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,

Sound wave propagation in a relativistic perfect fluid with a non-homogeneous isentropic flow is studied in terms of acoustic geometry. The sound wave equation turns out to be equivalent to the equation of motion for a massless scalar field propagating in a curved space-time geometry. The geometry is described by the acoustic metric tensor that depends locally on the equation of state and the four-velocity of the fluid. For a relativistic supersonic flow in curved space-time the ergosphere and acoustic horizon may be defined in a way analogous to the non-relativistic case. A general-relativistic expression for the acoustic analog of surface gravity has been found.

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.

Transonic accretion onto astrophysical objects is a unique example of analogue black hole realized in nature. In the framework of acoustic geometry we study axially symmetric accretion and wind of a rotating astrophysical black hole or of a neutron star assuming isentropic flow of a fluid described by a polytropic equation of state. In particular we analyze the causal structure of multitransonic configurations with two sonic points and a shock. Retarded and advanced null curves clearly demonstrate the presence of the acoustic black hole at regular sonic points and of the white hole at the shock. We calculate the analogue surface gravity and the Hawking temperature for the inner and outer acoustic horizons.

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