1,721,007 research outputs found
Weakly broken Galileon symmetry in cosmology
No full textOne of the main challenges of modern cosmology consists of confirming, or even disproving, inflation. In the simplest models, a single scalar field drives the acceleration of the early Universe thanks to a flat potential or derivative self-interactions. In the latter case, however, in order to avoid possible instabilities, only single derivatives acting on the field are usually considered in the Lagrangians.
In the present work, using an effective field theory point of view, we explore theories of single-field inflation where higher derivative operators become relevant, affecting in a novel way the dynamics and therefore the observations. For instance, concerning the scalar spectrum, they allow for measurable equilateral non-Gaussianity, whose amplitude can differ signifi- cantly from the predictions of other existing models.
Moreover, we show that the stability and the consistency of such theories are ensured by an approximate Galileon symmetry. Indeed, being generi- cally possible to build an invariant theory under Galileon transformations in flat space-time, it is instead well known that such a symmetry is un- avoidably broken by gravity. In principle, this might ruin the nice and in- teresting properties of the Galileons in flat backgrounds, such as the non- renormalization theorem. However, we find that this does not happen if the Galileon invariance is broken only weakly, in a well defined sense, by a suit- able coupling to gravity, providing therefore an extension of the quantum non-renormalization properties in curved space-times.
Hence, besides discussing the phenomenological consequences and the observational predictions for inflation, we apply such Galileon theories to the context of the late-time acceleration of the Universe.
In the last part, in order to probe non-standard primordial scenarios, they are also employed in a cosmology where the Big Bang singularity is smoothed down and the Universe emerges from a Minkowski space-time, in a well defined extension at all times of the Galilean Genesis scenario
Constraints on single-field inflation
Full text linkMany alternatives to canonical slow-roll inflation have been proposed over
the years, one of the main motivations being to have a model, capable of
generating observable values of non-Gaussianity. In this work, we (re-)explore
the physical implications of a great majority of such models within a single,
effective field theory framework (including novel models with large
non-Gaussianity discussed for the first time below.) The constraints we
apply---both theoretical and experimental---are found to be rather robust,
determined to a great extent by just three parameters: the coefficients of the
quadratic EFT operators and , and the
slow-roll parameter . This allows to significantly limit the
majority of single-field alternatives to canonical slow-roll inflation. While
the existing data still leaves some room for most of the considered models, the
situation would change dramatically if the current upper limit on the
tensor-to-scalar ratio decreased down to r < 10^-2. Apart from inflationary
models driven by plateau-like potentials, the single-field model that would
have a chance of surviving this bound is the recently proposed slow-roll
inflation with weakly-broken galileon symmetry. In contrast to
extitcanonical slow-roll inflation, the latter model can support r <
10^-2 even if driven by a convex potential, as well as generate observable
values for the amplitude of non-Gaussianity.Many alternatives to canonical slow-roll inflation have been proposed over the years, one of the main motivations being to have a model, capable of generating observable values of non-Gaussianity. In this work, we (re-)explore the physical implications of a great majority of such models within a single, effective field theory framework (including novel models with large non-Gaussianity discussed for the first time below). The constraints we apply - both theoretical and experimental - are found to be rather robust, determined to a great extent by just three parameters: the coefficients of the quadratic EFT operators (δN)2 and δNδE, and the slow-roll parameter ". This allows to significantly limit the majority of single-field alternatives to canonical slow-roll inflation. While the existing data still leaves some room for most of the considered models, the situation would change dramatically if the current upper limit on the tensor-to-scalar ratio decreased down to r < 10-2. Apart from inflationary models driven by plateau-like potentials, the single-field model that would have a chance of surviving this bound is the recently proposed slow-roll inflation with weakly-broken galileon symmetry. In contrast to canonical slow-roll inflation, the latter model can support r < 10-2 even if driven by a convex potential, as well as generate observable values for the amplitude of non-Gaussianity
Stability of geodesically complete cosmologies
Full text linkWe study the stability of spatially flat FRW solutions which are geodesically
complete, i.e. for which one can follow null (graviton) geodesics both in the
past and in the future without ever encountering singularities. This is the
case of NEC-violating cosmologies such as smooth bounces or solutions which
approach Minkowski in the past. We study the EFT of linear perturbations around
a solution of this kind, including the possibility of multiple fields and
fluids. One generally faces a gradient instability which can be avoided only if
the operator is present and its coefficient changes sign
along the evolution. This operator (typical of beyond-Horndeski theories) does
not lead to extra degrees of freedom, but cannot arise starting from any theory
