50 research outputs found
Sp-brane accelerating cosmologies
We investigate time dependent solutions (S-brane solutions) for product
manifolds consisting of factor spaces where only one of them is non-Ricci-flat.
Our model contains minimally coupled free scalar field as a matter source. We
discuss a possibility of generating late time acceleration of the Universe. The
analysis is performed in conformally related Brans-Dicke and Einstein frames.
Dynamical behavior of our Universe is described by its scale factor. Since the
scale factors of our Universe are described by different variables in both
frames, they can have different dynamics.
Indeed, we show that with our S-brane ansatz in the Brans-Dicke frame the
stages of accelerating expansion exist for all types of the external space
(flat, spherical and hyperbolic). However, applying the same ansatz for the
metric in the Einstein frame, we find that a model with flat external space and
hyperbolic compactification of the internal space is the only one with the
stage of the accelerating expansion. Scalar field can prevent this
acceleration. It is shown that the case of hyperbolic external space in
Brans-Dicke frame is the only model which can satisfy experimental bounds for
the fine structure constant variations. We obtain a class of models where a
pare of dynamical internal spaces have fixed total volume. It results in fixed
fine structure constant. However, these models are unstable and external space
is non-accelerating.Comment: 17 pages, 4 figures, accepted in PR
Acceleration from M theory and Fine-tuning
The compactification of M theory with time dependent hyperbolic internal
space gives an effective scalar field with exponential potential which provides
a transient acceleration in Einstein frame in four dimensions. Ordinary matter
and radiation are present in addition to the scalar field coming from
compactification. We find that we have to fine-tune the initial conditions of
the scalar field so that our Universe experiences acceleration now. During the
evolution of our Universe, the volume of the internal space increases about 12
times. The time variation of the internal space results in a large time
variation of the fine structure constant which violates the observational
constraint on the variation of the fine structure constant. The large variation
of the fine structure constant is a generic feature of transient acceleration
models.Comment: 9 pages, 3 figures, use iopart, v2; references updated, accepted for
publication in Class. Quantum Gra
Variations in characteristics of the barents branch of the Atlantic Water in the Nansen Basin under the influence of atmospheric circulation over the Barents Sea
The thermohaline structure of the Arctic Basin (AB) of the Arctic Ocean (AO) is determined to a great extent by an intermediate water layer existing under ice at a depth varying from 100 to 700–1000 m. The water layer is formed by warm North Atlantic Water (AW), which enters the AB by two ways: through Fram Strait and the Barents Sea (Fig. 1). The AW arriving to the AB via Fram Strait extends further eastward along the continental slope of the Eurasian Arctic region and forms the Fram Branch (FBAW). The Barents Branch of the AW (BBAW) was formed by the North Atlantic Water entering the Barents Sea between the Spitsbergen Archipelago and the Scandinavian Peninsula. Both branches merge in the northern Kara Sea
Dynamical dark energy from extra dimensions
We consider multidimensional cosmological model with a higher-dimensional
product manifold M = R x R^{d_0} x H^{d_1}/\Gamma where R^{d_0} is
d_0-dimensional Ricci-flat external (our) space and H^{d_1}/\Gamma is
d_1-dimensional compact hyperbolic internal space. M2-brane solution for this
model has the stage of accelerating expansion of the external space. We apply
this model to explain the late time acceleration of our Universe. Recent
observational data (the Hubble parameter at the present time and the redshift
when the deceleration parameter changes its sign) fix fully all free parameters
of the model. As a result, we find that considered model has too big size of
the internal space at the present time and variation of the effective
four-dimensional fine structure constant strongly exceeds the observational
limits.Comment: 5 pages, 3 figures, LaTex, a few remarks and reference adde
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
Modified Gravity and Cosmology
In this review we present a thoroughly comprehensive survey of recent work on
modified theories of gravity and their cosmological consequences. Amongst other
things, we cover General Relativity, Scalar-Tensor, Einstein-Aether, and
Bimetric theories, as well as TeVeS, f(R), general higher-order theories,
Horava-Lifschitz gravity, Galileons, Ghost Condensates, and models of extra
dimensions including Kaluza-Klein, Randall-Sundrum, DGP, and higher
co-dimension braneworlds. We also review attempts to construct a Parameterised
Post-Friedmannian formalism, that can be used to constrain deviations from
General Relativity in cosmology, and that is suitable for comparison with data
on the largest scales. These subjects have been intensively studied over the
past decade, largely motivated by rapid progress in the field of observational
cosmology that now allows, for the first time, precision tests of fundamental
physics on the scale of the observable Universe. The purpose of this review is
to provide a reference tool for researchers and students in cosmology and
gravitational physics, as well as a self-contained, comprehensive and
up-to-date introduction to the subject as a whole.Comment: 312 pages, 15 figure