The Hubble tension is the name given to a persistent discrepancy between two ways of measuring how fast the universe is expanding. The present-day rate of expansion is known as the Hubble constant, named after Edwin Hubble, the American astronomer who published observations of distant galaxies in 1929 showing the universe was expanding. The fact that the expansion is accelerating was discovered in 1998 and honoured with a Nobel prize in 2011.
Measuring the modern universe directly—working out how far away galaxies are and how fast they are receding—gives a value of about 73 kilometres per second per megaparsec (km/s/mpc). Analysing the cosmic microwave background radiation (CMB), an aftershock of the Big Bang, and feeding it into cosmological models gives a value closer to 67km/s/mpc. A megaparsec is the distance light travels in about 3.3m years; a value of 73 means that objects one megaparsec away recede from an observer at 73km per second.
The tension "has got stronger every year for the past decade", according to Dan Scolnic, an astronomer at Duke University. It makes it impossible to calculate an exact age for the universe or to be certain of its exact size.
This approach relies on the cosmic distance ladder, in which several measurement techniques are chained together. The lowest rungs use trigonometry for nearby stars; higher rungs use "standard candles" such as Cepheid variable stars and certain supernovae, whose absolute brightness is known. Interstellar dust absorption, the "metallicity" of individual Cepheids and the small sample size of suitable supernovae can all introduce errors.
Wendy Freedman, an astronomer at the University of Chicago, argues that the evidence that two bulletproof sets of measurements disagree is not yet "extraordinary" enough. Adam Riess, an astronomer at the Space Telescope Science Institute in Baltimore and one of the 2011 Nobel winners, counters that every rung of the distance ladder has been double-checked with different standard candles and the tension persists.
A paper published in June 2025 bypassed the distance ladder entirely. By examining how gravity from massive objects causes different beams of light from quasars to arrive at different times, it obtained a value very similar to distance-ladder studies—suggesting any unknown confounder would have to be distorting several fundamentally different methods at once.
The CMB has been measured and re-measured with increasing accuracy by satellites since the 1990s and by ground-based telescopes in Chile and at the South Pole. The suspicion among some astronomers is not that the CMB readings are wrong, but that the standard cosmological model (Lambda-CDM) into which those measurements are fed may be incomplete.
Lambda-CDM holds that visible matter—galaxies, planets, starlight—makes up just 5% of the total universe, with the rest split between dark energy (which opposes gravity and drives expansion) and cold dark matter (inferred from gravitational effects on galaxies). The model is highly successful at predicting everything from element abundances to galaxy distribution and CMB patterns, making it difficult to replace.
Proposed alternatives include the possibility that dark energy's potency changes over time, or that the Milky Way sits within a giant, comparatively empty region of space that inflates the local Hubble constant.
The Vera Rubin Observatory in Chile and the Nancy Grace Roman Space Telescope, due to fly no later than May 2027, may help resolve the tension. But if past decades are any guide, new observations are as likely to re-confirm the discrepancy as to resolve it.
For every complex problem, there is a solution that is simple, neat, and wrong.