arXiv subjects

G. Kofinas

Publications and source records attributed to G. Kofinas.

12 recordsLinked to original sources

arXiv2010

Classical ultrarelativistic bremsstrahlung in extra dimensions

The emitted energy and the cross-section of classical scalar bremsstrahlung in massive particle collisions in D=4+d dimensional Minkowski space M_D as well as in the brane world M_4 \times T^d is computed to leading ultra-relativistic order. The particles are taken to interact in the first case via the exchange of a bulk massless scalar field Φand in the second with an additional massless scalar ϕconfined together with the particles on the brane. Energy is emitted as Φradiation in the bulk and/or ϕradiation on the brane. In contrast to the quantum Born approximation, the classical result is unambiguous and valid in a kinematical region which is also specified. For D=4 the results are in agreement with corresponding expressions in classical electrodynamics.

hep-th↗

arXiv2006

Brane-bulk energy exchange : a model with the present universe as a global attractor

The role of brane-bulk energy exchange and of an induced gravity term on a single braneworld of negative tension and vanishing effective cosmological constant is studied. It is shown that for the physically interesting cases of dust and radiation a unique global attractor which can realize our present universe (accelerating and 0<Omega_{m0}<1) exists for a wide range of the parameters of the model. For Omega_{m0}=0.3, independently of the other parameters, the model predicts that the equation of state for the dark energy today is w_{DE,0}=-1.4, while Omega_{m0}=0.03 leads to w_{DE,0}=-1.03. In addition, during its evolution, w_{DE} crosses the w_{DE}=-1 line to smaller values.

hep-th↗

arXiv2004

Gravitational collapse in braneworld models with curvature corrections

We study the collapse of a homogeneous braneworld dust cloud in the context of the various curvature correction scenarios, namely, the induced-gravity, the Gauss-Bonnet, and the combined induced-gravity and Gauss-Bonnet. In accordance to the Randall-Sundrum model, and contrary to four-dimensional general relativity, we show in all cases that the exterior spacetime on the brane is non-static.

gr-qc↗

arXiv2004

Cosmology with exponential potentials

We examine in the context of general relativity the dynamics of a spatially flat Robertson-Walker universe filled with a classical minimally coupled scalar field ϕof exponential potential ~ e^{-μϕ} plus pressureless baryonic matter. This system is reduced to a first-order ordinary differential equation, providing direct evidence on the acceleration/deceleration properties of the system. As a consequence, for positive potentials, passage into acceleration not at late times is generically a feature of the system, even when the late-times attractors are decelerating. Furthermore, the structure formation bound, together with the constraints on the present values of Ω_{m}, w_ϕ provide, independently of initial conditions and other parameters, necessary conditions on μ. Special solutions are found to possess intervals of acceleration. For the almost cosmological constant case w_ϕ ~ -1, as well as, for the generic late-times evolution, the general relation Ω_ϕ(w_ϕ) is obtained.

gr-qc↗

arXiv2003

Brane cosmology with curvature corrections

We study the cosmology of the Randall-Sundrum brane-world where the Einstein-Hilbert action is modified by curvature correction terms: a four-dimensional scalar curvature from induced gravity on the brane, and a five-dimensional Gauss-Bonnet curvature term. The combined effect of these curvature corrections to the action removes the infinite-density big bang singularity, although the curvature can still diverge for some parameter values. A radiation brane undergoes accelerated expansion near the minimal scale factor, for a range of parameters. This acceleration is driven by the geometric effects, without an inflaton field or negative pressures. At late times, conventional cosmology is recovered.

hep-th↗

arXiv2003

Cosmological evolution with brane-bulk energy exchange

The consequences for the brane cosmological evolution of energy exchange between the brane and the bulk are analysed in detail, in the context of a non-factorizable background geometry with vanishing effective cosmological constant on the brane. A rich variety of brane cosmologies is obtained, depending on the precise mechanism of energy transfer, the equation of state of brane-matter and the spatial topology. An accelerating era is generically a feature of our solutions. In the case of low-density flat universe more dark matter than in the conventional FRW picture is predicted. Spatially compact solutions are found to delay their recollapse.

hep-th↗

arXiv2002

Black Holes on the Brane with Induced Gravity

An analysis of a spherically symmetric braneworld configuration is performed when the intrinsic curvature scalar is included in the bulk action. In the case when the electric part of the Weyl tensor is zero, all the exterior solutions are found; one of them is of the Schwarzschild-(A)dS(4) form, which is matched to a modified Oppenheimer-Volkoff interior solution. In the case when the electric part of the Weyl tensor is non zero, the exterior Schwarzschild-(A)dS(4) black hole solution is modified receiving corrections from the non-local bulk effects. A non-universal gravitational constant arises, depending on the density of the considered object and the Newton's law is modified for small and large distances; however, the conventional limits are easily obtained.

