It ocurred to me this morning that it probably doesn’t make sense to discuss solutions to the Horizon Problem without first giving a sketch of the Big Bang model. Here’s the sketch:
The Big Bang model posits that the universe began as a superhot, superdense singularity, and has been expanding and cooling ever since. Although we don’t know a whole lot about the initial singularity , there is very strong evidence for the rest of the model.
1. The expansion of the universe: it’s been known since 1929 that far-away galaxies are receding away from each other at a speed proportional to their distance (Hubble’s Law). This is surprisingly easy to measure as the light emitted by moving galaxies is red-shifted by the Doppler effect. Following the famous Hubble graph back down to the origin led Georges Lemaitre to the idea of a Universe bursting out from a tiny volume (the primeval atom) . Nowadays, we say the Universe began as a superhot, superdense explosion of space, time , matter and radiation, expanding and cooling as time goes on
2. The composition of the elements: if the universe began as some sort of tiny fireball, it would have been too hot for atoms to form at first. Calculations show that a universe made up of about 75% Hydrogen and 25 % Helium should then have evolved. Guess what – the figures match our universe (all the other elements are formed in dying stars)
3. The cosmic backround radiation: the physicist George Gamow first pointed out that radiation left over from the very early universe might still be observable. (The reason is once atoms began to form, the scattering of light becomes reduced and the universe becomes transparent). Such radiation would be hugely redshifted and freezing cold, but it should be there. Just such radiation was found by Penzias and Wilson, on building the world’s first radiotelescope (although they didn’t know what it was). Since the background radiation offers a real glimpse of the very early universe, most of modern cosmology is concerned with getting more and more accurate measurements of this ‘cosmic fossil’, using fancy satellite telescopes such as COBE and WMAP.
And that’s the model..