37 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
Identifying strongly lensed gravitational waves through their phase consistency
Strongly lensed gravitational waves (GWs) from binary coalescence manifest as
repeated chirps from the original merger. At the detectors, the phase of the
lensed GWs and its arrival time differences will be consistent modulo a fixed
constant phase shift. We develop a fast and reliable method to efficiently
reject event pairs that are not-lensed copies and appropriately rank the most
interesting candidates. Our method exploits that detector phases are the best
measured GW parameter, with errors only of a fraction of a radian and
differences across the frequency band that are better measured than the chirp
mass. The arrival time phase differences also avoid the shortcomings of looking
for overlaps in highly non-Gaussian sky maps. Our basic statistic determining
the consistency with lensing is the distance between the phase posteriors of
two events and it directly provides information about the lens-source geometry
which helps inform electromagnetic followups. We demonstrate that for simulated
signals of not-lensed binaries with many shared parameters none of the pairs
have phases closer than , and most cases reject the lensing hypothesis
by . Looking at the latest catalog, GWTC3, we find that only of
the pairs are consistent with lensing at confidence level. Moreover, we
reject about half of the pairs that would otherwise favor lensing by their
parameter overlaps and demonstrate good correlation with detailed joint
parameter estimation results. This reduction of the false alarm rate will be of
paramount importance in the upcoming observing runs and the eventual discovery
of lensed GWs. Our code is publicly available and could be applied beyond
lensing to test possible deviations in the phase evolution from modified
theories of gravity and constrain GW birefringence.Comment: 14+7 pages, 12+7 figures, 2 tables, code at
https://github.com/ezquiaga/phaza
Dark Energy in Light of Multi-Messenger Gravitational-Wave Astronomy
Gravitational waves (GWs) provide a new tool to probe the nature of dark energy (DE) and the fundamental properties of gravity. We review the different ways in which GWs can be used to test gravity and models for late-time cosmic acceleration. Lagrangian-based gravitational theories beyond general relativity (GR) are classified into those breaking fundamental assumptions, containing additional fields and massive graviton(s). In addition to Lagrangian based theories we present the effective theory of DE and the ÎŒ-ÎŁ parametrization as general descriptions of cosmological gravity. Multi-messenger GW detections can be used to measure the cosmological expansion (standard sirens), providing an independent test of the DE equation of state and measuring the Hubble parameter. Several key tests of gravity involve the cosmological propagation of GWs, including anomalous GW speed, massive graviton excitations, Lorentz violating dispersion relation, modified GW luminosity distance and additional polarizations, which may also induce GW oscillations. We summarize present constraints and their impact on DE models, including those arising from the binary neutron star merger GW170817. Upgrades of LIGO-Virgo detectors to design sensitivity and the next generation facilities such as LISA or Einstein Telescope will significantly improve these constraints in the next two decades
Primordial black holes and their gravitational-wave signatures
In the recent years, primordial black holes (PBHs) have emerged as one of the
most interesting and hotly debated topics in cosmology. Among other
possibilities, PBHs could explain both some of the signals from binary black
hole mergers observed in gravitational wave detectors and an important
component of the dark matter in the Universe. Significant progress has been
achieved both on the theory side and from the point of view of observations,
including new models and more accurate calculations of PBH formation,
evolution, clustering, merger rates, as well as new astrophysical and
cosmological probes. In this work, we review, analyse and combine the latest
developments in order to perform end-to-end calculations of the various
gravitational wave signatures of PBHs. Different ways to distinguish PBHs from
stellar black holes are emphasized. Finally, we discuss their detectability
with LISA, the first planned gravitational-wave observatory in space.Comment: 161 pages, 47 figures, comments welcom