Wednesday, March 16, 2005

Beyond the Bang

My Dad works in a warehouse in Wigan in the North-West of England. I'll have some more about him in other posts. A few days ago he forwarded a cosmology question to me from Paul, a former co-worker of his. It's one of the classics that cosmologists get pretty often. The question is "What happened before the big bang?" My Dad's friend began wondering about this after reading an article in the Daily Express in which Michio Kaku said that this is the most embarrassing question to ask physicists. People are fascinated by this stuff and the question is a good one. However, I don't think of this as an embarrassing question; rather just one to which, at present, we don't know the answer.

Physicists arrive at the idea of the big bang by first making observations of the universe today and understanding how these are described by well-established theories of gravity and particle physics. We then extrapolate back in time to infer what the early universe must have been like, and then test the theory by working out what new predictions result and checking whether they agree with observations. This methodology works remarkably well and has provided us with an extremely well tested, self-consistent and coherent understanding of the universe.

The central result that arises from this work is that the universe is expanding - all distant galaxies are moving away from us and the further away they are, the faster they are moving. Of course, this means that in the past all galaxies were closer together. When you get far enough back in time, what inevitably results is that one has a very high density of matter in a very small region. This means that one is no longer able to use gravity (describing the physics of space and time and understood by Einstein's General Relativity) and quantum mechanics (describing the physics of the very small) separately, but is forced to take into account their mutual effects. However, at present we do not have a theoretical framework that allows us to answer questions about gravity and particle physics working together in this way. There are some promising attempts to develop such a theory, such as string theory, but so far this has not allowed us to provide a definitive answer to the question.

If one just blindly continues to use General Relativity past this point of its breakdown (not something that should really make sense), one reaches a mathematically unacceptable result known as a singularity. At this point, time, space and matter do not exist. The emergence of time, space and matter after this is what we loosely refer to as the big bang. Since we don't have a theory to understand these extremely early times (much less than a nanosecond after the big bang) the right answer to Paul's question is that we don't know.

This in no way diminishes the remarkable progress in cosmology during the last century. It is just a natural part of science - there are things we're still working on. To be precise, the big bang theory only refers to the fact that the observable universe originated as a very small, highly energetic region about 13.7 billion years ago. The evidence for this is overwhelming and it is a firmly established pillar of modern science. Of course, one can speculate about the earliest times - maybe time has no meaning before this and the question is really meaningless, or perhaps the universe has to be understood in the context of a multi-universe theory, or maybe the expanding universe we see today arose after an earlier collapsing phase. These speculations are crucial to eventually extending our firm understanding to earlier and earlier times. However, for now, all I can say is that we're working on it and it is great fun.
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