Astronomy
Cosmic Expansion
Quick fact
The expansion of the universe is not like an explosion sending galaxies outward through space; rather, space itself is stretching, carrying galaxies along as if they were dots on an inflating balloon.
Why this is interesting
You look up at the stars and they seem fixed in place, yet every distant galaxy is racing away from us. Why is the entire universe stretching like a rubber band?
Read the full explanation
Understanding Cosmic Expansion
Imagine baking a loaf of raisin bread. As the dough rises, the raisins move apart from one another. They don't move because they are pushing each other; the dough itself expands, carrying the raisins along. Cosmic expansion works similarly: space itself is 'the dough', and galaxies are the 'raisins'. The farther apart two galaxies are, the more space there is between them, so they appear to recede faster. This is captured by Hubble's Law: a galaxy's speed away from us is proportional to its distance. The expansion does not happen within our solar system or galaxy because gravity holds those structures together — it only appears on the largest scales.
A deeper explanation
Cosmic expansion is described by the metric expansion of space. In general relativity, the universe is modeled by the Friedmann–Lemaître–Robertson–Walker (FLRW) metric, which allows the scale factor of the universe to change over time. Observations show that this expansion is accelerating, attributed to a mysterious dark energy that makes up about 68% of the universe's energy density. The expansion began with the Big Bang about 13.8 billion years ago, and its rate is characterized by the Hubble constant. Understanding this expansion explains why distant galaxies have redshifts (stretched wavelengths), why the cosmic microwave background is nearly uniform, and why the universe is growing cooler and emptier over time. The fate of the universe — whether it continues expanding forever, slows, or recollapses — depends on the density of matter and dark energy.