Universe Expands at 73 km/s/Mpc — New Insights into Cosmic History

The Hubble constant, written H0, captures how fast the universe stretches per unit distance. Astronomers express it in kilometers per second per megaparsec, where one megaparsec equals about 3.26 million light-years. Two precision methods now produce values that disagree by roughly 5%, and the gap refuses to shrink as data improves.

Two methods, two numbers

The local distance-ladder approach builds outward from nearby objects. The SH0ES team, led by Adam Riess, calibrates Cepheid variable stars in Milky Way and Magellanic Cloud galaxies, then uses Cepheids in farther galaxies that also host Type Ia supernovae. The 2022 SH0ES result is 73.04 ± 1.04 km/s/Mpc.

The other method reads the cosmic microwave background. The Planck satellite's 2018 analysis, run through the standard ΛCDM model, gives 67.4 ± 0.5 km/s/Mpc. The two error bars do not overlap; the discrepancy clears 5σ, which is far above statistical noise.

Why this is called a tension

• If the local value is right, ΛCDM may be missing a piece of physics in the early universe.
• If the CMB value is right, something subtle could be biasing Cepheid or supernova distances.
• Both teams have spent years hunting for systematics and have not erased the gap.

JWST entered this debate in 2023 and 2024. Its infrared eye re-checked Cepheid distances that earlier work measured in visible light, where dust and crowding can distort readings. The JWST cross-checks largely confirmed the SH0ES local value, which deepened rather than dissolved the disagreement.

A short history of a moving number

Edwin Hubble first measured galactic recession in 1929 and got roughly 500 km/s/Mpc. That figure was wildly off because his Cepheid calibration was wrong; later astronomers showed that two distinct populations of Cepheids had been mixed together. The HST Key Project pulled the value down to about 72 km/s/Mpc by 2001, and Planck and SH0ES then sharpened the picture into today's split.

Implications for the age of the universe

The Hubble constant sets a clock. A value near 67.4 km/s/Mpc points to an age of about 13.8 billion years, the figure most textbooks still quote. A value near 73 km/s/Mpc shortens that estimate to roughly 13 billion years. Several hundred million years is a real difference for stellar-population models, the timing of reionization, and the formation of the first galaxies.

Possible new physics

Theorists have floated several routes that could pull the two numbers together:

• Early dark energy, a brief burst of repulsive energy before recombination that would change how the CMB is interpreted.
• Modified gravity, in which general relativity needs corrections on cosmological scales.
• Decaying dark matter, a model in which a fraction of dark matter converts to radiation over time.

None of these has been confirmed, and each predicts other signals that current data do not yet show. Upcoming surveys from the Vera Rubin Observatory, the Euclid space telescope, and continued JWST campaigns will tighten distance measurements and push the tension toward resolution.

The Hubble tension is the rare cosmological problem where both sides do excellent work and still cannot agree, which is why it has become the central open question in observational cosmology.