What then are the limits of extrapolation for cosmology?
Just as we talked about temperature in the example above, we can talk equivalently about energy or density in cosmology. We have some direct experience of energies at particle accelerators up to about one trillion electron volts, roughly analogous to a temperature of 1016 degrees. We also believe we have a reasonable understanding from laboratory experiments of the behavior of matter at densities up to that occuring in atomic nuclei, about 1014 grams per cubic centimeter. The main extrapolations we make to the early universe that provide support for the Big Bang cosmological model concern the cosmic microwave background and its properties when its temperature was 3000 degrees, and primordial nucleosynthesis which occurred when the temperature was about 1010 degrees and the matter density about 10 grams per cubic centimeter, all well within our confidence range. This is not to say we know everything about what happened back to those epochs, but that we have good reason to believe the br
