How did the Big Bang Work?

Most recent answer: 07/26/2012

I have a question which has bothered me for quite a number of years and I just have no one to turn to ask. I hope that you can take a few minutes to help me understand. My question has to do with the basic structure of the universe. My understanding is that most physicists buy into the big bang theory - as supported by Hubble's observations of the red light shift indicating that the universe is constantly expanding. However, to put this concept in terms that I can understand, this would mean that from the moment of the big bang to now, the universe would constantly expand from a given point and that all matter would travel outward from that point at some particular speed - as with all explosions. After several billion years, this would create something of a balloon-like structure - relatively empty at the center - but with most matter falling within a certain zone at a constantly expanding radius from that central point - and allowing for differential speeds, collisions, etc. - a fuzzy balloon perhaps. This is the only mechanism I can understand that proves the red light shift is universal. However, it's also my understanding that when the galaxies have been mapped, they suggest the structure of the universe is actually more of a non-ending sponge-like structure with strings and clusters of galaxies linked together with gaps in between. This structure would seem to indicate a steady-state universe over a big bang/fuzzy balloon universe. The obvious conclusion is that the observable universe is completely at odds with the theoretical concept of the big bang. Is there something that ties these two concepts together? Or, if the observation of the sponge-like nature of the universe is correct, is there something fundamentally wrong with the big bang concept?
- jeffrey (age ~50)
Yes, it's hard to initially grasp this, but the particular issues you're concerned about actually work out fine in the current picture. Let's look at the key points.

In the BB picture, the universe expands out not from one "given point" but from any given point. Stand anywhere. You'll see the stuff near you moving away, the farther the faster. Think of how things look from somebody else's point of view. They see the same thing.

One illustration often used is a raisin muffin expanding as it cooks. From any raisin's point of view, the other raisins are moving away. There's no particular place that gets especially empty, so there's nothing balloon-like about it. The red light shift is approximately proportional to distance, so it covers a huge range.

As for the current structure of the galaxies, on a fairly large scale it is indeed more spongy and irregular than one would get from well-stirred muffin dough. However, on a very large scale it looks quite uniform.

There is indeed a close tie-in between the BB picture and the current distribution of matter. At one stage, there were just small ripples in the density. These are still visible as small ripples in the cosmic microwave background (CMB) coming in from different directions. Over time, those ripples would tend to grow because regions with a little extra mass pull more mass in via gravity. That process can be simulated on a computer using ordinary gravitational dynamics. It turns out that the slight ripples in the CMB imply that now the matter distribution should have unevenness very close to what we see in the galaxies. So it really does all tie together.

The major remaining uncertainties concern what happened at even earlier stages. There the evidence starts getting thinner. For example, although the ripples in the CMB are close to what's expected from quantum fluctuations, we don't know for sure what was going on when those fluctuations set in. The main picture has been "inflation", a period of rapid exponential expansion, expected from General Relativity for certain types of transient physical states. (We're currently in a period of much slower inflation.) Problems with that picture have led to alternatives, including collisions between entire 3D spaces in some higher dimension. Weird as all that may sound, the Planck satellite is currently measuring details of the CMB ripples, in an attempt to sort out those possibilities.

Mike W.

(published on 07/26/2012)

Follow-up on this answer