Alternative explanations of “dark energy” in cosmology

I consider alternative explanations for the data suggesting detection of dark energy (or a cosmological constant Λ). Apart from alternative gravity theories and some form of ‘quintessence’, one proposal to explain a small non‐zero value of Λ is a multiverse, but this is not testable in the usual sense. Other possibilities are that it may at least in part be due to the backreaction from and observational effects of small scale inhomogeneity, or it might be completely explained by large scale spatial inhomogeneity with zero Λ. The latter proposal can be observationally tested in a number of ways.