OK, take a bunch of photons (say, 1 kg) and let them go. They will go in all directions, right? So you may call this "expanding Universe". As they go away from each other, they gain gravitational potential energy thus have to lose their kinetic energy (red shift). Total energy as well as total number of photons in this "Universe" stays the same all the time.

So it is quite possible that in the "early days" (well, close to Plank time) the Universe indeed was filled with radiation only. But at that high energies you immediately have pair creation/ destruction activity all over the place. Moreover, as forces unify there is no distinction between photons, leptons and baryons - they all look the same at Plank energy.

• Re: Gravity. - yanniru - February 8, 2001 - 16:35 UTC
  • Re: Re: Gravity. - alexander - February 8, 2001 - 18:17 UTC