R. de Ritis

Some aspects of the cosmological conformal equivalence between the `Jordan frame' and the `Einstein frame'

The conformal equivalence between the Jordan and the Einstein frames can be used in order to search for exact solutions in general theories of gravity in which scalar fields are coupled with geometry in a standard or in a nonstandard way. In the cosmological arena a relevant role is played by the time parameter in which dynamics is described. In this paper we discuss the problem of conformal equivalence between a given nonstandard coupled model and the corresponding standard one. We analyse in detail the cosmological case and we see that, together with this, and through a careful analysis of the cosmological parameters and , it is possible to contribute to the discussion on which is the physical system.

Microlensing search towards M31

We present the first results of the analysis of data collected during the 1998-99 observational campaign at the 1.3 meter McGraw-Hill Telescope, towards the Andromeda galaxy (M 31), aimed to detect gravitational microlensing effects as a probe for the presence of dark matter in our Galaxy and in the M 31 halo. The analysis is performed using the pixel lensing technique, which consists of the study of flux variations of unresolved sources and has been proposed and implemented by the AGAPE collaboration. We carry out a shape analysis by demanding that the detected flux variations be achromatic and compatible with a Paczynski light curve. We apply the Durbin-Watson hypothesis test to the residuals. Furthermore, we consider the background of variables sources. Finally five candidate microlensing events emerge from our selection. Comparing with the predictions of a Monte Carlo simulation, assuming a standard spherical model for the M 31 and Galactic haloes, and typical values for the MACHO mass, we find that our events are only marginally consistent with the distribution of observable parameters predicted by the simulation.

Recovering the effective cosmological constant in extended gravity theories

In the framework of extended gravity theories, we discuss the meaning of a time-dependent “cosmological constant” and give a set of conditions to recover an asymptotic de Sitter behaviour for a class of cosmological models independently of initial data. To this purpose we introduce a time-dependent (effective) quantity which asymptotically becomes the true cosmological constant. We will deal with scalar-tensor, fourth and higher than fourth-order theories.

Distances in inhomogeneous quintessence cosmology

We investigate the properties of cosmological distances in locally inhomogeneous universes with pressureless matter and dark energy (quintessence), with constant equation of state, , . We give exact solutions for angular diameter distances in the empty beam approximation. In this hypothesis, the distance–redshift equation is derived from the multiple-lens-plane theory. The case of a flat universe is considered with particular attention. We show how this general scheme makes distances degenerate with respect to wX and the smoothness parameters αi, accounting for the homogeneously distributed fraction of energy of the i-components. We analyse how this degeneracy influences the critical redshift where the angular diameter distance takes its maximum, and discuss future prospects for measuring the smoothness parameter of the pressureless matter, αM.

Approximate angular diameter distance in a locally inhomogeneous universe with nonzero cosmological constant

We derive and solve exactly the Dyer-Roeder equation in a Friedman-Robertson-Walker cosmological model with non zero cosmological constant. To take into account non homogeneous distribution of matter we use the phenomenological clumpiness parameter. We propose also a general form of an approximate solution which is simple enough to be useful in practical applications and sufficiently accurate in the interesting range of redshifts.We discuss the general and approximate angular diameter distance in the Friedman-Robertson-Walker cosmological models with nonzero cosmological constant. We modify the equation for the angular diameter distance by taking into account the fact that locally the distribution of matter is non homogeneous. We present exact solutions of this equation in a few special cases. We propose an approximate analytic solution of this equation which is simple enough and sufficiently accurate to be useful in practical applications.

Recovering the Cosmological Constant in Scalar-Tensor Cosmologies

We extend the cosmic no-hair theorem to a general class of scalar-tensor nonminimally coupled theories of gravity where ordinary matter is also present in the form of a perfect fluid. We give a set of conditions for obtaining an asymptotic de Sitter expansion, independently of any initial data, by a sort of time-dependent (effective) cosmological constant. Finally we apply the results to some specific models.

Nöther’s symmetries in fourth-order cosmologies

SummaryWe apply the so-called Nöther Symmetry Approach to a generic fourth-order theory of gravityf(R) to integrate the dynamics connected with the pointlike FRW cosmological Lagrangian. We get interesting results whenf(R)≽R3/2, since in this case the equations of motion are exactly solvable and solutions are physically relevant.

Scalar fields and matter cosmologies

Abstract We have extended the FRW scalar field cosmology including matter. We have discussed exact solutions for potentials like V(ϕ)=0 and (only in presence of dust) V(ϕ)=αeηϕ+e-ηϕ.

EXACT SOLUTIONS IN BRANS-DICKE MATTER COSMOLOGIES

We investigate Brans-Dicke cosmology with perfect-fluid matter and give a method for selecting exact solutions parametrized by the constant γ of the state equation p=(γ−1)ρ and the parameter ξ of the nonminimal coupling in which the Brans-Dicke theory can be recast. In this sense, we can classify the solutions having and not having inflationary behavior in the presence of ordinary matter.

New exact solutions of cosmological equations with considerations upon the cosmological constant

Abstract The cosmological equations of a gravitational field, minimally coupled witha scalar field, are exactly integrated, provided that the potential of the scalar field belongs to a special class of exponentials. A particular solution is given, exhibiting the damping of the effective cosmological constant and a power-law inflation.