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S. E. Joras

Publications and source records attributed to S. E. Joras.

8 recordsLinked to original sources
arXiv2013

$γ$ gravity: Steepness control

We investigate a simple generalization of the metric exponential $f(R)$ gravity theory that is cosmologically viable and compatible with solar system tests of gravity. We show that, as compared to other viable $f(R)$ theories, its steep dependence on the Ricci scalar $R$ facilitates agreement with structure constraints, opening the possibility of $f(R)$ models with equation-of-state parameter that could be differentiated from a cosmological constant ($w_{de}=-1$) with future surveys at both background and perturbative levels.

astro-ph.CO↗
arXiv2009

Trans-Planckian Physics from a Nonlinear Dispersion Relation

We study a particular nonlinear dispersion relation $ω_p(k_p)$ -- a series expansion in the physical wavenumber $k_p$ -- for modeling first-order corrections in the equation of motion of a test scalar field in a de Sitter spacetime from trans-Planckian physics in cosmology. Using both a numerical approach and a semianalytical one, we show that the WKB approximation previously adopted in the literature should be used with caution, since it holds only when the comoving wavenumber $k\gg aH$. We determine the amplitude and behavior of the corrections on the power spectrum for this test field. Furthermore, we consider also a more realistic model of inflation, the power-law model, using only a numerical approach to determine the corrections on the power spectrum.

gr-qc↗
arXiv2003

Entropy perturbations in quartessence Chaplygin models

We show that entropy perturbations can eliminate instabilities and oscillations, in the mass power spectrum of the quartessence Chaplygin models. Our results enlarge the current parameter space of models compatible with large scale structure and cosmic microwave background (CMB) observations.

astro-ph↗
arXiv2002

Improved semiclassical density matrix: taming caustics

We present a simple method to deal with caustics in the semiclassical approximation to the thermal density matrix of a particle moving on the line. For simplicity, only its diagonal elements are considered. The only ingredient we require is the knowledge of the extrema of the Euclidean action. The procedure makes use of complex trajectories, and is applied to the quartic double-well potential.

quant-ph↗
arXiv1999

Dynamical Viscosity of Nucleating Bubbles

We study the viscosity corrections to the growth rate of nucleating bubbles in a first order phase transition in scalar field theory. We obtain the non-equilibrium equation of motion of the coordinate that describes small departures from the critical bubble and extract the growth rate consistently in weak coupling and in the thin wall limit. Viscosity effects arise from the interaction of this coordinate with the stable quantum and thermal fluctuations around a critical bubble. In the case of 1+1 dimensions we provide an estimate for the growth rate that depends on the details of the free energy functional. In 3+1 dimensions we recognize robust features that are a direct consequence of the thin wall approximation and give the leading viscosity corrections.These are long-wavelength hydrodynamic fluctuations that describe surface waves, quasi-Goldstone modes which are related to ripples on interfaces in phase ordered Ising-like systems. We discuss the applicability of our results to describe the growth rate of hadron bubbles in a quark-hadron first order transition.

hep-ph↗
arXiv1998

Semiclassical Series at Finite Temperature

We derive the semiclassical series for the partition function of a one-dimensional quantum-mechanical system consisting of a particle in a single-well potential. We do this by applying the method of steepest descent to the path-integral representation of the partition function, and we present a systematic procedure to generate the terms of the series using the minima of the Euclidean action as the only input. For the particular case of a quartic anharmonic oscillator, we compute the first two terms of the series, and investigate their high and low temperature limits. We also exhibit the nonperturbative character of the terms, as each corresponds to sums over infinite subsets of perturbative graphs. We illustrate the power of such resummations by extracting from the first term an accurate nonperturbative estimate of the ground-state energy of the system and a curve for the specific heat. We conclude by pointing out possible extensions of our results which include field theories with spherically symmetric classical solutions.

quant-ph↗
arXiv1994

Minimal Closed Set of Observables in the Theory of Cosmological Perturbations

The theory of perturbation of Friedman-Robertson-Walker (FRW) cosmology is analysed exclusively in terms of observable quantities. Although this can be a very complete and general procedure we limit our presentation here to the case of irrotational perturbations for simplicity. We show that the electric part of Weyl conformal tensor {\bf $E$} and the shear {\bf $Σ$} constitute the two basic perturbed variables in terms of which all remaining observable quantities can be described. Einstein\rq s equations of General Relativity reduce to a closed set of dynamical system for {\bf $E$} and {\bf $Σ$}. The basis for a gauge-invariant Hamiltonian treatment of the Perturbation Theory in the FRW background is then set up.

gr-qc↗