We explore the cosmological implications of an ultra-light pseudo-NambuGoldstone boson. With global spontaneous symmetry breaking scale f 10 18 GeV and explicit breaking scale comparable to MSW neutrino masses, M 10 3 eV, such a eld, which acquires a mass m M 2 =f H 0 , w ould have become dynamical at recent epochs and currently dominate the energy density of the universe. The eld acts as an e ective cosmological constant for several expansion times and then relaxes into a condensate of coherent non-relativistic bosons. Such a model can reconcile dynamical estimates of the density parameter, m 0:2, with a spatially at universe, and can yield an expansion age H 0 t 0 1 while remaining consistent with limits from gravitational lens statistics. PACS numbers: 98.80. Cq, 98.70.Vc Typeset using REVT E X 1

Despite the interest in dark matter and dark energy, it has never been shown that they are in fact two separate substances. We provide the first strong evidence that they are separate by ruling out a broad class of so-called unified dark matter models that have attracted much recent interest. We find that they produce oscillations or exponential blowup of the matter power spectrum inconsistent with observation. For the particular case of generalized Chaplygin gas models, 99.999% of the previously allowed parameter space is excluded, leaving essentially only the standard Lambda-CDM limit allowed

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A cosmological model that aims at solving the coincidence problem should show that dark energy and dark matter follow the same scaling solution from some time onward. At the same time, the model should contain a sufficiently long matter-dominated epoch that takes place before acceleration in order to guarantee a decelerated epoch and structure formation. So a successful cosmological model requires the occurrence of a sequence of epochs, namely a radiation era, a matter-dominated era and a final accelerated scaling attractor with Ωϕ ≃ 0.7. In this paper we derive the generic form of a scalar-field Lagrangian that possesses scaling solutions in the case where the coupling Q between dark energy and dark matter is a free function of the field ϕ. We then show, rather surprisingly, that the aforementioned sequence of epochs cannot occur for a vast class of generalized coupled scalar field Lagrangians that includes, to our knowledge, all scaling models in the current literature.

We explore the implications of type Ia supernovae (SNIa) observations on flat cosmological models whose matter content is an exotic fluid with equation of state, p = −M 4(α+1) /ρ α . In this scenario, a single fluid component may drive the Universe from a nonrelativistic matter dominated phase to an accelerated expansion phase behaving, first, like dark matter and in a more recent epoch as dark energy. We show that these models are consistent with current SNIa data for a rather broad range of parameters. However, future SNIa experiments will place stringent constraints on these models, and could safely rule out the special case of a Chaplygin gas (α = 1) if the Universe is dominated by a true cosmological constant.

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We use type Ia supernovae (SN Ia) data in combination with recent baryonic acoustic oscillations (BAO) and cosmic microwave background (CMB) observations to constrain a kink-like parametrization of the deceleration parameter (q). This q-parametrization can be written in terms of the initial (q i ) and present (q 0 ) values of the deceleration parameter, the redshift of the cosmic transition from deceleration to acceleration (z t ) and the redshift width of such transition (τ ). By assuming a flat space geometry, q i = 1/2 and adopting a likelihood approach to deal with the SN Ia data we obtain, at the 68% confidence level (C.L.), that: z t = 0.56 +0.13 −0.10 , τ = 0.47 +0.16 −0.20 and q 0 = −0.31 +0.11 −0.11 when we combine BAO/CMB observations with SN Ia data processed with the MLCS2k2 light-curve fitter. When in this combination we use the SALT2 fitter we get instead, at the same C.L.: z t = 0.64 +0.13 −0.07 , τ = 0.36 +0.11 −0.17 and q 0 = −0.53 +0.17 −0.13 . Our results indicate, with a quite general and model independent approach, that MLCS2k2 favors Dvali-Gabadadze-Porrati-like cosmological models, while SALT2 favors ΛCDM-like ones. Progress in determining the transition redshift and/or the present value of the deceleration parameter depends crucially on solving the issue of the difference obtained when using these two light-curve fitters.