734 research outputs found
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
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
Where does the physics of extreme gravitational collapse reside?
The gravitational collapse of massive stars serves to manifest the most
severe deviations of general relativity with respect to Newtonian gravity: the
formation of horizons and spacetime singularities. Both features have proven to
be catalysts of deep physical developments, especially when combined with the
principles of quantum mechanics. Nonetheless, it is seldom remarked that it is
hardly possible to combine all these developments into a unified theoretical
model, while maintaining reasonable prospects for the independent experimental
corroboration of its different parts. In this paper we review the current
theoretical understanding of the physics of gravitational collapse in order to
highlight this tension, stating the position that the standard view on
evaporating black holes stands for. This serves as the motivation for the
discussion of a recent proposal that offers the opposite perspective,
represented by a set of geometries that regularize the classical singular
behavior and present modifications of the near-horizon Schwarzschild geometry
as the result of the propagation of non-perturbative ultraviolet effects
originated in regions of high curvature. We present an extensive exploration of
the necessary steps on the explicit construction of these geometries, and
discuss how this proposal could change our present understanding of
astrophysical black holes and even offer the possibility of detecting genuine
ultraviolet effects on future gravitational wave experiments.Comment: 43 pages, 1 figure. Review article with new results on the black to
white hole transition. Prepared for the special issue "Open Questions in
Black Hole Physics" edited by Gonzalo J. Olm
The lifetime problem of evaporating black holes: mutiny or resignation
It is logically possible that regularly evaporating black holes exist in
nature. In fact, the prevalent theoretical view is that these are indeed the
real objects behind the curtain in astrophysical scenarios. There are several
proposals for regularizing the classical singularity of black holes so that
their formation and evaporation do not lead to information-loss problems. One
characteristic is shared by most of these proposals: these regularly
evaporating black holes present long-lived trapping horizons, with absolutely
enormous evaporation lifetimes in whatever measure. Guided by the discomfort
with these enormous and thus inaccessible lifetimes, we elaborate here on an
alternative regularization of the classical singularity, previously proposed by
the authors in an emergent gravity framework, which leads to a completely
different scenario. In our scheme the collapse of a stellar object would result
in a genuine time-symmetric bounce, which in geometrical terms amounts to the
connection of a black-hole geometry with a white-hole geometry in a regular
manner. The two most differential characteristics of this proposal are: i) the
complete bouncing geometry is a solution of standard classical general
relativity everywhere except in a transient region that necessarily extends
beyond the gravitational radius associated with the total mass of the
collapsing object; and ii) the duration of the bounce as seen by external
observers is very brief (fractions of milliseconds for neutron-star-like
collapses). This scenario motivates the search for new forms of stellar
equilibrium different from black holes. In a brief epilogue we compare our
proposal with a similar geometrical setting recently proposed by Haggard and
Rovelli.Comment: 20 pages, 2 figures; v2: published version, references adde
Schwarzschild geometry counterpart in semiclassical gravity
We investigate the effects of vacuum polarization on vacuum static
spherically-symmetric spacetimes. We start from the Polyakov approximation to
the renormalized stress-energy tensor (RSET) of a minimally coupled massless
scalar field. This RSET is not regular at , so we define a regularized
version of the Polyakov RSET. Using this Regularized RSET, and under the
previous symmetry assumptions, we find all the solutions to the semiclassical
field equations in vacuum. The resulting counterpart to the Schwarzschild
classical geometry substitutes the presence of an event horizon by a wormhole
throat that connects an external asymptotically flat region with an internal
asymptotic region possessing a naked singularity: there are no semiclassical
vacuum solutions with well-defined Cauchy surfaces. We also show that the
Regularized Polyakov RSET allows for wormhole geometries of arbitrarily small
throat radius. This analysis paves the way to future investigations of proper
stellar configurations with an internal non-vacuum region.Comment: 22 pages, 4 figures, v2: references and minor changes added to match
published versio
Electromagnetism as an emergent phenomenon: a step-by-step guide
We give a detailed description of electrodynamics as an emergent theory from
condensed-matter-like structures, not only {\it per se} but also as a warm-up
for the study of the much more complex case of gravity. We will concentrate on
two scenarios that, although qualitatively different, share some important
features, with the idea of extracting the basic generic ingredients that give
rise to emergent electrodynamics and, more generally, to gauge theories. We
start with Maxwell's mechanical model for electrodynamics, where Maxwell's
equations appear as dynamical consistency conditions. We next take a superfluid
He-like system as representative of a broad class of fermionic quantum
systems whose low-energy physics reproduces classical electrodynamics (Dirac
and Maxwell equations as dynamical low-energy laws). An important lesson that
can be derived from both analyses is that the vector potential has a
microscopic physical reality and that it is only in the low-energy regime that
this physical reality is blurred in favour of gauge invariance, which in
addition turns out to be secondary to effective Lorentz invariance.Comment: 41 pages, 4 figures; v2: references added, version accepted for
publicatio
Hybrid classical-quantum formulations ask for hybrid notions
We reappraise some of the hybrid classical-quantum models proposed in the
literature with the goal of retrieving some of their common characteristics. In
particular, first, we analyze in detail the Peres-Terno argument regarding the
inconsistency of hybrid quantizations of the Sudarshan type. We show that to
accept such hybrid formalism entails the necessity of dealing with additional
degrees of freedom beyond those in the straight complete quantization of the
system. Second, we recover a similar enlargement of degrees of freedom in the
so-called statistical hybrid models. Finally, we use Wigner's quantization of a
simple model to illustrate how in hybrid systems the subsystems are never
purely classical or quantum. A certain degree of quantumness (classicality) is
being exchanged between the different sectors of the theory, which in this
particular unphysical toy model makes them undistinguishable.Comment: 13 pages, 3 figures (minor changes to match the published version
- …