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 H0 ∼ 71 ± 1 km/s/Mpc. Since various local determinations may be converging to H0 ∼ 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."How often have I said to you that when you have eliminated the impossible, whatever remains, however improbable, must be the truth?" -Sherlock Holmes

On Larger $H_0$ Values in the CMB Dipole Direction

“…Nevertheless, cosmological tensions point to something being amiss. Here, we present evidence that FLRW is suspect [10] (see [11] for earlier comments).…”

“…From our end, FIG. 3 is reminiscent of similar features in strong lensing time delay [10] and Type Ia SN [34]. It remains to address caveats.…”

“…In particular, Hubble tension [1][2][3][4][5] casts a spotlight on H 0 = H(z = 0). Here, we build on earlier observations for strongly lensed QSOs [10] and Type Ia SN [34] that H 0 values in the direction of the CMB dipole, loosely defined, are larger. Similar variations of H 0 across the sky have been reported for scaling relations in galaxy clusters [35,36].…”

“…It is hence plausible that the Universe is anisotropic. As explained in [10], future observations of strongly lensed QSOs [49,50] and potentially lensed SN [51] may settle the issue to everyone's satisfaction.…”

Aether, Dark Energy, and String Compactifications

“…The idea that dark energy can emerge from string-theory as a byproduct of cosmological compactifications to FLRW universes with an accelerated cosmic expansion powered by some ideal fluid with w 1 has now run into difficulties, which go beyond those mentioned here; see e.g. [50].…”