11 research outputs found
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
Observational Constraints on Theories with a Blue Spectrum of Tensor Modes
Motivated by the string gas cosmological model, which predicts a blue tilt of
the primordial gravitational wave spectrum, we examine the constraints imposed
by current and planned observations on a blue tilted tensor spectrum. Starting
from an expression for the primordial gravitational wave spectrum normalized
using cosmic microwave background observations, pulsar timing, direct detection
and nucleosynthesis bounds are examined. If we assume a tensor to scalar ratio
on scales of the CMB which equals the current observational upper bound, we
obtain from these current observations constraints on the tensor spectral index
of , , and
respectively.Comment: 12 pages, 1 figure, 2 references added, relationship of this work
with Ref. 20 adde
Producing a Scale-Invariant Spectrum of Perturbations in a Hagedorn Phase of String Cosmology
We study the generation of cosmological perturbations during the Hagedorn
phase of string gas cosmology. Using tools of string thermodynamics we provide
indications that it may be possible to obtain a nearly scale-invariant spectrum
of cosmological fluctuations on scales which are of cosmological interest
today. In our cosmological scenario, the early Hagedorn phase of string gas
cosmology goes over smoothly into the radiation-dominated phase of standard
cosmology, without having a period of cosmological inflation.Comment: 4 pages, 1 figur
Cosmological Perturbations in Non-Commutative Inflation
We compute the spectrum of cosmological perturbations in a scenario in which
inflation is driven by radiation in a non-commutative space-time. In this
scenario, the non-commutativity of space and time leads to a modified
dispersion relation for radiation with two branches, which allows for
inflation. The initial conditions for the cosmological fluctuations are
thermal. This is to be contrasted with the situation in models of inflation in
which the accelerated expansion of space is driven by the potential energy of a
scalar field, and in which the fluctuations are of quantum vacuum type. We find
that, in the limit that the expansion of space is almost exponential, the
spectrum of fluctuations is scale-invariant with a slight red tilt. The
magnitude of the tilt is different from what is obtained in a usual
inflationary model with the same expansion rate during the period of inflation.
The amplitude also differs, and can easily be adjusted to agree with
observations.Comment: 7 pages, 1 figur
Inaccessible Singularities in Toral Cosmology
The familiar Bang/Crunch singularities of classical cosmology have recently
been augmented by new varieties: rips, sudden singularities, and so on. These
tend to be associated with final states. Here we consider an alternative
possibility for the initial state: a singularity which has the novel property
of being inaccessible to physically well-defined probes. These singularities
arise naturally in cosmologies with toral spatial sections.Comment: 10 pages, version to appear in Classical and Quantum Gravit
Near Scale Invariance with Modified Dispersion Relations
We describe a novel mechanism to seed a nearly scale invariant spectrum of
adiabatic perturbations during a non-inflationary stage. It relies on a
modified dispersion relation that contains higher powers of the spatial
momentum of matter perturbations. We implement this idea in the context of a
massless scalar field in an otherwise perfectly homogeneous universe. The
couplings of the field to background scalars and tensors give rise to the
required modification of its dispersion relation, and the couplings of the
scalar to matter result in an adiabatic primordial spectrum. This work is meant
to explicitly illustrate that it is possible to seed nearly scale invariant
primordial spectra without inflation, within a conventional expansion history.Comment: 7 pages and no figures. Uses RevTeX
Lectures on Cosmic Inflation and its Potential Stringy Realizations
These notes present a brief introduction to Hot Big Bang cosmology and Cosmic
Inflation, together with a selection of some recent attempts to embed inflation
into string theory. They provide a partial description of lectures presented in
courses at Dubrovnik in August 2006, at CERN in January 2007 and at Cargese in
August 2007. They are aimed at graduate students with a working knowledge of
quantum field theory, but who are unfamiliar with the details of cosmology or
of string theory.Comment: 68 pages, lectures given at Dubrovnik, Aug 2006; CERN, January 2007;
and Cargese, Aug 200
Fractional Brane State in the Early Universe
In the early Universe matter was crushed to high densities, in a manner
similar to that encountered in gravitational collapse to black holes. String
theory suggests that the large entropy of black holes can be understood in
terms of fractional branes and antibranes. We assume a similar physics for the
matter in the early Universe, taking a toroidal compactification and letting
branes wrap around the cycles of the torus. We find an equation of state
p_i=w_i rho, for which the dynamics can be solved analytically. For black
holes, fractionation can lead to non-local quantum gravity effects across
length scales of order the horizon radius; similar effects in the early
Universe might change our understanding of Cosmology in basic ways.Comment: 40 pages, 18 figures, references adde
Non-perturbative gravity, Hagedorn bounce and the cosmic microwave background.
In Biswas et al (2006 J. Cosmol. Astropart. Phys. JCAP03(2006)009 [hep-th/0508194]) it was shown how non-perturbative corrections to gravity can resolve the big bang singularity, leading to a bouncing universe. Depending on the scale of the non-perturbative corrections, the temperature at the bounce may be close to or higher than the Hagedorn temperature. If matter is made up of strings, then massive string states will be excited near the bounce, and the bounce will occur inside (or at the onset of) the Hagedorn phase for string matter. As we discuss in this paper, in this case cosmological fluctuations can be generated via the string gas mechanism recently proposed in Nayeri et al (2005 Preprint hep-th/0511140). In fact, the model discussed here demonstrates explicitly that it is possible to realize the assumptions made in Nayeri et al (2005 Preprint hep-th/0511140) in the context of a concrete set of dynamical background equations. We also calculate the spectral tilt of thermodynamic stringy fluctuations generated in the Hagedorn regime in this bouncing universe scenario. Generally we find a scale-invariant spectrum with a red tilt which is very small but does not vanish