Louis Leblond

Super-Hubble de Sitter fluctuations and the dynamical RG

Perturbative corrections to correlation functions for interacting theories in de Sitter spacetime often grow secularly with time, due to the properties of fluctuations on super-Hubble scales. This growth can lead to a breakdown of perturbation theory at late times. We argue that Dynamical Renormalization Group (DRG) techniques provide a convenient framework for interpreting and resumming these secularly growing terms. In the case of a massless scalar field in de Sitter with quartic self-interaction, the resummed result is also less singular in the infrared, in precisely the manner expected if a dynamical mass is generated. We compare this improved infrared behavior with large-N expansions when applicable.

Breakdown of Semiclassical Methods in de Sitter Space

Massless interacting scalar fields in de Sitter space have long been known to experience large fluctuations over length scales larger than Hubble distances. A similar situation arises in condensed matter physics in the vicinity of a critical point, and in this better-understood situation these large fluctuations indicate the failure in this regime of mean-field methods. We argue that for non-Goldstone scalars in de Sitter space, these fluctuations can also be interpreted as signaling the complete breakdown of the semi-classical methods widely used throughout cosmology. By power-counting the infrared properties of Feynman graphs in de Sitter space we find that for a massive scalar interacting through a λ 4 interaction, control over the loop approximation is lost for masses smaller than m √λ H/2π, where H is the Hubble scale. We briefly discuss some potential implications for inflationary cosmology.

Scale dependent local non-gaussianity from loops

We analyze multi-field inflationary systems which yield strongly scale dependent non-Gaussianity with a shape that is very close to the local shape. As in usual multi-field models, the non-Gaussianity arises from the non-linear transfer of scalar field fluctuations to curvature perturbations. Here we consider models in which higher order terms (loops) dominate over the lowest order source of non-linearity. The magnitude of non-Gaussianity depends on an infrared cutoff which is determined by our observational probes measuring non-Gaussianity. In our models, the running is positive and large (nNG ~ 0.2) on CMB scales. The magnitude of the bispectrum is maximally of order (100), and grows on small scales. This can lead to interesting signals for large scale structure.

Inflation from wrapped branes

We show that the use of higher dimensional wrapped branes can significantly extend the inflaton field range compared to brane inflation models which use

Simple Bounds from the Perturbative Regime of Inflation

D3

Resonant trispectrum and a dozen more primordial N-point functions

-branes. We construct a simple inflationary model in terms of 5-branes wrapping a 2-cycle and traveling towards the tip of the Klebanov-Strassler throat. Inflation ends when the branes reach the tip of the cone and self-annihilate. Assuming a quadratic potential for the brane it is possible to match the cosmic microwave background data in the Dirac-Born-Infeld regime, but we argue that the backreaction of the brane is important and cannot be neglected. This scenario predicts a strong non-Gaussian signal and possibly detectable gravitational waves.

Gravitational waves from broken cosmic strings: The bursts and the beads

We examine the conditions under which a perturbative expansion around an inflating background is valid. When inflation is driven by a single field with a general sound speed, we find a lower limit on the sound speed related to the amplitude of the inflationary power spectrum. Generalizing the sound speed constraints to include scale dependence can limit the number of e-folds obtained in the perturbative regime and restrict otherwise apparently viable models. We also show that, for models with a low sound speed, eternal inflation cannot occur in the perturbative regime.

Cosmic necklaces from string theory

We compute all N-point primordial curvature correlation functions from inflation at tree-level up to N of order ten or more depending on the choice of parameters. This is achieved for resonant inflationary models in which the inflaton potential has a periodic modulation on top of a slow-roll flat term. These models find a natural UV completion in string theory implementation of axion monodromy. Key to the success of our computation is the observation that gravitational interactions among the perturbations can be neglected, which we argue is justified for any model of single-field inflation with parametrically large non-Gaussianity. We provide a comprehensive review and detailed derivations of known consistency relations for squeezed and collinear limits, and generalize them to any N-point function.

Constructing Infrared Finite Propagators in Inflating Space-time

We analyze the gravitational wave signatures of a network of metastable cosmic strings. We consider the case of cosmic string instability to breakage, with no primordial population of monopoles. This scenario is well motivated from grand unified theories and string-theoretic models with an inflationary phase below the grand unified theories/string scale. The network initially evolves according to a scaling solution, but with breakage events resulting from confined monopoles (beads) being pair produced and accelerated apart. We find these ultrarelativistic beads to be a potent source of gravitational wave bursts, detectable by initial LIGO, advanced LIGO, and LISA. Indeed, advanced LIGO could observe bursts from strings with tensions as low as

Cosmology of the tachyon in brane inflation

G\ensuremath{\mu}\ensuremath{\sim}{10}^{\ensuremath{-}12}