Avoidance of Strong Coupling in General Relativity Solutions with a Timelike Scalar Profile in a Class of Ghost-Free Scalar-Tensor Theories.

Modified gravity theories can accommodate exact solutions, for which the metric has the same form as the one in general relativity, i.e., stealth solutions.

Ghosts without Runaway Instabilities.

We present a simple class of mechanical models where a canonical degree of freedom interacts with another one with a negative kinetic term, i.e., with a ghost.

Graviton Mass Might Reduce Tension between Early and Late Time Cosmological Data.

The standard cold dark matter model with a cosmological constant (Λ-CDM) predicts a growth of structures which tends to be higher than the values of redshift space distortion (RSD) measurements if the cosmological parameters are fixed by the cosmic microwave background data. In this Letter, we point out that this discrepancy can be resolved or understood if we assume that the graviton has a small but nonzero mass. In the context of the minimal theory of massive gravity (MTMG), due to infrared Lorentz violations measurable only at present cosmological scales, the graviton acquires a mass without being haunted by unwanted extra degrees of freedom.

Strict limit on CPT violation from polarization of γ-ray bursts.

We report the strictest observational verification of CPT invariance in the photon sector, as a result of γ-ray polarization measurement of distant gamma-ray bursts (GRBs), which are the brightest stellar-sized explosions in the Universe. We detected γ-ray polarization of three GRBs with high significance levels, and the source distances may be constrained by a well-known luminosity indicator for GRBs. For the Lorentz- and CPT-violating dispersion relation E(±)(2) = p(2) ± 2ξp(3)/M(Pl), where ± denotes different circular polarization states of the photon, the parameter ξ is constrained as |ξ| View Article and Find Full Text PDF

Massive gravity: nonlinear instability of a homogeneous and isotropic universe.

We argue that all homogeneous and isotropic solutions in nonlinear massive gravity are unstable. For this purpose, we study the propagating modes on a Bianchi type-I manifold. We analyze their kinetic terms and dispersion relations as the background manifold approaches the homogeneous and isotropic limit.