Steven Gratton

Conditions for generating scale-invariant density perturbations

We analyze the general conditions on the equation of state

Hurdles for recent measures in eternal inflation

w

Impact of Galactic polarized emission on B-mode detection at low multipoles

required for quantum fluctuations of a scalar field to produce a scale-invariant spectrum of density perturbations, including models which (in the four dimensional effective description) bounce from a contracting to an expanding phase. We show that there are only two robust cases:

Prospects for constraining neutrino mass using Planck and Lyman-{alpha} forest data

w\approx -1

Cosmological perturbations from the no boundary Euclidean path integral

(inflation) and

Measures on Transitions for Cosmology from Eternal Inflation

w \gg 1

Closed universes from cosmological instantons

(the ekpyrotic/cyclic scenario). All other cases, including the

Observational test of quantum cosmology

w \approx 0

Path Integral for Stochastic Inflation: Non-Perturbative Volume Weighting, Complex Histories, Initial Conditions and the End of Inflation

case considered by some authors, require extreme fine-tuning of initial conditions and/or the effective potential. For the ekpyrotic/cyclic (

Cosmology: Beyond the inflationary border

w \gg 1