Accelerating Universe

Brian Sand · #1 (**permalink**) 19-March-2004, 04:03 AM

In the last few years, we have been hearing that not only is the universe expanding - it is also speeding up.

From what I read, it seems that the people writing those articles sometimes forget that the universe is (at least) a three-dimensional object. Scaling down, if the earth were to suddenly explode, The United States and Canada would go in similar directions while China and India would go in a totally opposite direction. As the explosion progressed, Canada and the United States would become further and further apart at a much slower rate than compared to the rate at which they were being separated from India and China.

Could it be that we are unwittingly calculating the acceleration of the expanding universe without accounting for this? I suppose the fact that nobody is able to point at the sky in a general direction and say "Somewhere over there is about where the Big Bang should have went off" makes me a little sceptical of such claims. Am I missing something? Any thoughts or explanations would be greatly appreciated.

Jeepien · #2 (**permalink**) 19-March-2004, 05:40 AM

No, they're not forgetting.

In your scenario, closer objects are not separating as rapidly as distant objects are, and that's to be expected. In fact, it's exactly what we observe in the universe.

The question is, though, whether the separation rate between any two given objects slows down over time (as you would expect if they were attracted by gravity), or remains the same over time, or speeds up over time. If it speeds up over time (outward acceleration), that's new, unexpected, and intriguing, because it raises the question of what force could possibly be responsible for it.

The reason nobody can point to a spot in the sky and say "that's where the big bang took place" is simply that the big bang took place everywhere! Although the universe was a LOT smaller back then, it was still the same universe, and it was still the WHOLE universe, and has been expanding ever since. So every place that now exists was once part of the big bang, not just one spot "over there".

Fraser · #3 (**permalink**) 19-March-2004, 03:06 PM

The Big Bang is an explosion "of space", not an explosion "in space". It's the Universe that's expanding, while everything else, including Canada and the United States are being carried along for the ride. So, they're not expanding away from each other.

Take a balloon, and attach little stickers to it and then inflate the balloon. From the vantage point of any sticker, it looks like all the other stickers are moving away from you. But, the top of the sticker isn't moving away from the bottom of the sticker. So, the balloon is space, and the stickers are objects, like the Earth, our Galaxy, etc. So to be really crude, imagine one half of the sticker is Canada and the other is the US. They aren't getting farther away from each other, even though other stickers look like they're moving away.

The recent discovery of dark energy is just another force that's working on this expansion of the Universe. Instead of slowing down, the expansion of the Universe is accelerating. So, some mysterious energy is pushing the Universe apart. It's pushing at a constant rate, though, which counters the constant rate of deceleration due to gravity.

Brian Sand · #4 (**permalink**) 21-March-2004, 05:27 AM

From what you guys are saying, I am not quite sure I understand the Big Bang concept as well as I thought. I was under the impression that the Universe was at one time an infinitesimally small point that hit some critical point of mass or whatever and exploded kinda like a supernova - creating space, time and everything. The point at which that speck busted into more than one piece is when space was first defined. Once there were two objects in motion, time could be defined. When light was first emmitted, the expansion of the universe accellerated to the speed of that first light (photons crashing the boundries of nothingness). Of course, what my original question was asking about was the speed of galaxies and stuff.

In a 3-D model of an explosion, things go up, down, and in every possible direction. That is why I used the Earth in my example. But I guess if I have the whole concept messed up, then my question isn't really relevant. I am just hung up on India going in the opposite direction - of course it would seem faster. Just like two cars on the highway passing each other - combined speeds are always additive in a collision. Are they actually looking accross the center of the universe and combining the two speeds?

Even the balloon has a theoretical center, and the sticker on the other side of the balloon would still be going twice as fast from us than we are from the center.

Am I still way off base? I actually hope this going to be like the time my mom told me that the trees don't make the wind. Man that blew my mind (I was only 3yrs. old) hehehe

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