hep-th↗

arXiv2002

Black Hole Solutions in Braneworlds with Induced Gravity

We extent our previous study on spherically symmetric braneworld solutions with induced gravity, including non-local bulk effects. We find the most general static four-dimensional black hole solutions with g_{tt}=-g_{rr}^{-1}. They satisfy a closed system of equations on the brane and represent the strong-gravity corrections to the Schwarzschild-(A)dS_4 spacetime. These new solutions have extra terms which give extra attraction relative to the Newtonian-(A)dS_4 force; however, the conventional limits are easily obtained. These terms, when defined asymptotically, behave like AdS_4 in this regime, while when defined at infinitely short distances predict either an additional attractive Newtonian potential or an attractive potential which scales approximately as sqrt(r). One of the solutions found gives extra deflection of light compared to Newtonian deflection.

hep-th↗

arXiv2002

Spherically Symmetric Braneworld Solutions with R_{4} term in the Bulk

An analysis of a spherically symmetric braneworld configuration is performed when the intrinsic curvature scalar is included in the bulk action; the vanishing of the electric part of the Weyl tensor is used as the boundary condition for the embedding of the brane in the bulk. All the solutions outside a static localized matter distribution are found; some of them are of the Schwarzschild-(A)dS_{4} form. Two modified Oppenheimer-Volkoff interior solutions are also found; one is matched to a Schwarzschild-(A)dS_{4} exterior, while the other does not. A non-universal gravitational constant arises, depending on the density of the considered object; however, the conventional limits of the Newton's constant are recovered. An upper bound of the order of TeV for the energy string scale is extracted from the known solar system measurements (experiments). On the contrary, in usual brane dynamics, this string scale is calculated to be larger than TeV.

hep-th↗

arXiv2001

Conditional Symmetries and Phase Space Reduction towards G.C.T. Invariant Wave Functions, for the Class A Bianchi Type VI & VII Vacuum Cosmologies

The quantization of Class A Bianchi Type VI and VII geometries -with all six scale factors, as well as the lapse function and the shift vector present- is considered. A first reduction of the initial 6-dimensional configuration space is achieved by the usage of the information furnished by the quantum form of the linear constraints. Further reduction of the space in which the wave function -obeying the Wheeler-DeWitt equation- lives, is accomplished by revealing a classical integral of motion, tantamount to an extra symmetry of the corresponding classical Hamiltonian. This symmetry generator -member of a larger group- is linear in momenta and corresponds to G.C.T.s through the action of the automorphism group -especially through the action of the outer automorphism subgroup. Thus, a G.C.T. invariant wave function is found, which depends on one combination of the two curvature invariants --which uniquely and irreducibly characterizes the hypersurfaces t=const.

gr-qc↗

arXiv2001

Time-Dependent Automorphism Inducing Diffeomorphisms In Vacuum Bianchi Cosmologies And The Complete Closed Form Solutions For Type II & V

We investigate the set of spacetime general coordinate transformations (G.C.T.) which leave the line element of a generic Bianchi Type Geometry, quasi-form invariant; i.e. preserve manifest spatial Homogeneity. We find that these G.C.T.'s, induce special time-dependent automorphic changes, on the spatial scale factor matrix $γ_{αβ}(t)$ -along with corresponding changes on the lapse function $N(t)$ and the shift vector $N^α(t)$. These changes, which are Bianchi Type dependent, form a group and are, in general, different from those induced by the group SAut(G) -advocated in earlier investigations as the relevant symmetry group-, they are used to simplify the form of the line element -and thus simplify Einstein's equations as well-, without losing generality. As far as this simplification procedure is concerned, the transformations found, are proved to be essentialy unique. For the case of Bianchi Types II and V, where the most general solutions are known -Taub's and Joseph's, respectively-, it is explicitly verified that our transformations and only those, suffice to reduce the generic line element, to the previously known forms. It becomes thus possible, -for these Types- to give in closed form, the most general solution, containing all the necessary ``gauge'' freedom.

gr-qc↗

arXiv2000

The untilted diffuse matter Bianchi V Universe

A diffuse matter filled Type V Universe is studied. The anisotropic behaviour, the distortion caused to the CMBR and the parameter region allowed by present cosmological bounds are examined. It is shown how the overall sky pattern of temperature anisotropies changes under a non-infinitesimal spatial coordinate transformation that preserves the Type V manifest homogeneity.

gr-qc↗