Models with a scalar field coupled to the Gauss-Bonnet Lagrangian appear naturally from Kaluza-Klein compactifications of pure higher-dimensional gravity. We study linear, cosmological perturbations in the limits of weak coupling and slow-roll, and derive simple expressions for the main observable sub-horizon quantities: the anisotropic stress factor, the time-dependent gravitational constant, ...

A covariant scalar-tensor-vector gravity theory is developed which allows the gravitational constant G, a vector field coupling ω and the vector field mass μ to vary with space and time. The equations of motion for a test particle lead to a modified gravitational acceleration law that can fit galaxy rotation curves without non-baryonic dark matter. The theory is consistent with solar system obs...

Non-linear effects in the evolution of the axion field in the early Universe may lead to the formation of gravitationally bound clumps of axions, known as “miniclusters.” Minicluster masses and radii should be in the range Mmc ∼ 10M⊙ and Rmc ∼ 10cm, and in plausible early-Universe scenarios a significant fraction of the mass density of the Universe may be in the form of axion miniclusters. If s...

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)dS4...

We present an orbifold compactification of the minimal seven-dimensional supergravity. The vacuum is a slice of seven-dimensional anti-de Sitter space where six-branes of opposite tension are located at the orbifold fixed points. The cancellation of gauge and gravitational anomalies restricts the gauge group and matter content on the boundaries. In addition, anomaly cancellation fixes the bound...

We studied the variation of Newton ́s gravitational constant G, using the experimental data and the scalar-tensor theory for gravitation. We search for a fine tunning to match the datas and find possible scenarios for the history of this constant.

We investigate the geometry of lightsheets comprised of null geodesics near a brane. Null geodesics which begin parallel to a brane a distance d away are typically gravitationally bound to the brane, so that the maximum distance from the geodesic to the brane never exceeds d. The geometry of resulting lightsheets is similar to that of the brane if one coarse grains over distances of order d. We...

A natural system of fundamental physical units c, fi, and wp is discussed, where с is the velocity of light, a is Planck's constant, and mv is the Planck mass, which is related to the Newtonian gravitational constant by the equation m\ = fic/G. In a natural system of units, such questions as: "How does the anthropic nature of the physical universe arise? Is it unique, or does an infinite set of...