The universe is expanding with the speed of light..

Originally posted by purplemer
reply to post by zatara

 

If you drive a car from point zero in a line at 60 mph you would say that the gap between point zero and the car is expanding at 60 mph. If instead you now have two cars going opposite directions from the point zero you would say the gap between them is expanding at 120 mph. The same applies for the speed of light travelling in two different directions at the same time. So it is possible that the universe is expanding at faster than the speed of light.

Thanks for that explanation..but it feels like cheating. I think the reply from ATS member Argitrageur is correct when it comes to this example. Probably when calculating your example with energy equations for one single car and two cars will not have the same outcome. I do not know much about relativity theories but that is probably what Argitrageur is refering to.

Originally posted by zatara
Thanks for that explanation..but it feels like cheating. I think the reply from ATS member Argitrageur is correct when it comes to this example.

Purplemer's math is absolutely correct regarding cars. We can apply the same relativity equations to cars going 60mph but the results are not noticeably different between relativity math and Newton's classical math at that speed. The difference between the classical math and the relativity math only becomes apparent at greater velocities.

I've been trying to get my head around the idea that everything in the universe is expanding away from each other since the big bang and that there is no "centre" to the Big Bang. I've read about the raiins in a loaf of bread analogy and it just doesn't make sense that there is no centre.

I am starting to understand that the big bang, although it started as a singularity expanded faster incredibly fast to begin with. I assume there was no speed of light yet, so the material expanded across a vast tract of land, perhaps thousands of times bigger than the observable universe, before it setle down into normal space. So as I understand it, the bit of observable space we can see, is a small patch of the actual universe. We'll never be able to see beyond it, because light will never travel fast enough to get from the part of the universe beyond what we see now.

What I don't understand is, why now that the universe has settled into real matter and space that is governed by the speed of light, why it isn't a 3-D object, like a sphere expanding outward.

Why do we have to have a flat sheet or donut analogy? Nothing will ever go fast enough to go from one side of the universe to the other. It's expanding as fast as light. So whatever edges there are are unreachable.

Why can't it just be an ever expanding ball, going out at the speed of light?
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A real life solid object I can understand, however, a flat sheet universe, that doesn't follow 3-D space, even thought we're in 3-D space, is hard for me to grasp, and seems an un-necessary analogy.
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If everything is expanding outward, then the farther away something is, the faster it must be expanding out from you. If you are on one "side" of the universe that is going away from the "Centre" at the speed of light, and the other side of the universe is going away from the "centre" at the speed of light, then the universe should be expanding at twice the speed of light. That is space is being created at twice the speed of light, because it is being created at the speed of light on both sides. Nothing is travelling at the speed of light.

So the analogy of two cars travelling away from each other makes sense to me. Someone in the middle who can see the two cars (or the objects travelling away from the centre of the universe) would see each car going at 60 MPH (or each edge of the universe going away at the speed of light.

The observer from one side of the universe would never see the observer at the other side of the universe, because the distance (the space between) is expanding faster than the speed of light, so light will never be able to reach the other side. In fact it will never reach the centre, because nothing can exceed the speed of light and the distance to the centre and beyond keeps growing.

Nothing is actually travelling faster than the speed of light. Space isn't going faster, the distance between each galaxy is getting bigger and if you add all the spaces from one end of the universe to the other, you find the space is increasing at twise the speed of light, but it's space expanding, spread out across the universe. Not one point expanding at the speed of light, the whole universe, perhaps 100's of billions of lightyears across expanding at twice the speed of light. Most of it well below the speed of light, gradually increasing in speed as you go out to the edges which are going at nearly the speed of light.

Mind you, the edges may be light going out, well ahead of any matter that is beyind it, going at a far slower speed..
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I figure there must be some point you can declare as a centre, relative to everything else, even if it wasn't intentionally created by the big bang. It must be out there somewhere.

I also think that since the edges are going out at the speed of light we have a 3-D (sphere or whatever shape we started with) going out at the speed of light and that whatever galaxies are expanding away from each other, the limit of expansion is the speed of light. So there must be an upper limit to the size of the universe, as it can't expand faster than that over the course of 13.78 billion years.

So If we observe the speed at which galaxies are receeding and extrapolate that out to faster and faster galaxies, farther and farther away, there must be a point where galaxies approach the speed of light and we can say, the universe is roughly this big.

What do you think?