This is, perhaps, one of the most important problems in modern cosmology. In fact, all other problems can be considered, essentially, solved in modern astronomy, with this key problem the only remaining holdout! 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 NASA's, COBE and WMAP satellites. The universe is awash in what is called the cosmic microwave background radiation - a relic from a time when matter and the radiation produced in the Big Bang were in intimate contact with each other. This means that the matter and radiation were physically interacting with one another, and 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 300,000 years after the Big Bang. Then, matter became neutral and non-ionized, and the fireball radiation saw the universe as a transparent void. COBE studied this radiation carefully for four years and discovered that to one part in 10,000 its intensity in the sky was the same in every direction after local effects were subtracted. The material that came out of the Big Bang, at least at the large angular scales studied by COBE, was remarkably smooth, except for some very large-scale fluctuations at a level of one part in 100,000 or so which were over 100 million light years across. COBE was not able to search for any irregularities that would be comparable in size to what we now see as galaxies. The WMAP mission, however, could study these irregularities (see image above) and in 2003 the WMAP scientists announced spectacular findings about the age and composition of the cosmos from even higher-resolution measurements of the cosmic microwave background.

Theoreticians studying the clumpiness generated by gravity in the matter in the Big Bang have been able to determine that, under the gravitational conditions thought to prevail just AFTER the universe became transparent to the fireball radiation, there are two scales of fluctuations in the density of matter that would have survived. The first scale contains clumps of matter equal to 1000 billion times the mass of the Sun, the second mass scale equals about 100,000 times the mass of the Sun. The first is about the size of a very large galaxy or a cluster of galaxies; the second is about the mass of a globular star cluster. This material would have been a mixture of only hydrogen and helium, in the proportion appropriate to their cosmological abundances. There would have been no other elements. So already we see that the infant galaxies and their first generations of stars would have been very different from those born today. Astronomers are still debating when the Galaxy Formation Era began, but WMAP measurements suggest that the first stars were forming about 280 million years after the Big Bang.

This answer was updated in 2011. See my books: The Astronomy Cafe (1998) and Back to the Astronomy Cafe (2003) for more FAQs in printed form. Author: Dr. Sten Odenwald, Copyright 2011 Return to Ask the Astronomer