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 $w=p/\rho=-1.02(^{+0.13}_{-0.19})$. The conventional quintessence models for dark energy are restricted to the range $-1\le w < 0$, with the cosmological constant corresponding to $w=-1$. Conformally coupled quintessence models are the simplest ones compatible with the marginally allowed superaccelerated regime ($w<-1$). 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) $\omega$ 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 ($\omega_{0}$ and $\omega_{a}$) 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 $\gamma$ and it can be parameterized by $\gamma_{0} + \gamma_{a} (1 - a)$ in general. We use the scalar-tensor theories of gravity (STG) and show that the discernment between models is possible only when $\gamma_a$ 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 $\omega$ and $\gamma$. From this method, we can rule out or prove the viable STG in future observations. When we use $Z(\phi) =1$, $F$ shows the convex shape of evolution in a viable STG model. The viable STG models with $Z(\phi) = 1$ 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