59 research outputs found
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
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 μ-Σ 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
Kinetic mixing in scalar-tensor theories of gravity
[EN] Kinetic mixing between the metric and scalar degrees of freedom is an essential ingredient in contemporary scalar-tensor theories. This often makes it hard to understand their physical content, especially when derivative mixing is present, as is the case for Horndeski action. In this work we develop a method that allows us to write a Ricci-curvature-free scalar field equation, and we discuss some of the advantages of such a rephrasing in the study of stability issues in the presence of matter, the existence of an Einstein frame, and the generalization of the disformal screening mechanism. For quartic Horndeski theories, such a procedure leaves, in general, a residual coupling to the curvature, given by the Weyl tensor. This gives rise to a binary classification of scalar-tensor theories into stirred theories, in which the curvature can be substituted, and shaken theories, in which a residual coupling to the curvature remains. Quite remarkably, we have found that generalized Dirac-Born-Infeld Galileons belong to the first class. Finally, we discuss kinetic mixing in quintic theories, in which nonlinear mixing terms appear, and in the recently proposed theories beyond Horndeski that display a novel form of kinetic mixing, in which the field equation is sourced by derivatives of the energy-momentum tensor
A spectre is haunting the cosmos: Quantum stability of massive gravity with ghosts
Many theories of modified gravity with higher order derivatives are usually
ignored because of serious problems that appear due to an additional ghost
degree of freedom. Most dangerously, it causes an immediate decay of the
vacuum. However, breaking Lorentz invariance can cure such abominable behavior.
By analyzing a model that describes a massive graviton together with a
remaining Boulware-Deser ghost mode we show that even ghostly theories of
modified gravity can yield models that are viable at both classical and quantum
levels and, therefore, they should not generally be ruled out. Furthermore, we
identify the most dangerous quantum scattering process that has the main impact
on the decay time and find differences to simple theories that only describe an
ordinary scalar field and a ghost. Additionally, constraints on the parameters
of the theory including some upper bounds on the Lorentz-breaking cutoff scale
are presented. In particular, for a simple theory of massive gravity we find
that a breaking of Lorentz invariance is allowed to happen even at scales above
the Planck mass. Finally, we discuss the relevance to other theories of
modified gravity.Comment: 18 pages, 3 figures, version published in JHE
Probing the foundations of the standerd cosmological model
Tesis doctoral inédita, leñida en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física Teórica. Fecha de lectura: 16-11-201
Lensing of gravitational waves: efficient wave-optics methods and validation with symmetric lenses
Gravitational wave (GW) astronomy offers the potential to probe the
wave-optics regime of gravitational lensing. Wave optics (WO) effects are
relevant at low frequencies, when the wavelength is comparable to the
characteristic lensing time delay multiplied by the speed of light, and are
thus often negligible for electromagnetic signals. Accurate predictions require
computing the conditionally convergent diffraction integral, which involves
highly oscillatory integrands and is numerically difficult. We develop and
implement several methods to compute lensing predictions in the WO regime valid
for general gravitational lenses. First, we derive approximations for high and
low frequencies, obtaining explicit expressions for several analytic lens
models. Next, we discuss two numerical methods suitable in the intermediate
frequency range: 1) Regularized contour flow yields accurate answers in a
fraction of a second for a broad range of frequencies. 2) Complex deformation
is slower, but requires no knowledge of solutions to the geometric lens
equation. Both methods are independent and complement each other. We verify
sub-percent accuracy for several lens models, which should be sufficient for
applications to GW astronomy in the near future. Apart from modelling lensed
GWs, our method will also be applicable to the study of plasma lensing of radio
waves and tests of gravity.Comment: 21 pages, 9 figures. Matches PRD versio
Gravitational wave lensing as a probe of halo properties and dark matter
Just like light, gravitational waves (GWs) are deflected and magnified by
gravitational fields as they propagate through the Universe. However, their low
frequency, phase coherence and feeble coupling to matter allow for distinct
lensing phenomena, such as diffraction and central images, that are challenging
to observe through electromagnetic sources. Here we explore how these phenomena
can be used to probe features of gravitational lenses. We focus on two variants
of the singular isothermal sphere, with 1) a variable slope of the matter
density and 2) a central core. We describe the imprints of these features in
the wave- and geometric-optics regimes, including the prospect of detecting
central images. We forecast the capacity of LISA and advanced LIGO to study
strongly lensed signals and measure the projected lens mass, impact parameter
and slope or core size. A broad range of lens masses allows all parameters to
be measured with precision up to , despite large
degeneracies. Thanks to wave-optics corrections, all parameters can be
measured, even when no central image forms. Although GWs are sensitive to
projected quantities, we compute the probability distribution of lens redshift,
virial mass and projection scale given a cosmology. As an application, we
consider the prospect of constraining self-interacting and ultra-light dark
matter, showing the regions of parameter space accessible to strongly-lensed
GWs. The distinct GW signatures will enable novel probes of fundamental physics
and astrophysics, including the properties of dark matter and the central
regions of galactic halos.Comment: 43 pages, 27 figures. Matches PRD versio
Theoretical priors in scalar-tensor cosmologies: Shift-symmetric Horndeski models
Attempts at constraining theories of late time accelerated expansion often
assume broad priors for the parameters in their phenomenological description.
Focusing on shift-symmetric scalar-tensor theories with standard gravitational
wave speed, we show how a more careful analysis of their dynamical evolution
leads to much narrower priors. In doing so, we propose a simple and accurate
parametrisation of these theories, capturing the redshift dependence of the
equation of state, , and the kinetic braiding parameter, , with only two parameters each, and derive their statistical
distribution (a.k.a. theoretical priors) that fit the cosmology of the
underlying model. We have considered two versions of the shift-symmetric model,
one where the energy density of dark energy is given solely by the scalar
field, and another where it also has a contribution from the cosmological
constant. By including current data, we show how theoretical priors can be used
to improve constraints by up to an order of magnitude. Moreover, we show that
shift-symmetric theories without a cosmological constant are observationally
viable. We work up to quartic order in first derivatives of the scalar in the
action and our results suggest this truncation is a good approximation to more
general shift-symmetric theories. This work establishes an actionable link
between phenomenological parameterisations and Lagrangian-based theories, the
two main approaches to test cosmological gravity and cosmic acceleration.Comment: 18 pages, 13 figures; Version as accepted in PRD - minor changes
Obtención de biomasa de microalga Chlorella vulgaris en un banco de prueba de fotobiorreactores de columna de burbujeo
Dada la agudización de la situación socioeconómica y medioambiental que se enfrenta en la actualidad, los investigadores buscan nuevas alternativas para sustituir el combustible fósil convencional, siendo una salida, los biocombustibles obtenidos a partir de microalgas. El objetivo de esta investigación fue la obtención de biomasa en un banco de prueba de fotobiorreactores de columna de burbujeo, utilizando una cepa de Chlorella vulgaris en medio Bristol. Se dimensionó el fotobiorreactor y se evaluó la influencia de las variables pH y concentración de nitrógeno, con y sin presencia de oligoelementos, sobre la productividaddel crecimiento de la biomasa de microalgas. Seencontró que en el intervalo estudiado (pH entre 6 y 8 y concentración de NaNO3 entre 0,5 y 1 g/L) estas variables no tienen un efecto significativo en el crecimiento, mientras que la presencia de oligoelementos favorece este
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