The expanding universe
In the 1920s, astronomers discovered that the universe is expanding. This single fact transformed our understanding of cosmic history.
Redshifted galaxies
Vesto Slipher found in the 1910s that most spiral nebulae have redshifted spectra, meaning they are moving away from us. In 1929, Edwin Hubble, building on work by Georges Lemaître, combined these redshifts with distances and found that more distant galaxies recede faster.
Hubble's law
Hubble's law states that a galaxy's recession velocity is proportional to its distance: \(v = H_0 d\). The Hubble constant \(H_0\) is about 70 km/s per megaparsec (one megaparsec is about 3.26 million light-years). A galaxy 100 Mpc away recedes at about 7,000 km/s.
v = H_0 dWhat expansion means
Galaxies are not flying through space away from a centre; space itself is expanding, carrying galaxies apart. Every observer in any galaxy would see the same pattern: all distant galaxies moving away. A common analogy is raisins in rising bread dough: every raisin moves away from every other.
Cosmological redshift
As light travels through expanding space, its wavelength is stretched. Cosmological redshift (z) measures how much the universe has expanded since the light was emitted: if z = 1, wavelengths have doubled, and the universe was half its current size when the light left.
The age of the universe
Running the expansion backward, all galaxies would have been together at some point in the past. Roughly, 1/H0 gives the age; precise modern measurements combining many observations give an age of about 13.8 billion years.
The Hubble tension
Different methods give slightly different values of the Hubble constant: measurements of the early universe (the cosmic microwave background) give about 67-68 km/s/Mpc, while measurements using nearby Cepheids and supernovae give about 73. This 'Hubble tension' may point to new physics or unrecognised measurement errors and is a major open question.
Measuring cosmic distances
Astronomers use a 'distance ladder': radar and parallax for nearby objects, Cepheid variables for nearby galaxies, and Type Ia supernovae, which have predictable peak brightness, for distant galaxies. Each rung calibrates the next.
Worked example
A galaxy has a recession velocity of 14,000 km/s. Using \(d = \frac{v}{H_0} = \frac{14{,}000}{70} = 200\) Mpc, or about 650 million light-years. Light from it left about 650 million years ago, when life on Earth was still mostly simple and marine.
Key takeaways
Slipher found redshifted galaxies; Hubble and Lemaître showed that more distant galaxies recede faster. Hubble's law is v = H0 d, with H0 about 70 km/s/Mpc. Space itself expands; every observer sees the same pattern. Cosmological redshift measures expansion since light was emitted. Extrapolating backward gives an age of about 13.8 billion years. The Hubble tension between early and late measurements is unresolved. The distance ladder links parallax, Cepheids, and supernovae.
Flashcard
Found galaxy redshifts
Check your understanding
According to Hubble's law, a galaxy twice as far away as another recedes: