Dark Energy: A Unifying View

Different models of the cosmic substratum which pretend to describe the present stage of accelerated expansion of the Universe like the $\Lambda$CDM model or a Chaplygin gas, can be seen as special realizations of a holographic dark energy cosmology if the option of an interaction between pressurless dark matter and dark energy is taken seriously. The corresponding interaction strength parameter plays the role of a cosmological constant. Differences occur at the perturbative level. In particular, the pressure perturbations are intrinsically non-adiabatic.Comment: 9 pages, selected for "Honorable Mention" by the Gravity Research Foundatio

Cosmic evolution during primordial black hole evaporation

Primordial black holes with a narrow mass range are regarded as a nonrelativistic fluid component with an equation of state for dust. The impact of the black hole evaporation on the dynamics of the early universe is studied by resorting to a two-fluid model. We find periods of intense radiation reheating in the initial and final stages of the evaporation.Comment: 12 pages, Revtex, two figures, to appear in Phys.Rev.

Cosmological two-fluid thermodynamics

We reveal unifying thermodynamic aspects of so different phenomena as the cosmological electron-positron annihilation, the evaporation of primordial black holes with a narrow mass range, and the ``deflationary'' transition from an initial de Sitter phase to a subsequent standard Friedmann-Lema\^{\i}tre-Robertson-Walker begin (FLRW) behavior.Comment: 11 pages, submitted to GRG (issue in honor of Heinz Dehnen

Statefinder parameters for interacting dark energy

We argue that the recently introduced "statefinder parameters" (Sahni et al., JETP Lett. 77, 201 (2003)), that include the third derivative of the cosmic scale factor, are useful tools to characterize interacting quitessence models. We specify the statefinder parameters for two classes of models that solve, or at least alleviate, the coincidence problem.Comment: 11 pages, two encapsulated eps figures, key words: cosmology, accelerated expansion, statefinder parameter

Self-interacting gas in a gravitational wave field

We investigate a relativistic self-interacting gas in the field of an external {\it pp} gravitational wave. Based on symmetry considerations we ask for those forces which are able to compensate the imprint of the gravitational wave on the macroscopic 4-acceleration of the gaseous fluid. We establish an exactly solvable toy model according to which the stationary states which characterize such a situation have negative entropy production and are accompanied by instabilities of the microscopic particle motion. These features are similar to those which one encounters in phenomena of self-organization in many-particle systems.Comment: 17 pages, to be published in the GRG-Journa

Inflation in a self-interacting gas universe

We show that a de Sitter spacetime is a solution of Einstein's field equations with the energy momentum tensor of a self-interacting, classical Maxwell-Boltzmann gas in collisional equilibrium. The self-interaction is described by a four-force which is quadratic in the (spatially projected) particle four-momenta. This force does not preserve the particle number and gives rise to an exponential increase in the comoving entropy of the universe while the temperature of the latter remains constant. These properties of a gas universe are related to the existence of a ``projector-conformal'' timelike Killing vector representing a symmetry which is ``in between'' the symmetries characterized by a Killing vector and those characterized by a conformal Killing vector.Comment: 10 pages, Revtex, Journal reference: Phys.Rev.D58 063503 (1998

Holographic Dark Energy and Present Cosmic Acceleration

We review the notion of holographic dark energy and assess its significance in the light of the well documented cosmic acceleration at the present time. We next propose a model of holographic dark energy in which the infrared cutoff is set by the Hubble scale. The model accounts for the aforesaid acceleration and, by construction, is free of the cosmic coincidence problem.Comment: 6 pages. To appear in the Proceedings of the XXVIII Spanish Relativity Meeting. Key words: Cosmology, Holography, Late accelerated expansion, Dark energ