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Hubble's Law

Physics: AQA A Level, topic 9.3.2

Hubble's Law

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

Measuring distance

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  • In more recent times, this has been superseded by the use of type 1a supernovae.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

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

  • Hubble's law relates how fast a galaxy is moving away from us, v, to its distance from Earth, d:
  • v=Hdv = 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

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

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

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:
  • (2.5×10−19)−1(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

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.

Test yourself on Hubble's Law

  • What is the equation for Hubble's law?

  • If object X is twice as far away from the Earth as object Y, then which of the following is true?

  • Which of these statements are true about Hubble's law and the big bang?

  • If the big bang happened what evidence should we see?

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