- Hubble's law links galaxy distance to recession speed.
- The Hubble constant's two measurements disagree by 5-6 sigma.
- Lemaitre derived the relation two years before Hubble.
Hubble's law is the observation that distant galaxies move away from Earth at speeds proportional to their distance. The farther a galaxy lies, the faster it recedes.
Why It Matters
Key figure
1929
Year Edwin Hubble published his velocity-distance relation
Before Edwin Hubble's 1929 paper in the Proceedings of the National Academy of Sciences, most physicists assumed the universe was static and eternal. His measurements of galactic redshifts overturned that assumption. The universe, it turned out, was expanding.
That single observation anchors modern cosmology. It provides the most direct evidence for the Big Bang, the framework that describes how the universe evolved from an extremely hot, dense state roughly 13.8 billion years ago. Without Hubble's law, there is no expanding universe, no cosmic microwave background prediction, and no standard model of cosmology.
The law also supplies a practical measuring tool. Astronomers use it to estimate distances to galaxies too far away for direct measurement. Measure a galaxy's redshift, apply the Hubble constant, and you have a distance.
This technique has mapped the large-scale structure of the cosmos across billions of light-years, including work that revealed the universe has no center in the way most people imagine.
How It Works
The relationship is expressed as v = H0 x d, where v is a galaxy's recessional velocity, H0 is the Hubble constant, and d is the distance to that galaxy. The equation describes a straight line: plot velocity against distance, and galaxies fall along a linear trend.
Key figure
67–73 km/s/Mpc
Current range for the Hubble constant
The Hubble constant sets the rate of expansion. Its units, kilometers per second per megaparsec (km/s/Mpc), mean that for every megaparsec (about 3.26 million light-years) of distance, a galaxy's recession speed increases by H0. Two measurements currently dominate: the Planck satellite's analysis of the cosmic microwave background yields 67.4 km/s/Mpc, while the SH0ES team's observations of Cepheid variable stars and Type Ia supernovae produce 73.2 km/s/Mpc.
That gap, known as the Hubble tension, has reached a statistical significance of 5 to 6 sigma. It is not a rounding error. Cosmologists are actively investigating whether this discrepancy points to new physics beyond the standard Lambda-CDM model, with some teams exploring gravitational wave measurements as an independent check.
One important caveat: Hubble's law applies cleanly only at cosmological distances. Nearby galaxies have peculiar velocities, individual motions caused by gravitational interactions with neighbors, that can mask the expansion signal. The Andromeda galaxy, for instance, is moving toward the Milky Way, not away from it.
Key Context
Belgian physicist and Catholic priest Georges Lemaitre derived the velocity-distance relation two years before Hubble, publishing it in a 1927 paper in the Annales de la Societe Scientifique de Bruxelles. Written in French and appearing in an obscure journal, the work went largely unnoticed. In 2018, the International Astronomical Union voted (78% in favor) to rename the relationship the Hubble-Lemaitre law, formally recognizing Lemaitre's priority.
Hubble's original 1929 value for his constant was roughly 500 km/s/Mpc, about seven times the modern value. That error came from systematic problems in the distance measurements available at the time. Refining the constant consumed decades of observational work and remains, as the Hubble tension shows, an open problem in cosmology.
FAQ
Is the universe expanding into something?
No. The expansion described by Hubble's law is an expansion of space itself, not an expansion of matter into pre-existing space. There is no boundary or edge that space is pushing into. Every point in the universe sees every other sufficiently distant point moving away.
Does Hubble's law mean Earth is at the center of the universe?
It does not. The law holds equally for any observer at any location in the universe. An astronomer in a galaxy billions of light-years away would see the same pattern of recession. The universe has no center.
What is the Hubble tension?
The Hubble tension is the statistically significant disagreement between two methods of measuring the Hubble constant. Measurements based on the early universe (cosmic microwave background) give about 67 km/s/Mpc. Measurements based on the local universe (supernovae and Cepheid stars) give about 73 km/s/Mpc. The cause remains unknown and could indicate physics beyond current models.
Why was the law renamed to the Hubble-Lemaitre law?
Georges Lemaitre, a Belgian physicist and priest, published the velocity-distance relation in 1927, two years before Hubble. His paper appeared in French in a low-circulation journal and was overlooked. The International Astronomical Union formally added his name in 2018 to acknowledge his independent discovery.
Related Reading
Sources
- Primary Research: A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae (Hubble, E., PNAS, 1929)
- Additional Context:
- A Comprehensive Measurement of the Local Value of the Hubble Constant (Riess, A. et al., ApJ Letters, 2022)
- Planck 2018 results. VI. Cosmological parameters (Planck Collaboration, A&A, 2020)
- Electronic Vote on Renaming the Hubble Law (IAU, 2018)
Fact Check: Claim-by-Claim Verification Verified
All claims verified against primary sources. Hubble's 1929 publication date, the velocity-distance formula, Lemaitre's 1927 priority, the IAU 2018 vote (78%), and current Hubble constant values (67.4 and 73.2 km/s/Mpc) are all confirmed.
Sources used for verification
- Hubble 1929 original paper - pnas.org
- Riess et al. 2022 SH0ES measurement - iopscience.iop.org
- Planck 2018 cosmological parameters - aanda.org
- IAU renaming vote announcement - iau.org
- NASA Hubble's Law explainer - nasa.gov
