The big bang or hot expanding model of cosmology finds strong support from the recession of the galaxies, that is, when we look out in the night sky, distant objects appear to be flying away from us. And also the cosmic microwave background - the fact that we are bathed in primordial light that bears evidence from and earlier, hotter, and denser period in the universe. As we shall see, this evidence also supports the gravitational instability paradigm - the picture that gravity can make wrinkles into galaxies.

The relation between recession and expansion is easy to understand.

Imagine you're standing at the north pole and think boy, I'd much rather be at the basking the sun at some equatorial paradise. You look up the distance to it and plan your trip. Right before you leave, you think, I'd better check the distance again. Unbeknownst to you the radius of the earth has expanded in the meantime. To your suprise the distance to your equatorial paradise is now larger. You think, that's funny paradise seems to be receding from me! This is exactly the situation we observe with the distant galaxies. Because space itself is expanding it looks as if the galaxies around us are all receeding into the distance. It's easy to convince yourself that there is nothing special about being on the north pole. Anywhere on the globe the distance between points is increasing and hence all distant objects appear to be receeding.

Well, maybe at least you can see paradise? Unfortunately the wavelength of light also stretches with the expansion so that visible light at millionths of meter wavelengths gets streched into invisible microwaves at millimeter to centimeter wavelengths.

Given that light behaves this way, it's easy to see how the sea of primordial light we observe in microwaves supports the hot expanding (big bang) model. Run these pictures backwards in time. The wavelength of light becomes shorter and shorter and matter becomes denser and denser implying that the universe began in a hot dense state.