Thursday, February 24, 2005

Where we live

The desktop of my computer displays an image called the Hubble Ultra Deep Field. It is a photograph of a patch of sky, empty to the naked eye, that could be covered by a pencil eraser held at arm's length. It was produced by the Hubble Space Telescope, which is named for Edwin Hubble, the man who discovered the universe. Really. (Although he had lots of help).

The photo looks back in time 12 billion years. It contains thousands of galaxies, each consisting of 100 billion stars or more. Here's what this and other pictures and various kinds of evidence tell us.

There was a moment of creation, not perhaps of everything that is, but of everything that we can see. Everything in the observable universe, hundreds of billions of galaxies of hundreds of billions of stars, including our own, and all of space, occupied a volume the size of a single atom. (Maybe at one time it was even smaller than that, even infinitely hot and infinitely dense, but our mathematics won't allow us to look back that far.) Space started to expand. Over hundreds of millions of years the universe cooled enough that matter could condense out of energy, and atoms of hydrogen and helium could form. The universe became transparent to electromagnetic radiation, and the remnant of that moment, the so-called cosmic background radiation, is still observable today. Tiny ripples in the fabric of space created slightly denser than average regions, into which gravity pulled surrounding matter. Galaxies condensed out of the cosmic gas, and stars condensed out of the galaxies. In the first generation of stars, heavier elements were formed. Some of these stars ultimately exploded, expelling heavy elements out into the galaxies, so that as subsequent generations of stars condensed from the resulting gas clouds, planetary systems containing heavier elements formed around them.

Our own planet formed about 4 1/2 billion years ago. For the first billion and a half years or so it was largely molten, and under constant bombardment from meteors. Once it even got clobbered by another planet-sized object, knocking off a hunk of matter that formed the moon. But around 3 billion years ago, the earth was cool enough that stable complex chemical compounds could form on or near the surface. I'll want to talk about evolution later, but that's how we got here, 3 billion years later.

Now look back out into space. There is our star, one of 200 billion in our galaxy, which is 100,000 light years across. (Some people think there are 400 billion stars in our galaxy.) There is the local group of a few dozen galaxies bound together by gravity. Then there are something like 200 billion more galaxies within the volume of space that we can see. (The universe could be larger than what we can observe, but is not necessarily infinite. As space has expanded faster than the speed of light, there are probably regions of space whose light has not yet reached us. While it is true that nothing can travel faster than light, the expansion of space has nothing to do with motion.)

From our point of view, but from no other possible point of view, one of the most interesting facts about this universe is that it is utterly indifferent to our existence or our fate. If the earth were to be destroyed tomorrow, it would be of less importance to the universe than destroying a single grain of sand would be to all the beaches and ocean bottoms of the earth. Indeed, billions of planets are destroyed every day in cosmic cataclysms. Undoubtedly, many of them harbor life.

We matter only to ourselves. We know this because we have discovered it with our own senses, and our own reason. We know far more than the people who contemplated the cosmos thousands of years ago and made up stories to explain its mysteries. We will continue to learn, and perhaps one day we will even overturn some part of the story I have just told. But this quest for discovery is what gives our existence meaning -- only to us, to be sure, but that will have to be enough.