General Relativistic Tilt and Dark Energy

Cosmological data seem to imply dynamical behaviour different to that implied by standard cosmological models. This dynamical behaviour is often modeled by an exotic form of matter with a non-standard effective equation of state parameter called dark energy. We show that in general relativistic tilting perfect fluid cosmological models with an ultra-radiative equation of state, generically the tilt becomes extreme at late times and observers moving with the tilting fluid will experience dynamical behaviour similar to that of quintessence or phantom cosmology.