On the co-existence of matter dominated and accelerating solutions in f(G)-gravity

Working within the theory of modified Gauss-Bonnet gravity, we show that FLRW--like power--law solutions only exist for a very special class of f(G)theories. Furthermore, we point out that any transition from decelerated to accelerated expansion must pass through G=0, and no function f(G) that is differentiable at this point can admit both a decelerating power--law solution and any accelerating solution. This strongly constrains the cosmological viability of f(G)-gravity, since it may not be possible to obtain an expansion history of the universe which is compatible with observations. We explain why the same issue does not occur in f(R)-gravity and discuss possible caveats for the case of f(G)-gravity.