Table of Contents
Hubble's Law
Hubble's Law states that the recession velocity of a galaxy is directly proportional to its distance from us. This relationship, first observed by Edwin Hubble in 1929, is the primary evidence for the expansion of the universe. The proportionality constant, known as the Hubble constant (H0), represents the current rate of expansion.
The expansion does not mean galaxies are moving through space; rather, the fabric of space itself is expanding, carrying galaxies along with it. Nearby galaxies within our Local Group are gravitationally bound and do not recede, but beyond about 10 Mpc, expansion becomes the dominant motion.
Expansion Formulas
Key Measurements of H0
| Method | H0 (km/s/Mpc) | Year |
|---|---|---|
| Hubble (original) | ~500 | 1929 |
| HST Key Project | 72 ± 8 | 2001 |
| Planck CMB | 67.4 ± 0.5 | 2018 |
| SH0ES (Cepheids) | 73.0 ± 1.0 | 2022 |
Frequently Asked Questions
Can galaxies recede faster than light?
Yes. Galaxies beyond a certain distance recede faster than the speed of light. This does not violate relativity because the galaxies are not moving through space faster than light; instead, the space between us is expanding at a rate that effectively makes the separation increase superluminally.
What is the Hubble tension?
The Hubble tension refers to the discrepancy between H0 measured from the early universe (CMB, ~67.4) and from the local universe (Cepheids/supernovae, ~73). This 4-5 sigma disagreement might indicate new physics beyond the standard cosmological model.
How old is the universe?
The age can be approximated as 1/H0, giving about 14 billion years. More precise calculations accounting for the changing expansion rate yield 13.8 billion years, consistent with the oldest known stars and globular clusters.