9.3.2

# Hubble's Law

Test yourself

## Hubble's Law

Hubble investigated the relationship between the recession velocities of galaxies and their distance from the Milky Way.

### Measuring distance

• The hydrogen spectra of distant galaxies gave a measurement of how fast the galaxies were moving away from the Milky Way.
• The distance to the galaxies was measured by observation of the period of Cepheid variables as a standard candle.
• In more recent times, this has been superseded by the use of type 1a supernovae.

### Hubble's graph

• When Hubble plotted a graph of velocity against distance, he found that there was a directly proportional relationship.
• The further away a galaxy is, the faster it is moving away from us.

### Hubble's law

• Hubble's law relates how fast a galaxy is moving away from us, v, to its distance from Earth, d:
• $v = Hd$
• H is the Hubble constant. It is the gradient of the graph.

## Hubble's Law and the Big Bang

Hubble's discovery has some important implications.

### Expanding universe

• The consequence of this graph is profound.
• It is only explained by the expansion of the universe, as the galaxies furthest away are also the fastest moving.
• If the clock is run backwards, everything in the universe appears to have come from a single point.

### Hubble constant

• The gradient of this graph is known as the Hubble constant.
• It is an indicator of the age of the universe, assuming that the Hubble constant does not change.

### Hubble constant 2

• The current accepted value of the Hubble constant is 65 km s-1 Mpc-1.
• In SI units, it can be shown that this is 2.5 × 10-19 s-1.
• The age of the universe is therefore:
• $(2.5 \times 10^{-19})^{-1}$
• Which gives 4.0 × 1018 s or 12.4 billion years.
• Remember this is only a rough estimate.

### Correct predictions

• If the universe came from a single point, then during the subsequent formation of atoms and expansion of the universe, a Cosmological Microwave Background Radiation (CMBR) should result.
• This was detected in 1965 by Penzias and Wilson.
• Furthermore, theory predicts that the ratio of hydrogen to helium should be about 3 to 1, which is largely borne out by observation.