Thursday, July 04, 2013
Stretching the imagination
It was an epic discovery. In the late 1920s, Edwin Hubble, working at the Mount Wilson Observatory in California, observed that the other galaxies were moving away from us. Moreover, there was a direct proportionality between the distance of the galaxy and its speed of recession. The universe is expanding!
There is a neat way to demonstrate this that I used in class.
Get a long piece of rubber. A snipped rubber band with do if it's long enough and wide enough. Lay it out against the blackboard and have a student hold each end. With a marker, place a big dot at even intervals, say every inch. These are the galaxies.
Pick any dot at random to be our own galaxy. Hold it in place with one's finger. Now have the students slowly stretch the rubber band and watch what happens to the dots. When the next galaxy over from us, originally one inch away, has moved an inch, the galaxy that was originally two inches away will have moved two inches. Twice as far – relative to us -- in the same time. Twice the velocity. A galaxy originally three inches away will have moved three inches in the same time interval. And so on.
It doesn't matter which dot we pick to be us, or how long the rubber band is. You will observe Hubble' law from any galaxy, even if the rubber band is infinitely long.
This is rather harder to demonstrate, but imagine the rubber band shrinking rather than stretching, that is, running the clock in reverse. The dots get closer together, everywhere! Eventually the distance between the dots goes to zero –- the Big Bang. Not somewhere. Everywhere!
So here we are, riding the rubber band on our Milky Way sleigh. It was originally assumed that the stretching was slowing down, as the mutual gravity of the galaxies resisted the initial impetus. Like a ball tossed up into the air, the velocity of recession would eventually go to zero, and expansion would surrender to contraction The universe would end as it began, in a blaze of glory.
Now it seems that the expansion is accelerating, driven by something called dark energy. If so, then the universe will end in infinite dispersal, cold, dark and dead.
It is good to remember that all of human history is but a blink of an eye in cosmic time, near a random star in a random dot. To speculate on those aspects of cosmic history that we can observe and measure requires a certain amount of hubris, as I was always aware as we stretched the rubber band. To speculate on the larger questions of where it all came from and why we are here may be a hubris too far.