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Publication Metadata only Accelerated expansion of the universe `a la the Stueckelberg mechanism(Iop Publishing Ltd, 2014) Arik, Metin; Katirci, Nihan; Kavuk, Mehmet; Department of Physics; Akarsu, Özgür; Researcher; Department of Physics; College of Sciences; N/AWe investigate a cosmological model in which the Stueckelberg fields are nonminimally coupled to the scalar curvature in a gauge invariant manner. We present not only a solution that can be considered in the context of the late time acceleration of the universe but also a solution compatible with the inflationary cosmology. Distinct behaviors of the scalar and vector fields together with the real valued mass gained by the Stueckelberg mechanism lead the universe to go through the two different accelerated expansion phases with a decelerated expansion phase between them. On the other hand, in the solutions we present, if the mass is null then the universe is either static or exhibits a simple power law expansion due to the vector field potential.Publication Metadata only Comment on the possibility of a geometric constraint in the Schrodinger quantum mechanics(World Scientific Publ Co Pte Ltd, 2000) Department of Mathematics; Mostafazadeh, Ali; Faculty Member; Department of Mathematics; College of Sciences; 4231It is shown that the geometric constraint advocated in [R. S. Kaushal, Mod. Phys. Lett.A15, 1391 (2000)] is trivially satisfied. Therefore, such a constraint does not exist. We also point out another flaw in Kaushal's paper.Publication Metadata only Constraining scalar-tensor theories using neutron star mass and radius measurements(American Physical Society (APS), 2022) Tuna, Semih; N/A; Department of Physics; Ünlütürk, Kıvanç İbrahim; Ramazanoğlu, Fethi Mübin; PhD Student; Faculty Member; Department of Physics; Graduate School of Sciences and Engineering; College of Sciences; N/A; 254225We use neutron star mass and radius measurements to constrain the spontaneous scalarization phenomenon in scalar-tensor theories using Bayesian analysis. Neutron star structures in this scenario can be significantly different from the case of general relativity, which can be used to constrain the theory parameters. We utilize this idea to obtain lower bounds on the coupling parameter ?? for the case of massless scalars. These constraints are currently weaker than the ones coming from binary observations, and they have relatively low precision due to the approximations in our method. Nevertheless, our results clearly demonstrate the power of the mass-radius data in testing gravity, and can be further improved with future observations. The picture is different for massive scalars, for which the same data is considerably less effective in constraining the theory parameters in an unexpected manner. We identify the main reason for this to be a large high-likelihood region in the parameter space where deviations from general relativity are relatively small. We hope this initial study to be an invitation to use neutron star structure measurements more commonly to test alternative theories in general.Publication Metadata only Cosmology with hybrid expansion law: scalar field reconstruction of cosmic history and observational constraints(Institute of Physics (IOP) Publishing, 2014) Kumar, Suresh; Myrzakulov, R.; Sami, M.; Xu, Lixin; Department of Physics; Akarsu, Özgür; Researcher; Department of Physics; College of Sciences; N/AIn this paper, we consider a simple form of expansion history of Universe referred to as the hybrid expansion law − a product of power-law and exponential type of functions. The ansatz by construction mimics the power-law and de Sitter cosmologies as special cases but also provides an elegant description of the transition from deceleration to cosmic acceleration. We point out the Brans-Dicke realization of the cosmic history under consideration. We construct potentials for quintessence, phantom and tachyon fields, which can give rise to the hybrid expansion law in general relativity. We investigate observational constraints on the model with hybrid expansion law applied to late time acceleration as well as to early Universe a la nucleosynthesis.Publication Metadata only Entropy of the quantum fluctuations of fermionic instantons in the universe(World Scientific Publishing Co Pte Ltd, 2022) Akilli, Mahmut; Department of Physics; Yılmaz, Nazmi; Teaching Faculty; Department of Physics; College of Sciences; 178427The aim of this paper is to study the entropy of quantum fluctuations of fermionic instantons. For this purpose, we focus on the spinor-type instanton solution family of the massless pure fermionic Thirring model because it is a well-known quantized toy model in the quantum field theory since 1958. We calculate the Boltzmann-Gibbs-Shannon (BGS) entropy of the Thirring model by the normalized inner scalogram (NIS) and find that the entropy of quantum fluctuations of Thirring fermionic instantons increase slightly just after the big bang. This result leads us to discuss the relation between the increasing entropy in the Universe due to quantum fluctuations of the fermionic Thirring instantons during the creation of fermionic