New post-Newtonian parameter to test Chern-Simons gravity

We study Chern-Simons (CS) gravity in the parametrized post-Newtonian (PPN) framework through a weak-field solution of the modified field equations. We find that CS gravity possesses the same PPN parameters as general relativity, except for the inclusion of a new term, proportional to the CS coupling and the curl of the PPN vector potential. This new term leads to a modification of frame dragging and gyroscopic precession and we provide an estimate of its size. This correction might be used in experiments, such as Gravity Probe B, to bound CS gravity and test string theory.     

Quantum gravity stability of isotropy in homogeneous cosmology

It has been shown that anisotropy of homogeneous spacetime described by the general Kasner metric can be damped by quantum fluctuations coming from perturbative quantum gravity in one-loop approximation. Also, a formal argument, not limited to one-loop approximation, is put forward in favor of stability of isotropy in the exactly isotropic case.     

Cosmological tests of gravity

Modifications of general relativity provide an alternative explanation to dark energy for the observed acceleration of the universe. We review recent developments in modified gravity theories, focusing on higher-dimensional approaches and chameleon/f(R) theories. We classify these models in terms of the screening mechanisms that enable such theories to approach general relativity on small scales (and thus satisfy solar system constraints). We describe general features of the modified Friedman equation in such theories.The second half of this review describes experimental tests of gravity in light of the new theoretical approaches. We summarize the high precision tests of gravity on laboratory and solar system scales. We describe in some detail tests on astrophysical scales ranging from ∼ kpc (galaxy scales) to ∼ Gpc (large-scale structure). These tests rely on the growth and inter-relationship of perturbations in the metric potentials, density and velocity fields which can be measured using gravitational lensing, galaxy cluster abundances, galaxy clustering and the integrated Sachs-Wolfe effect. A robust way to interpret observations is by constraining effective parameters, such as the ratio of the two metric potentials. Currently tests of gravity on astrophysical scales are in the early stages — we summarize these tests and discuss the interesting prospects for new tests in the coming decade.     

Experimental tests of relativistic gravity

The confrontation between Einstein's gravitation theory and experimental results, notably binary pulsar data, is summarized and its significance discussed. Experiment and theory agree at the 10⁻³ level or better. All the basic structures of Einstein's theory (coupling of gravity to matter; propagation and self-interaction of the gravitational field, including in strong-field conditions) have been verified. However, the theoretical possibility that scalar couplings be naturally driven toward zero by the cosmological expansion suggests that the present agreement between Einstein's theory and experiment might be compatible with the existence of a long-range scalar contribution to gravity (such as the dilaton field, or a moduli field, of string theory). This provides a new theoretical paradigm, and new motivations for improving the experimental tests of gravity.     

On the post-Newtonian approximation of the Einstein-Cartan-Sciama-Kibble theory

The post-Newtonian approximation of the Einstein-Cartan-Sciama-Kibble or ECSK theory is developed. It is used to obtain a drastic reduction on the density of the source needed to detect the spin. It is also proved that if we know the structure of the source, up to order ¯v, the spin can be detected, and if we know it up to order ¯v² the theory can be verified.Supported by a scholarship from the Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET).     

Constraining extended theories of gravity using Solar System tests

Solar System tests give nowadays constraints on the estimated value of the cosmological constant, which can be accurately derived from different experiments regarding gravitational redshift, light deflection, gravitational time-delay and geodesic precession. Assuming that each reasonable theory of gravitation should satisfy Solar System tests, we use these limits on the estimated value of the cosmological constant to constrain extended theories of Gravity, which are nowadays studied as possible theories for cosmological models and provide viable solutions to the cosmological constant problem and the explanation of the present acceleration of the Universe. We obtain that the estimated values, from Solar System tests, for the parameters appearing in the extended theories of Gravity are orders of magnitude bigger than the values obtained in the framework of cosmologically relevant theories.     

On the post-Newtonian approximation of the ECSK theory II

We complete the PNA program for the ECSK theory. We find the general post-Newtonian equations of motion for the source. The different components of the complete affine connection, the torsion, and the energy-momentum tensor, as well as the conservation theorems of the theory, are developed for the case of an ideal fluid with spin in order to find the post-Newtonian trajectories of test particles exterior to the sources distribution. The main results are compared with the corresponding ones of general relativity.Supported by a scholarship from the Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET).     

Statistical isotropy of the cosmic microwave background

The breakdown of statistical homogeneity and isotropy of cosmic perturbations is a generic feature of ultra-large scale structure of the cosmos, in particular, of non-trivial cosmic topology. The statistical isotropy (SI) of the cosmic microwave background temperature fluctuations (CMB anisotropy) is sensitive to this breakdown on the largest scales comparable to, and even beyond the cosmic horizon. We propose a set of measures,K _l (l = 1, 2,3,...) which for non-zero values indicate and quantify statistical isotropy violations in a CMB map. We numerically compute the predictedK _l spectra for CMB anisotropy in flat torus universe models. Characteristic signatures of different models in theK _l spectrum are noted.     

Precision test of the isotropy of light propagation

We report on a test of the isotropy of light propagation performed by comparing the resonance frequencies of two orthogonal cryogenic optical resonators subject to Earths rotation over 1yr. The technical aspects of the experiment are discussed and the analysis of the data is presented in detail. For a possible anisotropy of the speed of light c, we obtain c/c₀=(2.6±1.7)×10^-15. Within the general extension of the standard model of particle physics, we extract limits on seven parameters at accuracies down to 10^-15, improving the best previous result by about two orders of magnitude. Within the Robertson–Mansouri–Sexl test theory, this implies an isotropy-violation parameter --1/2=(2.2±1.5)×10^-9, about three times lower than the best previous result. PACS 03.30.+p; 12.60.-i; 06.30.Ft     

Consistency of post-Newtonian waveforms with numerical relativity

General relativity predicts the gravitational wave signatures of coalescing binary black holes. Explicit waveform predictions for such systems, required for optimal analysis of observational data, have so far been achieved primarily using the post-Newtonian (PN) approximation. The quality of this treatment is unclear, however, for the important late-inspiral portion. We derive late-inspiral waveforms via a complementary approach, direct numerical simulation of Einstein's equations. We compare waveform phasing from simulations of the last approximately 14 cycles of gravitational radiation from equal-mass, nonspinning black holes with the corresponding 2.5PN, 3PN, and 3.5PN orbital phasing. We find phasing agreement consistent with internal error estimates for either approach, suggesting that PN waveforms for this system are effective until the last orbit prior to final merger.     

The weak interaction and the isotropy of the universe

We consider viscous dissipation of anisotropy and the corresponding entropy production during the lepton era of the universe, and investigate their sensitivity to the viscosity coefficients of the cosmological fluid.     

Chameleon fields: awaiting surprises for tests of gravity in space

We present a novel scenario where a scalar field acquires a mass which depends on the local matter density: the field is massive on Earth, where the density is high, but is essentially free in the solar system, where the density is low. All existing tests of gravity are satisfied. We predict that near-future satellite experiments could measure an effective Newton's constant in space different from that on Earth, as well as violations of the equivalence principle stronger than currently allowed by laboratory experiments.