Geometric Perfect Fluids and Dark Side of the Universe

Recently we showed that in FLRW cosmology, the contribution from higher curvature terms in any generic metric gravity theory to the energy-momentum tensor is of the perfect fluid form. Such a geometric perfect fluid can be interpreted as a fluid remaining from the beginning of the universe where the string theory is thought to be effective. Just a short time after the beginning of the Universe, it is known that the Einstein-Hilbert action is assumed to be modified by adding all possible curvature invariants. We propose that the observed late-time accelerating expansion of the Universe can be solely driven by this geometric fluid. To support our claim, we specifically study the quadratic gravity field equations in D-dimensions. We show that the field equations of this theory for the FLRW metric possess a geometric perfect fluid source containing two critical parameters σ_1 and σ_2. To analyze this theory concerning its parameter space (σ_1, σ_2), we obtain the general second-order nonlinear differential equation governing the late-time dynamics of the deceleration parameter q. Hence using some present-day cosmological data as our initial conditions, our findings for the σ_2=0 case are as follows: (i) In order to have a positive energy density for the geometric fluid ρ_g, the parameter σ_1 must be negative for all dimensions up to D = 11, (ii) For a suitable choice of σ_1, the deceleration parameter experiences signature changes in the past and future, and in the meantime it lies within a negative range which means that the current observed accelerated expansion phase of the Universe can be driven solely by the curvature of the spacetime, (iii) q experiences a signature change and as the dimension D of spacetime increases, this signature change happens at earlier and later times, in the past and future, respectively.