Centre for Mathematical Sciences, University of Cambridge
The usual approach in physics could be described as building from the bottom up. That is, one assumes some initial state for a system and then evolves it forward in time with the Hamiltonian and the Schrodinger equation. This approach is appropriate for laboratory experiments like particle scattering, where one can prepare the initial state and measure the final state. The bottom-up approach is more problematic in cosmology, however, because we do not know what the initial state of the Universe was, and we certainly cannot try out different initial states and see what kinds of universe they produce.
Different physicists react to this difficulty in different ways. Some - generally those brought up in the particle physics tradition - just ignore the problem. They feel the task of physics is to predict what happens in the laboratory, and they are convinced that string theory or M-theory can do this. All they think remains to be done is to identify a solution of M-theory, a Calabi-Yau or G2 manifold that will give the Standard Model as an effective theory in four dimensions. But they have no idea why the Universe should be 4-dimensional and have the Standard Model, with the values of the forty or so parameters that we observe. How can anyone believe that something so messy is the unique prediction of string theory? It amazes me that people can have such blinkered vision - that they can concentrate just on the final state of the Universe and not ask how and why it got there.
Those physicists that try to explain the Universe from the bottom up mostly belong to one of two schools, these being associated with either the inflationary or pre-big-bang scenarios. I now discuss these approaches in turn and show that neither of them is satisfactory.
Universe or Multiver.se?, ed. Bernard Carr. Published by Cambridge University Press. © Cambridge University Press 2007.
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