X iv : g r - qc / 0 51 20 85 v 2 6 J ul 2 00 6 Tilt and phantom cosmology

We show that in tilting perfect fluid cosmological models with an ultra-radiative equation of state, generically the tilt becomes extreme at late times and, as the tilt instability sets in, observers moving with the tilting fluid will experience singular behaviour in which infinite expansion is reached within a finite proper time, similar to that of phantom cosmology (but without the need for exotic forms of matter). Cosmological data, including galaxy, CMB and supernovae observations [1], seem to be consistent with a cosmological constant or a dark phantom energy with effective equation of state parameter γ < 0 [2]. We shall study cosmological models with a tilting, but otherwise conventional, perfect fluid, and show that the models have dynamical behaviour similar to that of phantom cosmology, but without the need for any exotic forms of matter and consequently avoiding the pathologies in these models, such as the existence of ghosts. For spatially homogeneous (SH) Bianchi cosmologies models, the universe is foliated into space-like hypersurfaces [3], and there are two naturally defined time-like vectors: the unit vector field, n a , normal to the group orbits, and the four-velocity, u a , of the perfect fluid. If u a is not aligned with n a , the model is called tilted (and non-tilted or orthogonal otherwise) [4]. Usually, the kinematical quantities associated with the normal congruence n a of the spatial symmetry surfaces, rather than the fluid flow u a , are used as variables. Following [4], a tilt variable v is introduced, so that in an orthonormal frame where n a = (1, 0, 0, 0), we have u a = 1 √ 1 − v 2 (1, v, 0, 0) .