We extend the world model of Kamenshchik et al. to large perturbations by formulating a Zeldovich-like approximation. We sketch how this model unifies dark matter with dark energy in a geometric setting reminiscent of M -theory.After nearly two decades of reign [1], the Einstein-de Sitter dust model has been swept aside by observations of high redshift supernovae [2] which suggest that the Hubble expansion is accelerating. When combined with the Boomerang/Maxima data [3] showing that the location of the first acoustic peak in the power spectrum of the microwave background is consistent with the inflationary prediction Ω = 1, the evidence for a net equation of state of the cosmic fluid lying in the range −1 ≤ w = P/ρ < −1/3 is compelling. Parametrically, w = P DE /(ρ DE + ρ DM ) = −Ω Λ gives a ratio of unclustered dark energy to clustered dark matter of order 7:3, thereby also resolving the longstanding Ω DM < 1 puzzle [1,4] implied by peculiar velocity fields. The theoretical implications of these dicoveries are profound. Simply appending a † Permanent address: Rudjer Bošković Institute,

We consider the prospects for dark matter/energy unification in k-essence type cosmologies. General mappings are established between the k-essence scalar field, the hydrodynamic and braneworld descriptions. We develop an extension of the general relativistic dust model that incorporates the effects of both pressure and the associated acoustic horizon. Applying this to a tachyon model, we show that this inhomogeneous "variable Chaplygin gas" does evolve into a mixed system containing cold dark matter like gravitational condensate in significant quantities. Our methods can be applied to any dark energy model, as well as to mixtures of dark energy and traditional dark matter.

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We construct new gravitational vacuum star solutions with a Born-Infeld phantom replacing the de Sitter interior. The model allows for a wide range of masses and radii required by phenomenology, and can be motivated from low-energy string theory.

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The Thomas-Fermi model at nite temperature is extended to describe a system of self-gravitating weakly interacting massive fermions in a general-relativistic framework. The existence and properties of the gravitational phase transition in this model are investigated numerically. It is shown that, by cooling a nondegenerate gas of weakly interacting massive fermions below some critical temperature, a condensed phase emerges, consisting of quasidegenerate fermion stars. For fermion masses of 10 to 25 keV, these fermion stars may very well provide an alternative explanation for the supermassive compact dark objects that are observed at galactic centers.

Heavy-neutrino (or neutralino) stars are studied using the general relativistic equations of hydrostatic equilibrium and the relativistic equation of state for degenerate fermionic matter. The Tolman-Oppenheimer-Volkoff equations are then generalized to include a system of degenerate neutrino and neutralino matter that is gravitationally coupled. The properties and implications of such an interacting astrophysical system are discussed in detail. 04.40.Dg, 97.20.Rp, 95.35.+d, 97.60.Jd, 98.35.Jk

It is shown that the matter concentration observed through stellar motion at the galactic center (Eckart & Genzel, 1997, MNRAS, 284, 576 and Genzel et al., 1996, ApJ, 472, 153) is consistent with a supermassive object of 2.5 × 10 6 solar masses composed of self-gravitating, degenerate heavy neutrinos, as an alternative to the black hole interpretation. According to the observational data, the lower bounds on possible neutrino masses are m ν ≥ 12.0 keV/c 2 for g = 2 or m ν ≥ 14.3 keV/c 2 for g = 1, where g is the spin degeneracy factor. The advantage of this scenario is that it could naturally explain the low X-ray and gamma ray activity of Sgr A * , i.e. the so called "blackness problem" of the galactic center.