The people behind it
Several people laid the groundwork. In 1912 Henrietta Swan Leavitt, one of the Harvard Observatory "computers," found that a Cepheid variable's pulsation period tracks its true brightness, creating a yardstick for vast distances. Around the same time Vesto Slipher measured redshifts showing that many "spiral nebulae" were moving away. In 1927 Belgian priest and physicist Georges Lemaître solved general relativity for an expanding universe and first published the distance–velocity relation in a French-language paper. In 1929 Edwin Hubble at Mount Wilson plotted Cepheid-based distances against Slipher's velocities and showed the proportionality observationally, and the law long bore his name alone. In 2018 the International Astronomical Union voted to recommend calling it the Hubble–Lemaître law.
What it means
The law is written v = H₀ × d, where H₀ is the Hubble constant. With H₀ = 70, a galaxy one megaparsec (about 3.26 million light-years) away recedes at 70 km/s, and one ten times farther at 700 km/s. That's not because we sit at the center, but because space itself is stretching, like raisins drifting apart in rising bread. Run the clock backward and everything converges, the starting point of the Big Bang theory, and 1/H₀ gives a rough age of about 14 billion years. Einstein, who had added a cosmological constant to keep the universe static, reportedly later called it his "biggest blunder."
The Hubble tension
Measuring H₀ precisely was a core goal of the Hubble Space Telescope, launched in 1990. Today, however, the local value from Cepheids and Type Ia supernovae (Adam Riess's team, about 73) and the early-universe value from the cosmic microwave background (Planck, about 67) disagree beyond their error bars. Whether it's a measurement problem or a sign of new physics, such as early dark energy or unexpected properties of dark matter, is unresolved, and the James Webb Space Telescope, launched in 2021, is now testing it.
FAQ
- Does expansion push Earth and the Sun apart?
- No. Gravitationally bound systems like the solar system or our galaxy are essentially unaffected; expansion shows up only on very large scales, between galaxy clusters.
- Can galaxies recede faster than light?
- Yes, very distant galaxies have apparent recession speeds above light speed. That's space itself stretching, not objects moving through space faster than light, so it doesn't contradict relativity.
Sources reviewed: 2026-10-26
References
- Hubble, E. (1929). A relation between distance and radial velocity among extra-galactic nebulae. PNAS, 15(3), 168–173.
- Lemaître, G. (1927). Un Univers homogène de masse constante et de rayon croissant. Annales de la Société Scientifique de Bruxelles, A47, 49–59.
- Riess, A. G., et al. (2022). A comprehensive measurement of the local value of the Hubble constant. Astrophysical Journal Letters, 934, L7.