Science 7–10 · Year 10

The expanding balloon: Hubble's law from dots on a surface

Science understanding: Earth and space sciences (ACARA Year 10 origin of the universe; NSW Science 7-10 (2023) has no content point on the expansion of the universe, so the Stage 5 outcome cited is the focus area Reactions, whose content group Nuclear reactions opens with how the first elements formed after the Big Bang)

Practical, model not builtLow risk

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The idea

If space itself stretches, every galaxy sees every other moving away at a speed in proportion to its distance, with no centre to the expansion, which is the pattern Hubble found and the big bang model explains.

What you need

1 round balloon that inflates to about 30 cm across, held closed with a peg or bulldog clip between inflations
a marker to draw 6 dots 2 to 3 cm apart, one labelled A (home galaxy) and the others B to F
a flexible tape measure, or string and a ruler, to measure along the curved surface
a stop clock and a results table of distances before and after

How to do it

Inflate the balloon to about 10 cm across, clip it, draw the six dots and label them.
Measure the distance along the surface from A to each other dot and record it.
Inflate the balloon steadily over 10 seconds to about 20 cm across, clip it and measure the distances from A again.
Calculate each dot's speed as its change in distance divided by 10 s and plot speed against the distance measured after the inflation, because Hubble's law compares a galaxy's speed with its present distance.
Repeat the measurements and the graph taking dot C as home.
Compare the gradient with Hubble's law and use the live model to find the recession speed and redshift of a galaxy 100 Mpc away, and the time 1 / H0.

What you should see

Every distance grows by the same factor (about 2 when the diameter doubles), so a dot twice as far from home moves twice as far in the same time: speed against distance is a straight line through the origin, whichever dot is home. If the diameter doubles in 10 s, speed against the distance measured after the inflation has a gradient of 0.05 per second, and its inverse, 20 s, is how long ago the dots would all have been together if the balloon had always grown at that steady rate. Plotting against the starting distance instead gives 0.10 per second, whose inverse, 10 s, runs back only as far as the start of the inflation and is not the analogue of 1 / H0. For the real universe the Planck 2018 value H0 = 67.4 km/s/Mpc gives a galaxy 100 Mpc away a recession speed of 6,740 km/s and a redshift of 0.0225, and 1 / H0 = 14.5 billion years, close to the age of about 13.7 billion years the CSIRO Australia Telescope National Facility (ATNF) gives from current observations (the two differ because the expansion rate has changed over time). Hubble's 1929 values were about 500 km/s/Mpc, which would give under 2 billion years; his distance scale was later corrected. The learner knows it worked when both home dots give straight lines through the origin with the same gradient within 10 percent.

What changes

What you change: distance of a dot from the home dot
What you measure: speed of the dot away from home
What you keep the same: inflation time
same balloon
measuring along the surface

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

The big bang was an explosion from a centre into empty space; every point sees the others moving away, so the expansion has no centre.
Galaxies themselves are growing; space between distant galaxies stretches while gravity holds each galaxy together.
Redshift means a galaxy looks red; it means its spectral lines are shifted to longer wavelengths.

Safety card

Low riskLearners carry it out

Hazards

latex allergy
a bursting balloon near the eyes

Controls

latex-free balloons for anyone with an allergy
inflate to no more than about 30 cm and keep the balloon away from faces

Note

No hazardous chemical and no flame or heating apparatus: a generic classroom risk assessment (CSIS 1.7 or RiskAssess) covers trips, spills, warm lamps and sharp edges.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science 7-10 Syllabus (2023), NSW Education Standards Authority (implemented from 2026; codes read at curriculum.nsw.edu.au on 22 and 23 September 2026)SC5-RXN-01 SC5-WS-04 SC5-WS-05
Australian Curriculum v9AC9S10U03AC9S10H01AC9S10I04

Sources

The pages the author read to write this activity.

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