1,332 research outputs found
Perturbations in a Bouncing Brane Model
The question of how perturbations evolve through a bounce in the Cyclic and
Ekpyrotic models of the Universe is still a matter of ongoing debate. In this
report we show that the collision between boundary branes is in most cases
singular even in the full 5-D formalism, and that first order perturbation
theory breaks down for at least one perturbation variable. Only in the case
that the boundary branes approach each other with constant velocity shortly
before the bounce, can a consistent, non singular solution be found. It is then
possible to follow the perturbations explicitly until the actual collision. In
this case, we find that if a scale invariant spectrum developed on the hidden
brane, it will get transferred to the visible brane during the bounce.Comment: 15 pages, minor modifications, a few typos correcte
A Radiation Bounce from the Lee-Wick Construction?
It was recently realized that matter modeled by the scalar field sector of
the Lee-Wick Standard Model yields, in the context of a homogeneous and
isotropic cosmological background, a bouncing cosmology. However, bouncing
cosmologies induced by pressure-less matter are in general unstable to the
addition of relativistic matter (i.e. radiation). Here we study the possibility
of obtaining a bouncing cosmology if we add not only radiation, but also its
Lee-Wick partner, to the matter sector. We find that, in general, no bounce
occurs. The only way to obtain a bounce is to choose initial conditions with
very special phases of the radiation field and its Lee-Wick partner.Comment: 11 page
Can universe exit from phantom inflation due to gravitational back reaction?
The effects of the gravitational back reaction of cosmological perturbations
are investigated in a phantom inflation model. The effective energy-momentum
tensor of the gravitational back reaction of cosmological perturbations whose
wavelengths are larger than the Hubble radius is calculated. Our results show
that the effects of gravitational back reaction will counteract that of the
phantom energy. It is demonstrated in a chaotic phantom inflation model that if
the phantom field at the end of inflation is larger than a critical value
determined by the necessary e-folds, the phantom inflation phase might be
terminated by the gravitational back reaction.Comment: 9 pages, Revtex4, to appear in JCA
Dynamical Relaxation of the Cosmological Constant and Matter Creation in the Universe
In this Letter we discuss the issues of the graceful exit from inflation and
of matter creation in the context of a recent scenario \cite{RHBrev} in which
the back-reaction of long wavelength cosmological perturbations induces a
negative contribution to the cosmological constant and leads to a dynamical
relaxation of the bare cosmological constant. The initially large cosmological
constant gives rise to primordial inflation, during which cosmological
perturbations are stretched beyond the Hubble radius. The cumulative effect of
the long wavelength fluctuations back-reacts on the background geometry in a
form which corresponds to the addition of a negative effective cosmological
constant to the energy-momentum tensor. In the absence of an effective scalar
field driving inflation, whose decay can reheat the Universe, the challenge is
to find a mechanism which produces matter at the end of the relaxation process.
In this Letter, we point out that the decay of a condensate representing the
order parameter for a ``flat'' direction in the field theory moduli space can
naturally provide a matter generation mechanism. The order parameter is
displaced from its vacuum value by thermal or quantum fluctuations, it is
frozen until the Hubble constant drops to a sufficiently low value, and then
begins to oscillate about its ground state. During the period of oscillation it
can decay into Standard Model particles similar to how the inflaton decays in
scalar-field-driven models of inflation.Comment: 6 page
On the Instability of the Lee-Wick Bounce
It was recently realized that a model constructed from a Lee-Wick type scalar
field theory yields, at the level of homogeneous and isotropic background
cosmology, a bouncing cosmology. However, bouncing cosmologies induced by
pressure-less matter are in general unstable to the addition of relativistic
matter (i.e. radiation). Here we study the possibility of obtaining a bouncing
cosmology if we add radiation coupled to the Lee-Wick scalar field. This
coupling in principle would allow the energy to flow from radiation to matter,
thus providing a drain for the radiation energy. However, we find that it takes
an extremely unlikely fine tuning of the initial phases of the field
configurations for a sufficient amount of radiative energy to flow into matter.
For general initial conditions, the evolution leads to a singularity rather
than a smooth bounce.Comment: 17 pages, 9 figure
Looking Beyond Inflationary Cosmology
In spite of the phenomenological successes of the inflationary universe
scenario, the current realizations of inflation making use of scalar fields
lead to serious conceptual problems which are reviewed in this lecture. String
theory may provide an avenue towards addressing these problems. One particular
approach to combining string theory and cosmology is String Gas Cosmology. The
basic principles of this approach are summarized.Comment: invited talk at "Theory Canada 1" (Univ. of British Columbia,
Vancouver, Canada, June 2 - 4, 2005) (references updated
Dilaton stabilization by massive fermion matter
The study started in a former work about the Dilaton mean field stabilization
thanks to the effective potential generated by the existence of massive
fermions, is here extended. Three loop corrections are evaluated in addition to
the previously calculated two loop terms. The results indicate that the Dilaton
vacuum field tend to be fixed at a high value close to the Planck scale, in
accordance with the need for predicting Einstein gravity from string theory.
The mass of the Dilaton is evaluated to be also a high value close to the
Planck mass, which implies the absence of Dilaton scalar signals in modern
cosmological observations. These properties arise when the fermion mass is
chosen to be either at a lower bound corresponding to the top quark mass, or
alternatively, at a very much higher value assumed to be in the grand
unification energy range. One of the three 3-loop terms is exactly evaluated in
terms of Master integrals. The other two graphs are however evaluated in their
leading logarithm correction in the perturbative expansion. The calculation of
the non leading logarithmic contribution and the inclusion of higher loops
terms could made more precise the numerical estimates of the vacuum field value
and masses, but seemingly are expected not to change the qualitative behavior
obtained. The validity of the here employed Yukawa model approximation is
argued for small value of the fermion masses with respect to the Planck one. A
correction to the two loop calculation done in the previous work is here
underlined.Comment: 18 pages, 5 figures, the study was extended and corrections on the
former calculations and redaction were done. The paper had been accepted for
publication in "Astrophysics and Space Science
On the new string theory inspired mechanism of generation of cosmological perturbations
Recently a non-inflationary mechanism of generation of scale-free
cosmological perturbations of metric was proposed by Brandenberger, Nayeri, and
Vafa in the context of the string gas cosmology. We discuss various problems of
their model and argue that the cosmological perturbations of metric produced in
this model have blue spectrum with a spectral index n = 5, which strongly
disagrees with observations. We conclude that this model in its present form is
not a viable alternative to inflationary cosmology.Comment: 11 pages, 1 figur
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