Astronomy
The Expanding Universe
Quick fact
The universe isn't expanding into anything; space itself is stretching, carrying galaxies along like raisins in a rising loaf of bread.
Why this is interesting
You've seen images of distant galaxies—but did you know they are all racing away from us? Why is the entire universe flying apart?
Read the full explanation
Understanding The Expanding Universe
Imagine a loaf of raisin bread baking. As the dough expands, every raisin moves away from every other raisin. Similarly, the universe's fabric—space—is stretching, causing galaxies to separate. In the 1920s, Edwin Hubble noticed that distant galaxies appear redder (redshifted), meaning they are moving away. The farther a galaxy, the faster it recedes—a relationship now called Hubble's Law. This doesn't mean we are at the center; from any galaxy, others appear to move away. This expanding motion is the key evidence for the Big Bang, where the universe began from an incredibly hot, dense state about 13.8 billion years ago and has been expanding ever since.
A deeper explanation
The expansion of the universe is not a motion through space but the expansion of space itself, as described by Einstein's general relativity. The metric of spacetime changes over time, increasing distances between cosmic structures. The rate of expansion is quantified by the Hubble constant (H₀). Importantly, the expansion is not slowing down as gravity would suggest; instead, it is accelerating due to a mysterious repulsive force called dark energy. Understanding this expansion allows cosmologists to trace the universe's history backward to the Big Bang, predict its eventual fate (e.g., heat death or Big Rip), and makes sense of the cosmic microwave background—the afterglow of the early universe. It is the cornerstone of modern cosmology.