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Abstract
Observational probes of dark energy converge on the fact that its equation of state w is close to −1. Some of these probes (for example the Gold SnIa sample) mildly support the possibility that w is time dependent and crosses the phantom divide line w = −1. Such a crossing is inconsistent with dark energy in the form of a single minimally coupled scalar field for any form of the field Lagrangian. The simplest theoretically motivated theories that are consistent with such crossing of the phantom divide are scalar-tensor extensions of general relativity. Therefore the crossing of the phantom divide if confirmed by future observations could be viewed as an indication hinting towards extensions of general relativity. Alternative signatures of extended gravity theories can be traced in the growth rate of density perturbations at recent redshifts as observed recently by the 2dFGRS.
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