Sarah Shandera

Effects of scale-dependent non-Gaussianity on cosmological structures

The detection of primordial non-Gaussianity could provide a powerful means to test various inflationary scenarios. Although scale-invariant non-Gaussianity (often described by the fNL formalism) is currently best constrained by the CMB, single-field models with changing sound speed can have strongly scale-dependent non-Gaussianity. Such models could evade the CMB constraints but still have important effects at scales responsible for the formation of cosmological objects such as clusters and galaxies. We compute the effect of scale-dependent primordial non-Gaussianity on cluster number counts as a function of redshift, using a simple ansatz to model scale-dependent features. We forecast constraints on these models achievable with forthcoming datasets. We also examine consequences for the galaxy bispectrum. Our results are relevant for the Dirac–Born–Infeld model of brane inflation, where the scale dependence of the non-Gaussianity is directly related to the geometry of the extra dimensions.

CMB-S4 Science Book, First Edition

This book lays out the scientific goals to be addressed by the next-generation ground-based cosmic microwave background experiment, CMB-S4, envisioned to consist of dedicated telescopes at the South Pole, the high Chilean Atacama plateau and possibly a northern hemisphere site, all equipped with new superconducting cameras. CMB-S4 will dramatically advance cosmological studies by crossing critical thresholds in the search for the B-mode polarization signature of primordial gravitational waves, in the determination of the number and masses of the neutrinos, in the search for evidence of new light relics, in constraining the nature of dark energy, and in testing general relativity on large scales.

Comparing Brane Inflation to WMAP

We compare the simplest realistic brane inflationary model to recent cosmological data, including Wilkinson Microwave Anisotropy Probe (WMAP) three-year cosmic microwave background (CMB) results, Sloan Digital Sky Survey Luminous Red Galaxies (SDSS LRG) power spectrum data and Supernovae Legacy Survey (SNLS) type 1a supernovae distance measures. Here, the inflaton is simply the position of a D3-brane which is moving towards a -brane sitting at the bottom of a throat (a warped, deformed conifold) in the flux compactified bulk in type IIB string theory. The analysis includes both the usual slow-roll scenario and the Dirac–Born–Infeld scenario of slow but relativistic rolling. Requiring that the throat is inside the bulk greatly restricts the allowed parameter space. We discuss possible scenarios in which a large tensor mode and/or non-Gaussianity may emerge. Here, the properties of a large tensor mode deviate from that in the usual slow-roll scenario, providing a possible stringy signature. Overall, within the brane inflationary scenario, the cosmological data are providing information about the properties of the compactification of the extra dimensions.

Observing brane inflation

Linking the slow-roll scenario and the Dirac–Born–Infeld scenario of ultra-relativistic roll (where, thanks to the warp factor, the inflaton moves slowly even with an ultra-relativistic Lorentz factor), we find that the KKLMMT D3–-brane inflation is robust; that is, enough e-folds of inflation is quite generic in the parameter space of the model. We show that the intermediate regime of relativistic roll can be quite interesting observationally. Introducing appropriate inflationary parameters, we explore the parameter space and give the constraints and predictions for the cosmological observables in this scenario. Among other properties, this scenario allows the saturation of the present observational bound of either the tensor/scalar ratio r (in the intermediate regime) or the non-Gaussianity fNL (in the ultra-relativistic regime), but not both.

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.

Weighing the giants – IV. Cosmology and neutrino mass

We employ robust weak gravitational lensing measurements to improve cosmological constraints from measurements of the galaxy cluster mass function and its evolution, using X-ray selected clusters detected in the ROSAT All-Sky Survey. Our lensing analysis constrains the absolute mass scale of such clusters at the 8 per cent level, including both statistical and systematic uncertainties. Combining it with the survey data and X-ray follow-up observations, we find a tight constraint on a combination of the mean matter density and late-time normalization of the matter power spectrum,

Breakdown of Semiclassical Methods in de Sitter Space

\sigma_8(\Omega_m/0.3)^{0.17}=0.81\pm0.03

Slow roll in brane inflation

, with marginalized, one-dimensional constraints of

A generalized local ansatz and its effect on halo bias

\Omega_m=0.26\pm0.03

Inflation from wrapped branes

and