92 research outputs found
Arbitrary p-form Galileons
We show that scalar, 0-form, Galileon actions --models whose field equations
contain only second derivatives-- can be generalized to arbitrary even p-forms.
More generally, they need not even depend on a single form, but may involve
mixed p combinations, including equal p multiplets, where odd p-fields are also
permitted: We construct, for given dimension D, general actions depending on
scalars, vectors and higher p-form field strengths, whose field equations are
of exactly second derivative order. We also discuss and illustrate their
curved-space generalizations, especially the delicate non-minimal couplings
required to maintain this order. Concrete examples of pure and mixed actions,
field equations and their curved space extensions are presented.Comment: 8 pages, no figure, RevTeX4 format, v2: minor editorial changes
reflecting the published version in PRD Rapid Communication
Covariant Galileon
We consider the recently introduced "galileon" field in a dynamical
spacetime. When the galileon is assumed to be minimally coupled to the metric,
we underline that both field equations of the galileon and the metric involve
up to third-order derivatives. We show that a unique nonminimal coupling of the
galileon to curvature eliminates all higher derivatives in all field equations,
hence yielding second-order equations, without any extra propagating degree of
freedom. The resulting theory breaks the generalized "Galilean" invariance of
the original model.Comment: 10 pages, no figure, RevTeX4 format; v2 adds footnote 1, Ref. [12],
reformats the link in Ref. [14], and corrects very minor typo
Quantum effects and superquintessence in the new age of precision cosmology
Recent observations of Type Ia supernova at high redshifts establish that the
dark energy component of the universe has (a probably constant) ratio between
pressure and energy density . The
conventional quintessence models for dark energy are restricted to the range
, with the cosmological constant corresponding to .
Conformally coupled quintessence models are the simplest ones compatible with
the marginally allowed superaccelerated regime (). However, they are
known to be plagued with anisotropic singularities.
We argue here that the extension of the classical approach to the
semiclassical one, with the inclusion of quantum counterterms necessary to
ensure the renormalization, can eliminate the anisotropic singularities
preserving the isotropic behavior of conformally coupled superquintessence
models. Hence, besides of having other interesting properties, they are
consistent candidates to describe the superaccelerated phases of the universe
compatible with the present experimental data.Comment: 7 pages. Essay selected for "Honorable Mention" in the 2004 Awards
for Essays on Gravitation, Gravity Research Foundatio
Light scalar field constraints from gravitational-wave observations of compact binaries
Scalar-tensor theories are among the simplest extensions of general
relativity. In theories with light scalars, deviations from Einstein's theory
of gravity are determined by the scalar mass m_s and by a Brans-Dicke-like
coupling parameter \omega_{BD}. We show that gravitational-wave observations of
nonspinning neutron star-black hole binary inspirals can be used to set lower
bounds on \omega_{BD} and upper bounds on the combination
m_s/\sqrt{\omega_{BD}}$. We estimate via a Fisher matrix analysis that
individual observations with signal-to-noise ratio \rho would yield
(m_s/\sqrt{\omega_{BD}})(\rho/10)<10^{-15}, 10^{-16} and 10^{-19} eV for
Advanced LIGO, ET and eLISA, respectively. A statistical combination of
multiple observations may further improve these bounds.Comment: 9 pages, 4 figures. Matches version accepted in Physical Review
Model-Independent Comparisons of Pulsar Timings to Scalar-Tensor Gravity
Observations of pulsar timing provide strong constraints on scalar-tensor
theories of gravity, but these constraints are traditionally quoted as limits
on the microscopic parameters (like the Brans-Dicke coupling, for example) that
govern the strength of scalar-matter couplings at the particle level in
particular models. Here we present fits to timing data for several pulsars
directly in terms of the phenomenological couplings (masses, scalar charges,
moment of inertia sensitivities and so on) of the stars involved, rather than
to the more microscopic parameters of a specific model. For instance, for the
double pulsar PSR J0737-3039A/B we find at the 68% confidence level that the
masses are bounded by 1.28 < m_A/m_sun < 1.34 and 1.19 < m_B/m_sun < 1.25,
while the scalar-charge to mass ratios satisfy |a_A| < 0.21, |a_B| < 0.21 and
|a_B - a_A| < 0.002$. These constraints are independent of the details of the
scalar tensor model involved, and of assumptions about the stellar equations of
state. Our fits can be used to constrain a broad class of scalar tensor
theories by computing the fit quantities as functions of the microscopic
parameters in any particular model. For the Brans-Dicke and quasi-Brans-Dicke
models, the constraints obtained in this manner are consistent with those
quoted in the literature.Comment: 19 pages, 7 figure
Constraints on scalar-tensor theories of gravity from observations
In spite of their original discrepancy, both dark energy and modified theory
of gravity can be parameterized by the effective equation of state (EOS)
for the expansion history of the Universe. A useful model independent
approach to the EOS of them can be given by so-called
Chevallier-Polarski-Linder (CPL) parametrization where two parameters of it
( and ) can be constrained by the geometrical
observations which suffer from degeneracies between models. The linear growth
of large scale structure is usually used to remove these degeneracies. This
growth can be described by the growth index parameter and it can be
parameterized by in general. We use the
scalar-tensor theories of gravity (STG) and show that the discernment between
models is possible only when is not negligible. We show that the
linear density perturbation of the matter component as a function of redshift
severely constrains the viable subclasses of STG in terms of and
. From this method, we can rule out or prove the viable STG in future
observations. When we use , shows the convex shape of evolution
in a viable STG model. The viable STG models with are not
distinguishable from dark energy models when we strongly limit the solar system
constraint.Comment: 19 pages, 20 figures, 2 tables, submitted to JCA
Generalized Gravity and a Ghost
We show that generalized gravity theories involving the curvature invariants
of the Ricci tensor and the Riemann tensor as well as the Ricci scalar are
equivalent to multi- scalar-tensor gravities with four derivatives terms. By
expanding the action around a vacuum spacetime, the action is reduced to that
of the Einstein gravity with four derivative terms, and consequently there
appears a massive spin-2 ghost in such generalized gravity theories in addition
to a massive spin-0 field.Comment: 8 pages, a reference adde
The Abnormally Weighting Energy Hypothesis: the Missing Link between Dark Matter and Dark Energy
We generalize tensor-scalar theories of gravitation by the introduction of an
abnormally weighting type of energy. This theory of tensor-scalar anomalous
gravity is based on a relaxation of the weak equivalence principle that is now
restricted to ordinary visible matter only. As a consequence, the convergence
mechanism toward general relativity is modified and produces naturally cosmic
acceleration as an inescapable gravitational feedback induced by the
mass-variation of some invisible sector. The cosmological implications of this
new theoretical framework are studied. From the Hubble diagram cosmological
test \textit{alone}, this theory provides an estimation of the amount of
baryons and dark matter in the Universe that is consistent with the independent
cosmological tests of Cosmic Microwave Background (CMB) and Big Bang
Nucleosynthesis (BBN). Cosmic coincidence is naturally achieved from a equally
natural assumption on the amplitude of the scalar coupling strength. Finally,
from the adequacy to supernovae data, we derive a new intriguing relation
between the space-time dependences of the gravitational coupling and the dark
matter mass, providing an example of crucial constraint on microphysics from
cosmology. This glimpses at an enticing new symmetry between the visible and
invisible sectors, namely that the scalar charges of visible and invisible
matter are exactly opposite.Comment: 24 pages, 6 figures, new version with extended discussions and added
references. Accepted for publication in JCAP (sept. 2008
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