124 citations · 250 across the 25 of their papers we have counts for
25 papers
Can the Universe Change Signature When Gravity is Dynamical?
Yaghoub Heydarzade
Can a universe undergo a regular Euclidean-Lorentzian signature transition when the gravitational coupling itself is dynamical? We address this question in scalar-tensor gravity wi…
Dynamical and Observational Analysis of Generalized Nash's Theory of Gravity
Amin Rezaei Akbarieh, Mohammad Amin Bolouri, Yaghoub Heydarzade
We investigate cosmic evolution in generalized Nash's theory of gravity involving the quadratic Ricci invariant . The analysis is divided into two complementary bra…
FLRW-Cosmology in Scalar-Vector-Tensor Theories of Gravity
Metin Gürses, Yaghoub Heydarzade
We generalize our previous theorem for FLRW spacetimes within the framework of generic metric gravity theories. In earlier work, we proved that, in the absence of matter fields, th…
Signature Change in Theory
Serkan Doruk Hazinedar, Yaghoub Heydarzade
We investigate a simple gravity model coupled to a scalar field and demonstrate that the theory admits classical degenerate metric solutions, analogous to those known i…
Quantum Cosmology in Theory with Schutz's Perfect Fluid
Serkan Doruk Hazinedar, Yaghoub Heydarzade, Shahram Jalalzadeh
The theory of gravity extends general relativity (GR) by allowing the gravitational Lagrangian to depend on both the Ricci scalar and the trace of the energy-momentum…
Generalized Vaidya Spacetime in Cotton and Conformal Killing Theories
Metin Gürses, Yaghoub Heydarzade, Çetin Şentürk
We demonstrate that the non-vacuum field equations of Cotton gravity and Conformal Killing gravity admit a generalized class of Vaidya-type solutions. In particular, beyond the sta…
Generalized Misner-Sharp energy in gravity
Amin Rezaei Akbarieh, Navid Safarzadeh Ilkhchi, Yaghoub Heydarzade
In this work, we explore the formulation of the Misner-Sharp energy within the framework of gravity, a modified theory incorporating the Ricci scalar and th…
Cosmological Singularities in Brane Gravity
R. Jalalzadeh, S. Jalalzadeh, Y. Heydarzade
We present a comprehensive study of cosmological singularities within the framework of Covariant Extrinsic Gravity (CEG), addressing both the initial Big Bang singularity and poten…
Dynamical Wormhole Solutions in Gravity
Yaghoub Heydarzade, Maryam Ranjbar
A class of theories extends the Einstein-Hilbert action by incorporating a general function of and , the Ricci scalar and the trace of the ordinary energy-momentum…
Non-Vacuum Solutions in Cotton Theory
Serkan Doruk Hazinedar, Yaghoub Heydarzade, Maryam Ranjbar
Cotton theory (CT) introduces a higher derivative extension of General Relativity (GR) characterized by third-rank field equations. Recently, key distinctions between CT and GR con…
Wave Metrics in the Cotton and Conformal Killing Gravity Theories
Metin Gürses, Yaghoub Heydarzade, Çetin Şentürk
We study wave metrics in the context of Cotton Gravity and Conformal Killing Gravity. First, we consider pp-wave metrics with flat and non-flat wave surfaces and show that they are…
Geometric Perfect Fluids and Dark Side of the Universe
Metin Gürses, Yaghoub Heydarzade, Çetin Şentürk
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 f…
Dynamical Photon Spheres in Charged Black Holes and Naked Singularities
Yaghoub Heydarzade, Vitalii Vertogradov
To understand the nature of a black hole shadow in dynamical spacetimes, we construct an analytical model of a dynamical photon sphere in the context of the Bonnor-Vaidya spacetime…
Dynamical Wormhole Solutions in Rastall Theory
Yaghoub Heydarzade, Maryam Ranjbar
Wormhole configurations in Einstein's general theory of relativity (GR) require exotic matter sources violating the weak energy condition (WEC). Rastall's theory is a generalizatio…
Hairy Kiselev Black Hole Solutions
Yaghoub Heydarzade, Maxim Misyura, Vitalii Vertogradov
In the realm of astrophysics, black holes exist within nonvacuum cosmological backgrounds, making it crucial to investigate how these backgrounds influence the properties of black…
Kerr-Schild-Kundt Metrics in Generic Einstein-Maxwell Theories
Metin Gurses, Yaghoub Heydarzade, Cetin Senturk
We study Kerr-Schild-Kundt class of metrics in generic gravity theories with Maxwell's field. We prove that these metrics linearize and simplify the field equations of generic grav…
Kerr-Schild-Kundt Metrics in Generic Gravity Theories with Modified Horndeski Couplings
Metin Gurses, Yaghoub Heydarzade, Cetin Senturk
The Kerr-Schild-Kundt (KSK) metrics are known to be one of the universal metrics in general relativity, which means that they solve the vacuum field equations of any gravity theory…
On the initial singularity in Kantowski-Sachs spacetime
Elham Ghorani, Yaghoub Heydarzade
The emergent universe scenario is a proposal for resolving the Big Bang singularity problem in the standard Friedmann-Lemaitre-Robertson-Walker cosmology. In the context of this sc…
FLRW-Cosmology in Generic Gravity Theories
Metin Gurses, Yaghoub Heydarzade
We prove that for the Friedmann-Lemaitre-Robertson-Walker metric, the field equations of any generic gravity theory in arbitrary dimensions are of the perfect fluid type. The cases…
Superposition of FLRW Universes
Metin Gurses, Yaghoub Heydarzade, Bayram Tekin
We show that (1) the Einstein field equations with a perfect fluid source admit a nonlinear superposition of two distinct homogenous Friedman-Lemaitre-Robertson-Walker (FLRW) metri…
Cosmological Singularities in Conformal Weyl Gravity
Yaghoub Heydarzade
In this work, we study the issue of the past and future cosmological singularities in the context of the fourth-order conformal Weyl gravity. In particular, we investigate the emer…
Black hole solutions and Euler equation in Rastall and generalized Rastall theories of gravity
H. Moradpour, Y. Heydarzade, C. Corda +2
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 simi…
NAT Black Holes
Metin Gurses, Yaghoub Heydarzade, Cetin Senturk
We study some physical properties of black holes in Null Aether Theory (NAT)--a vector-tensor theory of gravity. We first review the black hole solutions in NAT and then derive the…
New Classes of Spherically Symmetric, Inhomogeneous Cosmological Models
Metin Gurses, Yaghoub Heydarzade
We present two classes of inhomogeneous, spherically symmetric solutions of the Einstein-Maxwell-Perfect Fluid field equations with cosmological constant generalizing the Vaidya-Sh…