Gravitational Pressure and the Accelerating Universe
In the context of the Teleparallel Equivalent of General Relativity (TEGR) one can obtain an alternative insight into General Relativity, as has been shown in addressing properties as energy, momentum and angular momentum of the gravitational field. In this paper, we apply the definition, that arises from the field equation of the the TEGR, for the stress-energy-momentum tensor of the gravitational field, whose spatial components naturally lead to the definition of gravitational pressure, to compute the total space-time pressure, due to the gravitational and matter fields, over a spherical, space-like two-surface of a Friedman-Robertson-Walker (FRW) universe, for any curvature index. In particular, for a spatially flat FRW universe in the actual era (i.e., for "cold matter"), it resulted that the pressure (now due only to the gravitational field) is outwardly directed over any spherical, spatial two-surface. This surface can be, in particular, the apparent horizon of a spatially flat FRW universe (in this case, the apparent horizon coincides with the Hubble horizon). Assuming the validity of the first law of thermodynamics for matter and gravity, and taking into account the contribution of the gravitational field to both the energy and the pressure terms in the first law of the (gravitational) thermodynamics, as well as considering the thermal character of the apparent horizon of the spatially flat FRW universe, we have thus obtained a value of the gravitational pressure that is very close to the observed value. We interpret this result as a possibility that the accelerated expansion of the actual universe might be due to the effect of the pressure of the very gravitational field, instead of an totally unkown (dark) energy.