Cosmologists are grappling with one of astronomy’s biggest debates: how fast is the universe expanding? This rate is quantified by the Hubble constant (H₀), but estimates refuse to align producing the perplexing Hubble tension.

Using “distance ladder” measurements—tracking supernovae and pulsating “standard candle” stars—astronomers estimate an expansive rate of about 73 km/s/Mpc today. In stark contrast, values implied by Cosmic Microwave Background (CMB) observations suggest the universe should be expanding closer to 67 km/s/Mpc. This persistent mismatch has only widened despite technological advances in measurement.

These conflicting figures challenge the standard cosmological model. Some researchers suspect one set of observations might be flawed, but many believe the tension signals a missing piece in our cosmic understanding perhaps a new element of fundamental physics.

Further complicating the picture, the James Webb Space Telescope (JWST) has corroborated Hubble’s measurement, cementing that the disagreement is unlikely to stem from instrument error.