Savas Arapoglu

Istanbul Technical University · Department of Physics Engineering

We numerically examine the exterior solution of spherically symmetric and static configuration in scalar-tensor theories by using the nonminimally coupled scalar field with zero potential as our sample model. Our main purpose in this work is to fit the resulting data of the numerical solutions in the interested region by seeking for approximate ana...

We examine the exterior solution of spherically symmetric and static configuration in scalar-tensor theories by solving the differential equations numerically and fitting the resulting data in the interested region. Our main purpose in this work is to find out approximate analytical expressions which are independent of the parameters of a model as...

We investigate first-order vacuum phase transitions in the presence of a nonminimally coupled scalar field starting with the coupling effect on the initial dynamics of phase transitions by defining an effective potential for the scalar field and then performing three dimensional numerical simulations to observe any possible distinction in gravitati...

We investigate the effect of peculiar velocities of inhomogeneities and the spatial curvature of the universe on the shape of the gravitating potential. To this end, we consider scalar perturbations of the FLRW metric. The gravitational potential satisfies a Helmholtz-type equation which follows from the system of linearized Einstein equations. We...

We investigate the effect of peculiar velocities of inhomogeneities and the spatial curvature of the universe on the shape of the gravitational potential. To this end, we consider scalar perturbations of the FLRW metric. The gravitational potential satisfies a Helmholtz-type equation which follows from the system of linearized Einstein equations. W...

We study a three-component universe filled with dust-like matter in the form of discrete inhomogeneities (e.g., galaxies) and perfect fluids characterized by linear and nonlinear equations of state. Within the cosmic screening approach, we develop the theory of scalar and vector perturbations. None of the energy density contrasts associated with th...

We study a three-component universe filled with dust-like matter in the form of discrete inhomogeneities (e.g., galaxies) and perfect fluids characterized by linear and nonlinear equations of state. Within the cosmic screening approach, we develop the theory of scalar and vector perturbations. None of the energy density contrasts associated with th...

The viability of slow-roll approximation is examined by considering the structure of phase spaces in scalar-tensor theories of gravitation and the analysis is exemplified with a nonminimally coupled scalar field to the spacetime curvature. The slow-roll field equations are obtained in the Jordan frame in two ways: first using the direct generalizat...

The viability of slow-roll approximation is examined by considering the structure of phase spaces in scalar-tensor theories of gravitation and the analysis is exemplified with a nonminimally coupled scalar field to the spacetime curvature. The slow-roll field equations are obtained in the Jordan frame in two ways: first using the direct generalizat...

We study the structure of spherically symmetric and static objects in the presence of a nonminimally coupled scalar field having a potential of the form $V(\phi)=-\mu^2\phi^2/2+\lambda\phi^4/4$. We numerically solve equations of the system using two different realistic equations of state for neutron matter and give the mass-radius relations as well...

We study the structure of spherically symmetric and static objects in the presence of a nonminimally coupled scalar field having a potential of the form V(ϕ)=−μ2ϕ2∕2+λϕ4∕4. We numerically solve equations of the system using two different realistic equations of state for neutron matter and give the mass-radius relations as well as the radial profile...

We study the structure of neutron stars in scalar-tensor theories for the nonminimal coupling of the form (1+κξϕ2)R. We solve the hydrostatic equilibrium equations for two different types of scalar field potentials and three different equations of state representative of different degrees of stiffness. We obtain the mass-radius relations of the con...

We study the structure of neutron stars in scalar-tensor theories for the nonminimal coupling of the form $(1+\kappa \xi \phi^{2})\cal R$. We solve the hydrostatic equilibrium equations for two different types of scalar field potentials and three different equations of state representative of different degree of stiffness. We obtain the mass-radius...

A five-dimensional cosmological model including a single perfect fluid is studied in the framework of dynamical system analysis. All the critical points of the system are listed with their stability properties and some representative phase diagrams are explicitly shown. It is found that the stabilization of extra dimension is possible and the obser...

Dynamical system analysis of a universe model which contains matter, radiation, and quintessence with exponential potential, $V \!(\phi)=V_{\!o} \, exp(-\alpha \kappa \phi) \,$, is studied in the light of recent observations and the tensions between different datasets. The three-dimensional phase space is constructed by the energy density parameter...

We study the structure of relativistic stars in $\mathcal{R}+\alpha \mathcal{R}^{2}$ theory using the method of matched asymptotic expansion to handle the higher order derivatives in field equations arising from the higher order curvature term. We find solutions, parametrized by $\alpha$, for uniform density stars matching to the Schwarzschild solu...

The glitch activity of young pulsars arises from the exchange of angular momentum between the crust and the interior of the star. Recently it is inferred that the moment of inertia of the crust of a neutron star is not sufficient to explain the observed glitches. Such estimates presume Einstein's general relativity in describing the hydrostatic equ...

The slow-roll field equations for the case of non-minimally coupled scalar fields are obtained in two ways: first using the direct generalization of slow-roll conditions in the minimal coupling case to non-minimal one; and, second, conformal transforming the slow-roll field equations in the Einstein frame to the Jordan frame and then applying the g...

We correct a sign mistake in the work mentioned in the title; explore consequences on energy conditions in the relevant context, and make a suggestion on the introduced parameter. Comment: We have been informed on Dec.22, 2010 that an erratum was sent to PRL before we wrote the present comment. Due to a communication problem we were unaware of this...

We study the structure of neutron stars in perturbative f(R) gravity models with realistic equations of state. We obtain mass-radius relations in a gravity model of the form f(R)=R+\alpha R^2. We find that deviations from the results of general relativity, comparable to the variations due to using different equations of state (EoS'), are induced fo...

We consider a toy cosmological model in string theory involving the winding and momentum modes of (m,n) strings, i.e. bound states of m fundamental and n D-strings. The model is invariant under S-duality provided that m and n are interchanged. The dilaton is naturally stabilized due to S-duality invariance, which offers a new mechanism of moduli fi...

We consider a toy cosmological model with a gas of wrapped Dp-branes in 10-dimensional dilaton gravity compactified on a p-dimensional Ricci flat internal manifold. A consistent generalization of the low energy effective field equations in the presence of a conserved brane source coupled to dilaton is obtained. It is then shown that the compact dim...

We examine spherical p-branes in AdSm×Sn that wrap an Sp in either AdSm (p=m-2) or Sn (p=n-2). We first construct a two-spin giant solution expanding in Sn and has spins both in AdSm and Sn. For (m,n)={(5,5),(4,7),(7,4)}, it is 1/2 supersymmetric, and it reduces to the single-spin giant graviton when the AdS spin vanishes. We study some of its basi...

Using the method developed by Cherkis and Hashimoto we construct partially localized D3 perpendicular to D5(2), D4 perpendicular to D4(2) and M5 perpendicular to M5(3) supergravity solutions where one of the harmonic functions is given in an integral form. This is a generalization of the already known near-horizon solutions. The method fails for ce...

We study an RNS string with one end fixed on a $D0$-brane and the other end free as a qualitative guide to the spectrum of hadrons containing one very heavy quark. The mixed boundary conditions break half of the world-sheet supersymmetry. Boson-fermion masses can still be matched if space-time is 9 dimensional; thus SO(8) triality still plays a rol...