543 research outputs found
Parametrizing modified gravity for cosmological surveys
One of the challenges in testing gravity with cosmology is the vast freedom
opened when extending General Relativity. For linear perturbations, one
solution consists in using the Effective Field Theory of Dark Energy (EFT of
DE). Even then, the theory space is described in terms of a handful of free
functions of time. This needs to be reduced to a finite number of parameters to
be practical for cosmological surveys. We explore in this article how well
simple parametrizations, with a small number of parameters, can fit observables
computed from complex theories. Imposing the stability of linear perturbations
appreciably reduces the theory space we explore. We find that observables are
not extremely sensitive to short time-scale variations and that simple, smooth
parametrizations are usually sufficient to describe this theory space. Using
the Bayesian Information Criterion, we find that using two parameters for each
function (an amplitude and a power law index) is preferred over complex models
for 86% of our theory space.Comment: 10 pages, 5 figure
Nomarski imaging interferometry to measure the displacement field of MEMS
We propose to use a Nomarski imaging interferometer to measure the
out-of-plane displacement field of MEMS. It is shown that the measured optical
phase arises both from height and slope gradients. Using four integrating
buckets a more efficient approach to unwrap the measured phase is presented,
thus making the method well suited for highly curved objects. Slope and height
effects are then decoupled by expanding the displacement field on a functions
basis, and the inverse transformation is applied to get a displacement field
from a measure of the optical phase map change with a mechanical loading. A
measurement reproducibility of about 10 pm is achieved, and typical results are
shown on a microcantilever under thermal actuation, thereby proving the ability
of such a set-up to provide a reliable full-field kinematic measurement without
surface modification
Healthy theories beyond Horndeski
We introduce a new class of scalar-tensor theories that extend Horndeski, or
"generalized galileon", models. Despite possessing equations of motion of
higher order in derivatives, we show that the true propagating degrees of
freedom obey well-behaved second-order equations and are thus free from
Ostrogradski instabilities, in contrast to the standard lore. Remarkably, the
covariant versions of the original galileon Lagrangians-obtained by direct
replacement of derivatives with covariant derivatives-belong to this class of
theories. These extensions of Horndeski theories exhibit an uncommon,
interesting phenomenology: the scalar degree of freedom affects the speed of
sound of matter, even when the latter is minimally coupled to gravity.Comment: 5 pages, version accepted in PR
Resilience of the standard predictions for primordial tensor modes
We show that the prediction for the primordial tensor power spectrum cannot
be modified at leading order in derivatives. Indeed, one can always set to
unity the speed of propagation of gravitational waves during inflation by a
suitable disformal transformation of the metric, while a conformal one can make
the Planck mass time-independent. Therefore, the tensor amplitude unambiguously
fixes the energy scale of inflation. Using the Effective Field Theory of
Inflation, we check that predictions are independent of the choice of frame, as
expected. The first corrections to the standard prediction come from two parity
violating operators with three derivatives. Also the correlator
is standard and only receives higher derivative
corrections. These results hold also in multifield models of inflation and in
alternatives to inflation and make the connection between a (quasi)
scale-invariant tensor spectrum and inflation completely robust.Comment: 5 pages, reference added, version accepted in PR
Effective Theory of Dark Energy at Redshift Survey Scales
We explore the phenomenological consequences of general late-time
modifications of gravity in the quasi-static approximation, in the case where
cold dark matter is non-minimally coupled to the gravitational sector. Assuming
spectroscopic and photometric surveys with configuration parameters similar to
those of the Euclid mission, we derive constraints on our effective description
from three observables: the galaxy power spectrum in redshift space,
tomographic weak-lensing shear power spectrum and the correlation spectrum
between the integrated Sachs-Wolfe effect and the galaxy distribution. In
particular, with CDM as fiducial model and a specific choice for the
time dependence of our effective functions, we perform a Fisher matrix analysis
and find that the unmarginalized CL errors on the parameters describing
the modifications of gravity are of order --. We
also consider two other fiducial models. A nonminimal coupling of CDM enhances
the effects of modified gravity and reduces the above statistical errors
accordingly. In all cases, we find that the parameters are highly degenerate,
which prevents the inversion of the Fisher matrices. Some of these degeneracies
can be broken by combining all three observational probes.Comment: 41 pages, 5 figures, 2 tables, improved analysis of ISW-galaxy
correlation, matches published version on JCA
Modified gravity inside astrophysical bodies
Many theories of modified gravity, including the well studied Horndeski
models, are characterized by a screening mechanism that ensures that standard
gravity is recovered near astrophysical bodies. In a recently introduced class
of gravitational theories that goes beyond Horndeski, it has been found that
new derivative interactions lead to a partial breaking of the Vainshtein
screening mechanism inside any gravitational source, although not outside. We
study the impact of this new type of deviation from standard gravity on the
density profile of a spherically symmetric matter distribution, in the
nonrelativistic limit. For simplicity, we consider a polytropic equation of
state and derive the modifications to the standard Lane-Emden equations. We
also show the existence of a universal upper bound on the amplitude of this
type of modified gravity, independently of the details of the equation of
state.Comment: 11 pages, 6 figure
Exploring gravitational theories beyond Horndeski
We have recently proposed a new class of gravitational scalar-tensor theories
free from Ostrogradski instabilities, in arXiv:1404.6495. As they generalize
Horndeski theories, or "generalized" galileons, we call them G. These
theories possess a simple formulation when the time hypersurfaces are chosen to
coincide with the uniform scalar field hypersurfaces. We confirm that they
contain only three propagating degrees of freedom by presenting the details of
the Hamiltonian formulation. We examine the coupling between these theories and
matter. Moreover, we investigate how they transform under a disformal
redefinition of the metric. Remarkably, these theories are preserved by
disformal transformations that depend on the scalar field gradient, which also
allow to map subfamilies of G into Horndeski theories.Comment: 33 pages, added comments and corrected typos as in JCAP versio
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