414 research outputs found
Quantum diffusion beyond slow-roll: implications for primordial black-hole production
Primordial black-holes (PBH) can be produced in single-field models of
inflation with a quasi-inflection point in the potential. In these models, a
large production of PBHs requires a deviation from the slow-roll (SR)
trajectory. In turn, this SR violation can produce an exponential growth of
quantum fluctuations. We study the back-reaction of these quantum modes on the
inflationary dynamics using stochastic inflation in the Hamilton-Jacobi
formalism. We develop a methodology to solve quantum diffusion beyond SR in
terms of the statistical moments of the probability distribution. We apply
these techniques to a toy model potential with a quasi-inflection point. We
find that there is an enhancement of the power spectrum due to the dominance of
the stochastic noise in the phase beyond SR. Moreover, non-Gaussian corrections
become as well relevant with a large positive kurtosis. Altogether, this
produces a significant boost of PBH production. We discuss how our results
extend to other single-field models with similar dynamics. We conclude that the
abundance of PBHs in this class of models should be revisited including quantum
diffusion.Comment: 17+7 pages, 5 figures. Matches JCAP versio
Towards the most general scalar-tensor theories of gravity: a unified approach in the language of differential forms
We use a description based on differential forms to systematically explore
the space of scalar-tensor theories of gravity. Within this formalism, we
propose a basis for the scalar sector at the lowest order in derivatives of the
field and in any number of dimensions. This minimal basis is used to construct
a finite and closed set of Lagrangians describing general scalar-tensor
theories invariant under Local Lorentz Transformations in a pseudo-Riemannian
manifold, which contains ten physically distinct elements in four spacetime
dimensions. Subsequently, we compute their corresponding equations of motion
and find which combinations are at most second order in derivatives in four as
well as arbitrary number of dimensions. By studying the possible exact forms
(total derivatives) and algebraic relations between the basis components, we
discover that there are only four Lagrangian combinations producing second
order equations, which can be associated with Horndeski's theory. In this
process, we identify a new second order Lagrangian, named kinetic Gauss-Bonnet,
that was not previously considered in the literature. However, we show that its
dynamics is already contained in Horndeski's theory. Finally, we provide a full
classification of the relations between different second order theories. This
allows us to clarify, for instance, the connection between different
covariantizations of Galileons theory. In conclusion, our formulation affords
great computational simplicity with a systematic structure. As a first step we
focus on theories with second order equations of motion. However, this new
formalism aims to facilitate advances towards unveiling the most general
scalar-tensor theories.Comment: 28 pages, 1 figure, version published in PRD (minor changes
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Gravitational wave lensing beyond general relativity: Birefringence, echoes, and shadows
Gravitational waves (GW), as light, are gravitationally lensed by intervening matter, deflecting their trajectories, delaying their arrival and occasionally producing multiple images. In theories beyond general relativity, new gravitational degrees of freedom add an extra layer of complexity and richness to GW lensing. We develop a formalism to compute GW propagation beyond general relativity over general space-times, including kinetic interactions with new fields. Our framework relies on identifying the dynamical propagation eigenstates (linear combinations of the metric and additional fields) at leading order in a short-wave expansion. We determine these eigenstates and the conditions under which they acquire a different propagation speed around a lens. Differences in speed between eigenstates cause birefringence phenomena, including time delays between the metric polarizations (orthogonal superpositions of , ) observable without an electromagnetic counterpart. In particular, GW echoes are produced when the accumulated delay is larger than the signal's duration, while shorter time delays produce a scrambling of the waveform. We also describe the formation of GW shadows as nonpropagating metric components are sourced by the background of the additional fields around the lens. As an example, we apply our methodology to quartic Horndeski theories with Vainshtein screening and show that birefringence effects probe a region of the parameter space complementary to the constraints from the multimessenger event GW170817. In the future, identified strongly lensed GWs and binary black holes merging near dense environments, such as active galactic nuclei, will fulfill the potential of these novel tests of gravity
Sueños de la razón: genealogía de los paradigmas para una teoría del planeamiento en el pensamiento de Fernando Terán
Sueños de la razón: genealogía de los paradigmas para una teoría del planeamiento en el pensamiento de Fernando Terá
Cuando la rabia prende
"No existe eso que se llama sociedad" dijo una vez la exlíder del Partido Conservador Margaret Thatcher; lo único existente serían entonces individuos que, buscando de forma egoísta su beneficio, contribuirían al bienestar del Imperio. Treinta años después, las revueltas de este verano en Reino Unido insisten precisamente en lo contrario. La afirmación de la exlíder conservadora se desvela además como una afirmación cínica; los actuales estallidos de violencia se hacen eco de las premisas ocultas de la misma ideología conservadora, esto es, de la "vuelta a lo básico" que alentaba la campaña de dicho partido en la década de los ochenta y que suponía la consabida exaltación de la familia, el trabajo duro y la individualidad, así como su corolario de satisfacción de los instintos tribales y territoriales. También podía añadirse un efecto ideológico que habla de lo social y económico como del estado del tiempo: se logra invitar a la gente a abdicar de su responsabilidad política al sostener que no hay nada que hacer, tan sólo adaptarse
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