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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 Restricted Cosmological solutions of some string inspired modified theories of gravity(Koç University, 2015) Oflaz, Neslihan; Dereli, Tekin; 0000-0002-6244-6054; Koç University Graduate School of Sciences and Engineering; Physics; 201358Publication 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 Effect of pressure loss devices on the performance of hybrid rocket systems(International Astronautical Federation (IAF), 2018) N/A; Department of Mechanical Engineering; Karabeyoğlu, Mustafa Arif; Faculty Member; Department of Mechanical Engineering; College of Engineering; 114595Internal ballistic devices that are used to trip the gas flow such as blades, steps, diaphragms or screens are commonly implemented in hybrid rocket motors to improve the mixing in the system. Enhanced mixing typically leads to an improvement in the regression rates and combustion efficiencies. The major issues with the use of these elements are 1) the pressure drop associated with the gas flow over these obstacles, 2) additional weight and 3) additional system complexity and cost. Even though a total pressure drop leads to a reduction in the thrust specific fuel consumption for air breathing propulsion systems, rockets do not suffer a direct hit on their specific impulse. The objective of this paper is to outline a theoretical proof that any pressure drop encountered along the motor axis does not lead to a reduction in the thrust or specific impulse performance of the rocket system. The primary adverse effect of the pressure loss is on the structural mass fraction of the rocket system. Assuming that the pressure at the nozzle entrance does not change, pressure drop requires higher head end pressures leading to increased injector manifold and feed system pressures. Using some example cases, we have estimated the combustion efficiency improvement required to balance the structural mass fraction increase caused by the flow trip devices. Such analysis needs to be conducted to justify the use of pressure drop elements for each design. Even though these arguments were developed for hybrid rockets, the analysis and conclusions are valid for all other chemical rocket types as well (i.e. solids and liquids).Publication Metadata only Electric propulsion optimization of microsatellite moon missions preliminary design application on CubeSats and Turkish small satellite field(International Astronautical Federation (IAF), 2014) N/A; N/A; Department of Mechanical Engineering; Kara, Ozan; Karabeyoğlu, Mustafa Arif; PhD Student; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 114595Subsystems miniaturization of spacecraft is making scientific microsatellite missions feasible. Specifically interplanetary space exploration mission can be provided by onboard micro propulsion systems. Due to the low requirements, Moon is a feasible destination for a preliminary space mission that universities, companies and governments can perform. This paper addresses the optimization of an electric propulsion system for a potential microsatellite lunar mission. Optimization takes the thrust level as a free variable to find the minimum initial mass along with the associated total burn time. The initial thrust range is chosen between 0.5-6mN. For a given thrust value, corresponding specific impulse, thruster power and thruster mass are determined based on curve fittings. As the next step, the input thrust level is extended up to 42mN to observe optimization over a broader thruster size. A feasible low thrust continuous orbit transfer to the Moon requires a high ΔV which is over 7,000 m/s. Edelbaum's analysis with optimal control theory is utilized to estimate the ΔV value. Edelbaum presents an approach for two non-coplanar circular orbits without any perturbations and shadowing effects. Furthermore, the optimization approach is applied for the preliminary design of a CubeSat Moon Mission. The spacecraft is determined to have an initial mass of 12 kg and requires a total power up to 100W. The particular mission selected for the study starts at 700km LEO and finishes at 200 km LLO. In addition, preliminary mission design presents (1) mass and power budgets, (2) thermal analysis, (3) ADCS selection, (4) structure and array mechanisms, and (5) cost estimation. In this paper, previous and planned small satellite researches in Turkey have also been reviewed. Small satellite projects which are performed by government, universities and industries show that Turkey has a small but growing small satellite activity.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 Restricted f (R) theories of dark gravity(Koç University, 2009) Oflaz, Neslihan; Dereli, Tekin; 0000-0002-6244-6054; Koç University Graduate School of Sciences and Engineering; Physics; 201358
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