In our last tour of the universe, you learned about the
afterglow of the Big Bang, more properly called the cosmic
microwave background (CMB). These microwaves, which come from
all directions, sprayed forth about 380,000 years after the Big
Bang when protons and electrons settled down to form hydrogen,
allowing light to travel freely through space. The CMB started
out as visible and infrared light, but has since been stretched
into microwaves by the relentless expansion of space itself.
There’s an amazing feature of the cosmic microwave background…
it is extremely smooth and uniform. In fact, across the entire
sky, the CMB is exactly the same to better than 1 part in
10,000. Why? Is there any reason for the universe to appear so
smooth on such a large scale? Or is it just a coincidence? As it
turns out, it is not a coincidence. But until 1980, it was
impossible to explain. That’s because the standard theory of the
Big Bang said that a tiny, hot, region expanded to form all of
space and all the matter and ener