure Chern-Simons theories
The canonical structure of higher dimensional pure Chern-Simons theories is
analysed. It is shown that these theories have generically a non-vanishing
number of local degrees of freedom, even though they are obtained by means of a
topological construction. This number of local degrees of freedom is computed
as a function of the spacetime dimension and the dimension of the gauge group.Comment: 9 pages, RevTeX3.0 (LaTeX2.09), no figure
Supersymmetry and Polytopes
We make an imaginative comparison between the Minimal Supersymmetric Standard
Model and the 24-cell polytope in four dimensions, the Octacube.Comment: Presented to the Workshop on Geometry and Physics: Supersymmetry.
Bilbao, Spain. May 200
Some not-so-common ideas about gravity
Most of the approaches to the construction of a theory of quantum gravity
share some principles which do not have specific experimental support up to
date. Two of these principles are relevant for our discussion: (i) the
gravitational field should have a quantum description in certain regime, and
(ii) any theory of gravity containing general relativity should be relational.
We study in general terms the possible implications of assuming deviations from
these principles, their compatibility with current experimental knowledge, and
how can they affect future experiments.Comment: 12 pages (+ references). Invited talk at DICE2014, Castiglioncello,
September 201
The fate of non-trivial entanglement under gravitational collapse
We analyse the evolution of the entanglement of a non-trivial initial quantum
field state (which, for simplicity, has been taken to be a bipartite state made
out of vacuum and the first excited state) when it undergoes a gravitational
collapse. We carry out this analysis by generalising the tools developed to
study entanglement behaviour in stationary scenarios and making them suitable
to deal with dynamical spacetimes. We also discuss what kind of problems can be
tackled using the formalism spelled out here as well as single out future
avenues of research.Comment: 9 pages, 2 figures. v2: Added Journal reference and small changes to
match published versio
Weyl relativity: A novel approach to Weyl's ideas
In this paper we revisit the motivation and construction of a unified theory
of gravity and electromagnetism, following Weyl's insights regarding the
appealing potential connection between the gauge invariance of electromagnetism
and the conformal invariance of the gravitational field. We highlight that
changing the local symmetry group of spacetime permits to construct a theory in
which these two symmetries are combined into a putative gauge symmetry but with
second-order field equations and non-trivial mass scales, unlike the original
higher-order construction by Weyl. We prove that the gravitational field
equations are equivalent to the (trace-free) Einstein field equations, ensuring
their compatibility with known tests of general relativity. As a corollary, the
effective cosmological constant is rendered radiatively stable due to Weyl
invariance. A novel phenomenological consequence characteristic of this
construction, potentially relevant for cosmological observations, is the
existence of an energy scale below which effects associated with the
non-integrability of spacetime distances, and an effective mass for the
electromagnetic field, appear simultaneously (as dual manifestations of the use
of Weyl connections). We explain how former criticisms against Weyl's ideas
lose most of their power in its present reincarnation, which we refer to as
Weyl relativity, as it represents a Weyl-invariant, unified description of both
the Einstein and Maxwell field equations.Comment: 34 pages, no figure
Black holes turn white fast, otherwise stay black: no half measures
Recently, various authors have proposed that the first ultraviolet effect on
the gravitational collapse of massive stars to black holes is the transition
between a black-hole geometry and a white-hole geometry, though their proposals
are radically different in terms of their physical interpretation and
characteristic time scales [1,2]. Several decades ago, it was shown by Eardley
that white holes are highly unstable to the accretion of small amounts of
matter, being rapidly turned into black holes [3]. Studying the crossing of
null shells on geometries describing the black-hole to white-hole transition,
we obtain the conditions for the instability to develop in terms of the
parameters of these geometries. We conclude that transitions with long
characteristic time scales are pathologically unstable: occasional
perturbations away from the perfect vacuum around these compact objects, even
if being imperceptibly small, suffocate the white hole explosion. On the other
hand, geometries with short characteristic time scales are shown to be robust
against perturbations, so that the corresponding processes could take place in
real astrophysical scenarios. This motivates a conjecture about the transition
amplitudes of different decay channels for black holes in a suitable
ultraviolet completion of general relativity.Comment: 24 pages, 3 figures. V2: Minor changes and updated references.
Matches the published versio
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