One of the most puzzling issues of theoretical physics today is the problem of the cosmological dark energy. As string theory is the only currently viable model of quantum gravity, it should have something to say about it. Various approaches to cosmology have been proposed within string theory, though none has yet consistently described observations, while having a sound derivation from string theory. We propose to compare various resolutions of the problem of dark energy within string cosmologies: within the ``chameleon cosmology'' derived from string theory, within string gas cosmology, and within ``holographic cosmology''. The latter is an application of the AdS/CFT correspondence of Maldacena, relating weakly coupled gravity to strongly coupled field theory, but also vice versa, to cosmology, in the formulation of McFadden and Skenderis, based on the general set-up of Maldacena, through the relation $Z[\Phi]=\Psi(\Phi)$.
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