3 research outputs found
Kantowski-Sachs String Cosmologies
We present new exact solutions of the low-energy-effective-action string
equations with both dilaton and axion fields non-zero. The
background universe is of Kantowski-Sachs type. We consider the possibility of
a pseudoscalar axion field () that can be either time or
space dependent. The case of time-dependent reduces to that of a stiff
perfect-fluid cosmology. For space-dependent there is just one non-zero
time-space-space component of the axion field , and this corresponds to a
distinguished direction in space which prevents the models from isotropising.
Also, in the latter case, both the axion field and its tensor potential
() are dependent on time and space yet the energy-momentum tensor remains
time-dependent as required by the homogeneity of the cosmological model.Comment: 23 pages, REVTEX, 6 figures available on reques
Spatially Homogeneous String Cosmologies
We determine the most general form of the antisymmetric -field tensor
derived from a purely time-dependent potential that is admitted by all possible
spatially homogeneous cosmological models in 3+1-dimensional low-energy bosonic
string theory. The maximum number of components of the field that are left
arbitrary is found for each homogeneous cosmology defined by the Bianchi group
classification. The relative generality of these string cosmologies is found by
counting the number of independent pieces of Cauchy data needed to specify the
general solution of Einstein's equations. The hierarchy of generality differs
significantly from that characteristic of vacuum and perfect-fluid cosmologies.
The degree of generality of homogeneous string cosmologies is compared to that
of the generic inhomogenous solutions of the string field equations.Comment: 16 pages, Latex, assumptions clarified, calculations unchanged,
published in Phys. Rev.