arXiv: 8 Jan 2010

Theoretical Priors On Modified Growth Parametrisations
Authors: Yong-Seon Song, Lukas Hollenstein, Gabriela Caldera-Cabral, Kazuya Koyama
arXiv:1001.0969v1
Next generation surveys will observe the large-scale structure of the Universe with unprecedented accuracy. This will enable us to test the relationships between matter over-densities, the curvature perturbation and the Newtonian potential. Any large-distance modification of gravity or exotic nature of dark energy modifies these relationships as compared to those predicted in the standard smooth dark energy model based on General Relativity. In linear theory of structure growth such modifications are often parameterised by virtue of two functions of space and time that enter the relation of the curvature perturbation to, first, the matter over-density, and second, the Newtonian potential. We investigate the predictions for these functions in Brans-Dicke theory, clustering dark energy models and interacting dark energy models. We find that each theory has a distinct path in the parameter space of modified growth. Understanding these theoretical priors on the parameterisations of modified growth is essential to reveal the nature of cosmic acceleration with the help of upcoming observations of structure formations.

Relativistic Bose-Einstein Condensates: a New System for Analogue Models of Gravity
Authors: Serena Fagnocchi, Stefano Finazzi, Stefano Liberati, Marton Kormos, Andrea Trombettoni
arXiv:1001.1044v1
In this paper we propose to apply the analogy between gravity and condensed matter physics to relativistic Bose-Einstein condensates, i.e. condensates composed by relativistic constituents. While such systems are not yet subject of experimental realization, they do provide us with a very rich analogue model of gravity. In particular we show here that they are characterized by several novel features with respect to their non-relativistic counterpart. First they are characterized by two (rather than one) quasi-particle excitations, a massless and a massive one, the latter disappearing in the non-relativistic limit. Secondly, the metric associated to the massless mode is a generalization of the usual acoustic geometry allowing also for non-conformally flat spatial sections. This is relevant, as it implies that these systems can allow the simulation of a wider variety of geometries. Finally, while in standard Bose-Einstein condensates the transition is from Lorentzian to Galilean relativity, these systems represent an emergent gravity toy model where Lorentz symmetry is present (albeit with different limit speeds) at both low and high energies. Hence they could be used as a test field for better understanding the phenomenological implications of such milder form of Lorentz violation at intermediate energies.