Big Bang, and a NOT so infinite universe?

So over the past few days I have been listening to a podcast called Astronomy Cast. I would definitely recommend it to anyone who even remotely finds science interesting. Within every single show I swear your mind is blown by the so simple logic you never thought about. I want to talk about one of those now, the size and age of the universe, and how they know they are NOT infinite.

I guess to start the first thoughts that the universe has not always been here, and that it doesn't go on forever was first thought of way back in 1823. I don't remember the guys name, but it's in the podcast. And what is his logic for this. Simple, there is darkness in the night sky.

Before we go into that, I have to set up a situation. Imagine we are standing here on earth and somebody flips God's switch that turns on and off light. It was probably an accident. But assume there is complete darkness. For the smarter readers who are saying this is wrong cause the light still would be coming from distance stars, let's just pretend there never was light.

So now we decide to turn the light back on in the universe. Since light travels at a finite speed, not everything would show up at once. The first thing we would see is the moon. Since it's about 7 light seconds away, we would see it in about 7 seconds. After eight minutes we would see the sun. (I realize you can't really see the moon without the sunlight reflecting off, but again just go with it). I want to reiterate the speed of light is fast, but still finite. That means it takes some time for light to get from one point to another.

We would have to wait 4.2 years before we would see any stars in the sky. (Alpha Centari is 4.2 light years away) With the speed of light being finite though, we are not seeing Alpha Centari at the present time, how it is at the moment. We are really seeing how it was 4.2 years ago.

So the light we see from the sun shows the sun as it was in the past, 8 minutes ago. If the moon were to explode, even if we were just staring at the moon, we wouldn't know until seven seconds after it happened.

After Alpha Centari, stars would pop up over the years, and this is were I come back to the first point. more and more stars would pop up taking up more of the darkness. If the universe has been around before the big bang, and went on forever, the night sky would just be lit up, opaque like a fluorescent bulb. All the light would be here.

That really only shows one or the other, finite age or finite size. What logic tells us with the big bang, which was thought of in the 1920's was that both are actually finite. The Astronomy Cast goes into tons of detail about all of this, but the cool thing is the reason the universe seems infinite. We obviously can't see the edge. We can't see all the way back to a star 13.7 +/- .2 billion years ago. This is because at a point during the expansion, the universe expanded faster than the speed of light. So even with the most powerful telescope ever we could only see to the light's end, and not into the darkness of the universe.

I guess if you want a good way to look at the universe, it's a sphere. The big band would have sent energy in all directions at the same speed. So to imagine being in a soccer ball or something. If you were in the inside you could see until the inside part of the ball, then there is some thickness, and finally the outer edge. This soccer ball is also still expanding and our galaxy is moving further away from ALL other galaxies. A mental image for this is a bunch of raisins in raisin bread. As the dough rises all the raisins slowly move apart from each other.

I know it was a long post, but the things were a little mind blowing, and I thought they might be fun for some of you to think about.