Observational constraints on power-law cosmologies

In a class of models designed to solve the cosmological constant problem by coupling scalar or tensor classical fields to the space-time curvature, the universal scale factor grows as a power law in the age, a∝t^α, regardless of the matter content or cosmological epoch. We investigate constraints on such ``power-law cosmologies'' from the present age of the Universe, the magnitude-redshift relation, and from primordial nucleosynthesis. Constraints from the current age of the Universe and from the high-redshift supernovae data require ``large'' α (≈1), while consistency with the inferred primordial abundances of deuterium and helium-4 forces α to lie in a very narrow range around a lower value (≈0.55). Inconsistency between these independent cosmological constraints suggests that such power-law cosmologies are not viable.