Massive graviton as a testable cold-dark-matter candidate

Milky Way Accelerometry via Millisecond Pulsar Timing

The temporal stability of millisecond pulsars is remarkable, rivaling even some terrestrial atomic clocks at long timescales. Using this property, we show that millisecond pulsars distributed in the galactic neighborhood form an ensemble of accelerometers from which we can directly extract the local galactic acceleration. From pulsar spin period measurements, we demonstrate acceleration sensitivity with about 1σ precision using 117 pulsars. We also present a complementary analysis using orbital periods of 13 binary pulsar systems that eliminates the systematics associated with pulsar braking and results in a local acceleration of (1.7±0.5)×10^{-10}  m/s^{2} in good agreement with expectations. This work is a first step toward dynamically measuring acceleration gradients that will eventually inform us about the dark matter density distribution in the Milky Way galaxy.

Massive Gravity

We review recent progress in massive gravity. We start by showing how different theories of massive gravity emerge from a higher-dimensional theory of general relativity, leading to the Dvali-Gabadadze-Porrati model (DGP), cascading gravity, and ghost-free massive gravity. We then explore their theoretical and phenomenological consistency, proving the absence of Boulware-Deser ghosts and reviewing the Vainshtein mechanism and the cosmological solutions in these models. Finally, we present alternative and related models of massive gravity such as new massive gravity, Lorentz-violating massive gravity and non-local massive gravity.

Constraints on massive-graviton dark matter from pulsar timing and precision astrometry

The effect of a narrow-band isotropic stochastic gravitational wave background on pulsar timing and astrometric measurements is studied. Such a background appears in some theories of gravity. We show that the existing millisecond pulsar timing accuracy (approximately 0.2 micros) strongly constrains possible observational consequences of theory of massive gravity with spontaneous Lorentz braking, essentially ruling out significant contribution of massive gravitons to the local dark halo density. The present-day accuracy of astrometrical measurements (approximately 100 microarc sec) sets less stringent constraints on this theory.

Green dyadic for the Proca fields

The dyadic Green functions for the Proca fields in free space are derived to include singular terms. Both the electric and magnetic types will be obtained with the results reduced back to those for the Maxwell fields in the limit of zero photon mass. Moreover, the singular terms are identical in both massless and massive electrodynamics. As an illustration, the results are applied to obtain the exact dynamical fields for an oscillating dipole which reduce back to the well-known expressions for static fields derived previously in the literature for massive electrodynamics.

Spontaneous lorentz breaking and massive gravity

Theories with an extra spin-two field coupled to gravity admit a massive phase with broken Lorentz symmetry. While the equivalence principle is respected, the Newtonian potentials are in general modified, but they may be protected by a scale symmetry of the coupling term. The gravitational waves phenomenology is quite rich: two gravitons, one massive and one massless, oscillate and propagate with distinct velocities, different from the speed of light. A time of flight difference between gravitons and photons from a common source would provide a clear signal of this theory.