Results 1  10
of
47
Modified Gravity and Cosmology
, 2012
"... In this review we present a thoroughly comprehensive survey of recent work on modified theories of gravity and their cosmological consequences. Amongst other things, we cover General Relativity, ScalarTensor, EinsteinAether, and Bimetric theories, as well as TeVeS, f(R), general higherorder theo ..."
Abstract

Cited by 28 (0 self)
 Add to MetaCart
In this review we present a thoroughly comprehensive survey of recent work on modified theories of gravity and their cosmological consequences. Amongst other things, we cover General Relativity, ScalarTensor, EinsteinAether, and Bimetric theories, as well as TeVeS, f(R), general higherorder theories, HořavaLifschitz gravity, Galileons, Ghost Condensates, and models of extra dimensions including KaluzaKlein, RandallSundrum, DGP, and higher codimension braneworlds. We also review attempts to construct a Parameterised PostFriedmannian formalism, that can be used to constrain deviations from General Relativity in cosmology, and that is suitable for comparison with data on the largest scales. These subjects have been intensively studied over the past decade, largely motivated by rapid progress in the field of observational cosmology that now allows, for the first time, precision tests of fundamental physics on the scale of the observable Universe. The purpose of this review is to provide a reference tool for researchers and students in cosmology and gravitational physics, as well as a selfcontained, comprehensive and uptodate introduction to the subject as a whole.
Dynamics with Infinitely Many Derivatives: The Initial Value Problem
, 2008
"... Differential equations of infinite order are an increasingly important class of equations in theoretical physics. Such equations are ubiquitous in string field theory and have recently attracted considerable interest also from cosmologists. Though these equations have been studied in the classical ..."
Abstract

Cited by 21 (0 self)
 Add to MetaCart
Differential equations of infinite order are an increasingly important class of equations in theoretical physics. Such equations are ubiquitous in string field theory and have recently attracted considerable interest also from cosmologists. Though these equations have been studied in the classical mathematical literature, it appears that the physics community is largely unaware of the relevant formalism. Of particular importance is the fate of the initial value problem. Under what circumstances do infinite order differential equations possess a welldefined initial value problem and how many initial data are required? In this paper we study the initial value problem for infinite order differential equations in the mathematical framework of the formal operator calculus, with analytic initial data. This formalism allows us to handle simultaneously a wide array of different nonlocal equations within a single framework and also admits a transparent physical interpretation. We show that differential equations of infinite order do not generically admit infinitely many initial data. Rather, each pole of the propagator contributes two initial data to the final solution. Though it is possible to find differential equations of infinite order which admit welldefined initial value problem with only two initial data, neither
padic Inflation
, 2006
"... We construct approximate inflationary solutions rolling away from the unstable maximum of padic string theory, a nonlocal theory with derivatives of all orders. Novel features include the existence of slowroll solutions even when the slowroll parameters, as usually defined, are much greater tha ..."
Abstract

Cited by 15 (5 self)
 Add to MetaCart
We construct approximate inflationary solutions rolling away from the unstable maximum of padic string theory, a nonlocal theory with derivatives of all orders. Novel features include the existence of slowroll solutions even when the slowroll parameters, as usually defined, are much greater than unity, as well as the need for the Hubble parameter to exceed the string mass scale ms. We show that the theory can be compatible with CMB observations if g 2 s/(ln p) ∼ 10 −15, where gs is the string coupling, and if ms < 10 −6 Mp. A redtilted spectrum is predicted, and an observably large tensor component is possible. The padic theory is shown to have identical inflationary predictions to a local theory with superPlanckian parameter values, but with the advantage that the padic theory is
Large Nongaussianity from Nonlocal Inflation
 JCAP 0707, 017 (2007) [arXiv:0704.3426 [hepth]]; “Predictions for Nongaussianity from Nonlocal Inflation,” arXiv:0802.3218 [hepth
"... Abstract: We study the possibility of obtaining large nongaussian signatures in the Cosmic Microwave Background in a general class of singlefield nonlocal hilltop inflation models. We estimate the nonlinearity parameter fNL which characterizes nongaussianity in such models and show that large nong ..."
Abstract

Cited by 14 (3 self)
 Add to MetaCart
(Show Context)
Abstract: We study the possibility of obtaining large nongaussian signatures in the Cosmic Microwave Background in a general class of singlefield nonlocal hilltop inflation models. We estimate the nonlinearity parameter fNL which characterizes nongaussianity in such models and show that large nongaussianity is possible. For the recently proposed padic inflation model we find that fNL ∼ 120 when the string coupling is order unity. We show that large nongaussianity is also possible in a toy model with an action similar to
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 ..."
Abstract

Cited by 9 (0 self)
 Add to MetaCart
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.
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 ..."
Abstract

Cited by 8 (3 self)
 Add to MetaCart
(Show Context)
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.
Arrow of Time in String Theory
 Nucl. Phys. B782
"... Inflation allows the problem of the Arrow of time to be understood as a question about the structure of spacetime: why was the intrinsic curvature of the earliest spatial sections so much better behaved than it might have been? This is really just the complement of a more familiar problem: what mech ..."
Abstract

Cited by 5 (4 self)
 Add to MetaCart
(Show Context)
Inflation allows the problem of the Arrow of time to be understood as a question about the structure of spacetime: why was the intrinsic curvature of the earliest spatial sections so much better behaved than it might have been? This is really just the complement of a more familiar problem: what mechanism prevents the extrinsic curvature of space from diverging, as classical General Relativity suggests? We argue that the stringy version of “creation from nothing”, sketched by Ooguri, Vafa, and Verlinde, solves both of these problems at once. The argument, while very simple, hinges on some of the deepest theorems in global differential geometry. These results imply that when a spatially toral spacetime is created from nothing, the earliest spatial sections are forced to be [quasiclassically] exactly locally isotropic. This local isotropy, in turn, forces the inflaton into its minimalentropy state. The theory explains why the Arrow does not reverse in black holes or in a cosmic contraction, if any. 1. The Arrow of Time: What Inflation Does For Us One of the deepest mysteries in physics is the origin of the Arrow of time. The first step towards understanding this mystery was identifying where the explanation is to be looked
Nonperturbative gravity, Hagedorn bounce and CMB,” arXiv:hepth/0610274. 16
 Extensions,” Phys. Rept
, 1992
"... Abstract: In [1] it was shown how nonperturbative corrections to gravity can resolve the big bang singularity, leading to a bouncing universe. Depending on the scale of the nonperturbative corrections, the temperature at the bounce may be close to or higher than the Hagedorn temperature. If matter ..."
Abstract

Cited by 4 (1 self)
 Add to MetaCart
(Show Context)
Abstract: In [1] it was shown how nonperturbative corrections to gravity can resolve the big bang singularity, leading to a bouncing universe. Depending on the scale of the nonperturbative 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 [2]. In fact, the model discussed here demonstrates explicitly that it is possible to realize the assumptions made in [2] 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 scaleinvariant spectrum with a red