18 research outputs found
Electric S-brane solutions corresponding to rank-2 Lie algebras: acceleration and small variation of G
Electric S-brane solutions with two non-composite electric branes and a set
of l scalar fields are considered. The intersection rules for branes correspond
to Lie algebras A_2, C_2 and G_2. The solutions contain five factor spaces. One
of them, M_0, is interpreted as our 3-dimensional space. It is shown that there
exists a time interval where accelerated expansion of our 3-dimensional space
is compatible with a small enough variation of the effective gravitational
constant G(\tau). This interval contains \tau_0, a point of minimum of the
function G(\tau). A special solution with two phantom scalar fields is analyzed
and it is shown that in the vicinity of the point \tau_0 the time variation of
G(\tau) (calculated in the linear approximation) decreases in the sequence of
Lie algebras A_2, C_2 and G_2.Comment: 13 pages, LaTex, no figure
Models of G time variations in diverse dimensions
A review of different cosmological models in diverse dimensions leading to a
relatively small time variation of the effective gravitational constant G is
presented. Among them: 4-dimensional general scalar-tensor model,
multidimensional vacuum model with two curved Einstein spaces, multidimensional
model with multicomponent anisotropic "perfect fluid", S-brane model with
scalar fields and two form field etc. It is shown that there exist different
possible ways of explanation of relatively small time variation of the
effective gravitational constant G compatible with present cosmological data
(e.g. acceleration): 4-dimensional scalar-tensor theories or multidimensional
cosmological models with different matter sources. The experimental bounds on
G-dot may be satisfied ether in some restricted interval or for all allowed
values of the synchronous time variable.Comment: 27 pages, Late
Self-stabilization of extra dimensions
We show that the problem of stabilization of extra dimensions in Kaluza-Klein
type cosmology may be solved in a theory of gravity involving high-order
curvature invariants. The method suggested (employing a slow-change
approximation) can work with rather a general form of the gravitational action.
As examples, we consider pure gravity with Lagrangians quadratic and cubic in
the scalar curvature and some more complex ones in a simple Kaluza-Klein
framework. After a transition to the 4D Einstein conformal frame, this results
in effective scalar field theories with certain effective potentials, which in
many cases possess positive minima providing stable small-size extra
dimensions. Estimates made in the original (Jordan) conformal frame show that
the problem of a small value of the cosmological constant in the present
Universe is softened in this framework but is not solved completely.}Comment: 10 pages, 4 figures, revtex4. Version with additions and corrections,
accepted at Phys. Rev.
Cosmologies from nonlinear multidimensional gravity with acceleration and slowly varying G
We consider multidimensional gravity with a Lagrangian containing the Ricci
tensor squared and the Kretschmann invariant. In a Kaluza-Klein approach with a
single compact extra space of arbitrary dimension, with the aid of a
slow-change approximation (as compared with the Planck scale), we build a class
of spatially flat cosmological models in which both the observed scale factor
and the extra-dimensional one, , grow exponentially at large
times, but grows slowly enough to admit variations of the effective
gravitational constant within observational limits. Such models predict a
drastic change in the physical laws of our Universe in the remote future due to
further growth of the extra dimensions.Comment: 5 pages, to be published in Grav. Cosmo
Possible variations of the fine structure constant and their metrological significance
We briefly review the recent experimental results on possible variations of
the fine structure constant on the cosmological time scale and its
position dependence. We outline the theoretical grounds for the assumption that
might be variable, mention some phenomenological models incorporating
a variable into the context of modern cosmology and discuss the
significance of possible variations for theoretical and practical
metrology.Comment: Latex, 17 pages, brief review. References updated, minor errors
remove
Brane world corrections to Newton's law
We discuss possible variations of the effective gravitational constant with
length scale, predicted by most of alternative theories of gravity and unified
models of physical interactions. After a brief general exposition, we review in
more detail the predicted corrections to Newton's law of gravity in diverse
brane world models. We consider various configurations in 5 dimensions (flat,
de Sitter and AdS branes in Einstein and Einstein-Gauss-Bonnet theories, with
and without induced gravity and possible incomplete graviton localization), 5D
multi-brane systems and some models in higher dimensions. A common feature of
all models considered is the existence of corrections to Newton's law at small
radii comparable with the bulk characteristic length: at such radii, gravity on
the brane becomes effectively multidimensional. Many models contain superlight
perturbation modes, which modify gravity at large scale and may be important
for astrophysics and cosmology.Comment: Brief review, 16 pages, 92 references. Some description and
references adde