Expert: Philip Stahl - 6/11/2010

Question
Hi Philip,
I have a question about Hubble's discovery and the big bang theory.
How does Hubble's discovery of the expansion of the universe imply that there was a Big Bang?

Thanks!

Answer
Hello,

One can think of the visible expansion of the universe, in terms of the recession of galaxy clusters obeying the Hubble law (v = HR),  as a kind of four -dimensional cosmic videotape. As with a prosaic videotape we can run it backwards, and see what is happening in respect to any event.

For example, if a normal video tape shows a kinetic (motion) effect with pieces flying in all different directions, one can run the tape backward and see how (or if) they all come together to a single source.

In a similar fashion, we can use the existing expansion of the universe, based on varied recessional velocities, to compute what the universe would have looked like at a very early phase. (Of course, to do this one must also incorporate known laws of thermodynamics!)

When we do this sort of thing (you can find a beautifully done version in the Appendix of the book 'The First Three Minutes' by Stephen Weinberg, pp. 166-76) we see that it leads to a primeval superdense state. This superdense state has come to be known as "the Big Bang" because based on the math and kinematics (rewinding the "tape" of expansion) we believe it to be the initial state of the cosmos.

Further, the background temperature we detect now in all directions (associated with the 2.7 K microwave radiation)is exactly what we'd expect the cosmos to have "cooled off" to, if our rewinding videotape image is correct!

Hope this helps!