The “Hubble tension” refers to a long-standing discrepancy in measurements of the universe’s expansion rate — specifically, between values calculated from nearby observations (like Cepheid variable stars and Type Ia supernovae) and those inferred from the early universe via the cosmic microwave background (CMB).
The universe’s expansion over time. Credit: NASA/WMAP Science Team/ Art by Dana Berry
Initially, the differences were small enough that the error bars overlapped, leaving room for agreement. But as measurements became more precise, the gap widened. This raised exciting possibilities: perhaps our understanding of cosmic expansion was incomplete, or maybe new physics was waiting to be discovered.
Now, a new study has revisited this question using refined distance measurements from both the Hubble Space Telescope and the James Webb Space Telescope. The results show a local expansion rate of 70.4 kilometers per second per megaparsec, with a 3% margin of error. In contrast, the Planck satellite’s measurement from the CMB is 67.4 km/s/Mpc, with a much tighter 0.7% margin. Importantly, these updated margins now overlap—suggesting the tension may have been resolved after all.
This illustration shows the three basic steps astronomers use to calculate how fast the universe expands over time, a value called the Hubble constant. All the steps involve building a strong “cosmic distance ladder,” by starting with measuring accurate distances to nearby galaxies and then moving to galaxies farther and farther away. This “ladder” is a series of measurements of different kinds of astronomical objects with an intrinsic brightness that researchers can use to calculate distances. Among the most reliable for shorter distances are Cepheid variables, stars that pulsate at predictable rates that indicate their intrinsic brightness. Astronomers recently used the Hubble Space Telescope to observe 70 Cepheid variables in the nearby Large Magellanic Cloud to make the most precise distance measurement to that galaxy. Astronomers compare the measurements of nearby Cepheids to those in galaxies farther away that also include another cosmic yardstick, exploding stars called Type Ia supernovas. These supernovas are much brighter than Cepheid variables. Astronomers use them as “milepost markers” to gauge the distance from Earth to far-flung galaxies. Each of these markers build upon the previous step in the “ladder.” By extending the ladder using different kinds of reliable milepost markers, astronomers can reach very large distances in the universe. Astronomers compare these distance values to measurements of an entire galaxy’s light, which increasingly reddens with distance, due to the uniform expansion of space. Astronomers can then calculate how fast the cosmos is expanding: the Hubble constant. NASA, ESA and A. Feild (STScI)
If this holds up, it means the standard model of cosmology still stands firm. That’s scientifically reassuring, but perhaps a little disappointing to those hoping this tension would open a window to new physics—just as the discovery of the Higgs boson in particle physics confirmed existing models rather than overturning them. For now, it seems the universe is playing it safe.
