1. An accidental discovery
The CMB was a key prediction of the Big Bang theory, so many people looked for it. It was eventually found in 1964 by radio astronomers looking for something else.
2. Space itself is expanding
Although we say the universe is expanding, it’s really the fabric of space itself. Any two galaxies that aren’t tied to each other by gravity are getting further apart.
3. Faster than light
Physical objects can’t travel faster than light, but that doesn’t apply to the stretching of space. During the universe’s inflationary phase, space expanded much faster than light.
4. The start of everything
Stephen Hawking believed that both space and time were created at the Big Bang. Before that, neither time nor space existed.
5. The missing link
The reason why there are so many competing theories of the universe is that physicists don’t know how to combine gravity and quantum physics. Until they do, it’s all guesswork.