SH0ES (2022): A Hubble Constant of 73 and a 5-Sigma Tension

The SH0ES Hubble constant, published by Adam Riess and the SH0ES team in 2022, measured today’s cosmic expansion rate at 73.04 ± 1.04 km/s per megaparsec. That is 5 sigma away from the value of about 67.4 predicted from the early universe by Planck, turning the Hubble tension into one of the biggest open problems in cosmology.

The question before the paper

There are two ways to get the Hubble constant, the expansion rate first measured by Edwin Hubble in 1929. One reads it from the early universe: the Planck satellite’s 2018 analysis of the cosmic microwave background, assuming the standard Lambda-CDM model, predicts 67.4 ± 0.5. The other measures it directly in the nearby universe with a “distance ladder” of stars and supernovae. For years the direct values came out higher. Was the gap real, or an artifact of the ladder?

What the paper found

• H₀ = 73.04 ± 1.04 km/s per megaparsec, including systematic uncertainties, an error of about 1.4%.
• A 5-sigma difference from the Planck plus Lambda-CDM prediction, the conventional threshold for treating a discrepancy as real.
• Robust to choices. The team explored about 70 variants of the analysis, and none removed the gap.
• No explanation found. The authors conclude that the source of the long-standing discrepancy “remains unknown.”

How they did it

The distance ladder has three rungs.

1. Geometry. Cepheid variable stars, whose pulsation period reveals their true brightness, were calibrated with purely geometric distances: Gaia EDR3 parallaxes in the Milky Way, masers (natural microwave lasers) in the galaxy NGC 4258, and eclipsing binary stars in the Large Magellanic Cloud.
2. Cepheids to supernovae. Using more than 1,000 orbits of the Hubble Space Telescope, the team measured Cepheids in the host galaxies of 42 type Ia supernovae, exploding white dwarfs of nearly uniform brightness.
3. Supernovae to the Hubble flow. Calibrated supernovae farther out, where galaxies move mainly with cosmic expansion, give the expansion rate.

Why it mattered

The paper pushed the Hubble tension to 5 sigma. If the measurements are right, the standard model of cosmology is missing something, perhaps in the physics of the early universe or the behavior of dark energy. If not, there is an unrecognized error somewhere in one of the most scrutinized measurements in astronomy. Either answer matters, which is why the result is so widely cited: about 2,100 times in Semantic Scholar as of October 2026.

What we know now

The tension has not gone away. The James Webb Space Telescope is now being used to observe the same galaxies and check the Cepheid measurements. Meanwhile DESI’s 2025 results hint that dark energy may evolve over time, one of the ideas proposed to reshape the expansion history. Our Hubble tension explainer tracks the latest numbers.

Who wrote it

Adam Riess is first author, writing with Wenlong Yuan, Lucas Macri and the rest of the SH0ES team. Riess shared the 2011 Nobel Prize in Physics for the 1998 discovery of cosmic acceleration, made with the same tool, type Ia supernovae.

Read the original

Riess, A. G., Yuan, W., Macri, L. M., et al. (2022). “A Comprehensive Measurement of the Local Value of the Hubble Constant with 1 km s⁻¹ Mpc⁻¹ Uncertainty from the Hubble Space Telescope and the SH0ES Team.” The Astrophysical Journal Letters. doi:10.3847/2041-8213/ac5c5b. Free on arXiv: 2112.04510.