Narrow Results

The phenomenon of accelerated expansion of the present universe and a cosmic transit aspect is explored in the framework of a modified gravity theory known as f(R, T) gravity (where R is the Ricci scalar and T is the trace of the energy–momentum tensor of the matter content). The cosmic transit phenomenon signifies a signature flipping behavior of the deceleration parameter. We employ a periodic varying deceleration parameter and obtained the exact solution of field equations. The dynamical features of the model including the oscillatory behavior of the EOS parameter are studied. We have also explored the obvious violation of energy–momentum conservation in f(R, T) gravity. The periodic behavior of energy conditions for the model are also discussed with a wide range of the free parameters.

The Friedmann–Robertson–Walker (FRW) metric expressed, in terms of comoving coordinates (r, t), always looks nonstatic. But by employing the recently derived curvature/Schwarzschild form, (R, T), of FRW metric (A. Mitra, Gravit. Cosmol. 19 (2013) 134), we show here that FRW metric can assume static forms when the net energy density (ρ_e) is solely due to the vacuum contribution. Earlier this question was explored by Florides (Gen. Relativ. Gravit. 12 (1980) 563) whose approach was complex and of purely mathematical nature. Also, unlike Florides, we do not assume any a priori separability of T(r, t) = F(r)G(t) and thus our treatment is truly general and yet simpler. More interestingly, even if the net energy density involved in a certain FRW model may appear to be nonzero from its algebric appearance, it may still be possible that tacitly ρ_e = 0 and the model actually corresponds to a vacuum Minkowski metric. For instance, it has been found that FRW universes which appear to be expanding with a fixed speed in comoving coordinates are intrinsically static universes. While such a linearly expanding universe having k = -1 is well-known as the Milne universe, the corresponding k = 0 case has recently been shown to be vacuum in disguise (A. Mitra, Mon. Not. Roy. Astron. Soc. 442 (2014) 382). In addition, here we show that even the k = +1 linearly "expanding" universe (in comoving coordinates) tacitly corresponds to Einstein's static universe.

We consider the newly proposed gravitational modifications that go beyond Horndeski’s theory, named as theories with extended nonminimal derivative couplings. By these modifications, the coefficient functions depend on the scalar field and its kinetic energy. These theories become ghost-free in cosmological background. We consider the flat FRW universe and explore the equation-of-state parameter, ${\omega }_{\text{tot}}$–${\omega }_{\text{tot}}^{\prime }$ plane and the squared speed of sound. The equation-of-state parameter exhibits phantom behavior of the universe, ${\omega }_{\text{tot}}$–${\omega }_{\text{tot}}^{\prime }$ plane represents the freezing region of the universe while the squared speed of sound denotes the stability of the model for the specific choice of constant parameters. Also, we investigate the validity of generalized second law of thermodynamics on the Hubble horizon taking into account the Bekenstein, power-law, Renyi and logarithmic corrections to the horizon entropy.