1,767 research outputs found
Gravitational waves in non-singular string cosmologies
We study the evolution of tensor metric fluctuations in a class of
non-singular models based on the string effective action, by including in the
perturbation equation the higher-derivative and loop corrections needed to
regularise the background solutions. We discuss the effects of such
higher-order corrections on the final graviton spectrum, and we compare the
results of analytical and numerical computations.Comment: 24 pages, 7 figure
The string of variable density: perturbative and non-perturbative results
We obtain systematic approximations for the modes of vibration of a string of
variable density, which is held fixed at its ends. These approximations are
obtained iteratively applying three theorems which are proved in the paper and
which hold regardless of the inhomogeneity of the string. Working on specific
examples we obtain very accurate approximations which are compared both with
the results of WKB method and with the numerical results obtained with a
collocation approach. Finally, we show that the asymptotic behaviour of the
energies of the string obtained with perturbation theory, worked to second
order in the inhomogeinities, agrees with that obtained with the WKB method and
implies a different functional dependence on the density that in two and higher
dimensions.Comment: 28 pages, 3 tables, 6 figure
Metric Perturbations in Dilaton-Driven Inflation
We compute the spectrum of scalar and tensor metric perturbations generated,
as amplified vacuum fluctuations, during an epoch of dilaton-driven inflation
of the type occurring naturally in string cosmology. In the tensor case the
computation is straightforward while, in the scalar case, it is made delicate
by the appearance of a growing mode in the familiar longitudinal gauge. In
spite of this, a reliable perturbative calculation of perturbations far outside
the horizon can be performed by resorting either to appropriate gauge invariant
variables, or to a new coordinate system in which the growing mode can be
"gauged down". The simple outcome of this complicated analysis is that both
scalar and tensor perturbations exhibit nearly Planckian spectra, whose common
"temperature" is related to some very basic parameters of the string-cosmology
background.Comment: 34 pages, latex, no figure
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