How did galaxies form from a smooth, uniformly-expanding Big Bang state?
This is, perhaps, one of the most important problems in modern cosmology, second only to determining whether the universe is infinite, or finite. In fact, all other problems can be considered, essentially, solved in modern astronomy, with this key problem the only remaining hold-out! The Big Bang was a very smooth affair. We know this thanks to the results of many investigations and in particular the recent dramatic data from the NASA, COBE satellite. The universe is awash in what is called the cosmic microwave background radiation which is a relic from a time when matter and the radiation produced in the Big Bang were in equilibrium. This means that the matter and radiation were physically interacting with one another, and this arose because the matter was fully ionized. The universe was opaque to its own radiation. This circumstance ended once the universe had expanded and cooled to a temperature of about 3000 K which occurred around 700,000 years after the Big Bang. Then, matter bec
