The universe.  That’s as in the totality of everything that exists.  Everything.  Hair, hide, fur, feathers, and all.  I’m not going to get into any arguments about whether or not this universe is the “only” universe; let’s just say it’s everything we can physically encompass in this dimension.  That which is currently known.

That should do for a working definition.  But what are the physical parameters of “everything”?  Does it begin?  Does it end?  If it ends, what’s beyond the “end”?  What is “it”, exactly?

A very small window on infinity - brough to you by Hubble

You know what?  I wish I knew.  We’ve gained so much knowledge in the last 100,000 years, sometimes we have to pause a moment, because we really know very little about the universe in which we live.  We’re like the residents of a tiny island in a big, big ocean.  Early explorers here thought that if you reached the end of the ocean (or the end of the land, for that matter), you simply fell off.  We’re at about the same place in thinking about the universe.  Some people think there is a form of “boundary” there; some that it’s impossible to reach the “end” because there is no end.  Some think that if you go far enough in one direction, you’ll end up back where you started… just like on Earth.

There is so much we don’t know.  What we do know is usually lumped together in the catchphrase “known universe”.  That’s a polite reminder that any ideas we hold dear today could be blown out of the water tomorrow.  You’ll also hear the phrase “observable universe”, which is (as it sounds) the part of the universe that is possible for us to observe.  The observable universe is limited by what light has had time to reach us.

NASA - WMAP Cosmic Microwave Background; one representation of the universe

One thing we DO know about the universe is that it’s big.  No, I mean it’s really big.  Bigger than that.  At the far end of our observations, we’re seeing things that are about 46.5 billion light years away.  Don’t plotz on me; I know the age of the universe is believed to be about 13.7 billion years old, but we are seeing things more distant due to the continuous expansion of the universe.  Just think about it; 46,500,000,000 light years away.  In any direction.  That’s about 14 billion parsecs.  A parsec is about 19 trillion miles (19,000,000,000,000 miles), or about 31 trillion kilometers (31,000,000,000,000 km).  Since the observable universe is about 46.5 billion light years away in any direction, you can infer that the observable universe is a sphere with a diameter of about 93 billion light years, or about 24 billion parsecs.  Now, you go multiply 24 billion by 19 trillion, and you’ll know about how many miles you’re dealing with.

You know, distances like that don’t really have any meaning to the average person.  There is simply no way we can relate to that.  There’s also no way we can relate to what a very tiny part we are to the universe.  I can try it with the old “grains of sand on a beach” analogy, but you’re going to have to work a little.  How about this:  If one grain of sand from all the beaches and deserts on Earth was a galaxy, and you were sitting on a planet around a star in this galaxy, looking at a grain of sand… there you are!  You’re on that second grain of sand, which is your solar system.

You didn't expect me to pass up this opportunity, did you?

Now, just imagine that the observable universe, just what we can see, is that second grain of sand.  That’s probably closer to the true size of the universe.