If Big Bang theory suggests that the universe started out as a homogeneous mass that spread uniformly in all directions, how did we get 'lumps' [stars, galaxies, etc] in the universe?

Asked by: Mike Myers

Answer

The most popular idea at the moment is quantum fluctuations in the inflation field. It is thought that the universe underwent a rapid period of expansion. This solves some of the so called classical problems of the Big Bang cosmology. In order to drive this inflation a new field was proposed; the inflation field. The quantum fluctuations in this field could be responsible for "seeding" the universe. These seeds appear as small fluctuations in the CMBR. This gives a test to check the theories. Other ideas have been proposed. For example cosmic strings have been suggested as the mechanism for seeding. However, detailed analysis of the theory and the CMBR do not agree. It is thus unlikely that cosmic strings are responsible. Other mechanisms via M-theory may be the answer.
Answered by: Andrew Bruce, Grad student, UK

Answer

Before the expansion, the entire universe was compressed into (nearly) infinitely small dimensions that were so small that the entire universe was subject to quantum mechanical uncertainty. One of the things that follows from the uncertainty principle is that as the number of possible positions an object can take is reduced, the number of possible momentums (or velocities) increases. In other words, the more certain you can be about an object's position, the less certain you can be about its speed. If you take a single picture of an event, you know nothing about the movement of the objects in the picture. If you take two pictures and know the time between them, you will be more certain about the movement of the objects, but you will be less certain about the position of the objects. This works in space too. As the entire universe collapses to a point, there becomes a nearly infinite number of possibilities for the trajectories of every object in that universe. Randomness is buried inside a tiny point. But then the inflationary period occurred extremely quickly instants after the big bang. It occurred so quickly that all of those random movements were blown up to sizes large enough for their momenta to settle down. In essence, the inflationary period took a "picture" of the quantum mechanical randomness that was buried in the tiny dot within which the entire universe was held just before the big bang. And thus we have a primarily homogeneous universe with small deviations which reflect the inherent quantum mechanical randomness at the beginning of time and space.
Answered by: Ted Pavlic, B.S., Electrical Engineering Grad Student, Ohio State U.

Answer

This is one of the many profound questions in Cosmology. The textbook answer is that in the early universe things were extremely close together. All mass was so close to all other mass, that simple quantum aberrations and statistics caused it to "lump" together. These lumps gained more and more in mass since they were now of a greater mass than the surrounding mass, and as the universe expanded and "cooled" these became our galaxies. This means that as we gaze out upon our beautiful universe, we are seeing quantum aberrations from billions of years ago. The Big Bang theory though, only guesses at a homogeneous mass for the beginning universe. There is no real theoretical basis for this except for the fact that we know as time increases, entropy increases. Therefore, the early universe had to start out as a very ordered place in order for it to end up how it is now. Especially since when you zoom out on a cosmological scale, the universe seems more or less homogeneous still!
Answered by: Justin Wilson, Physics Undergrad Student, TAMU, College Station