Speaker
Description
We study the periapsis shift of a quasi-circular orbit in general static spherically symmetric spacetimes. We derive two formulae in full order with respect to the gravitational field, one in terms of the gravitational mass m and the Einstein tensor and the other in terms of the orbital angular velocity and the Einstein tensor. These formulae reproduce the well-known ones for the forward shift in the Schwarzschild spacetime. In a general case, the shift deviates from that in the vacuum spacetime due to a particular combination of the components of the Einstein tensor at the radius r of the orbit. The formulae give a backward shift due to the extended-mass effect in Newtonian gravity. In general relativity, in the weak-field and diffuse regime, the active gravitational mass density, ρA=(ϵ+pr+2pt)/c2, plays an important role, where ϵ, pr and pt are the energy density, the radial stress and the tangential stress of the matter field, respectively. We show that the shift is backward if ρA is beyond a critical value ρc≃2.8×10−15g/cm3(m/M⊙)2(r/a.u.)−4, while a forward shift greater than that in the vacuum spacetime instead implies ρA<0, i.e. the violation of the strong energy condition, and thereby provides evidence for dark energy. We obtain new observational constraints on ρA in the Solar System and the Galactic Centre.
