In the 1920s, Edwin Hubble made a landmark discovery by measuring the distances to distant galaxies and analysing the spectra of their light. He found that the light from almost all galaxies was redshifted — the spectral lines were shifted toward longer (redder) wavelengths compared to laboratory sources.
This redshift arises from the Doppler effect: as a galaxy moves away from Earth, the light waves it emits are stretched, increasing their wavelength. The greater the recessional speed, the larger the redshift.
By plotting recessional velocity against distance for many galaxies, Hubble discovered a linear relationship now known as Hubble's Law:
where:
The Hubble Constant represents the rate of expansion of the universe. Its accepted value is approximately:
The SI unit of is , but it is conventionally quoted in .
If the universe has been expanding at a constant rate since the Big Bang, then the time elapsed (the Hubble Time) is:
This gives an upper estimate for the age of the universe of approximately 14 billion years.
A galaxy is at a distance of from Earth. Using , calculate its recessional velocity.
Hubble's Law implies that all galaxies are moving away from each other — the universe is expanding. Running this expansion backward in time leads to the conclusion that all matter in the universe was once concentrated at a single point of infinite density. This initial event is called the Big Bang.
Key evidence supporting the Big Bang theory:
| Evidence | Description |
|---|---|
| Hubble's Law / Redshift | Galaxies recede at speeds proportional to their distance, implying an expanding universe originating from a single point. |
| Cosmic Microwave Background (CMB) | Uniform microwave radiation filling all of space, interpreted as the afterglow of the Big Bang. |
| Abundance of light elements | The observed ratio of hydrogen to helium (~75%:25%) matches predictions from Big Bang nucleosynthesis. |