The Big Bang theory was arrived at by building as comprehensive as possible a picture of the present universe, then "running time backward" as far as our present knowledge of physics permits. The process started with what we know about the present structure of the Universe and the observation that it is expanding. Running time backward allowed us to see what the Universe would have looked like at successively earlier stages in its evolution, at each stage being guided exclusively by what the science of physics has taught us about how matter and energy behave under a wide range of conditions. Thus, we were able to "watch" the Universe as it contracted, the galaxies and galaxy clusters comprising it contract and revert successively into cold gas and dust clouds, into the first stars, into cold clouds of nearly pure hydrogen too cold to produce any radiation, into a hot plasma of protons, neutrons, mesons, gluons, electrons, and corresponding antimatter particles that broke down even further into an even hotter mix of quarks, mesons, and electrons, and finally into incredibly hot photons - pure energy - with the spectral distribution corresponding to a black body with a temperature increasing without limit while contracting with no known mechanism for preventing it from contracting to a point. Our knowledge of physics guides us back in time only to about 10 exp -43 second (exp denotes exponentiation) after the expansion began.

I envisage the point at which this radiaton appeared as located in a four-dimensional space about which we know nothing. We cannot even speculate about what caused the huge concentration of pure energy to appear at that point. We also have no basis for speculating what preceded that dramatic event.

I described in the opening entry in the thread that I started entitled "The Shape of the Universe" my view of how the Universe started from a point in that four-dimensional space and expanded into that space to become our Universe we find ourselves in today.