The expanding universe — Astronomy, 11–13 years
The universe is not simply an empty room with galaxies moving inside it. Space itself has been stretching, carrying galaxies farther apart over time.
Space can stretch
Imagine dots drawn on a stretchy sheet. As the sheet expands, every dot can become farther from the others, even though no dot is crawling across the sheet. Galaxies behave in a similar way: on very large scales, the space between them expands. The expansion does not mean that everything, including people and planets, is growing.
What problem did expansion solve?
If the universe had always stayed the same, distant galaxies should not show a simple pattern of moving away. Observations found that light from many galaxies is shifted towards red, and more distant galaxies usually show a larger shift. Expansion explained this shared pattern better than treating every galaxy as moving randomly through a fixed space.
A simple redshift model
Draw three dots on a rubber band at 1, 2 and 3 centimetres. Stretch the band so every distance doubles: the gaps become 2, 4 and 6 centimetres. The farthest dot has gained the most distance from the chosen starting dot. In astronomy, a larger redshift usually tells us that a galaxy has been carried farther by expansion.
An explosion into empty space?
It is natural to picture the Big Bang as a bomb exploding at one place and throwing galaxies into pre-existing emptiness. That picture is misleading. The early universe was hot and dense, and space itself began expanding everywhere; there was no known central point from which everything flew outward into an outside universe.
Reading the universe’s history
Astronomers use redshift to estimate how fast distant galaxies are receding and how far away they are. Looking farther away also means looking further into the past, because light takes time to arrive. Together, these clues let scientists compare the young universe with the one we observe today.
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