Cosmological equations and Thermodynamics on Apparent Horizon in Thick Braneworld

We derive the generalized Friedmann equation governing the cosmological evolution inside the thick brane model in the presence of two curvature correction terms: a four-dimensional scalar curvature from induced gravity on the brane, and a five-dimensional Gauss-Bonnet curvature term. We find two effective four-dimensional reductions of the Friedmann equation in some limits and demonstrate that they can be rewritten as the first law of thermodynamics on the apparent horizon of thick braneworld.Comment: 25 pages, no figure, a definition corrected, several references added, more motivation and discussio

Screening of cosmological constant for De Sitter Universe in non-local gravity, phantom-divide crossing and finite-time future singularities

We investigate de Sitter solutions in non-local gravity as well as in non-local gravity with Lagrange constraint multiplier. We examine a condition to avoid a ghost and discuss a screening scenario for a cosmological constant in de Sitter solutions. Furthermore, we explicitly demonstrate that three types of the finite-time future singularities can occur in non-local gravity and explore their properties. In addition, we evaluate the effective equation of state for the universe and show that the late-time accelerating universe may be effectively the quintessence, cosmological constant or phantom-like phases. In particular, it is found that there is a case in which a crossing of the phantom divide from the non-phantom (quintessence) phase to the phantom one can be realized when a finite-time future singularity occurs. Moreover, it is demonstrated that the addition of an $R^2$ term can cure the finite-time future singularities in non-local gravity. It is also suggested that in the framework of non-local gravity, adding an $R^2$ term leads to possible unification of the early-time inflation with the late-time cosmic acceleration.Comment: 42 pages, no figure, version accepted for publication in General Relativity and Gravitatio

Black holes and wormholes in RS2 type brane worlds

We review some recent results concerning the properties of static, spherically symmetric configurations in RS2 type brane worlds: 1) Solving the trace of the effective 4D gravity equations, we obtain general classes of black hole and wormhole metrics, including families which unify symmetric wormholes, extremal black holes and non-extremal non-singular black holes. 2) Assuming Eμ ν = 0 (where Eμ ν is a part of the 5D Weyl tensor contributing to the 4D gravity equations), we show that material scalar fields confined on the brane may violate the no-hair theorem, support wormholes or form particlelike objects, but only at super-nuclear matter densities in the strong field region. 3) Solutions to the bulk field equations are found in case Eμ ν ≡ 0, among them solutions with a nonzero 4D cosmological constant, generalizing the so-called "black string". © World Scientific Publishing Company

Black holes and wormholes in RS2 type brane worlds

We review some recent results concerning the properties of static, spherically symmetric configurations in RS2 type brane worlds: 1) Solving the trace of the effective 4D gravity equations, we obtain general classes of black hole and wormhole metrics, including families which unify symmetric wormholes, extremal black holes and non-extremal non-singular black holes. 2) Assuming Eμ ν = 0 (where Eμ ν is a part of the 5D Weyl tensor contributing to the 4D gravity equations), we show that material scalar fields confined on the brane may violate the no-hair theorem, support wormholes or form particlelike objects, but only at super-nuclear matter densities in the strong field region. 3) Solutions to the bulk field equations are found in case Eμ ν ≡ 0, among them solutions with a nonzero 4D cosmological constant, generalizing the so-called "black string". © World Scientific Publishing Company

On spherically symmetric minimally coupled brane worlds

For a static, spherically symmetric brane in the framework of the RS2 concept, we study the conditions under which the 4D tensor Eμν, arising from the 5D Weyl tensor, vanishes on the brane. Gravity on the brane is then decoupled from the bulk geometry, it is the so-called minimally coupled brane world (MCBW). Assuming Eμν = 0 in the whole bulk, we try to solve the 5D Einstein equations GAB + Λ5gAB = 0 and obtain an overdetermined set of equations for functions of the radial coordinate. Some special solutions are found, among which are the well-known "black string" solution with the Schwarzschild metric on the brane and its generalizations with Schwarzschild-(A)dS on-brane metrics. It is concluded that a MCBW can be embedded, in general, in a bulk where Eμ νis not identically zero but only vanishes on the brane. We also present some previous results on the general properties of scalar fields on the brane and give an example of a wormhole supported by a scalar field in a MCBW

Scalar field in a minimally coupled brane world: No-hair and other no-go theorems

