The Fate of the Universe: Dark Energy Dilution?

We study the possibility that dark energy decays in the future and the universe stops accelerating. The fact thatthe cosmological observations prefer an equation of state of dark energy smaller than -1 can be a signal that dark energy will decay in the future. This conclusion is based in interpreting a w<-1 as a signal of dark energy interaction with another fluid. We determine the interaction through the cosmological data and extrapolate it into the future. The resulting energy density for dark energy becomes rho=a^{-3(1+w)}e^{-\beta(a-1)}, i.e. it has an exponential suppression for a >> a_o=1. In this scenario the universe ends up dominated by this other fluid, which could be matter, and the universe stops accelerating at some time in the near future.Comment: 5 pages, 3 figure

Inflation from superstrings

We investigate the possibility of obtaining inflationary solutions of the slow roll type from a low energy Lagrangian coming from superstrings. The advantage of such an approach is that in these theories the scalar potential has only one free parameter (the Planck scale) and therefore no unnatural fine tuning may be accommodated. We find that in any viable scheme the dilaton and the moduli fields have to be stabilized and that before this happens, no other field may be used as the inflaton. Then inflation may occur due to chiral matter fields. Demanding that the potential terms associated with the chiral fields do not spoil the dilaton and moduli minimization leads to severe constraints on the magnitude of the density fluctuations.Comment: 22 pages, no figures, latex file We have corrected the magnitude of the density fluctuations, which become smaller than the COBE ones. Some references have also been added, and a few misprints correcte

A Realistic Particle Physics Dark Energy Model

We present a realistic dark energy model derived from particle physics. Our model has essentially no free parameters and has an equivalent fit to the observational data (CMB, SN1a and LSS) as LCDM and a better fit than the best effective $w(z)$ model. With the lack of a clear determination of the cosmological parameters theoretical considerations should be taken seriously to distinguish between dark energy models.Comment: 5 pages, RevTex, 6 figure

Interacting Tachyon: generic cosmological evolution for a tachyon and a scalar field

We study the cosmological evolution of a tachyon scalar field T with a Dirac-Born-Infeld type lagrangian and potential V(T) coupled to a canonically normalized scalar field \phi with an arbitrary interaction term B(T,\phi) in the presence of a barotropic fluid \rb, which can be matter or radiation. The force between the barotropic fluid and the scalar fields is only gravitational. We show that the dynamics is completely determine by only three parameters L1 = - V_T/ V^{3/2}, L2= - B_T /B^{3/2} and L3 =-B_{\phi}/B. We determine analytically theconditions for \lm_i under which the energy density of T, \phi and \rb have the same redshift. We study the behavior of T and \phi in the asymptotic limits for L_i and we show the numerical solution for different interesting cases. The effective equation of state for the tachyon field changes due to the interaction with the scalar field and we show that it is possible for a tachyon field to redshift as matter in the absence of an interaction term B and as radiation when B is turned on. This result solves then the tachyonic matter problem.Comment: 13 pages, 5 figure