6,371 research outputs found
Dynamics of Dimensions in Factor Space Cosmology
We consider multidimensional cosmological models with a generalized
space-time manifold M = R x M_1 ...x M_n, composed from a finite number of
factor spaces M_i, i=1,..n. While usually each factor space M_i is considered
to be some Riemannian space of integer dimension d_i, here it is, more
generally, a fractal space, the dimension of which is a smooth function d_i(t)
of time. Hence, besides the scale factor exponents ln a_i and their
derivatives, we consider also the dimensions d_i of the factor spaces as
classical dynamical variables. The classical equation of motions and the
corresponding Wheeler-de Witt equation are set up generally, and the
qualitative behaviour of the system is discussed for some specific model with 2
factor spaces.Comment: 19 pages, latex, no figure
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
New exact solutions for power-law inflation Friedmann models
We consider the spatially flat Friedmann model. For a(t) = t^p, especially,
if p is larger or equal to 1, this is called power-law inflation. For the
Lagrangian L = R^m with p = - (m - 1)(2m - 1)/(m - 2), power-law inflation is
an exact solution, as it is for Einstein gravity with a minimally coupled
scalar field Phi in an exponential potential V(Phi) = exp(mu Phi) and also for
the higher-dimensional Einstein equation with a special Kaluza-Klein ansatz.
The synchronized coordinates are not adapted to allow a closed-form solution,
so we use another gauge. Finally, special solutions for the closed and open
Friedmann model are found.Comment: 9 pages, LaTeX, reprinted from Astron. Nachr. 311 (1990) 16
Composite p-branes on Product of Einstein Spaces
A multidimensional gravitational model with several scalar fields, fields of
forms and cosmological constant is considered. When scalar fields are constant
and composite p-brane monopole-like ansatz for the fields of forms is adopted,
a wide class of solutions on product of n+1 Einstein spaces is obtained. These
solutions are the composite p-brane generalizations of the Freund-Rubin
solution. Some examples including the AdS_m x S^k x... solutions are
considered.Comment: 11 pages, Late
Multidimensional Extremal Dilatonic Black Holes in String-like Model with Cosmological Term
A string-like model with the "cosmological constant" is considered.
The Maki-Shiraishi multi-black-hole solution \cite{MS1} is generalized to
space-times with a Ricci-flat internal space. For the obtained
solution in the one-black-hole case is shown to coincide with the extreme limit
of the charged dilatonic black hole solution \cite{BI,BM}. The Hawking
temperature for the solution \cite{BI,BM} is presented and its extreme
limit is considered. For the string value of dilatonic coupling the temperature
does not depend upon the internal space dimension.Comment: 8 pages, LaTex. Submitted to Physics Letters
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