758 research outputs found
A possible contribution to CMB anisotropies at high l from primordial voids
We present preliminary results of an analysis into the effects of primordial
voids on the cosmic microwave background (CMB). We show that an inflationary
bubble model of void formation predicts excess power in the CMB angular power
spectrum that peaks between 2000 < l < 3000. Therefore, voids that exist on or
close to the last scattering surface at the epoch of decoupling can contribute
significantly to the apparent rise in power on these scales recently detected
by the Cosmic Background Imager (CBI).Comment: 5 pages, 3 figures. MNRAS accepted versio
Extended Inflation from Strings
We study the possibility of extended inflation in the effective theory of
gravity from strings compactified to four dimensions and find that it strongly
depends on the mechanism of supersymmetry breaking. We consider a general class
of string--inspired models which are good candidates for successful extended
inflation. In particular, the --problem of ordinary extended inflation
is automatically solved by the production of only very small bubbles until the
end of inflation. We find that the inflaton field could belong either to the
untwisted or to the twisted massless sectors of the string spectrum, depending
on the supersymmetry breaking superpotential.Comment: 18p
Tachyon Condensates and String Theoretic Inflation
Cosmological solutions of the beta function equations for the background
fields of the closed bosonic string are investigated at the one-loop level.
Following recent work of Kostelecky and Perry, it is assumed that the spatial
sections of the space-time are conformally flat. Working in the sigma-model
frame, the non-trivial tachyon potential is utilized to determine solutions
with sufficient inflation to solve the smoothness and flatness problems. The
graceful exit and density perturbation constraints can also be successfully
implemented.Comment: FERMI-PUB 93/074-T, harvmac, 16 page
Induced-gravity Inflation and the Density Perturbation Spectrum
Recent experimental determinations of the spectral index describing the
scalar mode spectrum of density perturbations encourage comparison with
predictions from models of the very early universe. Unlike extended inflation,
Induced-gravity Inflation predicts a power spectrum with , in close agreement with the experimental measurements.Comment: 11pp, no figures. Plain LaTeX. HUTP-94/A011. Revised edition --
Forthcoming in Physics Letters
Phase space geometry in scalar-tensor cosmology
We study the phase space of spatially homogeneous and isotropic cosmology in
general scalar-tensor theories. A reduction to a two-dimensional phase space is
performed when possible-in these situations the phase space is usually a
two-dimensional curved surface embedded in a three-dimensional space and
composed of two sheets attached to each other, possibly with complicated
topology. The results obtained are independent of the choice of the coupling
function of the theory and, in certain situations, also of the potential.Comment: 18 pages, 9 figures, latex, to appear in Ann. Phys. (NY
Asymptotic Freedom Cosmology
For a general class of scalar--tensor gravity theories, we discuss how to
recover asymptotic freedom regimes when cosmic time . Such a
feature means that the effective gravitational coupling , while
cosmological solutions can asymptotically assume de Sitter or power--law
behaviours. In our opinion, through this mechanism, it is possible to cure some
shortcomings in inflationary and in string--dilaton cosmology.Comment: 9 pages, Latex, submitted to Phys. Lett.
Conditions for Successful Extended Inflation
We investigate, in a model-independent way, the conditions required to obtain
a satisfactory model of extended inflation in which inflation is brought to an
end by a first-order phase transition. The constraints are that the correct
present strength of the gravitational coupling is obtained, that the present
theory of gravity is satisfactorily close to general relativity, that the
perturbation spectra from inflation are compatible with large scale structure
observations and that the bubble spectrum produced at the phase transition
doesn't conflict with the observed level of microwave background anisotropies.
We demonstrate that these constraints can be summarized in terms of the
behaviour in the conformally related Einstein frame, and can be compactly
illustrated graphically. We confirm the failure of existing models including
the original extended inflation model, and construct models, albeit rather
contrived ones, which satisfy all existing constraints.Comment: 8 pages RevTeX file with one figure incorporated (uses RevTeX and
epsf). Also available by e-mailing ARL, or by WWW at
http://star-www.maps.susx.ac.uk/papers/infcos_papers.html; Revised to include
extra references, results unchanged, to appear Phys Rev
Constraints in the Context of Induced-gravity Inflation
Constraints on the required flatness of the scalar potential for a
cousin-model to extended inflation are studied. It is shown that, unlike
earlier results, Induced-gravity Inflation can lead to successful inflation
with a very simple lagrangian and , rather than
as previously reported. A second order phase transition further
enables this model to escape the \lq big bubble' problem of extended inflation,
while retaining the latter's motivations based on the low-energy effective
lagrangians of supergravity, superstring, and Kaluza-Klein theories.Comment: 19 pp; 3 figures (not included -- available from author). Plain
LaTeX. In press in Physical Review
Scalar-Tensor Cosmological Models
We analyze the qualitative behaviors of scalar-tensor cosmologies with an
arbitrary monotonic function. In particular, we are interested
on scalar-tensor theories distinguishable at early epochs from General
Relativity (GR) but leading to predictions compatible with solar-system
experiments. After extending the method developed by Lorentz-Petzold and
Barrow, we establish the conditions required for convergence towards GR at
. Then, we obtain all the asymptotic analytical solutions
at early times which are possible in the framework of these theories. The
subsequent qualitative evolution, from these asymptotic solutions until their
later convergence towards GR, has been then analyzed by means of numerical
computations. From this analysis, we have been able to establish a
classification of the different qualitative behaviors of scalar-tensor
cosmological models with an arbitrary monotonic function.Comment: uuencoded compressed postscript file containing 41 pages, with 9
figures, accepted for publication in Physical Review
Non-Equilibrium Evolution of Scalar Fields in FRW Cosmologies I
We derive the effective equations for the out of equilibrium time evolution
of the order parameter and the fluctuations of a scalar field theory in
spatially flat FRW cosmologies.The calculation is performed both to one-loop
and in a non-perturbative, self-consistent Hartree approximation.The method
consists of evolving an initial functional thermal density matrix in time and
is suitable for studying phase transitions out of equilibrium. The
renormalization aspects are studied in detail and we find that the counterterms
depend on the initial state. We investigate the high temperature expansion and
show that it breaks down at long times. We also obtain the time evolution of
the initial Boltzmann distribution functions, and argue that to one-loop order
or in the Hartree approximation, the time evolved state is a ``squeezed''
state. We illustrate the departure from thermal equilibrium by numerically
studying the case of a free massive scalar field in de Sitter and radiation
dominated cosmologies. It is found that a suitably defined non-equilibrium
entropy per mode increases linearly with comoving time in a de Sitter
cosmology, whereas it is {\it not} a monotonically increasing function in the
radiation dominated case.Comment: 29 pages, revtex 3.0, 11 figures available upon request, PITT-93-6;
LPTHE-93-52; CMU-HEP-93-2
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