In general relativity, the energy conditions are invoked to restrict general energy-momentum tensors Tµν in different frameworks, and to derive general results that hold in a variety of general contexts on physical grounds. We show that in the standard Friedmann-Lemaître-Robertson-Walker (FLRW) approach, where the equation of state of the cosmological fluid is unknown, the energy conditions provide model-independent bounds on the behavior of the distance modulus of cosmic sources as a function of the redshift for any spatial curvature. We use the most recent type Ia supernovae (SNe Ia) observations, which include the new Hubble Space Telescope SNe Ia events, to carry out a model-independent analysis of the energy conditions violation in the context of the standard cosmology. We show that both the null (NEC), weak (WEC) and dominant (DEC) conditions, which are associated with the existence of the so-called phantom fields, seem to have been violated only recently (z 0.2), whereas the condition for attractive gravity, i.e., the strong energy condition (SEC) was firstly violated billions of years ago, at z 1.PACS numbers: 98.80. Es, 98.80.Jk

In general relativity, the energy conditions are invoked to restrict general energy-momentum tensors on physical grounds. We show that in the standard Friedmann-Lemaître-Robertson-Walker (FLRW) approach to cosmological modeling, where the energy and matter components of the cosmic fluid are unknown, the energy conditions provide model-independent bounds on the behavior of the lookback time of cosmic sources as a function of the redshift for any value of the spatial curvature. We derive and confront such bounds with a lookback time sample which is built from the age estimates of 32 galaxies lying in the interval 0.11 z 1.84 and by assuming the total expanding age of the Universe to be 13.7 ± 0.2 Gyr, as obtained from current cosmic microwave background experiments. In agreement with previous results, we show that all energy conditions seem to have been violated at some point of the recent past of cosmic evolution.

The observed late-time acceleration of the Universe may be the result of unknown physical processes involving either modifications of gravitation theory or the existence of new fields in high energy physics. In the former case, such modifications are usually related to the possible existence of extra dimensions (which is also required by unification theories), giving rise to the so-called brane cosmology. In this paper we investigate the viability of this idea by considering a particular class of brane scenarios in which a large scale modification of gravity arises due to a gravitational leakage into extra dimensions. To this end, differently from other recent analyses, we combine orthogonal age and distance measurements at intermediary and high redshifts. We use observations of the lookback time to galaxy clusters, indirect estimates of the age of the Universe from the most recent Large-Scale Structure (LSS) and Cosmic Microwave Background (CMB) data, along with the recent detection of the baryon acoustic oscillations at z = 0.35. In agreement with other recent analyses we show that, although compatible with these age and distance measurements, a spatially closed scenario is largely favoured by the current observational data. By restricting our analysis to a spatially flat universe, we also find that the standard ΛCDM model is favoured over the particular braneworld scenario here investigated.

Cosmic acceleration may be the result of unknown physical processes involving either new fields in high energy physics or modifications of gravitation theory. In the latter case, such modifications are usually related to the existence of extra dimensions (which is also required by unification theories), giving rise to the so-called brane cosmology. In this paper we investigate the phenomenon of the acceleration of the Universe in a particular class of brane scenarios in which a large scale modification of gravity arises due to a gravitational \emph{leakage} into extra dimensions. By using the most recent supernova observations we study the transition (deceleration/acceleration) epoch as well as the constraints imposed on the parameters characterizing the model. We show that these models provide a good description for the current supernova data, which may be indicating that the existence of extra dimensions play an important role not only in fundamental physics but also in cosmology.Comment: 4 pages, 3 figures, to appear in Phys. Rev.

In this paper, we use the cosmokinematics approach to study the accelerated expansion of the Universe. This is a model independent approach and depends only on the assumption that the Universe is homogeneous and isotropic and is described by the FRW metric. We parametrize the deceleration parameter, q(z), to constrain the transition redshift (z t ) at which the expansion of the Universe goes from a decelerating to an accelerating phase. We use three different parametrizations of q(z) namely, q I (z) = q 1 + q 2 z, q II (z) = q 3 + q 4 ln(1 + z) and q III (z) = 1 2 + q 5 (1+z) 2 . A joint analysis of the age of galaxies, strong lensing and supernovae Ia data indicates that the transition redshift is less than unity i.e. z t < 1. We also use a nonparametric approach (LOESS+SIMEX) to constrain z t . This too gives z t < 1 which is consistent with the value obtained by the parametric approach.