133 research outputs found
Ambiguous Tests of General Relativity on Cosmological Scales
There are a number of approaches to testing General Relativity (GR) on linear
scales using parameterized frameworks for modifying cosmological perturbation
theory. It is sometimes assumed that the details of any given parameterization
are unimportant if one uses it as a diagnostic for deviations from GR. In this
brief report we argue that this is not necessarily so. First we show that
adopting alternative combinations of modifications to the field equations
significantly changes the constraints that one obtains. In addition, we show
that using a parameterization with insufficient freedom significantly tightens
the apparent theoretical constraints. Fundamentally we argue that it is almost
never appropriate to consider modifications to the perturbed Einstein equations
as being constraints on the effective gravitational constant, for example, in
the same sense that solar system constraints are. The only consistent
modifications are either those that grant near-total freedom, as in
decomposition methods, or ones which map directly to a particular part of
theory space
Technically natural dark energy from Lorentz breaking
We construct a model of dark energy with a technically natural small
contribution to cosmic acceleration, i.e. this contribution does not receive
corrections from other scales in the theory. The proposed acceleration
mechanism appears generically in the low-energy limit of gravity theories with
violation of Lorentz invariance that contain a derivatively coupled scalar
field Theta. The latter may be the Goldstone field of a broken global symmetry.
The model, that we call Theta-CDM, is a valid effective field theory up to a
high cutoff just a few orders of magnitude below the Planck scale. Furthermore,
it can be ultraviolet-completed in the context of Horava gravity. We discuss
the observational predictions of the model. Even in the absence of a
cosmological constant term, the expansion history of the Universe is
essentially indistinguishable from that of Lambda-CDM. The difference between
the two theories appears at the level of cosmological perturbations. We find
that in Theta-CDM the matter power spectrum is enhanced at subhorizon scales
compared to Lambda-CDM. This property can be used to discriminate the model
from Lambda-CDM with current cosmological data.Comment: A few equations in the Appendix correcte
A Class of Effective Field Theory Models of Cosmic Acceleration
We explore a class of effective field theory models of cosmic acceleration
involving a metric and a single scalar field. These models can be obtained by
starting with a set of ultralight pseudo-Nambu-Goldstone bosons whose couplings
to matter satisfy the weak equivalence principle, assuming that one boson is
lighter than all the others, and integrating out the heavier fields. The result
is a quintessence model with matter coupling, together with a series of
correction terms in the action in a covariant derivative expansion, with
specific scalings for the coefficients. After eliminating higher derivative
terms and exploiting the field redefinition freedom, we show that the resulting
theory contains nine independent free functions of the scalar field when
truncated at four derivatives. This is in contrast to the four free functions
found in similar theories of single-field inflation, where matter is not
present. We discuss several different representations of the theory that can be
obtained using the field redefinition freedom. For perturbations to the
quintessence field today on subhorizon lengthscales larger than the Compton
wavelength of the heavy fields, the theory is weakly coupled and natural in the
sense of t'Hooft. The theory admits a regime where the perturbations become
modestly nonlinear, but very strong nonlinearities lie outside its domain of
validity.Comment: 43 pages, 2 figures; Version 3 publication versio
The Distinguishability of Interacting Dark Energy from Modified Gravity
We study the observational viability of coupled quintessence models with
their expansion and growth histories matched to modified gravity cosmologies.
We find that for a Dvali-Gabadadze-Porrati model which has been fitted to
observations, the matched interacting dark energy models are observationally
disfavoured. We also study the distinguishability of interacting dark energy
models matched to scalar-tensor theory cosmologies and show that it is not
always possible to find a physical interacting dark energy model which shares
their expansion and growth histories.Comment: 8 pages, 5 figure
Fundamental constants and tests of general relativity - Theoretical and cosmological considerations
The tests of the constancy of the fundamental constants are tests of the
local position invariance and thus of the equivalence principle. We summarize
the various constraints that have been obtained and then describe the
connection between varying constants and extensions of general relativity. To
finish, we discuss the link with cosmology, and more particularly with the
acceleration of the Universe. We take the opportunity to summarize various
possibilities to test general relativity (but also the Copernican principle) on
cosmological scales.Comment: Proceedings of the workshop ``The nature of gravity, confronting
theory and experiment in space'', ISSI, Bern, october 200
The acceleration of the universe and the physics behind it
Using a general classification of dark enegy models in four classes, we
discuss the complementarity of cosmological observations to tackle down the
physics beyond the acceleration of our universe. We discuss the tests
distinguishing the four classes and then focus on the dynamics of the
perturbations in the Newtonian regime. We also exhibit explicitely models that
have identical predictions for a subset of observations.Comment: 18 pages, 18 figure
Non-minimally coupled dark matter: effective pressure and structure formation
We propose a phenomenological model in which a non-minimal coupling between
gravity and dark matter is present in order to address some of the apparent
small scales issues of \lcdm model. When described in a frame in which gravity
dynamics is given by the standard Einstein-Hilbert action, the non-minimal
coupling translates into an effective pressure for the dark matter component.
