396 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
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
Probing lens-induced gravitational-wave birefringence as a test of general relativity
Theories beyond general relativity (GR) modify the propagation of
gravitational waves (GWs). In some, inhomogeneities (aka. gravitational lenses)
allow interactions between the metric and additional fields to cause
lens-induced birefringence (LIB): a different speed of the two linear GW
polarisations ( and ). Inhomogeneities then act as non-isotropic
crystals, splitting the GW signal into two components whose relative time delay
depends on the theory and lens parameters. Here we study the observational
prospects for GW scrambling, i.e when the time delay between both GW
polarisations is smaller than the signal's duration and the waveform recorded
by a detector is distorted. We analyze the latest LIGO-Virgo-KAGRA catalog,
GWTC-3, and find no conclusive evidence for LIB. The highest log Bayes factor
that we find in favour of LIB is for GW, a particularly loud but
short event. However, when accounting for false alarms due to (Gaussian) noise
fluctuations, this evidence is below 1-. The tightest constraint on the
time delay is ms (90% C.L.) from GW. From the
non-observation of GW scrambling, we constrain the optical depth for LIB,
accounting for the chance of randomly distributed lenses (eg. galaxies) along
the line of sight. Our LIB constraints on a (quartic) scalar-tensor Horndeski
theory are more stringent than solar system tests for a wide parameter range
and comparable to GW170817 in some limits. Interpreting GW190521 as an AGN
binary (i.e. taking an AGN flare as a counterpart) allows even more stringent
constraints. Our results demonstrate the potential and high sensitivity
achievable by tests of GR, based on GW lensing.Comment: 18 pages, 10 figure
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