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- R Brandenberger, C Vafa
- 1989

We investigate some aspects of thermodynamics and cosmology for superstrings. By a rather delicate computation using the microcanonical ensemble we show that the ther-modynamic description of strings is sound (speciic heat is positive at large energies) only for strings propagating in spaces where all the spatial directions are compact. Using this result… (More)

The theory of cosmological perturbations is the main tool which connects theories of the early Universe (based on new fundamental physics such as string theory) with cosmological observations. In these lectures, I will provide an introduction to this theory, beginning with an overview of the Newtonian theory of fluctuations, moving on to the analysis of… (More)

An introduction to modern theories for the origin of structure in the Universe is given. After a brief review of the growth of cosmological perturbations in an expanding Universe and a summary of some important observational results, the lectures focus on the inflationary Universe scenario and on topological defect models of structure formation. A summary… (More)

These lecture notes 1 intend to form a short pedagogical introduction to inflationary cosmology, highlighting selected areas of recent progress such as reheating and the theory of cosmological perturbations. Problems of principle for inflationary cosmology are pointed out, and some new attempts at solving them are indicated, including a nonsingular Universe… (More)

- Ali Nayeri, Robert H Brandenberger, Cumrun Vafa
- Physical review letters
- 2006

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… (More)

The COBE satellite has discovered anisotropies in the cosmic microwave background (CMB) consistent with a scale invariant spectrum of density perturbations1). As reviewed in this lecture, topological defect models of structure formation generically produce such a spectrum. We give a summary of the cosmic string and global texture models, focusing on… (More)

Recently, Khoury et al. [1] suggested a new cosmological scenario which they called the Ekpyrotic Universe, a model motivated by string theory (more precisely on Horava-Witten theory [2]). The authors claim it solves the well-known problems of standard cosmology such as the horizon, flatness and monopole problems (see e.g. [3]) without the need to invoke a… (More)

- Robert H Brandenberger, Ali Nayeri, Subodh P Patil, Cumrun Vafa
- Physical review letters
- 2007

It has recently been shown that a Hagedorn phase of string gas cosmology can provide a causal mechanism for generating a nearly scale-invariant spectrum of scalar metric fluctuations, without the need for an intervening period of de Sitter expansion. In this Letter, we compute the spectrum of tensor metric fluctuations (gravitational waves) in this scenario… (More)

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 codimension 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… (More)

The hypothesis that the Universe underwent a period of exponential expansion at very early times has become the most popular theory of the early Universe. Not only does it solve some of the problems of standard big bang cosmology, but it also provides a causal theory for the origin of primordial density fluctuations which may explain the observed density… (More)