Speaker
Colin MacLaurin
(University of Queensland)
Description
The 'Big Rip' is a proposed hypothetical end to the universe where the Hubble parameter increases to infinity in a finite time. It assumes the universe contains a fluid ('dark energy' say) with an equation of state state w<-1 (pressure + density < 0). Caldwell et al give just 20 Gyr until the catastrophe, under assumptions including perfect homogeneity and isotropy, however we show inhomogeneity can prevent the Big Rip. As a rough intuition consider the edge of a galaxy, with negligible expansion inside but the Hubble expansion rate outside, say. This gradient in expansion rate implies shear, but in the Raychaudhuri equation shear reduces the expansion rate. We assume a silent universe, analyse the kinematic parameters as a dynamical system, and obtain exact qualitative results in the parameter space under certain approximations. For a local system the possibilities include collapse, a Big Rip, oscillating expansion and contraction, and various asymptotic cases. Collapse is a good thing because the Rip is prevented, it is then hoped that adding rotation will virialise the system against collapse. Research in conjunction with Krzysztof Bolejko.
