Inflation of the Universe by the Non-minimal Y(R)F2Y(R)F^2 Models

The cosmological solutions of the non-minimal $Y (R)F^2$ theory which are compatible with FRW space-time are investigated. In order to avoid the isotropy violation of a vector field, it can be considered that the presence of a triplet of vector fields satisfying $SO(3)$ symmetry, or the average of randomly oriented electromagnetic fields over the sufficiently large volume. By considering the symmetry arguments, the inflation driven by the non-minimally coupled vector fields is studied. Then the cosmological solutions and corresponding models are obtained with the power law, hyperbolic and hybrid scale functions.Comment: 16 pages, accepted in MPL

Regular Black Hole Solutions of the Non-minimally Coupled Y(R)F2Y(R)F^2 Gravity

In this study we investigate regular black hole solutions of the non-minimally coupled $Y(R)F^2$ gravity model. We give two regular black hole solutions and the corresponding non-minimal model for both electrically or magnetically charged cases. We calculate all the energy conditions for these solutions.Comment: 11 pages, some typos are correcte

Holographic Superconductors with the General RF2RF^2 type Couplings

We explore the effects of the general non-minimally coupled $RF^2$-type couplings on the holographic s-wave superconductors numerically in the Schwarzschild-AdS background. We calculate the condensation and conductivity of the model for the coupling parameters $a_1$ and $\beta$. We obtain that the bigger deviations of the parameter $a_1$ from the minimal case lead to the larger deviations of the gap frequency from the universal value $\omega_g/T_c \approx 8$. Moreover the smaller $\beta$ and $a_1$ cause to gradually stronger and narrower coherence peak.Comment: 12 pages, 14 figure

Non-minimal RF2RF^2-type corrections to holographic superconductor

We study $(2+1)$-dimensional holographic superconductors in the presence of non-minimally coupled electromagnetic field to gravity by considering an arbitrary linear combination of $RF^2$-type invariants with three parameters. Our analytical analysis shows that the non-minimal couplings affect the condensate and the critical temperature.Comment: Accepted for publication in Modern Physics Letters

Spinor coupling to the weak Poincare gauge theory of gravity in three dimensions

Minimal coupling of a Dirac field to gravity with the most general non-propagating torsion is considered in (1+2)-dimensions. The field equations are obtained from a lagrangian by a variational principle. The space-time torsion is calculated algebraically in terms of a quadratic spinor condensate plus coupling coefficients. Firstly we give circularly symmetric rotating exact solutions that collapse to $AdS_3$ geometry in the absence of the Dirac condensate. Secondly we investigate a BTZ-like solution.Comment: We added a new subsection, corrected syntax errors, extended dicussion. To be published in Phys. Rev.

Compact Stars in the Non-minimally Coupled Electromagnetic Fields to Gravity

We investigate the gravitational models with the non-minimal $Y(R)F^2$ coupled electromagnetic fields to gravity, in order to describe charged compact stars, where $Y(R)$ denotes a function of the Ricci curvature scalar $R$ and $F^2$ denotes the Maxwell invariant term. We determine two parameter family of exact spherically symmetric static solutions and the corresponding non-minimal model without assuming any relation between energy density of matter and pressure. We give the mass-radius, electric charge-radius ratios and surface gravitational redshift which are obtained by the boundary conditions. We reach a wide range of possibilities for the parameters $k$ and $\alpha$ in these solutions. Lastly we show that the models can describe the compact stars even in the the more simple case $\alpha=3$.Comment: 17 pages, 14 figure

Anisotropic cosmological solutions to the Y(R)F2Y(R)F^2 gravity

We investigate anisotropic cosmological solutions of the theory with non-minimal couplings between electromagnetic fields and gravity in $Y(R) F^2$ form. After we derive the field equations by the variational principle, we look for spatially flat cosmological solutions with magnetic fields or electric fields. Then we give exact anisotropic solutions by assuming the hyperbolic expansion functions. We observe that the solutions approach to the isotropic case in late-times.Comment: 16 pages, 5 figure

The Couplings Of Electromagnetic And Dirac Spinor Fields To Gravity

Tez (Doktora) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 2011Thesis (PhD) -- İstanbul Technical University, Institute of Science and Technology, 2011Gravitasyon teorisine yeni öngörüler elde edebilmek için, elektromanyetik ve spinör alanlarının gravitasyona bağlanmalarını inceliyoruz. İlk olarak, herhangi bir d-boyutta diferansiyel formların dış cebirini kullanarak Einstein-Cartan gravitasyon teorisini özetledikten sonra, dört boyutta elektromanyetik alanların gravitasyona minimal olmayan bağlanmalarını içeren teorileri araştırıyoruz. Özel olarak, bu teorilere analitik, düzlem yüzlü dalga ve küresel simetrik, durgun manyetik tek-kutup çözümleri veriyoruz. İkinci olarak, (1+2)-boyutta Einstein-Cartan-Dirac teorisini dış diferensiyel form hesabını kullanarak inceliyoruz. Uzay-zaman burulmasını Dirac yoğunlaşmış alanları cinsinden elde ederek Dirac spinörünün yokluğunda durgun metriğe dönüşen durağan çembersel simetrik tam çözümleri veriyoruz.In order to obtain new insights to gravity, we investigate the couplings of electromagnetic and spinor fields to gravity. Firstly, after we summarized Einstein-Cartan Gravity in d-dimensions using the algebra of exterior differential forms, we investigate non-minimal couplings of electromagnetism to gravity in four dimensions. In particular, we give a class of exact plane wave solutions and static, spherically symmetric magnetic monopole solutions. Secondly, we formulate Einstein-Cartan-Dirac theory in (1+2)-dimensions using the algebra of exterior differential forms. We determine the space-time torsion to be given algebraically in terms of the Dirac condensate field. We give circularly symmetric, stationary, exact solutions that collapse to static AdS3 geometry in the absence of a Dirac spinor.DoktoraPh

The non-minimally coupled symmetric teleparallel gravity with electromagnetic field

We construct a symmetric teleparallel gravity model which is non-minimally coupled with electromagnetic field in four dimensions inspired by its Riemannian equivalent. We derive the field equations by taking the variation of this model, which is written here for the first time. Then, we find some classes of spherically symmetric static solutions by the coincident gauge of symmetric teleparallel spacetime.Comment: 16 page