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41
String gas cosmology and structure formation  A brief review
, 2007
"... For suitable cosmological backgrounds, thermal fluctuations of a gas of strings can generate a scaleinvariant spectrum of cosmological fluctuations without requiring a phase of inflationary expansion. We highlight the key points of this mechanism, and discuss cosmological backgrounds in which this ..."
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Cited by 25 (7 self)
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For suitable cosmological backgrounds, thermal fluctuations of a gas of strings can generate a scaleinvariant spectrum of cosmological fluctuations without requiring a phase of inflationary expansion. We highlight the key points of this mechanism, and discuss cosmological backgrounds in which this scenario can be realized. The spectrum of cosmological perturbations has a small red tilt (like in scalar fielddriven inflation) but (unlike in inflation) there is a small blue tilt of the spectrum of gravitational waves.
Lectures on the theory of cosmological perturbations
 Lect. Notes Phys. 646
, 2004
"... Summary. The theory of cosmological perturbations has become a cornerstone of modern quantitative cosmology since it is the framework which provides the link between the models of the very early Universe such as the inflationary Universe scenario (which yield causal mechanisms for the generation of ..."
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Cited by 23 (8 self)
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Summary. The theory of cosmological perturbations has become a cornerstone of modern quantitative cosmology since it is the framework which provides the link between the models of the very early Universe such as the inflationary Universe scenario (which yield causal mechanisms for the generation of fluctuations) and the wealth of recent highprecision data on the spectrum of density fluctuations and cosmic microwave anisotropies. In these lectures, I provide an overview of the classical and quantum theory of cosmological fluctuations. Crucial points in both the current inflationary paradigm [1, 2] of the early Universe and in proposed alternatives such as the PreBigBang [3] and Ekpyrotic [4] scenarios are that, first, the perturbations are generated on microscopic scales as quantum vacuum fluctuations, and, second, that via an accelerated expansion of the background geometry (or by a contraction of the background), the wavelengths of the fluctuations become much larger than the Hubble radius for a long period of cosmic evolution. Hence, both Quantum Mechanics and General Relativity are required in order to understand the generation and evolution of fluctuations. As a guide to develop the physical intuition for the evolution of inhomogeneities, I begin with a discussion of the Newtonian theory of fluctuations. applicable at late times and on scales smaller than the Hubble radius. The analysis of superHubble fluctuations requires a general relativistic analysis. I first review the classical relativistic theory of fluctuations, and then discuss their quantization. I conclude with a brief overview of two applications of the theory of cosmological fluctuations: the transPlanckian “problem ” of inflationary cosmology and the current status of the study of the backreaction of cosmological fluctuations on the background spacetime geometry. Most of this article is based on the review [5] to which the reader is referred to for the details omitted in these lecture notes. 1
Magnetogenesis and the primordial nongaussianity
, 810
"... Abstract. The primordial density fluctuation inevitably couples to all forms of matter via loop corrections and depends on the ambient conditions while inflation was ongoing. This gives us an opportunity to observe processes which were in progress while the universe was inflating, provided they were ..."
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Cited by 10 (4 self)
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Abstract. The primordial density fluctuation inevitably couples to all forms of matter via loop corrections and depends on the ambient conditions while inflation was ongoing. This gives us an opportunity to observe processes which were in progress while the universe was inflating, provided they were sufficiently dramatic to overcome suppression by powers of (H/MP) 2 ≈ 10 −9, where H is the Hubble scale during inflation and MP is the Planck mass. As an example, if a primordial magnetic field was synthesized during inflation, as suggested by some interpretations of the apparently universal 10 −6 gauss field observed on galactic scales, then this could leave traces in inflationary observables. In this paper, I compute corrections to the spectrum and bispectrum generated by a varying electromagnetic coupling during inflation, assuming that the variation in this coupling is mediated by interaction with a collection of light scalar fields. If the mass scale associated with this interaction is too far below the Planck scale then the stability of perturbation theory can be upset. For the massscale which is relevant in the standard magnetogenesis scenario, however, the theory is stable and the model is apparently consistent with observational constraints.
String Gas Cosmology
, 2004
"... String gas cosmology is a string theorybased approach to early universe cosmology which is based on making use of robust features of string theory such as the existence of new states and new symmetries. A first goal of string gas cosmology is to understand how string theory can effect the earliest ..."
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Cited by 9 (0 self)
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String gas cosmology is a string theorybased approach to early universe cosmology which is based on making use of robust features of string theory such as the existence of new states and new symmetries. A first goal of string gas cosmology is to understand how string theory can effect the earliest moments of cosmology before the effective field theory approach which underlies standard and inflationary cosmology becomes valid. String gas cosmology may also provide an alternative to the current standard paradigm of cosmology, the inflationary universe scenario. Here, the current status of string gas cosmology is reviewed.
