Does Hubble tension signal a breakdown in FLRW cosmology?

Abstract The tension between early and late Universe probes of the Hubble constant has motivated various new FLRW cosmologies. Here, we reanalyse the Hubble tension with a recent age of the Universe constraint. This allows us to restrict attention to matter and a dark energy sector that we treat without assuming a specific model. Assuming analyticity of the Hubble parameter H ( z ), and a generic low redshift modification to flat ΛCDM, we find that low redshift data ( z ≲ 2.5) and well-motivated priors only permit a dark energy sector close to the cosmological constant Λ. This restriction rules out late Universe modifications within FLRW. We show that early Universe physics that alters the sound horizon can yield an upper limit of H 0 ∼ 71 ± 1 km s −1 Mpc −1 . Since various local determinations may be converging to H 0 ∼ 73 km s −1 Mpc −1 , a breakdown of the FLRW framework is a plausible resolution. We outline how future data, in particular strongly lensed quasar data, could also provide further confirmations of such a resolution.