2,199 research outputs found
Generalized energy conditions in Extended Theories of Gravity
Theories of physics can be considered viable if the initial value problem and
the energy conditions are formulated self-consistently. The former allow a
uniquely determined dynamical evolution of the system, and the latter guarantee
that causality is preserved and that "plausible" physical sources have been
considered. In this work, we consider the further degrees of freedom related to
curvature invariants and scalar fields in Extended Theories of Gravity (ETG).
These new degrees of freedom can be recast as effective perfect fluids that
carry different meanings with respect to the standard matter fluids generally
adopted as sources of the field equations. It is thus somewhat misleading to
apply the standard general relativistic energy conditions to this effective
energy-momentum, as the latter contains the matter content and a geometrical
quantity, which arises from the ETG considered. Here, we explore this subtlety,
extending on previous work, in particular, to cases with the contracted Bianchi
identities with diffeomorphism invariance and to cases with generalized
explicit curvature-matter couplings, which imply the non-conservation of the
energy-momentum tensor. Furthermore, we apply the analysis to specific ETGs,
such as scalar-tensor gravity, gravity and modified Gauss-Bonnet
gravity. Interesting results appear such as matter that may exhibit unusual
thermodynamical features, for instance, and gravity that retains its attractive
character in the presence of negative pressures; or alternatively, we verify
that repulsive gravity may occur for standard matter.Comment: 12 pages, version accepted for publication in Phys.Rev.
Energy conditions in modified gravity
We consider generalized energy conditions in modified theories of gravity by
taking into account the further degrees of freedom related to scalar fields and
curvature invariants. The latter are usually recast as generalized {\it
geometrical fluids} that have different meanings with respect to the standard
matter fluids generally adopted as sources of the field equations. More
specifically, in modified gravity the curvature terms are grouped in a tensor
and a coupling that can be reorganized in effective
Einstein field equations, as corrections to the energy-momentum tensor of
matter. The formal validity of such inequalities does not assure some basic
requirements such as the attractive nature of gravity, so that the energy
conditions have to be considered in a wider sense.Comment: 4 pages. V2: 5 pages; version to appear in Physics Letters B. V3:
typo in Eq. (4) correcte
Newtonian limits of warp drive spacetimes
We find a class of warp drive spacetimes possessing Newtonian limits, which
we then determine. The same method is used to compute Newtonian limits of the
Schwarzschild solution and spatially flat Friedmann-Robertson-Walker
cosmological models.Comment: 9 pages; v2: major changes; v3: misprints correcte
Gravitational induced particle production through a nonminimal curvature-matter coupling
We consider the possibility of a gravitationally induced particle production
through the mechanism of a nonminimal curvature-matter coupling. An interesting
feature of this gravitational theory is that the divergence of the
energy-momentum tensor is nonzero. As a first step in our study we reformulate
the model in terms of an equivalent scalar-tensor theory, with two arbitrary
potentials. By using the formalism of open thermodynamic systems, we interpret
the energy balance equations in this gravitational theory from a thermodynamic
point of view, as describing irreversible matter creation processes. The
particle number creation rates, the creation pressure, and the entropy
production rates are explicitly obtained as functions of the scalar field and
its potentials, as well as of the matter Lagrangian. The temperature evolution
laws of the newly created particles are also obtained. The cosmological
implications of the model are briefly investigated, and it is shown that the
late-time cosmic acceleration may be due to particle creation processes.
Furthermore, it is also shown that due to the curvature--matter coupling,
during the cosmological evolution a large amount of comoving entropy is also
produced.Comment: 15 pages; accepted for publication in the European Physical Journal
Cosmological solutions in generalized hybrid metric-Palatini gravity
We construct exact solutions representing a
Friedmann-Lema\^itre-Robsertson-Walker (FLRW) universe in a generalized hybrid
metric-Palatini theory. By writing the gravitational action in a scalar-tensor
representation, the new solutions are obtained by either making an ansatz on
the scale factor or on the effective potential. Among other relevant results,
we show that it is possible to obtain exponentially expanding solutions for
flat universes even when the cosmology is not purely vacuum. We then derive the
classes of actions for the original theory which generate these solutions.Comment: 14 pages, 17 figure
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