Hubble's Law

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Hubble's Law

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

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  • In more recent times, this has been superseded by the use of type 1a supernovae.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

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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.
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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.
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Hubble constant 2

  • The current accepted value of the Hubble constant is 73.5 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:
    • (2.5×1019)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.
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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.

Jump to other topics

1Measurements & Errors

2Particles & Radiation


4Mechanics & Materials


6Further Mechanics & Thermal Physics (A2 only)

7Fields & Their Consequences (A2 only)

8Nuclear Physics (A2 only)

9Option: Astrophysics (A2 only)

10Option: Medical Physics (A2 only)

11Option: Engineering Physics (A2 only)

12Option: Turning Points in Physics (A2 only)

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