The impact of the Universe's expansion rate on constraints on modified growth of structure

Understand

In the context of modified gravity, at linear level, the growth of structure in the Universe will be affected by modifications to the Poisson equation and by the background expansion rate of the Universe.

• It has been shown that these two effects lead to a degeneracy which must be properly accounted for if one is to place reliable constraints on new forces on large scales or, equivalently, modifications to General Relativity.
• In this paper we show that current constraints are such that assumptions about the background expansion have little impact on constraints on modifications to gravity.
• We do so by considering the background of a $\Lambda$ Cold Dark Matter ($\Lambda$CDM) universe, a universe with a more general equation of state for the dark energy, and finally, a general, model-independent, expansion rate.