Question: How did the big-bang happen ?

David Pyle answered on 13 Mar 2011:

Wow.. if only I knew!

Actually this is a little far from my field, and I am not sure that I properly understand the current ideas. Anyway, here is a very short summary:

There are three pieces of evidence that give us the clues to what happened ‘in the beginning’ – during the Big Bang. First, the distribution of galaxies: the galaxies that are furthest away are also those that are travelling fastest. This would fit with the idea that all galaxies formed at the same place, some long time in the past. Second, there is a background ‘glow’ to the Universe, of radiation at micro-wave (mm) wavelengths. This glow is similar in all directions, and the wavelength of the radiation suggests a ‘background’ temperature of a little under 3 K (- 270 C!). Finally, the relative amounts of the light elements (Hydrogen, [H-1] and its isotope, Deuterium [H-2]; Helium isotopes [He-3 and He-4], Lithium and Beryllium) in the universe: there is one atom of He-4 for every 12 atoms of H in the universe; and about 1 atoms of H-2 for every 50,000 atoms of H.

Together this evidence fits with a theory of the Big Bang that starts as a massively dense and hot point, which then rapidly expands and cools, and has continued to expand since then [this explains the distribution of galaxies, and the background microwave glow]. At an early stage (after a few micro-seconds) hydrogen nuclei begin to form, and the first nuclear reactions of these H nuclei together with neutrons and protons then start to form H-2, He, Li and Be. These reactions stop when the early universe is sufficiently cool, and expanded, that high energy collisions between neutrons and atomic nuclei become relatively rare. This might be as soon at 3 minutes into the Big Bang. There’s a lot of complicated maths, physics (and philosophy!) that goes into the theory of these first three minutes; leave alone the difficulty of imagining a universe that was infinitely small and infinitely hot which arose out of nothing!