Browsing by Subject "Modified gravity"

Now showing 1 - 2 of 2

Open Access
Black hole solutions and Euler equation in Rastall and generalized Rastall theories of gravity
(World Scientific Publishing, 2019) Moradpour, H.; Heydarzade, Yaghoub; Corda, C.; Ziaie, A. H.; Ghaffari, S.
Focusing on the special case of generalized Rastall theory, as a subclass of the non-minimal curvature-matter coupling theories in which the field equations are mathematically similar to the Einstein field equations in the presence of cosmological constant, we find two classes of black hole (BH) solutions including (i) conformally flat solutions and (ii) non-singular BHs. Accepting the mass function definition and by using the entropy contents of the solutions along with thermodynamic definitions of temperature and pressure, we study the validity of Euler equation on the corresponding horizons. Our results show that the thermodynamic pressure, meeting the Euler equation, is not always equal to the pressure components appeared in the gravitational field equations and satisfies the first law of thermodynamics, a result which in fact depends on the presumed energy definition. The requirements of having solutions with equal thermodynamic and Hawking temperatures are also studied. Additionally, we study the conformally flat BHs in the Rastall framework. The consequences of employing generalized Misner–Sharp mass in studying the validity of the Euler equation are also addressed.
Open Access
Geometric perfect fluids and the dark side of the universe
(American Physical Society, 2024-07-26) Gürses, Metin; Heydarzade, Yaghoub; Şentürk, Çetin
Recently, we showed that in Friedman-Lemaître-Robertson-Walker (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 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 σ₁ and σ₂. To analyze this theory concerning its parameter space (σ₁, σ₂), 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 σ₂ = 0 case are as follows: (i) To have a positive energy density for the geometric fluid ρᵍ, the parameter σ₁ must be negative for all dimensions up to D = 11. (ii) For a suitable choice of σ₁, 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 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.