Episode 704: The expanding universe

Ursa Major

The universe is expanding; don’t confuse this with the big bang (see episode 705: Cosmology).


  • Student activity: Looking at a galaxy (20 minutes)
  • Discussion: The scale of the universe (10 minutes)
  • Discussion: Hubble’s observations (10 minutes)
  • Demonstrations: Expanding universe (20 minutes)
  • Student activity: Modelling Hubble’s law (20 minutes)
  • Discussion: Cosmological red shift (10 minutes)
  • Student questions: Red shift of quasars (30 minutes)

Student activity: Looking at a galaxy
In 1925 Hubble showed that the Andromeda nebula was a collection of stars (i.e. a galaxy) outside and quite distinct from our own Milky Way galaxy. Students can look for Andromeda using binoculars.

Andromeda nebula

Episode 704-1: Two million-year-old light: Seeing the Andromeda nebula (Word, 60 KB)

Discussion: The scale of the universe
Discuss the different ways in which astronomers determine distances in space, and the units used.

Episode 704-8: The ladder of astronomical distances (Word, 106 KB)

1 light-year = 1 ly = 9.46 ´ 1015 m

1 parsec = 1pc = 3.09 ´ 1016 m

1 Mpc = 3.09 1022 m

Episode 704-2: Distances in light travel time (Word, 42 KB)

Discussion: Hubble’s observations
Hubble measured 24 galaxies. 22 had red shifted light. He plotted recession speed v against distance d.

Speed was much easier to measure (from the Doppler shift) than distance. There are real problems in setting a length scale. Different methods are used at the ever increasing distances, each overlapping to allow a (hopefully) consistent calibration.

Hubble found v µ d; for each increase in distance of 1 Mpc the recessional speed of galaxies increases by 70 km s-1. This is the Hubble constant, Ho. The bigger Ho the faster the universe expands (and thus the younger it is) and vice versa.

This gives us an idea of the age of the universe:

Ho in km s-1 per km = 70/3.09 ´ 1019 = 2.26 ´ 10-18 s-1;

Age of universe = 1/2.26 ´ 10-18 s-1 = 4.4 ´ 1017 s = 15 My approx.

Episode 704-3: Hubble’s law and the age of the universe (Word, 200 KB)

Demonstrations: Expanding universe
It is useful to show that, although the whole of the universe is expanding, this does not imply that there is a single centre of expansion.

Inflate a balloon with sticky paper dots attached to it, representing galaxies. Note that the galaxies move apart, but they do not themselves get bigger (because gravity holds them together).

Episode 704-4: Relativity and the expanding universe (Word, 140 KB)

Draw up two OHP sheets, each with a matrix of dots. They have the same pattern, but one has a greater spacing. Overlay second OHP sheet – whichever dot on the bottom sheet you use as the origin to match to a dot on the second sheet, all the other dots move away from the chosen dot. There is no centre from which all dots move away from.

Student activity: Modelling Hubble’s law
Students could use a length cut from a wide rubber band. Mark dots to represent galaxies. Identify a ‘home’ galaxy. Stretch the rubber. A dot twice as far from the home galaxy moves twice the distance; this is the Hubble Law.

Stretching a 'rubber' universe

The Hubble law thus implies that the universe is expanding.

What’s it expanding into? Nothing! Space (or rather space-time) is being created as the universe expands.

If universe is expanding, why don’t we see it locally – e.g. in the solar system? Is it too small an effect? Yes; the expansion is overcome by gravity.

Discussion: Cosmological red shift
The so called cosmological red shift is not due to relative speeds as such – it’s due to the expansion of space itself, stretching the wavelength of light because space(time) is expanding but the resulting formula for red shift is the same.

A red shift of 1 corresponds to 7 ´ 109 years ago, i.e. the light was emitted when the universe was half as old as it is now. Red shifts > 5 have been observed.

Episode 704-5: Red shift (Word, 199 KB)

Episode 704-6: Red shifts of galactic spectra (Word, 156 KB)

Student questions: Red shift of quasars
Students can tackle some questions about the red shifts of quasars.

Episode 704-7: Red shifts of quasars (Word, 37 KB)

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Episode 704: The expanding universe (Word, 865 KB)