How the Hubble Constant Measures the Expansion of the Universe

Two Numbers That Cannot Both Be Right

The Hubble constant—H₀—tells us how fast the universe is expanding. Measure it using the cosmic microwave background (CMB) radiation left over from the Big Bang, and you get 67.4 ± 0.5 km/s/Mpc. Measure it using the distances and recession velocities of nearby galaxies, and you get 73.0 ± 1.0 km/s/Mpc. The two methods have been refined to extraordinary precision. Their error bars no longer overlap. The discrepancy, known as the Hubble tension, is either a systematic error that no one can find or a crack in the standard model of cosmology. Either outcome would reshape physics.

Edwin Hubble's Original Discovery

In 1929, Edwin Hubble published observations showing that galaxies are moving away from us and that more distant galaxies recede faster. The relationship is linear: a galaxy twice as far away recedes twice as fast. Hubble's original estimate of the constant was about 500 km/s/Mpc—wildly high by modern standards, because his distance measurements to galaxies were severely underestimated. But the proportionality was correct. The universe is expanding, and H₀ describes the rate.

• H₀ units: kilometers per second per megaparsec (1 Mpc = 3.26 million light-years)
• Interpretation: for every megaparsec of distance, galaxies recede an additional H₀ km/s
• At H₀ = 70 km/s/Mpc, a galaxy 100 Mpc away recedes at 7,000 km/s
• The reciprocal of H₀ gives the Hubble time—a rough estimate of the age of the universe
• Hubble's 1929 value: ~500 km/s/Mpc (distance calibration errors)

The Cosmic Distance Ladder

Measuring H₀ from local galaxies requires knowing two things: how fast a galaxy is moving away (easy—measure the redshift of its light) and how far away it is (hard). Distance measurement uses a chain of overlapping techniques called the cosmic distance ladder.

Each rung calibrates the next. Errors compound. If the Cepheid period-luminosity relationship is slightly miscalibrated, every subsequent distance measurement inherits the error. This is why the distance ladder result—73.0 km/s/Mpc, led by the SH0ES team under Adam Riess—has been subjected to relentless scrutiny.

The CMB Measurement—A View From the Beginning

The alternative approach bypasses the distance ladder entirely. The cosmic microwave background is radiation from 380,000 years after the Big Bang, when the universe cooled enough for atoms to form and light to travel freely. The European Space Agency's Planck satellite mapped tiny temperature fluctuations in this radiation with extraordinary precision.

These fluctuations encode the geometry, composition, and expansion history of the early universe. By fitting the data to the Lambda-CDM model (the standard model of cosmology), physicists extract H₀ = 67.4 ± 0.5 km/s/Mpc. This value assumes that the standard model correctly describes the universe's expansion from the CMB era to the present day.

• Planck satellite operated from 2009 to 2013, mapping the full sky in microwave wavelengths
• Temperature fluctuations are measured to parts per million
• The angular size of the acoustic peaks in the CMB power spectrum constrains H₀
• The result depends on the cosmological model assumed—change the model, change H₀
• Independent CMB measurements (WMAP, ACT, SPT) yield consistent results near 67–68 km/s/Mpc

The Hubble Tension—5 Sigma and Growing

The disagreement between the two methods has exceeded 5 sigma—the threshold particle physicists use to declare a discovery. Multiple independent teams have confirmed both measurements using different techniques and datasets.

The TRGB measurement by Wendy Freedman's team falls between the two extremes and has been cited by both sides. Gravitational lensing and megamaser measurements independently support the higher value. No known systematic error in any measurement accounts for the gap.

JWST—The Tiebreaker That Wasn't

When the James Webb Space Telescope began science operations in 2022, one of its primary targets was resolving the Hubble tension. JWST can observe Cepheid variables in distant galaxies with far greater clarity than Hubble, potentially revealing calibration errors caused by crowded stellar fields ("blending"). Early JWST results from the SH0ES team confirmed the Cepheid distances measured by Hubble, ruling out blending as the source of the tension. The local measurement stands.

Freedman's CCHP team, using JWST observations with the TRGB and a newer method called J-region asymptotic giant branch (JAGB) stars, initially reported values closer to the Planck result. The debate continues with new data releases, but the tension has not been resolved.

What New Physics Could Explain the Gap

If neither measurement is wrong, the standard model of cosmology is incomplete. Proposed modifications include early dark energy (a brief burst of accelerated expansion before the CMB era), varying dark energy (dark energy that changes strength over time), additional neutrino species, and modified gravity theories. Each proposal has consequences that can be tested with upcoming surveys from the Euclid satellite, the Vera Rubin Observatory, and the Dark Energy Spectroscopic Instrument. The Hubble tension may be the first observational evidence that the universe operates under rules we have not yet written down.