17 research outputs found
On the Generation of a Scale-Invariant Spectrum of Adiabatic Fluctuations in Cosmological Models with a Contracting Phase
In Pre-Big-Bang and in Ekpyrotic Cosmology, perturbations on cosmological
scales today are generated from quantum vacuum fluctuations during a phase when
the Universe is contracting (viewed in the Einstein frame). The backgrounds
studied to date do not yield a scale invariant spectrum of adiabatic
fluctuations. Here, we present a new contracting background model (neither of
Pre-Big-Bang nor of the Ekpyrotic form) involving a single scalar field coupled
to gravity in which a scale-invariant spectrum of curvature fluctuations and
gravitational waves results. The equation of state of this scalar field
corresponds to cold matter. We demonstrate that if this contracting phase can
be matched via a nonsingular bounce to an expanding Friedmann cosmology, the
scale-invariance of the curvature fluctuations is maintained. We also find new
background solutions for Pre-Big-Bang and for Ekpyrotic cosmology, which
involve two scalar fields with exponential potentials with background values
which are evolving in time. We comment on the difficulty of obtaining a
scale-invariant spectrum of adiabatic fluctuations with background solutions
which have been studied in the past.Comment: 8 pages, revised version without the section on perturbations,
matching the version published on Phys. Rev. D. For cosmological
perturbations in the two field model see astro-ph/021127
Classical Stabilization of Homogeneous Extra Dimensions
If spacetime possesses extra dimensions of size and curvature radii much
larger than the Planck or string scales, the dynamics of these extra dimensions
should be governed by classical general relativity. We argue that in general
relativity, it is highly nontrivial to obtain solutions where the extra
dimensions are static and are dynamically stable to small perturbations. We
also illustrate that intuition on equilibrium and stability built up from
non-gravitational physics can be highly misleading. For all static, homogeneous
solutions satisfying the null energy condition, we show that the Ricci
curvature of space must be nonnegative in all directions. Much of our analysis
focuses on a class of spacetime models where space consists of a product of
homogeneous and isotropic geometries. A dimensional reduction of these models
is performed, and their stability to perturbations that preserve the spatial
symmetries is analyzed. We conclude that the only physically realistic examples
of classically stabilized large extra dimensions are those in which the
extra-dimensional manifold is positively curved.Comment: 25 pages; minor changes, improved reference
The Trans-Planckian Problem of Inflationary Cosmology
In most current models of inflation based on a weakly self-coupled scalar
matter field minimally coupled to gravity, the period of inflation lasts so
long that, at the beginning of the inflationary period, the physical
wavelengths of comoving scales which correspond to the present large-scale
structure of the Universe were smaller than the Planck length. Thus, the usual
computations of the spectrum of fluctuations in these models involve
extrapolating low energy physics (both in the matter and gravitational sector)
into regions where this physics is not applicable. In this paper we demonstrate
that the usual predictions of inflation for the spectrum of cosmological
fluctuations do indeed depend on the hidden assumptions about super-Planck
scale physics. We introduce a class of modified dispersion relations to mimic
possible effects of super-Planck scale physics, and show that in some cases
important deviations from the usual predictions of inflation are obtained. Some
implications of this result for the unification of fundamental physics and
early Universe cosmology are discussed.Comment: 16 pages, 2 figures. One important correction in the Corley/Jacobson
case with b_m>0 and some misprints corrected. Version published in PR
On the Construction of Nonsingular Pre-Big-Bang and Ekpyrotic Cosmologies and the Resulting Density Perturbations
We consider the construction of nonsingular Pre-Big-Bang and Ekpyrotic type
cosmological models realized by the addition to the action of specific
higher-order terms stemming from quantum corrections. We study models involving
general relativity coupled to a single scalar field with a potential motivated
by the Ekpyrotic scenario. We find that the inclusion of the string loop and
quantum correction terms in the string frame makes it possible to obtain
solutions of the variational equations which are nonsingular and bouncing in
the Einstein frame, even when a negative exponential potential is present, as
is the case in the Ekpyrotic scenario. We analyze the spectra of perturbations
produced during the bouncing phase and find that the spectrum of curvature
fluctuations in the model proposed originally to implement the Ekpyrotic
scenario has a large blue tilt (). Except for instabilities introduced
on small scales, the result agrees with what is obtained by imposing continuity
of the induced metric and of the extrinsic curvature across a constant scalar
field (up to corrections equal to the constant energy density) matching
surface between the contracting and the expanding Einstein Universes. We also
discuss nonsingular cosmological solutions obtained when a Gauss-Bonnet term
with a coefficient suitably dependent on the scalar matter field is added to
the action in the Einstein frame with a potential for the scalar field present.Comment: 29 pages, 9 figures, accepted for publication in Physical Review D,
reference added, discussion section expande
Inflation and Brane Gases
We investigate a new way of realizing a period of cosmological inflation in
the context of brane gas cosmology. It is argued that a gas of co-dimension one
branes, out of thermal equilibrium with the rest of the matter, has an equation
of state which can - after stabilization of the dilaton - lead to power-law
inflation of the bulk. The most promising implementation of this mechanism
might be in Type IIB superstring theory, with inflation of the three large
spatial dimensions triggered by ``stabilized embedded 2-branes''. Possible
applications and problems with this proposal are discussed.Comment: 7 pages, uses REVTeX, version to appear in Phys. Rev.
Holographic studies of quasi-topological gravity
Quasi-topological gravity is a new gravitational theory including
curvature-cubed interactions and for which exact black hole solutions were
constructed. In a holographic framework, classical quasi-topological gravity
can be thought to be dual to the large limit of some non-supersymmetric
but conformal gauge theory. We establish various elements of the AdS/CFT
dictionary for this duality. This allows us to infer physical constraints on
the couplings in the gravitational theory. Further we use holography to
investigate hydrodynamic aspects of the dual gauge theory. In particular, we
find that the minimum value of the shear-viscosity-to-entropy-density ratio for
this model is .Comment: 45 pages, 6 figures. v2: References adde
Lee Strasberg's Method
Lee Strasberg (1901–1982) is a divisive figure whose method of acting is often misunderstood. Few people really know what it is or how it works, or what is its true relationship to the Stanislavski System. This is due to a number of misconceptions about the nature of his training with its emphasis on the development of the sensory imagination. This paper aims to challenge some of the myths about the Method, and to clarify Strasberg’s unique contribution to actor training