Wednesday, March 6, 2013

See It to Believe It

We've all heard that the universe is expanding, right?  But what does that really mean?

A common model for understanding universal expansion is the dough model.  Imagine a lump of dough with raisins in it.  All of the raisins are stationary in the dough, but as the dough rises and expands all of the raisins move apart from each other.  Two raisins that are really close to each other wouldn't be moving apart very fast, but two raisins at opposite ends of the loaf would be moving apart at a much higher rate.

Now when we think about the universe, it's more than just the distance between objects (the raisins) that's increasing, the fabric of "space-time" itself - the dough - is expanding.  Matter can't move faster than the speed of light, right?  But that's in relation to the space-time it's in, not to us.

As we look further and further away into space, space time is moving faster and faster away from us.  Eventually, you get to a point where space time is moving away from us at light speed.  This is called the event horizon.  You can think of it as the "point of no return" because anything beyond that point is moving away from us faster than the speed of light, and so is lost to us forever.  If someone on a planet beyond the event horizon shone a laser at us, that laser light would never reach us ever!  Since space-time is moving away faster than light-speed, the laser (traveling at light speed), even thought it was pointed at us, would be moving away from us. A similar phenomenon occurs around black holes - that's why nothing that goes in ever gets out.

In other words, anything that is beyond the event horizon or that crosses the event horizon is lost to us - forever.  It becomes undetectable.  As far as we're concerned, it may as well not exist, right?  But it does exist!  We know it!  We just can't prove it.  Or provide any evidence of it.

So how do we know that there are things out there that we can't observe?  I guess we'll just have to take it on faith.

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