The expanding universe — Astronomy, 14–17 years
Distant galaxies are generally moving away from one another because space itself is expanding. Their light is stretched to longer, redder wavelengths, giving us evidence about the universe’s history.
Space can expand
The universe is not expanding into an empty room. The distances within space are growing, so many galaxies become farther apart. A useful picture is dots on a stretching sheet, although no picture captures every detail.
The problem of galaxy motion
Astronomers noticed that the light from most distant galaxies is shifted towards red. Hubble found that more distant galaxies usually recede faster. Expansion explained this shared pattern better than a simple explosion through fixed space.
Using Hubble’s relation
Use a simplified value of 70 kilometres per second per megaparsec. A galaxy 100 megaparsecs away would then recede at about 70 × 100 = 7,000 kilometres per second. This is a model, not a precise forecast for every galaxy.
Not an explosion from one point
It is reasonable to imagine the Big Bang as debris flying through pre-existing space. That picture suggests a centre, but expansion happens throughout space: every distant observer sees other galaxies receding, with no special central location.
Measuring cosmic history
Astronomers compare galaxy distances, speeds and light spectra to estimate how fast the universe expands. These measurements test models of its age, contents and future, rather than merely making a picture of the night sky.
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