Abstract
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).
Highlights
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]
Let T be the proper time as measured along the fluid congruence, and let us define the quantity where τ is a dynamical time variable defined in terms of the clock time t and the by dτ dt is finite, the fluid congruence is future incomplete: the fluid observers will reach infinite expansion within finite proper time
It is known that expanding non-type-IX Bianchi models with a positive cosmological constant isotropize to the future [14], and that this result applies to tilted models, the isotropization is with respect to the congruence normal to the homogeneous symmetry surfaces – not the fluid congruence [15]
Summary
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]. It is known that expanding non-type-IX Bianchi models with a positive cosmological constant isotropize to the future (cosmic no hair theorem) [14], and that this result applies to tilted models, the isotropization is with respect to the congruence normal to the homogeneous symmetry surfaces – not the fluid congruence [15].
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