The expanding Universe: recession without motion

In 1929 Edwin Hubble noticed something unsettling: nearly every galaxy he pointed his telescope at is moving away from Earth. And by a strikingly simple rule — twice as far means twice as fast. For a moment it could seem we sit at the centre of a cosmic explosion. Drag the time slider in the figure and click any other galaxy — you will see why that conclusion was wrong.

Raisins in rising dough

Picture raisin dough rising in the oven. Every raisin sees exactly the same thing: all the others move away from it, the distant ones faster — because there is simply more rising dough between them. No raisin is "the centre of the rising". So it is with galaxies: click a different one in the figure and you will see the identical law of recession. Every observer in the Universe has the impression that everything flees from them.

Recession without motion

The hardest part: galaxies are not flying through space like shrapnel after an explosion. They are essentially standing still — it is the distances between them that grow, because space itself is expanding. That is why remote galaxies can "recede" faster than light without breaking any law of physics: nothing moves quickly through space; there is simply more and more space.

The light travelling towards us is stretched too. A wave sent out blue arrives shifted towards the red — the longer its journey, the more so. This redshift is the astronomer’s ruler: it says directly how much the Universe has grown since the light set off.

The Big Bang did not happen somewhere. It happened everywhere at once.

Run the film backwards

If everything is moving apart, it must once have been closer together. Rewinding the expansion takes us to a moment 13.8 billion years ago, when the entire observable Universe was denser and hotter than the inside of a star. The trace of that epoch can still be seen: the cosmic microwave background, an afterglow filling the whole sky — the oldest light in existence, stretched a thousandfold by the expansion.

A simplificationThe drawing is flat and ignores the galaxies’ own motions (we fake them as a slight scatter of the points), the gravitational binding of clusters, and the fact that for a few billion years the expansion has been accelerating — the work of dark energy, still not understood. The law v = H·d itself, though, remains exactly what Hubble measured.