H₀ from the matter–radiation equality scale
The “Hubble tension” — the disagreement between local measurements of the Universe’s expansion rate, $H_0$, and the value inferred from the Cosmic Microwave Background (CMB) and galaxy surveys through observations of Baryon Acoustic Oscillations (BAO) — has been one of cosmology’s most discussed open puzzles. $H_0$ measurements from the CMB and BAO, while done at very different epochs, share a calibration: the scale of the sound horizon at recombination, which depends on early-universe physics. If the tension reflects something missing in our early-universe model, those measurements all move together.
This project develops an independent route to $H_0$ from galaxy surveys that does not rely on the sound horizon at all. It uses the matter–radiation equality scale — a different feature imprinted in the matter power spectrum — as a standard ruler.
What we’ve shown
- In a paper led by Oliver Philcox (arXiv:2008.08084, Phys. Rev. D 2020) we applied a preliminary version of our new methodology to galaxy clustering data from BOSS.
- Perspectives paper (arXiv:2112.10749, Phys. Rev. D 2022) — I further refined the method and led forecasts showing that upcoming spectroscopic galaxy surveys (DESI, Euclid, MegaMapper) can deliver sub-percent precision on $H_0$ from the equality scale alone.
- Philcox, Farren, Sherwin, Baxter & Brout (arXiv:2204.02984, Phys. Rev. D 2022) — combining galaxy surveys, CMB lensing, and Type-Ia supernovae to deliver a 3.6% sound-horizon-independent constraint on $H_0$.
The result is an alternative angle on the Hubble tension that future high-redshift spectroscopic data will sharpen further.
Where it has been applied
The method we developed has since been picked up across the field and the Dark Energy Spectroscopic Instrument (DESI) recently reported new percent-level constraints on $H_0$, independent of the sound horizon (Zaborowski et al. arXiv:2411.16677, JCAP 2025 & arXiv:2510.19149, JCAP 2026).