type particles.Publication Metadata only Exact solutions in five-dimensional axi-dilaton gravity with euler-poincare term(Iop Publishing Ltd, 2007) Aliev, A. N.; Cebeci, H.; Department of Physics; Dereli, Tekin; Faculty Member; Department of Physics; College of Sciences; 201358We examine the effective field equations that are obtained from the axi-dilaton gravity action with a second-order Euler-Poincare term and a cosmological constant in all higher dimensions. We solve these equations for five-dimensional-spacetimes possessing homogeneity and isotropy in their three-dimensional subspaces. For a number of interesting special cases, we show that the solutions fall into two main classes: the first class consists of time-dependent solutions with spherical or hyperboloidal symmetry which require certain fine-tuning relations between the coupling constants of the model and the cosmological constant. Solutions in the second class are locally static and prove the validity of Birkhoff's staticity theorem in the axi-dilaton gravity. We also give a class of static solutions, among them the well known charged black hole solutions with a cosmological constant in which the usual electric charge is superseded by an axion charge.Publication Open Access Gravitational Higgs mechanism and resulting observational effects(American Physical Society (APS), 2020) Krall, Verena; Kokkotas, Kostas D.; Department of Physics; Coates, Andrew; Researcher; Department of Physics; College of SciencesRecently, a toy model was introduced to demonstrate that screening mechanisms in alternative theories of gravitation can hide additional effects. In this model a scalar field is charged under a U(1) symmetry. In sufficiently compact objects the scalar field spontaneously grows, i.e., the object scalarizes, spontaneously breaking the U(1) symmetry. Exactly as in the U(1) Higgs mechanism this leads to the emergence of a mass for the gauge field. The aim of this paper is to provide an example of the physical consequences if we consider this toy model as a prototype of Weak Equivalence Principle (WEP) violations. We model neutron stars with a dipolar magnetic field to compare the magnetic field behavior of stars in Einstein-Maxwell theory on the one hand and in scalar-tensor theory with the, so-called, gravitational Higgs mechanism on the other hand.Publication Metadata only Hilbert space structures on the solution space of Klein-Gordon-type evolution equations(Iop Publishing Ltd, 2003) N/A; Department of Mathematics; Mostafazadeh, Ali; Faculty Member; Department of Mathematics; College of Sciences; 4231We use the theory of pseudo-Hermitian operators to address the problem of the construction and classification of positive-definite invariant inner-products on the space of solutions of a Klein-Gordon-type evolution equation. This involves dealing with the peculiarities of formulating a unitary quantum dynamics in a Hilbert space with a time-dependent inner product. We apply our general results to obtain possible Hilbert space structures on the solution space of the equation of motion for a classical simple harmonic oscillator, a free Klein-Gordon equation and the Wheeler-DeWitt equation for the FRW-massive-real-scalar-field models.Publication Metadata only Inducing gravity from connections and scalar fields(Iop Publishing Ltd, 2019) N/A; Azri, Hemza; Resarcher; Graduate School of Sciences and Engineering; N/AWe propose an approach to induced gravity, or Sakharov's 'metrical elasticity', which requires only an affine spacetime that accommodates scalar fields. The setup provides the induction of metric gravity from a pure (One action, and it is established in two possible ways: (i) at the classical level, the Einstein-Hilbert action arises with both, metric and Newton's constant, from the nonzero potential energy of the background field (ii) at the quantum level (quantized matter), gravity scale is induced from the one-loop effective action by integrating out the scalar degrees of freedom. In the former, the cosmological constant is absorbed leading to the gravitational sector, however, the fact remains that quantum corrections induce a large cosmological constant. This new approach adds a crucial feature to induced gravity which is the fact that the metric structure is not imposed from the scratch, but it is an outcome of the primary theory.Publication Metadata only Neutrino fields in a sandwich gravitational wave background(Institute of Physics (IOP) Publishing, 2022) Gürtuğ, Özay; Halilsoy, Mustafa; Şenikoğlu, Yorgo; Department of Physics; Dereli, Tekin; Faculty Member; Department of Physics; College of Sciences; 201358Sandwich gravitational waves are given globally in terms of step functions at the boundaries. Linearized Einstein-Weyl equations are solved exactly in this background in Rosen coordinates. Depending on the geometry and composition of the sandwich wave, the neutrino's energy-momentum redistributes itself. At the test field level, since the background will not change, the neutrino's energy density in particular will show variations between positive and negative extrema when crossing the sandwich wave. This may reveal facts about the weakly interacting neutrinos in cosmology.