Astronomy
Universe Expansion
Quick fact
The farther away a galaxy is, the faster it appears to recede—this relationship is Hubble's law, discovered in 1929 by Edwin Hubble.
Why this is interesting
Look at the night sky—almost every galaxy appears to be rushing away from us. But are we at the center of a cosmic explosion, or is something far stranger happening to space itself?
Read the full explanation
Understanding Universe Expansion
Imagine a loaf of raisin bread baking in an oven—as the dough expands, every raisin moves away from every other raisin. Similarly, the universe is like that dough, with galaxies as the raisins. The expansion is not galaxies moving through space; instead, space itself is stretching, carrying galaxies along. This means that on large scales, the distance between any two faraway galaxies increases over time. Astronomers measure this expansion using redshift: light from distant galaxies is shifted to longer wavelengths because the space it traveled through expanded. The expansion rate is described by Hubble's constant, and it implies that the universe was once much smaller and hotter—the Big Bang.
A deeper explanation
The expansion is described by the metric expansion of space, a solution to Einstein's equations of general relativity applied to a homogeneous, isotropic universe. The scale factor of the universe increases with time, as captured in the Friedmann equations. In the 1990s, observations of distant supernovae revealed that this expansion is accelerating, not slowing down—a surprising discovery that led to the concept of dark energy, a mysterious form of energy counteracting gravity. Without expansion, we could not explain the cosmic microwave background radiation, the abundance of light elements, or the large-scale structure of galaxies. Understanding universe expansion is central to cosmology: it tells us about the past, present, and future fate of the cosmos, whether it will expand forever, slow to a halt, or eventually contract.