Does Hubble Tension Signal a Breakdown in FLRW Cosmology?
  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 $\Lambda$CDM, we find that low
redshift data ($z \lesssim 2.5$) and well-motivated priors only permit a dark
energy sector close to the cosmological constant $\Lambda$. 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 $H_0 \sim 71
\pm 1$ km/s/Mpc. Since various local determinations may be converging to $H_0
\sim 73$ km/s/Mpc, 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.
