Science Puzzle
The Cosmic Balloon
Astronomers observe that every distant galaxy is moving away from us, and the further away it is, the faster it recedes. This seems to suggest that we are at the centre of the universe and everything is exploding away from us.
But we are not at the centre. How can every galaxy be moving away from every other galaxy without there being a single central point?
The Answer
Think of dots drawn on the surface of a balloon. As you inflate the balloon, every dot moves away from every other dot. No single dot is at the centre because the expansion is happening in the surface itself, not outward from any point on the surface.
The universe expands in the same way: space itself is stretching. Galaxies are not moving through space away from a central explosion; the space between them is growing. From any galaxy in the universe, all other galaxies appear to be receding, because the fabric they all sit in is expanding uniformly.
This is metric expansion, a property of spacetime described by general relativity. The Big Bang was not an explosion into pre-existing space; it was the beginning of space, time and matter together, with no centre and no edge.
The principle: Metric expansion. The universe has no centre because space itself is expanding uniformly. Every point recedes from every other point, just as every dot on an inflating balloon moves away from every other dot.