Never has a measurement of cosmic expansion achieved such precision. It reveals that the universe is expanding faster than what the standard model of cosmology predicts, worsening the famous Hubble tension. This finding suggests that an ingredient is missing from our current understanding of the cosmos.
Researchers traditionally use two very different methods to determine the universe's expansion rate. One focuses on relatively nearby objects by measuring our distance to certain other stars and galaxies. The other traces back to the early universe, using the cosmic microwave background to estimate what the expansion rate should be today according to the standard model of cosmology.
Artist's interpretation of the cosmic distance ladder — a succession of overlapping methods to measure distances in the universe, where each rung of the ladder provides information to determine distances on the next rung.
Credit: CTIO/NOIRLab/DOE/NSF/AURA/J. Pollard Image processing: D. de Martin & M. Zamani (NSF NOIRLab) In principle, the two approaches should agree. In practice, they do not. Observations of the local universe consistently indicate a faster expansion rate — around 45.4 miles per second per megaparsec (73 km/s/Mpc) — while estimates based on the early universe give lower values, close to 41.6 or 42.3 miles per second per megaparsec (67 or 68 km/s/Mpc).
To refine the measurement, astronomers combined decades of observations into a single, coordinated system. This effort, led by the H0 Distance Network (H0DN) collaboration, produced the most precise direct measurement of the local expansion rate. In a paper published April 10 in
Astronomy & Astrophysics, the team reports a Hubble constant of 45.66 ± 0.50 miles per second per megaparsec (73.50 ± 0.81 km/s/Mpc), achieving a precision of just over 1%. This result is not just a new value; it is a community-built framework that brings together decades of independent distance measurements in a transparent and accessible way.
Instead of relying on a single technique, the team created a "distance network" that links several independent methods of measuring cosmic distances. These methods include Cepheid variable stars, red giant stars of known luminosity, type Ia supernovae, and certain types of galaxies. This network allows scientists to cross-check results in multiple ways.
For its part, the slower measured expansion rate depends on the standard model of cosmology, which describes how the universe has evolved since the Big Bang. If this model is incomplete — for example, if it does not fully capture the behavior of dark energy, unknown particles, or possible modifications to gravity — its predictions for the current expansion rate could be inaccurate.
This could therefore indicate that the Hubble tension is evidence that our current model of the universe is missing an important element.