Can pre-recombination physics raise the Hubble constant to 0.73?
In plain words
One proposed fix changes physics in the first 380,000 years so that the standard ruler imprinted in the cosmic microwave background is shorter. The question is whether any such change still fits all the detailed data.
Precise statement
Determine whether any model that reduces the sound horizon $r_d$ by $\sim 7\%$ (early dark energy, extra radiation $\Delta N_{\mathrm{eff}}$, varying electron mass, primordial magnetic fields) fits Planck, ACT DR6 and SPT-3G CMB spectra plus DESI DR2 BAO with $h \ge 0.72$ and total $\chi^2$ no worse than LCDM. Answer yes with an explicit model or no with a general argument.
What would settle it
Joint fits of candidate models to current CMB and BAO data, plus a model-independent bound on $r_{d} h$ from BAO and supernovae.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
2025 ACT DR6 extended-model analyses found no preference for early dark energy or extra radiation at the level needed (moderate confidence).
Parent problems
- Special case of What causes the Hubble tension? · open · from revision 1 The parent was revised since revision 1: it now states another question.
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