655 research outputs found
Solar System experiments do not yet veto modified gravity models
The dynamical equivalence between modified and scalar-tensor gravity theories
is revisited and it is concluded that it breaks down in the limit to general
relativity. A gauge-independent analysis of cosmological perturbations in both
classes of theories lends independent support to this conclusion. As a
consequence, the PPN formalism of scalar-tensor gravity and Solar System
experiments do not veto modified gravity, as previously thought.Comment: 7 pages, latex, submitted to Phys. Rev.
Reconstructing the universe history, from inflation to acceleration, with phantom and canonical scalar fields
We consider the reconstruction technique in theories with a single or
multiple (phantom and/or canonical) scalar fields. With the help of several
examples, it is demonstrated explicitly that the universe expansion history,
unifying early-time inflation and late-time acceleration, can be realized in
scalar-tensor gravity. This is generalized to the theory of a scalar field
coupled non-minimally to the curvature and to a Brans-Dicke-like theory.
Different examples of unification of inflation with cosmic acceleration, in
which de Sitter, phantom, and quintessence type fields play the fundamental
role--in different combinations--are worked out. Specifically, the frame
dependence and stability properties of de Sitter space scalar field theory are
studied. Finally, for two-scalar theories, the late-time acceleration and
early-time inflation epochs are successfully reconstructed, in realistic
situations in which the more and more stringent observational bounds are
satisfied, using the freedom of choice of the scalar field potential, and of
the kinetic factor.Comment: 22 pages, revtex, no figures, version to appear in Phys. Rev.
The final state and thermodynamics of dark energy universe
As it follows from the classical analysis, the typical final state of the
dark energy universe where dominant energy condition is violated is finite
time, sudden future singularity (Big Rip). For a number of dark energy
universes (including scalar phantom and effective phantom theories as well as
specific quintessence model) we demonstrate that quantum effects play the
dominant role near Big Rip, driving the universe out of future singularity (or,
at least, making it milder). As a consequence, the entropy bounds with quantum
corrections become well-defined near Big Rip. Similarly, black holes mass loss
due to phantom accretion is not so dramatic as it was expected: masses do not
vanish to zero due to transient character of phantom evolution stage. Some
examples of cosmological evolution for negative, time-dependent equation of
state are also considered with the same conclusions. The application of
negative entropy (or negative temparature) occurence in the phantom
thermodynamics is briefly discussed.Comment: LaTeX file 36 pages, version to appear in PR
Generalization of Einstein-Lovelock theory to higher order dilaton gravity
A higher order theory of dilaton gravity is constructed as a generalization
of the Einstein-Lovelock theory of pure gravity. Its Lagrangian contains terms
with higher powers of the Riemann tensor and of the first two derivatives of
the dilaton. Nevertheless, the resulting equations of motion are quasi-linear
in the second derivatives of the metric and of the dilaton. This property is
crucial for the existence of brane solutions in the thin wall limit. At each
order in derivatives the contribution to the Lagrangian is unique up to an
overall normalization. Relations between symmetries of this theory and the
O(d,d) symmetry of the string-inspired models are discussed.Comment: 18 pages, references added, version to be publishe
On the way from matter-dominated era to dark energy universe
We develop the general program of the unification of matter-dominated era
with acceleration epoch for scalar-tensor theory or dark fluid. The general
reconstruction of single scalar-tensor theory is fulfilled. The explicit form
of scalar potential for which the theory admits matter-dominated era,
transition to acceleration and (asymptotically deSitter) acceleration epoch
consistent with WMAP data is found. The interrelation of the epochs of
deceleration-acceleration transition and matter dominance-dark energy
transition for dark fluids with general EOS is investigated. We give several
examples of such models with explicit EOS (using redshift parametrization)
where matter-dark energy domination transition may precede the
deceleration-acceleration transition. As some by-product, the reconstruction
scheme is applied to scalar-tensor theory to define the scalar potentials which
may produce the dark matter effect. The obtained modification of Newton
potential may explain the rotation curves of galaxies.Comment: LaTeX 12 pages, 1 figure, extended version to appear in PR
Generalized black hole entropy in two dimensions
The Bekenstein-Hawking entropy of a black hole is proportional to its horizon
area, hence in spacetime dimensions it is constant because the horizon
degenerates into two points. This fact is consistent with Einstein's gravity
becoming topological in two dimensions. In gravity, which is non-trivial
even in , we find that the entropy is constant, as for Bekenstein-Hawking.
As shown in EPL 139 (2022) no.6, 69001 (arXiv:2208.10146), two-dimensional
gravity is equivalent to Jackiw-Teitelboim gravity, in turn equivalent
to the Sachdev-Ye-Kitaev model where the entropy becomes constant in the large
limit. Several recently proposed entropies are functions of the
Bekenstein-Hawking entropy and become constant in , but in two-dimensional
dilaton gravity entropies are not always constant. We study general dilaton
gravity and obtain arbitrary static black hole solutions for which the
non-constant entropies depend on the mass, horizon radius, or Hawking
temperature, and constitute new proposals for a generalized entropy.Comment: 12 pages, latex, to appear in Int. J. Geom. Meth. Mod. Phy
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