“There is physically, absolutely zero way that we will ever know how large the universe is,” says Sara Webb, an astrophysicist at Swinburne University of Technology. Yet what astronomers can measure is staggering: the observable universe stretches for at least 93 billion light-years across – that figure inflated far beyond the cosmos’s 13.8-billion-year age by the continued stretching of space since the Big Bang. Light from the earliest galaxies has travelled for 13.8 billion years, yet the expansion of space means those sources are now roughly 46.5 billion light-years away.
This expansion was first detected in the early 20th century through the optical analogue of the Doppler effect-in which light from receding galaxies shifts toward longer, redder wavelengths. Edwin Hubble’s observations of Cepheid variable stars provided the first precise distances to other galaxies, cementing the notion that the universe is not static but growing. Along with other “standard candles”-such as Type Ia supernovae and tip-of-the-red-giant-branch stars-Cepheids remain the backbone of the so-called cosmic distance ladder, a hierarchical set of measurements that tie distance scales across the universe.
The expansion rate, otherwise known as the Hubble constant (H₀), is at the heart of cosmology. But here lies the perpetual “Hubble Tension”: two extremely precise measures yield incompatible values. Measurements based on nearby standard candles give H₀ at around 73 km/s/Mpc, while analyses of the cosmic microwave background from the early universe predict about 67 km/s/Mpc. The difference, roughly 8-9%, is too large to dismiss as mere statistical noise.
For years, astronomers suspected subtle observational biases crowded stellar fields blending light, dust absorption skewing brightness, or calibration errors could account for the mismatch. The James Webb Space Telescope, with its sharp infrared vision, has now largely eliminated those concerns. By resolving individual Cepheids in distant galaxies and slicing through obscuring dust, JWST has confirmed Hubble’s measurements with less than 2% deviation. “We can rule out a measurement error as the cause of the Hubble Tension with very high confidence,” says Adam Riess, the Nobel laureate and lead investigator on the SH₀ES team.
These findings span the full second rung of the cosmic distance ladder-from Cepheids in NGC 4258, a galaxy with a well-established geometric distance, to hosts of Type Ia supernovae as far as 130 million light-years away. JWST’s precision has also allowed cross-checks using carbon stars and the brightest red giants, each giving a convergence on expansion rates into the low 70s km/s/Mpc. Even independent approaches are not supporting the conventional model: gravitationally lensed quasars from the TDCOSMO collaboration are yielding values above 72 km/s/Mpc, too.
New JWST observations may ease the tension, according to some researchers, such as University of Chicago cosmologist Wendy Freedman. Her team’s multi-method analysis, including Cepheids, red giants, and carbon stars, produced an H₀ of 70 km/s/Mpc, which overlaps the CMB-derived figure within error margins. Others, however, like Dan Scolnic of Duke University, remain unconvinced and cite full datasets that push the constant higher. If measurement errors are truly ruled out, the discrepancy may hint at physics beyond the standard cosmological model. Possibilities range from “early dark energy” that briefly accelerated expansion after the Big Bang to exotic dark matter properties, to shifts in fundamental constants like electron mass or even primordial magnetic fields.
“Theorists have license to get pretty creative.” says Marc Kamionkowski of Johns Hopkins. Meanwhile, dark energy itself-already a mysterious force driving accelerated expansion-could be uneven across space, subtly altering local and distant measurements. Such non-uniformity would challenge the assumption that dark energy acts identically everywhere, forcing a rethink on how general relativity works on cosmic scales. Therefore, resolving the Hubble Tension may shine light not just on how fast the universe grows but why it does so in the first place and whether there are still hidden components of reality waiting to be found.