In the brane-world framework, we consider static, spherically symmetric configurations of a scalar field with the Lagrangian (∂φ)2/2 - V (φ), confined on the brane. We use the 4D Einstein equations on the brane obtained by Shiromizu et al., containing the conventional stress tensor Tμ ν, the tensor IIμ ν which is quadratic in Tμ ν, and Eμ ν describing interaction with the bulk. For models under study, the tensor IIμ ν has zero divergence, allowing one to consider Eμ ν = 0. Such a brane, whose 4D gravity is decoupled from the bulk geometry, may be called minimally coupled. Assuming Eμ ν = 0, we try to extend to brane worlds some theorems valid for scalar fields in general relativity (GR). Thus, the list of possible global causal structures in all models under consideration is shown to be the same as is known for vacuum with a cosmological constant in GR: Minkowski, Schwarzschild, (anti-) de Sitter and Schwarzschild-(anti-)de Sitter. A no-hair theorem, saying that, given a potential V ≥ 0, asymptotically flat black holes cannot have nontrivial external scalar fields, is proved under certain restrictions. Some objects, forbidden in GR, are allowed on the brane, e.g, traversable wormholes supported by a scalar field, but only at the expense of enormous matter densities in the strong field region

On spherically symmetric minimally coupled brane worlds

For a static, spherically symmetric brane in the framework of the RS2 concept, we study the conditions under which the 4D tensor Eμν, arising from the 5D Weyl tensor, vanishes on the brane. Gravity on the brane is then decoupled from the bulk geometry, it is the so-called minimally coupled brane world (MCBW). Assuming Eμν = 0 in the whole bulk, we try to solve the 5D Einstein equations GAB + Λ5gAB = 0 and obtain an overdetermined set of equations for functions of the radial coordinate. Some special solutions are found, among which are the well-known "black string" solution with the Schwarzschild metric on the brane and its generalizations with Schwarzschild-(A)dS on-brane metrics. It is concluded that a MCBW can be embedded, in general, in a bulk where Eμ νis not identically zero but only vanishes on the brane. We also present some previous results on the general properties of scalar fields on the brane and give an example of a wormhole supported by a scalar field in a MCBW

Inhomogeneous compact extra dimensions

We show that an inhomogeneous compact extra space possesses two necessary features - their existence does not contradict the observable value of the cosmological constant Lambda(4) in pure f(R) theory, and the extra dimensions are stable relative to the "radion mode" of perturbations, the only mode considered. For a two-dimensional extra space, both analytical and numerical solutions for the metric are found, able to provide a zero or arbitrarily small Lambda(4). A no-go theorem has also been proved, that maximally symmetric compact extra spaces are inconsistent with 4D Minkowski space in the framework of pure f(R) gravity

Inhomogeneous compact extra dimensions

We show that an inhomogeneous compact extra space possesses two necessary features - their existence does not contradict the observable value of the cosmological constant Lambda(4) in pure f(R) theory, and the extra dimensions are stable relative to the "radion mode" of perturbations, the only mode considered. For a two-dimensional extra space, both analytical and numerical solutions for the metric are found, able to provide a zero or arbitrarily small Lambda(4). A no-go theorem has also been proved, that maximally symmetric compact extra spaces are inconsistent with 4D Minkowski space in the framework of pure f(R) gravity

Performance Assessment in the PILOT Experiment On Board Space Stations Mir and ISS

BACKGROUND: The aim of this investigation into the performance and reliability of Russian cosmonauts in hand-controlled docking of a spacecraft on a space station (experiment PILOT) was to enhance overall mission safety and crew training efficiency. The preliminary findings on the Mir space station suggested that a break in docking training of about 90 d significantly degraded performance. METHODS: Intensified experiment schedules on the International Space Station (ISS) have allowed for a monthly experiment using an on-board simulator. Therefore, instead of just three training tasks as on Mir, five training flights per session have been implemented on the ISS. This experiment was run in parallel but independently of the operational docking training the cosmonauts receive. RESULTS: First, performance was compared between the experiments on the two space stations by nonparametric testing. Performance differed significantly between space stations preflight, in flight, and postflight. Second, performance was analyzed by modeling the linear mixed effects of all variances (LME). The fixed factors space station, mission phases, training task numbers, and their interaction were analyzed. Cosmonauts were designated as a random factor. All fixed factors were found to be significant and the interaction between stations and mission phase was also significant. DISCUSSION: In summary, performance on the ISS was shown to be significantly improved, thus enhancing mission safety. Additional approaches to docking performance assessment and prognosis are presented and discusse