BRANEWORLD MODELS · Investigation of the viability and cosmological consequences of the curvature corrections of the minimal Randall-Sundrum model.

Final Activity Report Summary - BRANEWORLD MODELS (Investigation of the viability and cosmological consequences of the curvature corrections of the minimal Randall-Sundrum model)

During recent years cosmology has become one of the most successful fields in physics. This has been largely driven by huge improvement in quality, quantity and the scope of cosmological observations. In consistency with observational constraints from the background evolution of the universe as well as with those from the formation of large scale structures, a simple "concordance model'' has emerged. All the parameters of this model are now accurate to a few percent and will be measured even more precisely with ongoing and planned experiments. The estimation of the parameters is only the first step. The challenge that lies ahead would be to connect them with physics. Thus, on a theoretical level our understanding has remained poor. We have no satisfactory answer to the questions: What is dark matter? What is dark energy? What is the physics of inflation? How can we resolve the singularities of classical general relativity? The precise property of the cold dark matter remains an open problem, and direct detection and identification of the cold dark matter particle candidates is needed. While interpreting in a simple parametrisation the cosmological constant as vacuum energy it implies an extreme form of fine-tuning. If the equation of state of dark energy is finally -1, one will need to review the cosmological constant problem again in order to understand why this is so small in order to dominate the expansion dynamics at precisely the present epoch. If the equation of state is different than -1 or is shown to be time-dependent, then searches for evolving dark energy models will need to be examined deeply in the light of developments both in high energy physics and in gravitational theory (superstring/M theory, extra dimensions). On the other side of particle physics, the upcoming experiments with the LHC at CERN will hopefully shed light on fundamental issues. Is there the Higgs particle? Is supersymmetry used in nature to prevent or at least soften unnatural fine tunings? Are the superpartners of the known standard model particles in the Tev scale? Is the neutralino, the most popular candidate for cold dark matter, going to show up in LHC? Obviously, apart from its relevance in particle physics, it will provide interesting possibilities for cosmology. All alternative possibilities for particle physics and cosmology models should be investigated, to be prepared for a proper interpretation of what will be seen. A great amount of theoretical work has already been devoted and will do in the future to use cosmology in order to gain information on the fundamental laws underlying the Standard Model. In this direction, braneworld models, motivated by string theories with extra dimensions, have proposed various scenarios, where for example both early and late time acceleration can be successfully unified within a single scheme, the effective equation of state of dark energy can be "phantom-like'' or "quintessence-like'', the present acceleration can be a transient phenomenon, bounce at early times can occur avoiding the big-bang singularity, etc. In the fulfilled project, such scenarios have been investigated in the codimension two frameworks where two large extra dimensions exist beyond the three plus one dimensions of the observed universe, and in the five-dimensional context where a mechanism of energy exchange between our universe and the unobserved dimensions has been proposed. This scenario, assuming a phenomenological law for the brane-bulk interaction, gives an explanation to the coincidence problem of cosmology (i.e. why is it today that the dark energy is comparable to the dark matter) by realising the present universe being close to a accelerating global attractor during the cosmic evolution, and also predicts a "phantom-like'' equation of state for the dark energy (smaller than -1) which is favoured by various analyses of the astronomical data.

Data: CORDIS, © European Union