Open questionCosmology · Universe

The Hubble tension

Two independent ways of measuring how fast the universe is expanding give different answers. The mismatch has hardened rather than faded as the data improved.

Open question · late-universe vs early-universe measurements

What it is: A mismatch between two measurements of the Hubble constant, H0
Local value: About 73 km/s per megaparsec
Early-universe value: About 67 km/s per megaparsec
Methods: Cepheids, supernovae, the CMB, baryon acoustic oscillations
Significance: Roughly 4 to 5 sigma, unlikely to be pure chance
Status: Unresolved in the latest data releases

The same star, two telescopes

NASA, ESA, CSA, STScI, A. Riess

Two rulers for the same number

The local measurement starts close by. Cepheid variable stars set the distances to nearby galaxies, type Ia supernovae carry the ladder farther out, and the result is about 73 km/s per megaparsec. The early-universe measurement runs the other way, from the cosmic microwave background and baryon acoustic oscillations through the standard model, and lands near 67.

The two routes overlap in principle, so they should agree. The fact that they do not is the tension.

Why the gap matters

If the mismatch is real, the standard model is missing something. Candidates include early dark energy, extra relativistic particles, a changing dark energy, or a subtle error in how we read the distance ladder. Each possibility points to new physics in a different place.

A plain measurement error is still possible, but the checks so far have not found one large enough to close the gap.

Where it stands

JWST was expected to settle the question by testing whether Cepheid stars were crowded in Hubble's images. Instead, its sharper view largely confirmed the local value, which deepens the puzzle rather than resolving it. Upcoming surveys will keep pressing both sides of the measurement.

Key publications

Year	Paper	Venue	Link
2022	Riess et al., A Comprehensive Measurement of the Local Value of the Hubble Constant with 1 km/s/Mpc Uncertainty from the Hubble Space Telescope and the SH0ES Team	ApJL 934, L7	DOI
2020	Planck Collaboration, Planck 2018 results. VI. Cosmological parameters	A&A 641, A6	DOI
2019	Verde, Treu & Riess, Tensions between the early and late Universe	Nature Astronomy 3, 891	DOI

Selected papers for further reading. DOI links open the publisher's record; preprints are linked on arXiv.

Worth knowing

The two routes agree on almost everything else. Only the expansion rate splits them.
JWST was expected to close the gap and instead sharpened the local value.
A 4 to 5 sigma gap is far too large to dismiss.