Thursday, July 10, 2008

Humanity versus the Universe

Have I mentioned that one of my hobbies is cosmology? I enjoy pondering the ultimate big picture: how big is the universe, how did it start, how will it end, and why is it like it is?

For today's post, I thought I'd describe just how big the universe appears to be. It is likely very much larger than the Observable universe, that part close enough to us that the evidence of it is visible. Another way of saying that is, how far back in time can we see? The age of the universe is currently estimated at 13.7 billion years, so we can, in principle, see light emitted 13.7 billion years ago in all directions. However, that light came from matter moving away from us, so scientists estimate that the current size of the observable universe is about 46 billion light-years in all directions.

But that is just the physical size of the portion we can, in principle, see. And it is by far empty space. The galaxies and stars are the visible part.

A better question is how many stars are in the observable universe? After all, stars are what we see in the night sky, stars (or rather the solar systems around them) are where life must evolve, and stars are likely where humanity will always congregate.

I have two ways of describing the size of the visible universe (the number of stars) in terms that some of us might comprehend.

THE HAND METHOD: Go to the beach, and pick up a handful of sand. Choose an average size grain. Imagine that our entire solar system is represented by that single grain of sand.  How many grains of sand does it take to be equivalent to a Universe full of solar systems? A bucket full? A cubic yard? A dump truck full?

The actual answer is astounding. Take all of the sand in places like Daytona Beach and Waikiki, larger places like Florida and the deserts of California, Nevada and Arizona, and throw in the really big deserts like the Sahara. Australian, Arabian, and Gobi, all of them combined. Then add the rest of it, the off shore sand of buried beaches.  If our sun is a single grain of sand, then the universe is equivalent to all of the sand on the entire planet Earth

THE EYEBALL METHOD: My screen background is the Hubble Ultra Deep Field image, where nearly every visible spec is yet another galaxy. Look at the image, which reveals about 10,000 galaxies, each much like our own Milky Way.

How big of a piece of the sky is in that Hubble image? Take a dime out of your pocket or purse. Hold it at arms length. No, that's too big. You see where it says "IN GOD WE TRUST" under President Roosevelt's chin?  Take a tiny drill, and drill out the center of the "O" in "GOD". Now hold the dime at arm's length, and look through that hole.

The Hubble Space Telescope sees 10,000 galaxies through that hole. 

If your arms are much longer than average, you might have to drill out the entire "O" instead of just the center. And remember, most galaxies are a bit smaller than the Milky Way which contains about a trillion suns (a heaping cubic yard of sand in the earlier example). But a hundred billion stars in an average galaxy, times 10,000 galaxies in that tiny fraction of the sky works out to a hell of a lot of stars.

Are you feeling a bit insignificant yet?

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