PROLOGUE:THE BIG PICTURE
As the Universe expands, galaxies get separated from one another, and the average density of matter over a large volume of space is reduced. If we imagine playing the cosmic movie in reverse and tracing this evolution backward in time, we can infer that there must have been an instant when the density of matter was infinite. This moment in time is the “Big Bang,” before which we cannot reliably extrapolate our history. But even before we get all the way back to the Big Bang, there must have been a time when stars like our Sun and galaxies like our Milky Way∗ did not exist, because the Universe was denser than they are. If so, how and when did the first stars and galaxies form?
Primitive versions of this question were considered by humans for thousands of years, long before it was realized
∗A star is a dense, hot ball of gas held together by gravity and powered by nuclear fusion reactions. A galaxy consists of a luminous core made of stars or cold gas surrounded by an extended halo of dark matter (see section 2.7).