Engineering topics

Gobbetti, Roberto

Publications and source records attributed to Gobbetti, Roberto.

4 records

DOE OSTI2015

On the three primordial numbers

Cosmological observations have provided us with the measurement of just three numbers that characterize the very early universe: 1−n{sub s} , N and ln Δ{sub R}{sup 2}. Although each of the three numbers individually carries limited information about the physics of inflation, one may hope to extract non-trivial information from relations among them. Invoking minimality, namely the absence of ad hoc large numbers, we find two viable and mutually exclusive inflationary scenarios. The first is the well-known inverse relation between 1− n{sub s} and N. The second implies a new relation between 1−n{sub s} and ln Δ{sub R}{sup 2}, which might provide us with a handle on the beginning of inflation and predicts the intriguing lower bound on the tensor-to-scalar ratio r > 0.006 (95% CL)

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2013

Large-scale anomalies from primordial dissipation

We analyze an inflationary model in which part of the power in density perturbations arises due to particle production. The amount of particle production is modulated by an auxiliary field. Given an initial gradient for the auxiliary field, this model produces a hemispherical power asymmetry and a suppression of power at low multipoles similar to those observed by WMAP and Planck in the CMB temperature. It also predicts an additive contribution to δT with support only at very small l that is aligned with the direction of the power asymmetry and has a definite sign, as well as small oscillations in the power spectrum at all l.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2013

Unwinding inflation

Higher-form flux that extends in all 3+1 dimensions of spacetime is a source of positive vacuum energy that can drive meta-stable eternal inflation. If the flux also threads compact extra dimensions, the spontaneous nucleation of a bubble of brane charged under the flux can trigger a classical cascade that steadily unwinds many units of flux, gradually decreasing the vacuum energy while inflating the bubble, until the cascade ends in the self-annihilation of the brane into radiation. With an initial number of flux quanta Q{sub 0}∼>N, this can result in N efolds of inflationary expansion while producing a scale-invariant spectrum of adiabatic density perturbations with amplitude and tilt consistent with observation. The power spectrum has an oscillatory component that does not decay away during inflation, relatively large tensor power, and interesting non-Gaussianities. Unwinding inflation fits naturally into the string landscape, and our preliminary conclusion is that consistency with observation can be attained without fine-tuning the string parameters. The initial conditions necessary for the unwinding phase are produced automatically by bubble formation, so long as the critical radius of the bubble is smaller than at least one of the compact dimensions threaded by flux.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

Analyzing cosmic bubble collisions

We develop a set of controlled, analytic approximations to study the effects of bubble collisions on cosmology. We expand the initial perturbation to the inflaton field caused by the collision in a general power series, and determine its time evolution during inflation in terms of the coefficients in the expansion. In models where the observer's bubble undergoes sufficient slow-roll inflation to solve the flatness problem, in the thin wall limit only one coefficient in the expansion is relevant to observational cosmology, allowing nearly model-independent predictions. We discuss two approaches to determining the initial perturbation to the inflaton and the implications for the sign of the effect (a hot or cold spot on the Cosmic Microwave Background temperature map). Lastly, we analyze the effects of collisions with thick-wall bubbles, i.e. away from the thin-wall limit.

79 ASTRONOMY AND ASTROPHYSICS↗