Episode 704: The expanding universe
Lesson for 16-19
- Activity time 120 minutes
- Level Advanced
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.
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 (1 pc) = 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 ∝ d; for each increase in distance of 1 Mpc the recessional speed of galaxies increases by 70 km s-1. This is the H0 . The bigger H0 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:
H0 in km s-1 per km = 70 km s-13.09 × 1019 km
H0 = 2.26 × 10-18 s-1;
Age of universe = 12.26 × 10-18 s-1
Age of universe = 4.4 × 1017 s
So 15 My, more or less.
Episode 704-3: Hubble’s law and the age of the universe (Word, 200 KB)
Demonstration: 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.
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)