Dark Energy - Dark Matter Unification: Generalized Chaplygin Gas Model

We review the main features of the generalized Chaplygin gas (GCG) proposal for unification of dark energy and dark matter and discuss how it admits an unique decomposition into dark energy and dark matter components once phantom-like dark energy is excluded. In the context of this approach we consider structure formation and show that unphysical oscillations or blow-up in the matter power spectrum are not present. Moreover, we demonstrate that the dominance of dark energy occurs about the time when energy density fluctuations start evolving away from the linear regime.Comment: 15 pages, 5 figures; Talk presented at the V New Worlds in Astroparticle Physics, Faro, Portugal, 8-10 January 2005. Reference and numbering of equations fixe

Cosmological thinking: cultural heritage and challenge

The limitations of current technology do not allow one to foresee the expansion of the humankind beyond our planet for at least a few decades. Furthermore, the laws of physics, as for as they are known, preclude any form of traveling beyond the speed of light, as well as any viable and stable space-time shortcuts (wormholes, warp-drives, etc) that would facilitate cosmic traveling. Given the vastness of the Universe these are insurmountable obstacles for any {\it in situ} exploration of the cosmos beyond our most immediate cosmic neighbourhood. Nevertheless, the Universe is transparent and contains countless sources of visible light. Actually, in the last decades, technological developments have made possible to observe the cosmos throughout most of the electromagnetic spectrum as well as to perform dynamical studies that allow perceiving the presence of invisible components such as black holes, dark matter and dark energy. In this respect, humankind has then been given the opportunity to unravel the inner workings of the cosmos and through this process be part of the cosmic habitat. In this contribution various forms of cosmological thinking will be discussed, from some myths of creation till some of the latest scientific discoveries.Comment: 18 pages, 2 figures. Talk delivered at the Third International Symposium "Fronteiras da Ci\^encia: A Humanidade e o Cosmos", 13 - 14 November 2009, Universidade Fernando Pesssoa, Oporto, Portuga

Threshold Effects and Lorentz Symmetry

Evidence on the violation of Lorentz symmetry arises from the observation of cosmic rays with energies beyond the GZK cutoff, $E_{GZK} \simeq 4 \times 10^{19} eV$, from the apparent transparency of the Universe to the propagation of high energy gamma radiation and from the stability of pions in air showers. These three paradoxes can be explained through deformations of the relativistic dispersion relation. Theoretical ideas aimed to understand how Lorentz symmetry may be broken and phenomenologically interesting deformations of the relativistic dispersion relation may arise are briefly discussed.Comment: 7 pages. Talk presented at the ``Decoherence, Information, Complexity and Entropy 2002'', Piombino, Italy, September 200

The adventures of Spacetime

We discuss how developments in physics often imply in the need that spacetime acquires an increasingly richer and complex structure. General Relativity was the first theory to show us the way to connect space and time with the physical world. Since then, scrutinizing the ways spacetime might exist is, in a way, the very essence of physics. Physics has thus given substance to the pioneering work of scores of brilliant mathematicians who speculated on the geometry and topology of spaces.Comment: Chapter in the book Relativity and the Dimensionality of the World to be published in the Springer series Fundamental Theories of Physics. Volume Editor: Vesselin Petkov. 18 pages plus macros; updated reference