Table of Contents
What Is Hubble's Law?
Hubble's Law (also called the Hubble-Lemaitre Law) describes the observation that galaxies are moving away from us at velocities proportional to their distance. Discovered by Edwin Hubble in 1929, this relationship was the first observational evidence for the expansion of the universe and remains one of the foundational principles of modern cosmology.
The law states that the farther a galaxy is from us, the faster it appears to be receding. This is not because galaxies are physically moving through space at high speeds, but rather because the fabric of space itself is expanding, stretching the distances between galaxies over time.
The Formula
Where v is the recessional velocity in km/s, H₀ is the Hubble constant in km/s/Mpc, and d is the distance in megaparsecs (Mpc). One megaparsec equals approximately 3.26 million light-years.
The Hubble Constant
| Method | H₀ Value (km/s/Mpc) | Year |
|---|---|---|
| Hubble (original) | ~500 | 1929 |
| Planck CMB | 67.4 ± 0.5 | 2018 |
| SH0ES (Cepheids) | 73.0 ± 1.0 | 2022 |
| Consensus range | 67 - 74 | Current |
Frequently Asked Questions
Why is there tension in the Hubble constant?
Different measurement methods give slightly different values. Measurements from the cosmic microwave background (Planck satellite) give about 67.4 km/s/Mpc, while local distance ladder measurements (using Cepheid variables and supernovae) give about 73 km/s/Mpc. This discrepancy is known as the "Hubble tension" and may point to new physics.
Does Hubble's Law mean Earth is the center of the universe?
No. Every observer in the universe would see the same pattern of galaxies receding proportionally to their distance. This is because space itself is expanding uniformly, like dots on an inflating balloon moving apart from each other.
What is a megaparsec?
A megaparsec (Mpc) is one million parsecs, or about 3.26 million light-years, equivalent to 3.086 x 10^22 meters. It is the standard unit of distance used in extragalactic astronomy and cosmology.