The vacuum expectation values of the energy-momentum tensor are investigated for massless scalar fields satisfying Dicichlet or Neumann boundary conditions, and for the electromagnetic field with perfect conductor boundary conditions on two infinite parallel plates moving by uniform proper acceleration through the Fulling-Rindler vacuum. The scalar case is considered for general values of the curvature coupling parameter and in an arbitrary number of spacetime dimension. The mode-summation method is used with combination of a variant of the generalized Abel-Plana formula. This allows to extract manifestly the contributions to the expectation values due to a single boundary. The vacuum forces acting on the boundaries are presented as a sum of the self-action and interaction terms. The first one contains well known surface divergences and needs a further regularization. The interaction forces between the plates are always attractive for both scalar and electromagnetic cases. An application to the 'Rindler wall' is discussed.

Positive frequency Wightman function and vacuum expectation value of the energymomentum tensor are computed for a massive scalar field with general curvature coupling parameter subject to Robin boundary conditions on two parallel plates located on (D + 1)-dimensional AdS background. The general case of different Robin coefficients on separate plates is considered. The mode summation method is used with a combination of a variant of the generalized Abel-Plana formula for the series over zeros of combinations of cylinder functions. This allows us to extract manifestly the parts due to the AdS spacetime without boundaries and boundary induced parts. The asymptotic behavior of the vacuum densities near the plates and at large distances is investigated. The vacuum forces acting on the boundaries are presented as a sum of the self-action and interaction forces. The first one contains well-known surface divergences and needs further regularization. The interaction forces between the plates are attractive for Dirichlet scalar. We show that there is a region in the space of parameters defining the boundary conditions in which the interaction forces are repulsive for small distances and attractive for large distances. An application to the Randall-Sundrum braneworld with arbitrary mass terms on the branes is discussed.

Cosmological solutions are examined in the proper representation of the JBD theory with a dominant nonminimally coupled scalar field. It is shown that only the introduction of a cosmological scalar that transforms to the ordinary cosmological constant in the Einstein representation enables a phase of evolution with a uniform and then an accelerated expansion of the universe over cosmological time scales.

Recent cosmological observations of large-scale structures (red shift of type Ia supernovae) confirm that the universe is currently expanding at an accelerating rate and its dominant component is dark energy. This has stimulated the development of the theory of gravity and led to many alternative variants, including tensor-scalar ones. This paper deals with the role of conformal transformations in the Jordan-BransDicke theory. Variants of intrinsic, conformally coupled, and Einstein representations are examined. In the Einstein representation an exact analytic solution for the standard cosmological model is obtained. Itis expressed in terms of the relative energy contributions of ordinary matter m Ω , the scalar field CK Ω , and a term Λ Ω related to the cosmological constant Λ . Information on the evolution of the universe for the case with a minimally coupled scalar field is given in the form of graphs.

We investigate the qualitative evolution of (D + 1)-dimensional cosmological models in f (R) gravity for the general case of the function f (R). The analysis is specified for various examples, including the (D + 1)-dimensional generalization of the Starobinsky model, models with polynomial and exponential functions. The cosmological dynamics are compared in the Einstein and Jordan representations of the corresponding scalar-tensor theory. The features of the cosmological evolution are discussed for Einstein frame potentials taking negative values in certain regions of the field space. IntroductionRecent observations of the cosmic microwave background, large scale structure and type Ia supernovae have provided strong evidence that at present epoch the expansion of the universe is accelerating [1]. Assuming that General Relativity correctly describes the large scale dynamics of the Universe, this means that the energy density is currently dominated by a form of energy having negative pressure. This type of a gravitational source is referred as dark energy. The simplest model for the latter, consistent with all observations to date, is a cosmological constant. From the cosmological point of view, a cosmological constant is equivalent to the vacuum energy in quantum field theory. However, the value of a cosmological constant inferred from cosmological observations is many orders of magnitude smaller than the value one might expect based on quantum field-theoretical considerations. This large discrepancy is one of the motivations to consider alternative models for dark energy. To account for the missing energy density, instead of a cosmological constant one could add a new component of matter, such as quintessence (see [2] for a review). The latter is modeled by slow-rolling scalar fields. However, because of very small mass of a scalar field responsible for the acceleration, it is generally difficult to construct viable potentials on the base of particle physics.More recently, it has been shown that suitable modifications of General Relativity can result in an accelerating expansion of the Universe at present epoch. These modifications fall into two general groups. The first one consists of scalar-tensor theories that are most widely considered extensions of General Relativity [3]. In addition to the metric tensor, these theories contain scalar fields in their gravitational sector and typically arise in the context of models with extra dimensions (Kaluza-Klein-type models, braneworld scenario) and within the framework of the low-energy string effective gravity. In the second group of models, the Ricci scalar R in the Einstein-Hilbert action is replaced by a general function f (R) (for recent reviews see [4]-[10]). One of the first models for inflation with quadratic in the Ricci scalar Lagrangian, proposed by Starobinsky [11], falls into this class of theories. An additional motivation for the f (R) theories comes from quantum field theory in classical curved