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| The Early Universe |
1. The Jeans mass is proportional to the temperature raised to the 3/2 power and inversely proportional to the square root of the number density of particles in the gas. Since density decreases faster than temperature, the Jeans mass will become larger as the Universe evolves, on average. Thus, we may expect that it is harder to make galaxies as time goes on because the average Jeans mass increases, meaning that larger and larger disturbances are required to initiate gravitational contraction.
3. To produce the pair requires 2 x 0.511 = 1.02 MeV. Solving for T = E/k, this gives about 6 x 109 K. Thus, in a gas at six billion degrees an average photon carries enough energy to create an electron-positron pair.
5. The Hubble Law applies on the large scale. Superposed on the Hubble expansion is local or "peculiar motion", which can offset the Hubble flow for nearby galaxies. Thus, because of gravitational attraction, galaxies in a group can collide, even as they participate in the overall expansion of the Universe.