By combining data from seven cosmic microwave background experiments (including the latest WMAP results) with large scale structure data, the Hubble parameter measurement from the Hubble Space Telescope and luminosity measurements of Type Ia supernovae we demonstrate the bounds on the dark energy equation of state wQ to be −1.38 < wQ < −0.82 at the 95% confidence level. Although our limit on wQ is improved with respect to previous analyses, cosmological data does not rule out the possibility that the equation of state parameter wQ of the dark energy Q is less than -1. We present a tracking model that ensures wQ ≤ −1 at recent times and discuss the observational consequences.

As yet, there is no underlying fundamental theory for the trans-Planckian regime. There is a need to address the issue of how the observables in our present Universe are affected by processes that may have occurred at super-Planckian energies ͑referred to as the trans-Planckian regime͒. Specifically, we focus on the impact the trans-Planckian regime has on two observables: namely, dark energy and the cosmic microwave background radiation ͑CMBR͒ spectrum. We model the trans-Planckian regime by introducing a 1-parameter family of smooth non-linear dispersion relations which modify the frequencies at very short distances. A particular feature of the family of dispersion functions chosen is the production of ultralow frequencies at very high momenta k ͑for kϾM P ). We name the range of the ultralow energy modes ͑of very short distances͒ that have frequencies equal to or less than the current Hubble rate H 0 as the tail modes. These modes are still frozen today due to the expansion of the Universe. We calculate their energy today and show that the tail provides a strong candidate for the dark energy of the Universe. During inflation, their energy is about 122 to 123 orders of magnitude smaller than the total energy, for any random value of the free parameter in the family of dispersion relations. For this family of dispersions, we present the exact solutions and show that the CMBR spectrum is that of a ͑nearly͒ blackbody, and that the adiabatic vacuum is the only choice for the initial conditions.

Recent analysis of the combined data of cosmic microwave background, galaxy clustering and supernovae type Ia observations have set strong constraints on the equation of state parameter wX . The upper bound wX < −0.82 at 95% c.l. rules out an important class of models, the domain walls (−2/3 < wX < −1/3). Here we revisit the issue of domain walls as a possible alternative to the standard Λ-CDM model by questioning the assumptions made in the choice of priors of the data analysis. The results of our investigation show that domain walls can provide a good fit to the WMAP data for a different choice of priors with "lower" values of the Hubble parameter (h < 0.65), (as indicated by Sunyaev-Zeldovich and time delays for gravitational lensing observations), and "higher" values of the matter density (Ωm > 0.35), (in agreement with recent measurements of the temperature-luminosity relation of distant clusters observed with the XMM-Newton satellite). In this new perspective, their existence would lead to important implications for the CMB constraints on cosmological and inflationary parameters.

A long-standing problem of theoretical physics is the exceptionally small value of the cosmological constant ⌳ϳ10 Ϫ120 measured in natural Planckian units. Here we suggest how this tiny number might arise from a toroidal string cosmology based on closed strings. In this picture the dark energy would arise from a correlation between momentum and winding modes that for short distances has an exponential fall-off with increasing values of the momenta. The freeze-out by the expansion of the background universe for these trans-Planckian modes might be interpreted as a frozen condensate of the closed-string modes in the three noncompactified spatial dimensions. Our qualitative arguments are heuristic in nature and published only because they may suggest an improved treatment of the connection between strings and cosmological expansion.

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