History of the Big Bang Theory

The strength of the Big Bang theory lies in the evidence for it, not the mechanism used to explain it. Actually, the theory has been revised a great deal since its first proposition and is probably not exactly as you think it is. I'll explain it in the order of historical development. My apologies if this is a bit technical - don't worry if you don't understand it all.

The theory was first proposed in the 1930s, based on Edwin Hubble's discovery that distant galaxies are receding. Hubble measured the distances to a large number of galaxies (based on the observed brightness of certain stars within them), and compared these distances with their electromagnetic spectra. As it turned out, more distant galaxies had the features in their spectra (spectral lines) shifted to lower frequencies in a linear manner: that is, more distant galaxies exhibit greater redshifts. The only known mechanism for generating a spectral shift is the Doppler effect, which means that distant galaxies are receding from us.

This suggests that in the past, galaxies were much closer to us than they are now: simply extrapolate the motion into the past. As it turns out, if this is performed, it indicates that all galaxies in the observable universe would have been at the same 'location' about 11 billion years ago: that is, all the matter in the universe originated from a single location. This is the (simplified) Big Bang theory. Actually, it's a little more complicated than that: according to general relativity, it's not really that distant galaxies are flying away from us, it's that space itself is expanding, increasing the distance. You can think of the universe as the surface of a balloon, with the balloon constantly expanding.

Not everyone believed ...

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...e realm of current science. We don't yet have a theory that can handle the conditions present at that time yet. However, from the era of inflation on, high-energy physicists and cosmologists are quite confident of the theory, as crazy as it sounds! In fact, this specific theory makes its own predictions regarding the exact distribution of fluctuations of the cosmic microwave background, which have so far been confirmed, though complete confirmation will have to wait until the MAP and FERMI satellites become operational later in this decade.

The bottom line is that historical sciences (in this case, cosmology) can make testable predictions: the existence of background radiation and its properties, the distribution of light isotopes, the presence of galaxy evolution, the age constraint, and so on. The same goes with evolutionary biology, another historical science.