arXiv2024
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.