1,844 research outputs found
Synchronous coordinates and gauge-invariant observables in cosmological spacetimes
We consider the relational approach to construct gauge-invariant observables
in cosmological perturbation theory using synchronous coordinates. We construct
dynamical synchronous coordinates as non-local scalar functionals of the metric
perturbation in the fully non-linear theory in an arbitrary gauge. We show that
the observables defined in this dynamical coordinate system are
gauge-independent, and that the full perturbed metric has the expected form in
these coordinates. Our construction generalises the familiar synchronous gauge
in linearised gravity, widely used in cosmological perturbation theory, to the
non-linear theory. We also work out the expressions for the gauge-invariant
Einstein equations, sourced either by an ideal fluid or a scalar field up to
second order in perturbation theory, and give explicit expressions for the
Hubble rate -- as measured by synchronous observers or by observers co-moving
with the matter field -- up to that order. Finally, we consider quantised
linear perturbations around Minkowski and de Sitter backgrounds, and compute
the two-point function of the gauge-invariant metric perturbation in
synchronous coordinates, starting with two-point function in a general linear
covariant gauge. Although the gauge-fixed two-point function contains gauge
modes, we show that the resulting gauge-invariant two-point function only
contains the physical tensor modes and it is thus positive, i.e., it has a
spectral representation.Comment: 52 pages. Comments are welcom
Awaking the vacuum with spheroidal shells
It has been shown that well-behaved spacetimes may induce the vacuum
fluctuations of some nonminimally coupled free scalar fields to go through a
phase of exponential growth. Here, we discuss this mechanism in the context of
spheroidal thin shells emphasizing the consequences of deviations from
spherical symmetry.Comment: 10 pages, 7 figures. Minor changes, version published on Phys. Rev.
From quantum to classical instability in relativistic stars
It has been shown that gravitational fields produced by realistic
classical-matter distributions can force quantum vacuum fluctuations of some
nonminimally coupled free scalar fields to undergo a phase of exponential
growth. The consequences of this unstable phase to the background spacetime
have not been addressed so far due to known difficulties concerning
backreaction in semiclassical gravity. It seems reasonable to believe, however,
that the quantum fluctuations will "classicalize" when they become large
enough, after which backreaction can be treated in the general-relativistic
context. Here we investigate the emergence of a classical regime out of the
quantum field evolution during the unstable phase. By studying the appearance
of classical correlations and loss of quantum coherence, we show that by the
time backreaction becomes important the system already behaves classically.
Consequently, the gravity-induced instability leads naturally to initial
conditions for the eventual classical description of the backreaction. Our
results give support to previous analyses which treat classically the
instability of scalar fields in the spacetime of relativistic stars, regardless
whether the instability is triggered by classical or quantum perturbations.Comment: 16 pages. Minor changes to match the published versio
L'immixtion et la trop grande expertise du propriétaire, cause d'exonération de responsabilité des architectes et des entrepreneurs sous l'article 1688 C.C.
In May 1977, the Supreme Court of Canada gave an authoritative judgment on the interpretation of article 1688 of the Civil Code dealing with the liability of the builder and the architect. In Davie Shipbuilding Ltd v. Cargill Grain Company Ltd and The Foundation Company of Canada Ltd, the court established a new method of rebutting the presumption of liability resulting from the article. Indeed, the Supreme Court found that where the owner intervened during the construction and was more qualified than the contractors in charge of the work to judge its quality, he could not invoke the presumption resulting from article 1688 C.C. and was responsible for any loss. The case is not an easy one — questions of law and facts are not simple in this dispute involving more than eleven million dollars
Graviton backreaction on the local cosmological expansion in slow-roll inflation
We compute the graviton one-loop correction to the expectation value of the local expansion rate in slow-roll inflation, with both slow-roll parameters finite. The calculation is based on a recent method to explicitly construct gauge-invariant observables in perturbative quantum gravity at all orders in perturbation theory, and it is particularly suited in cases of highly-symmetrical space-time backgrounds. Our analysis adds to recent calculations of that correction in de Sitter space-time and in single-field inflation with constant deceleration. In the former case a vanishing one-loop correction was found, while in the latter the quantum backreaction produces a secular effect that accelerates the expansion. The quantum correction we describe here produces a finite secular effect that can either accelerated or decelerate the background expansion, depending on the value of the slow-roll parameters
Non-commutative Geometry from Perturbative Quantum Gravity in de Sitter spacetime
We show that a non-commutative structure arises naturally from perturbative
quantum gravity in a de Sitter background metric. Our work builds on recent
advances in the construction of observables in highly symmetric background
spacetimes [Brunetti et al., JHEP 08, 032 (2016); Fr\"ob and Lima, Class.
Quant. Grav. 35, 095010 (2018)], where the dynamical coordinates that are
needed in the relational approach were established for such backgrounds to all
orders in perturbation theory. We show that these dynamical coordinates that
describe events in the perturbed spacetime are naturally non-commuting, and
determine their commutator to leading order in the Planck length. Our result
generalizes the causal non-commutative structure that was found using the same
approach in Minkowski space [Fr\"ob, Much and Papadopoulos, Phys. Rev. D 107,
064041 (2023)]
Product line architecture recovery with outlier filtering in software families: the Apo-Games case study
Software product line (SPL) approach has been widely adopted to achieve systematic reuse in families of software products. Despite its benefits, developing an SPL from scratch requires high up-front investment. Because of that, organizations commonly create product variants with opportunistic reuse approaches (e.g., copy-and-paste or clone-and-own). However, maintenance and evolution of a large number of product variants is a challenging task. In this context, a family of products developed opportunistically is a good starting point to adopt SPLs, known as extractive approach for SPL adoption. One of the initial phases of the extractive approach is the recovery and definition of a product line architecture (PLA) based on existing software variants, to support variant derivation and also to allow the customization according to customers’ needs. The problem of defining a PLA from existing system variants is that some variants can become highly unrelated to their predecessors, known as outlier variants. The inclusion of outlier variants in the PLA recovery leads to additional effort and noise in the common structure and complicates architectural decisions. In this work, we present an automatic approach to identify and filter outlier variants during the recovery and definition of PLAs. Our approach identifies the minimum subset of cross-product architectural information for an effective PLA recovery. To evaluate our approach, we focus on real-world variants of the Apo-Games family. We recover a PLA taking as input 34 Apo-Game variants developed by using opportunistic reuse. The results provided evidence that our automatic approach is able to identify and filter outlier variants, allowing to eliminate exclusive packages and classes without removing the whole variant. We consider that the recovered PLA can help domain experts to take informed decisions to support SPL adoption.This research was partially funded by INES 2.0; CNPq grants 465614/2014-0 and 408356/2018-9; and FAPESB grants JCB0060/2016 and BOL2443/201
Gravity-induced vacuum dominance
It has been widely believed that, except in very extreme situations, the
influence of gravity on quantum fields should amount to just small,
sub-dominant contributions. This view seemed to be endorsed by the seminal
results obtained over the last decades in the context of renormalization of
quantum fields in curved spacetimes. Here, however, we argue that this belief
is false by showing that there exist well-behaved spacetime evolutions where
the vacuum energy density of free quantum fields is forced, by the very same
background spacetime, to become dominant over any classical energy-density
component. This semiclassical gravity effect finds its roots in the infrared
behavior of fields on curved spacetimes. By estimating the time scale for the
vacuum energy density to become dominant, and therefore for backreaction on the
background spacetime to become important, we argue that this vacuum dominance
may bear unexpected astrophysical and cosmological implications.Comment: To appear in Phys. Rev. Lett
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