We discuss the effects of a (possibly) negative ( 1 + z ) 6 type contribution to the Friedmann equation in a spatially flat universe. No definite answer can be given as to the presence and magnitude of a particular mechanism, because any test using the general relation H ( z ) is able to estimate only the total of all sources of such a term. That is why we describe four possibilities: (1) geometric effects of loop quantum cosmology , (2) braneworld cosmology, (3) metric-affine gravity, and (4) cosmology with spinning fluid. We find the exact solutions for the models with ρ 2 correction in terms of elementary functions, and show all evolutional paths on their phase plane. Instead of the initial singularity, the generic feature is now a bounce.
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Stachowiak et al. (2007) studied this question.
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