Alfredo B. Henriques

Stochastic gravitational waves from a new type of modified Chaplygin gas

We propose a new scenario for the early Universe where there is a smooth transition between an early de Sitter-like phase and a radiation-dominated era. In this model, the matter content is modeled by a new type of generalized Chaplygin gas [6] for the early Universe, with an underlying scalar field description. We study the gravitational waves generated by the quantum fluctuations. In particular, we calculate the gravitational-wave power spectrum, as it would be measured today, following the method of the Bogoliubov coefficients. We show that the high frequencies region of the spectrum depends strongly on one of the parameters of the model. On the other hand, we use the number of e folds, along with the power spectra and spectral index of the scalar perturbations, to constrain the model observationally.

Loop quantum cosmology and the Wheeler–De Witt equation

We present some results concerning the large volume limit of loop quantum cosmology in the flat homogeneous and isotropic case. We derive the Wheeler–De Witt equation in this limit. Looking for the action from which this equation can also be obtained, we then address the problem of the modifications to be brought to the Friedman’s equation and to the equation of motion of the scalar field, in the classical limit.

Cosmic microwave background and parametric resonance in reheating

The variation of the perturbative 3-curvature parameter, \zeta, is investigated in the period of reheating after inflation. The two-field model used has the inflaton, with an extra scalar field coupled to it, and non-linear effects of both fields are included as well as a slow decay mechanism into the hydrodynamic fluid of the radiation era. Changes in \zeta occur and persist into the succeeding cosmic eras to influence the generation of the cosmic microwave background fluctuations.

Inflation in a simple Kantowski-Sachs model

Abstract In this work we study a cosmological model based on the Kantowski-Sachs metric with an exponential potential. We investigate the initial conditions leading to inflationary solutions, their asymptotic stability and the role played by viscous forces representing the production of particles by the scalar field. We find that viscosity enlarges the range of initial conditions leading to inflation.

Unification of inflation, dark energy, and dark matter within the Salam-Sezgin cosmological model

We investigate a cosmological model, based on the Salam-Sezgin six-dimensional supergravity theory and on previous work by Anchordoqui, Goldberg, Nawata, and Nunez. Assuming a period of warm inflation, we show that it is possible to extend the evolution of the model back in time, to include the inflationary period, thus unifying inflation, dark matter, and dark energy within a single framework. Like the previous authors, we were not able to obtain the full dark matter content of the universe from the Salam-Sezgin scalar fields. However, even if only partially successful, this work shows that present-day theories, based on superstrings and supergravity, may eventually lead to a comprehensive modeling of the evolution of the universe. We find that the gravitational-wave spectrum of the model has a nonconstant negative slope in the frequency range (10{sup -15}-10{sup 6}) rad/s, and that, unlike standard (cold) inflation models, it shows no structure in the MHz/GHz range of frequencies.

Exact calculation of the energy density of cosmological gravitational waves.

In this paper we calculate the Bogoliubov coefficients and the energy density of the stochastic gravitational wave background for a universe that undergoes inflation followed by radiation domination and matter domination, using a formalism that gives the Bogoliubov coefficients as continuous functions of time. By making a reasonable assumption for the equation of state during reheating, we obtain in a natural way the expected high frequency cutoff in the spectral energy density.

The stochastic gravitational - wave background and the inflation to radiation transition in the early universe

In this paper we calculate the gravitational-wave power-spectra corresponding to different types of transitions between the inflationary regime and the radiation era. We study four cases, where inflation is followed by a stiff matter phase, or by a dust dominated phase, or by a combination of the two, before the universe enters the radiation era. Use is made of differential equations for the Bogoliubov coefficients, allowing us to model all the transitions between the different eras as continuous. New features appear, for frequencies above 10^{7} rad/s in an otherwise flat part of the spectrum.

The Spectrum of Gravitational Waves in an f ( R ) Model with a Bounce

We present an inflationary model preceded by a bounce in a metric

The classical tests of cosmology with a simple Kantowski-Sachs model

f(R)

Boson-fermion stars: going to larger boson masses

theory. In this model, modified gravity affects only the early stages of the universe. We analyse the predicted spectrum of the gravitational waves in this scenario using the method of the Bogoliubov coefficients. We show that there are distinctive (oscillatory) signals on the spectrum for very low frequencies; i.e., corresponding to modes that are currently entering the horizon.