arXiv subjects

J. C. Carvalho

Publications and source records attributed to J. C. Carvalho.

21 records · Page 2Linked to original sources

arXiv2004

Nonsingular FRW cosmology and nonlinear electrodynamics

The possibility to avoid the cosmic initial singularity as a consequence of nonlinear effects on the Maxwell eletromagnetic theory is discussed. For a flat FRW geometry we derive the general nonsingular solution supported by a magnetic field plus a cosmic fluid and a nonvanishing vacuum energy density. The nonsingular behavior of solutions with a time-dependent $Λ(t)$-term are also examined. As a general result, it is found that the functional dependence of $Λ(t)$ can uniquely be determined only if the magnetic field remains constant. All these models are examples of bouncing universes which may exhibit an inflationary dynamics driven by the nonlinear corrections of the magnetic field.

astro-ph↗

arXiv1999

Quantum corrections to Maxwell electrodynamics in a homogeneous and isotropic universe with cosmological constant

Some cosmological consequences of first order quantum corrections to Maxwell electrodynamics are investigated in the context of a spatially flat homogeneous and isotropic universe driven by a magnetic field plus a cosmological term $Λ$. For a vanishing $Λ$, we derive the general solution corresponding to the particular one recently found by Novello et al. [gr-qc/9806076]. We also find a general solution for the case when $Λ$ is a non-vanishing constant. Both solutions describe a non-singular, bouncing universe. However, we show that the first order correction to the electromagnetic Lagrangean density, in which the analysis is based, fails to describe the dynamics near $a_{min}$. The time range where the first order approximation can be used is explicitly evaluated. These problems my be circumvented through the use of higher order terms in the effective Lagrangean, as numerical calculations performed by Novello et al. [gr-qc/9809080], for the vanishing $Λ$ case, have indicated. A third general solution corresponding to a constant magnetic field sustained by a time dependent $Λ$ is derived. This latter solution is capable of describing the whole cosmic history and describes a universe that, although with vanishing curvature (K=0), has a scale factor that reaches a maximum and then contracts back to an arbitrarilly small size. The cosmological term decays during the initial expansion phase and increases during the late contraction phase, so as to keep $B$ constant throughout.

astro-ph↗

arXiv1999

Cosmological gravitons and the expansion dynamics during the matter age

Extending previous results [Phys. Rev.D {\bf 56}, 6351 (1997)], we estimate the back reaction of cosmological gravitons in the expansion dynamics, during the matter age. Tensor perturbations with scales larger than the Hubble radius are created due to the inequivalence of vacua at different times. During noninflationary phases these perturbations become effective gravitational waves as they enter the Hubble radius, adding new contributions to the energy density of the subhorizon waves. During the radiation epoch the creation of these effective gravitons may lead to a departure from the standard behaviour $a(t)\propto t^{1/2}$, with possible consequences to several cosmic processes, such as primordial nucleosynthesis. We examine the implications of this phenomenom during the matter dominated era, assuming an initial inflationary period. The dynamical equation obeyed by the scale factor is derived and numerically solved for different values of the relevant parameters involved.

astro-ph↗