On the consistency of de Sitter vacua
 JHEP 0212 (2002) 025 [arXiv:hepth/0210058
"... In this paper the consistency of the de Sitter invariant αvacua, which have been introduced as simple tools to study the effects of transplanckian physics, is investigated. In particular possible non renormalization problems are discussed, as well as non standard properties of Greens functions. We ..."
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Cited by 9 (5 self)
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In this paper the consistency of the de Sitter invariant αvacua, which have been introduced as simple tools to study the effects of transplanckian physics, is investigated. In particular possible non renormalization problems are discussed, as well as non standard properties of Greens functions. We also discuss the non thermal properties of the αvacua and the necessity of α to change. The conclusion is that non of these problems necessarily exclude an application of the αvacua to inflation. The main driving force behind recent work on transplanckian physics in cosmology, is to find out whether physics beyond the Planck scale can give effects on the CMBR spectrum, [130]. A crucial ingredient in inflationary cosmology is that microscopic quantum fluctuations are magnified by inflation into macroscopic seeds for galaxy
Conceptual Problems of Inflationary Cosmology and a New Approach to Cosmological Structure Formation
 IN THE PROCEEDINGS OF INFLATION
, 2007
"... In spite of its great phenomenological success, current models of scalar fielddriven inflation suffer from important unresolved conceptual issues. New fundamental physics will be required to address these questions. String theory is a candidate for a unified quantum theory of all four forces of n ..."
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Cited by 8 (3 self)
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In spite of its great phenomenological success, current models of scalar fielddriven inflation suffer from important unresolved conceptual issues. New fundamental physics will be required to address these questions. String theory is a candidate for a unified quantum theory of all four forces of nature. As will be shown, string theory may lead to a cosmological background quite different from an inflationary cosmology, and may admit a new stringy mechanism for the origin of a roughly scaleinvariant spectrum of cosmological fluctuations.
Degravitation, Inflation and the Cosmological Constant as an Afterglow
, 801
"... Abstract: In this report, we adopt the phenomenological approach of taking the degravitation paradigm seriously as a consistent modification of gravity in the IR, and investigate its consequences for various cosmological scenarios. By considering a one parameter family of resonance graviton degravit ..."
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Cited by 7 (0 self)
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Abstract: In this report, we adopt the phenomenological approach of taking the degravitation paradigm seriously as a consistent modification of gravity in the IR, and investigate its consequences for various cosmological scenarios. By considering a one parameter family of resonance graviton degravitation models (with a degravitation scale much larger than the present horizon scale), we show that slow roll inflation, hybrid inflation and old inflation remain quantitatively unchanged. We also find that the degravitation mechanism inherits a memory of past energy densities in the present epoch in such a way that is likely significant for present cosmological evolution. For example, if the universe underwent inflation in the past due to it having tunneled out of some false vacuum (as is possible in some sort of a potential landscape), we find that degravitation implies a remnant ‘afterglow ’ cosmological constant, whose scale immediately afterwards is parametrically suppressed by the graviton mass (m) in Planck units Λ ∼ m/Mpl. We discuss circumstances through which this scenario reasonably yields the presently observed value for Λ ∼ O(10 −120). We also find that in a universe still currently trapped in some false vacuum state, resonance graviton models of degravitation only degravitate initially Planck or GUT scale energy densities down to the
Theory of cosmological perturbations and applications to superstring cosmology,” arXiv:hepth/0501033
"... 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 Newtoni ..."
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Cited by 6 (0 self)
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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 fluctuations in the realm of classical general relativity, and culminating with a discussion of the quantum theory of cosmological perturbations. I will illustrate the formalism with applications to inflationary cosmology. I will review the basics of inflationary cosmology and discuss why through the evolution of fluctuations inflation may provide a way of observationally testing Planckscale physics. 1.
ALTERNATIVES TO THE INFLATIONARY PARADIGM OF STRUCTURE FORMATION
, 902
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Cosmological Particle Production Without Bogolubov Coefficients
, 2003
"... We present an efficient new technique for calculating the amount of particle production in a cosmological background. The expectation value of the number operator for a given momentum mode can be extracted from the Feynman propagator. We demonstrate the computational economy of the technique by appl ..."
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Cited by 5 (0 self)
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We present an efficient new technique for calculating the amount of particle production in a cosmological background. The expectation value of the number operator for a given momentum mode can be extracted from the Feynman propagator. We demonstrate the computational economy of the technique by applying it to various cosmologies. We also show that the appropriate Feynman propagator, with boundary conditions that encode the initial state, can be constructed by the method of images. Our technique uses a firstquantized approach so, unlike conventional Bogolubov transformations, it may be amenable to a stringtheoretic generalization.