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image Now, the most amazing discovery of Hubble (which is why we named the space telescope after him). Most of you have probably heard of Hubble's law but it is one of the most confusing things in astronomy and cosmology and it is frequently misunderstood, misinterpreted, and mis-taught. Here is Hubble's law, I have just drawn several of these galaxies; here is the Milky Way. Here is another galaxy and then here is another one. What Hubble discovered was that you can measure the velocity of these galaxies through the Doppler shift. The fact that something that is traveling away from you is admitting light at a certain frequency but since it is traveling away it takes longer for those individual pulses to reach you, that's the Doppler shift. It is often called a "red shift" because many of these things are moving away, and as they move away you look in the spectral lines and they are at a longer wave length --- they are "red shifted". From this we can measure the velocity of recession. He measured this from many galaxies. He could also estimate the distance of the galaxies, very crudely if you just take the galaxy's apparent and assume most of the small ones and dim ones are further away.

There are many more sophisticated ways of estimating distance now. (In fact, my up-to-date information that I got actually last night, involves one of the ways of estimating distance.) What Hubble determined was that the galaxies that were further away were moving away from us, they were all moving away from us and the ones that were further away were moving away faster. The ones that were twice as far away moved away twice as fast.

This is disturbing to many people. Does this mean that we are at the center of universe? Everything is moving away from us! This is actually a very nice mathematical issue for those of you who love to work with vectors. There is a way of demonstrating that that is not true, but let me show you my preferred way of explaining what is going on. This is Hubble's law.

The Universe Before
The Big Bang
(Actual Size)

One way you might be tempted to explain Hubble's law is like this: assume that the universe began with all of the matter lumped into a tiny little clump which then exploded and the earth happened to remain at the center. In this Sidney Harris cartoon, the universe before the big bang, actual size. Then as the thing blew apart the objects that were moving faster are further away and that explains Hubble's law. This is the way many people interpret Hubble's law but not scientists who are knowledgeable about the field. There is just an enormity of evidence that say's this picture is not correct, and when I say there is misinformation, there is a wonderful quote attributed to Mark Twain that I use all the time. "The trouble with most folks isn't so much their ignorance, it's knowing so many things that ain't so". Actually I checked with the Mark Twain research program at UC Berkeley and they say that is actually not his quote. It was probably said first by Josh Billings, another humorist in the 1800's.


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