We consider some phenomenological examples and describe both background and
linear perturbations. We show that the presence of an effective pressure may
lead these scenarios to differ from \lcdm at the scales where the non-minimal
coupling (and therefore the pressure) is active. In particular two effects are
present: a pressure term for the dark matter component that is able to reduce
the growth of structures at galactic scales, possibly reconciling simulations
and observations; an effective interaction term between dark matter and baryons
that could explain observed correlations between the two components of the
cosmic fluid within Tully-Fisher analysis.Comment: 18 pages, 6 figures, references added. Published in JCA
Statefinder Diagnostic for Dilaton Dark Energy
Statefinder diagnostic is a useful method which can differ one dark energy
model from the others. The Statefinder pair is algebraically related
to the equation of state of dark energy and its first time derivative. We apply
in this paper this method to the dilaton dark energy model based on Weyl-Scaled
induced gravitational theory. We investigate the effect of the coupling between
matter and dilaton when the potential of dilaton field is taken as the Mexican
hat form. We find that the evolving trajectory of our model in the
diagram is quite different from those of other dark energy models.Comment: 6 pages, 4 figures, type errors corrected, reference no. changed,
accepted by Astrophysics and Space Scienc
Tracking Extended Quintessence
We study the cosmological role of a Tracking Field in Extended
Quintessence scenarios (TEQ), where the dynamical vacuum energy driving the
acceleration of the universe today is coupled with the Ricci scalar, , with
a term of the form , where . Tracker solutions for these NMC models, with
inverse power-law potentials, possess an initial enhancement of the scalar
field dynamics, named -boost, caused by the Ricci scalar in the Klein-Gordon
equation. During this phase the field performs a "gravitational" slow rolling
which we model analytically, with energy density scaling as . We
evolve linear perturbations in TEQ models assuming Gaussian scale-invariant
initial spectrum. We obtain significant changes in the Integrated Sachs Wolfe
effect and in the acoustic peaks locations on the Cosmic Microwave Background,
as well as in the turnover on the matter power spectrum. All these corrections
may assume positive as well as negative values, depending on the sign of the
NMC parameter . We give analytical formulas describing all these effects.
We show that they can be as large as with respect to equivalent
cosmological constant and ordinary tracking Quintessence models, respecting all
the existing experimental constraints on scalar-tensor theories of gravity.
These results demonstrate that the next decade data will provide deep
constraints on the nature of the dark energy in the Universe, as well as the
structure of the theory of gravity.Comment: 24 pages including 8 figures, final version to be published in
Phys.Rev.
On Physical Equivalence between Nonlinear Gravity Theories
We argue that in a nonlinear gravity theory, which according to well-known
results is dynamically equivalent to a self-gravitating scalar field in General
Relativity, the true physical variables are exactly those which describe the
equivalent general-relativistic model (these variables are known as Einstein
frame). Whenever such variables cannot be defined, there are strong indications
that the original theory is unphysical. We explicitly show how to map, in the
presence of matter, the Jordan frame to the Einstein one and backwards. We
study energetics for asymptotically flat solutions. This is based on the
second-order dynamics obtained, without changing the metric, by the use of a
Helmholtz Lagrangian. We prove for a large class of these Lagrangians that the
ADM energy is positive for solutions close to flat space. The proof of this
Positive Energy Theorem relies on the existence of the Einstein frame, since in
the (Helmholtz--)Jordan frame the Dominant Energy Condition does not hold and
the field variables are unrelated to the total energy of the system.Comment: 37 pp., TO-JLL-P 3/93 Dec 199
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