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View Full Version : Is there an end to our universe?



Heroin
2004-Jan-21, 01:15 AM
I was just wondering can something go on forever?

tuffel999
2004-Jan-21, 02:31 AM
13

AGN Fuel
2004-Jan-21, 02:43 AM
I was just wondering can something go on forever?

I remember a lecture I once attended on vector calculus......... :lol:

semi-sentient
2004-Jan-21, 02:54 AM
The universe will continue to exist forever (and probably has existed forever), but life will not. When the stars finally run out of fuel, that will pretty much be the end of it--at least until the next "brane" comes along and stirs things up again! 8-[

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Taibak
2004-Jan-21, 03:31 AM
Bear with me, this gets weird fast.

Current theory holds that the universe has no edge and has no center. One of the basic principles of cosmology is that the universe looks roughly the same in any direction. I say 'roughly' because no two portions of the universe will be identical (i.e., we don't see Orion in all parts of the sky at the same time). This may be a naive assumption, but it follows from relativity (that there is no preferred reference frame) and it seems to fit our observations of the universe.

Now, think about that in structural terms. This argument will work for any shape, but for simplicity assume the universe is shaped like a sphere. At the center of the sphere, you can look in any direction and it will all look roughly the same. However, if you're near the edge of the sphere, the universe will look totally different depending on whether or not you're looking towards the center or towards the edge. This violates the idea that the universe must look the same in any direction, therefore the universe is truly infinite.

That's the simple explanation. The more complex version has to do with the idea of lookback time. Remember, light travels at a finite speed, c, approximately equal to 300,000,000 meters per second. That means that the light from any object hits our retinas after a time, t, where t = d/c seconds after it left that object where d is the distance to that object and c is the speed of light. For instance, Alpha Centauri is four light years away, approximately. The light we see from Alpha Centauri left the star four years before it hits our eyes. Therefore, we can only see what Alpha Centauri looked like four years ago, the lookback time to Alpha Centauri.

Taking this a step further, the Universe is approximately 13 billion years old. (I'm rounding for simplicity, the actual age is more like 12.7 billion years old, but a few hundred million years isn't that important here). When we look at an object 13 billion light years away, we're seeing what that object looked like *when the Universe was created.* Now, assume there's a star 14 billion light years away. We won't be able to see it because its light has only been travelling 13 billion years - not enough time to reach us. Therefore, the *observable* universe, the universe we can possibly see, is a sphere with a radius equal to the age of the universe multiplied by the speed of light.

If you think about it, that would mean that the observable universe is a giant sphere with us at the center. The *actual* universe is much different. Again, this follows from the idea that the universe looks roughly the same from any observation point and the Copernican idea that the Earth is not situated in any special location. Moreover, we know that the universe is expanding - Hubble proved that 80 years ago and this has been confirmed both by general relativity and every observation made since then. If that were true, that would mean that the Big Bang was what happened when the universe stopped being a hollow shell and imploded inwards. However, that would mean the universe is contracting - not expanding, so it doesn't fit the observations. Moreover, what would it be imploding into? The Earth? If that were the case, why don't we see any weird physics taking place on our planet and nowhere else?

Now, go back to that star that's 14 billion light years away from us and assume it has a planet with astronomers on it. Those astronomers would see the univese the same way we would and would come to the conclusion that the *observable* universe is a sphere 13 billion light years across. Moreover, they can't see our Sun for the same reason we can't see the light from their's - the Sun's light hasn't reached them yet. However, there's no reason why both stars can't exist, nor is there any reason why there can't be another star 28 light years away from us in the same direction, and so on. Therefore, the universe *must* be infinite and can have neither a center nor an edge.

Part of the key to understanding this - and here comes the really weird part - is that you have to understand that when the Big Bang happened, the universe didn't explode in the same sense that a bomb or a star explodes. It wasn't just this ultra-condensed singularity (essentially, a point-like object) containing all the mass and energy in the universe. Prior to the Big Bang that singularity contained *time and space* as well as matter and energy. When it exploded, it was really the beginning of the expansion of time and space. This expansion also takes place in four non-Euclidean dimensions so we're not talking about anything that can be completely understood using traditional geometry. By way of example, in traditional, Euclidean geometry, the surface of a sphere is a three-dimensional object. In the geometry we're using, it's a two-dimensional object. Moreover, humans have a hard time thinking in three dimensions, let alone four dimensions. In other words, don't try to picture the actual shape of the universe in your mind and from that try to find a center and an edge simply because the human brain isn't equipped to visualize the geometry you'd need to do it.