with second-order equations of motion. The change of sign of this operator
prevents to set it to zero with a generalised disformal transformation
Black hole ringdown as a probe for dark energy
Full text linkUnder the assumption that a dynamical scalar field is responsible for the current acceleration of the Universe, we explore the possibility of probing its physics in black hole merger processes with gravitational wave interferometers. Remaining agnostic about the microscopic physics, we use an effective field theory approach to describe the scalar dynamics. We investigate the case in which some of the higher-derivative operators, that are highly suppressed on cosmological scales, instead become important on typical distances for black holes. If a coupling to the Gauss-Bonnet operator is one of them, a nontrivial background profile for the scalar field can be sourced in the surroundings of the black hole, resulting in a potentially large amount of "hair." In turn, this can induce sizeable modifications to the spacetime geometry or a mixing between the scalar and the gravitational perturbations. Both effects will ultimately translate into a modification of the quasinormal mode spectrum in a way that is also sensitive to other operators besides the one sourcing the scalar background. The presence of deviations from the predictions of general relativity in the observed spectrum can therefore serve as a window onto dark energy physics.Under the assumption that a dynamical scalar field is responsible for the current acceleration of the Universe, we explore the possibility of probing its physics in black hole merger processes with gravitational wave interferometers. Remaining agnostic about the microscopic physics, we use an effective field theory approach to describe the scalar dynamics. We investigate the case in which some of the higher-derivative operators, that are highly suppressed on cosmological scales, instead become important on typical distances for black holes. If a coupling to the Gauss-Bonnet operator is one of them, a nontrivial background profile for the scalar field can be sourced in the surroundings of the black hole, resulting in a potentially large amount of "hair." In turn, this can induce sizeable modifications to the spacetime geometry or a mixing between the scalar and the gravitational perturbations. Both effects will ultimately translate into a modification of the quasinormal mode spectrum in..
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Behind Horndeski: structurally robust higher derivative EFTs
Full text linkHigher derivative scalar interactions can give rise to interesting cosmological scenarios. We present a complete classification of such operators that can yield sizeable effects without introducing ghosts and, at the same time, define an effective field theory robust under the inclusion of quantum corrections. A set of rules to power count consistently the coefficients of the resulting Lagrangian is provided by the presence of an approximate global symmetry. The interactions that we derive in this way contain a subset of the so-called Horndeski and beyond Horndeski theories. Our construction therefore provides a structurally robust context to study their phenomenology. Applications to dark energy/modified gravity and geodesically complete cosmologies are briefly discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Inflation from Minkowski space
Full text linkWe propose a class of scalar models that, once coupled to gravity, lead to cosmologies that smoothly and stably connect an inflationary quasi-de Sitter universe to a low, or even zero-curvature, maximally symmetric spacetime in the asymptotic past, strongly violating the null energy condition (dot{H}≫H^2) at intermediate times. The models are deformations of the conformal galileon lagrangian and are therefore based on symmetries, both exact and approximate, that ensure the quantum robustness of the whole picture. The resulting cosmological backgrounds can be viewed as regularized extensions of the galilean genesis scenario, or, equivalently, as ‘early-time-complete’ realizations of inflation. The late-time inflationary dynamics possesses phenomenologically interesting properties: it can produce a large tensor-to-scalar ratio within the regime of validity of the effective field theory and can lead to sizeable equilateral nongaussianities
An analytic approach to quasinormal modes for coupled linear systems
Full text linkQuasinormal modes describe the ringdown of compact objects deformed by small
perturbations. In generic theories of gravity that extend General Relativity,
the linearized dynamics of these perturbations is described by a system of
coupled linear differential equations of second order. We first show, under
general assumptions, that such a system can be brought to a Schr\"odinger-like
form. We then devise an analytic approximation scheme to compute the spectrum
of quasinormal modes. We validate our approach using a toy model with a
controllable mixing parameter and showing that the analytic
approximation for the fundamental mode agrees with the numerical computation
when the approximation is justified. The accuracy of the analytic approximation
is at the (sub-) percent level for the real part and at the level of a few
percent for the imaginary part, even when is of order one. Our
approximation scheme can be seen as an extension of the approach of Schutz and
Will to the case of coupled systems of equations, although our approach is not
phrased in terms of a WKB analysis, and offers a new viewpoint even in the case
of a single equation.Comment: 30 pages. v2: matches published versio
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