Reconstructing the interaction term between dark matter and dark energy

We apply a parametric reconstruction method to a homogeneous, isotropic and spatially flat Friedmann-Robertson-Walker (FRW) cosmological model filled of a fluid of dark energy (DE) with constant equation of state parameter interacting with dark matter (DM). The reconstruction method is based on expansions of the general interaction term and the relevant cosmological variables in terms of Chebyshev polynomials which form a complete set orthonormal functions. This interaction term describes an exchange of energy flow between the DE and DM within dark sector. To show how the method works we do the reconstruction of the interaction function expanding it in terms of only the first three Chebyshev polynomials and obtain the best estimation for the coefficients of the expansion as well as for the DE equation of the state constant parameter w using the type Ia Supernova SCP Union data set (307 SNe-Ia). The preliminary reconstruction shows that in the best scenario there is an energy transfer from DM to DE which worsen the problem of the cosmic coincidence in comparison with the LCDM model. We conclude that this fact is an indication of a serious drawback for the existence of such interaction between dark components.Comment: 6 pages, 2 figures, Talk presented at the Conference "Gravitational Physics: Testing gravity from submillimeter to cosmic. Playa del Carmen, Mexico (2009). To appear in Proceedings of the VIII Mexican School on Gravitation and Mathematical Physics, AIP Conference Proceeding

Black Holes with Zero Mass

We consider the spacetimes corresponding to static Global Monopoles with interior boundaries corresponding to a Black Hole Horizon and analyze the behavior of the appropriate ADM mass as a function of the horizon radius r_H. We find that for small enough r_H, this mass is negative as in the case of the regular global monopoles, but that for large enough r_H the mass becomes positive encountering an intermediate value for which we have a Black Hole with zero ADM mass.Comment: 10 pages, 2 ps figures, REVTeX, some minor change

Constraining a matter-dominated cosmological model with bulk viscosity proportional to the Hubble parameter

We present and constrain a cosmological model where the only component is a pressureless fluid with bulk viscosity as an explanation for the present accelerated expansion of the universe. We study the particular model of a bulk viscosity coefficient proportional to the Hubble parameter. The model is constrained using the SNe Ia Gold 2006 sample, the Cosmic Microwave Background (CMB) shift parameter R, the Baryon Acoustic Oscillation (BAO) peak A and the Second Law of Thermodynamics (SLT). It was found that this model is in agreement with the SLT using only the SNe Ia test. However when the model is constrained using the three cosmological tests together (SNe+CMB+BAO) we found: 1.- The model violates the SLT, 2.- It predicts a value of H_0 \approx 53 km sec^{-1} Mpc^{-1} for the Hubble constant, and 3.- We obtain a bad fit to data with a \chi^2_{min} \approx 532. These results indicate that this model is viable just if the bulk viscosity is triggered in recent times.Comment: 5 pages, 2 eps figures. Work presented in the III International Meeting on Gravitation and Cosmology, Morelia, Mexico, May 26-30, 2008. Submitted to AIP Conference Proceedings of this conference. Related work: arXiv:0801.168

The violation of the weak energy condition, Is it generic of spontaneous scalarization ?

It was recently shown by Whinnett & Torres \cite{WT} that the phenomenon of spontaneous scalarization (SC) in compact objects (polytropes) was accompanied also by a {\it spontaneous violation of the weak energy condition} (WEC). Notably, by the encounter of negative-energy densities at several star places as measured by a static observer. Here we argue that the negativeness of such densities is not generic of scalar tensor theories of gravity (STT). We support this conclusion by numerical results within a class of STT and by using three realistic models of dense matter. However, we show that the ``angular parts'' of the additional conditions for the WEC to hold $\rho_{\rm eff} + T^{i ({\rm eff})}_{i} \geq 0$ tend to be ``slightly violated'' at the outskirts of the star.Comment: 9 pages; 3 figures; Submitted to Phys. Rev.