"A 2.4% Determination of the Local Value of the Hubble Constant" by Riess et al has led to some excited news stories recently. I don't see it discussed anywhere here. Looking for the essence of the paper, I note three things:
- The two measurements considered are "the Hubble constant ... measured locally" and "the sound horizon observed from the cosmic microwave background radiation"
- Possible causes of a discrepancy include "time-dependent or early dark energy, gravitational physics beyond General Relativity, additional relativistic particles, or nonzero curvature"
- "Systematic uncertainties in CMB measurements may [also] play a role in the tension"