Rhiannon Gwyn

On degenerate models of cosmic inflation

In this article we discuss the role of current and future CMB measurements in pinning down the model of ination responsible for the generation of primordial curvature pertur

Effective field theory of weakly coupled inflationary models

The application of Effective Field Theory (EFT) methods to inflation has taken a central role in our current understanding of the very early universe. The EFT perspective has been particularly useful in analyzing the self-interactions determining the evolution of comoving curvature perturbations (Goldstone boson modes) and their influence on low-energy observables. However, the standard EFT formalism, to lowest order in spacetime differential operators, does not provide the most general parametrization of a theory that remains weakly coupled throughout the entire low-energy regime. Here we study the EFT formulation by including spacetime differential operators implying a scale dependence of the Goldstone boson self-interactions and its dispersion relation. These operators are shown to arise naturally from the low-energy interaction of the Goldstone boson with heavy fields that have been integrated out. We find that the EFT then stays weakly coupled all the way up to the cutoff scale at which ultraviolet degrees of freedom become operative. This opens up a regime of new physics where the dispersion relation is dominated by a quadratic dependence on the momentum ω ∼ p 2 . In addition, provided that modes crossed the Hubble scale within this energy range, the predictions of inflationary observables —including non-Gaussian signatures— are significantly affected by the new scales characterizing it.

de Sitter vacua in type IIB string theory: classical solutions and quantum corrections

A bstractWe revisit the classical theory of ten-dimensional two-derivative gravity coupled to fluxes, scalar fields, D-branes, anti D-branes and Orientifold-planes. We show that such set-ups do not give rise to a four-dimensional positive curvature spacetime with the isometries of de Sitter spacetime. We further argue that a de Sitter solution in type IIB theory may still be achieved if the higher-order curvature corrections are carefully controlled. Our analysis relies on the derivation of the de Sitter condition from an explicit background solution by going beyond the supergravity limit of type IIB theory. As such this also tells us how the background supersymmetry should be broken and under what conditions D-term uplifting can be realized with non self-dual fluxes.

Gauge-gravity dualities, dipoles and new non-Kähler manifolds

Abstract In this work we explore many directions in the framework of gauge-gravity dualities. In type IIB theory we give an explicit derivation of the local metric for five branes wrapped on rigid two-cycles. Our derivation involves various interplays between warp factors, dualities and fluxes and the final result confirms our earlier predictions. We also find a novel dipole-like deformation of the background due to an inherent orientifold projection in the full global geometry. The supergravity solution for this deformation takes into account various things like the presence of a non-trivial background topology and fluxes as well as branes. Considering these, we manage to calculate the precise local solution using equations of motion. We also show that this dipole-like deformation has the desired property of decoupling the Kaluza–Klein modes from the IR gauge theory. Finally, for the heterotic theory we find new non-Kahler complex manifolds that partake in the full gauge-gravity dualities and study the mathematical structures of these manifolds including the torsion classes, Betti numbers and other topological data.

Dipole-deformed bound states and heterotic Kodaira surfaces

Abstract We study a particular N = 1 confining gauge theory with fundamental flavors realised as seven branes in the background of wrapped five branes on a rigid two-cycle of a non-trivial global geometry. In parts of the moduli space, the five branes form bound states with the seven branes. We show that in this regime the local supergravity solution is surprisingly tractable, even though the background topology is non-trivial. New effects such as dipole deformations may be studied in detail, including the full backreactions. Performing the dipole deformations in other ways leads to different warped local geometries. In the dual heterotic picture, which is locally given by a C * fibration over a Kodaira surface, we study details of the geometry and the construction of bundles. We also point out the existence of certain exotic bundles in our framework.

On the warped heterotic axion

We study the axion in a warped heterotic background. It is shown that the axion decay constant, f_a, is sensitive to the warped mass scale of the throat. As an explicit model, we construct a novel AdS-like geometry in heterotic string theory. We demonstrate that in this background f_a is given by the mass scale of the longest throat in the compactification. The question of obtaining f_a within the required bound 10^9--10^12 GeV is reduced to the construction of a throat inside the heterotic string theory compactification with warped mass scale in the above range. This provides a natural mechanism for realising the axion in heterotic string theory.

Resonant non-Gaussianity with equilateral properties

We discuss the effect of superimposing multiple sources of resonant non-Gaussianity, which arise for instance in models of axion inflation. The resulting sum of oscillating shape contributions can be used to ``Fourier synthesize different non-oscillating shapes in the bispectrum. As an example we reproduce an approximately equilateral shape from the superposition of (10) oscillatory contributions with resonant shape. This implies a possible degeneracy between the equilateral-type non-Gaussianity typical of models with non-canonical kinetic terms, such as DBI inflation, and an equilateral-type shape arising from a superposition of resonant-type contributions in theories with canonical kinetic terms. The absence of oscillations in the 2-point function together with the structure of resonant N-point functions give a constraint of fNL(5) for equilateral non-Gaussianity with resonant origin, but this constraint can be avoided when additional U(1)s are involved in the breaking of the shift symmetry. We comment on the questions arising from possible embeddings of this idea in a string theory setting.

Relations between canonical and non-canonical inflation

We look for potential observational degeneracies between canonical and non-canonical models of ination of a single eld . Non-canonical inationary mod- els are characterized by higher than linear powers of the standard kinetic term X in the eective Lagrangian p(X; ) and arise for instance in the context of the Dirac- Born-Infeld (DBI) action in string theory. An on-shell transformation is introduced that transforms non-canonical inationary theories to theories with a canonical ki- netic term. The 2-point function observables of the original non-canonical theory and its canonical transform are found to match in the case of DBI ination.

Axions from heterotic stringThis paper was presented at the Theory CANADA 4 conference, held at Centre de recherches mathématiques, Montréal, Québec, Canada on 4–7 June 2008.

We study the axion in a warped heterotic background. It is shown that the axion decay constant, fa, is sensitive to the warped mass scale of the throat. As an explicit model, we construct a novel AdS-like geometry in heterotic string theory. We demonstrate that in this background fa is given by the mass scale of the longest throat in the compactification. The question of obtaining fa within the required bound 109–1012 GeV is reduced to the construction of a throat inside the heterotic string theory compactification with warped mass scale in the above range. This provides a natural mechanism for realising the axion in heterotic string theory.

Braneworld graviton interactions in early universe phase transitions

This chapter discusses the Braneworld models that solve the hierarchy problem. The chapter focuses on Randall-Sundrum (RS) type models that embed one or two branes into a five dimensional AdS bulk. In RS 1, a static solution of the 5d Einstein equations is derived with two branes of opposite tension at the orbifold fixed points of the finite fifth dimension. Low-scale gravity leads to low reheating temperatures, and it is possible to obtain temperatures lower than the electroweak scale. This implies that the electroweak phase transition can take place before the completion of reheating. The chapter discusses how the gravitons couple in order to matter fields on the brane. The KK decomposition, graviton propagators and their coupling to matter and gauge fields are considered for toroidal extra dimensions and the analysis for the infinite AdS bulks have to be repeated.