# Thread: Universal Expansion Speed

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## Universal Expansion Speed

Is the universe expanding faster than light?

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It is expanding at about 70 km/s per megaparsec. So bits of it are receding from us at greater than the speed of light. It is hard to actually answer that question because there is no fixed rate the whole universe is expanding at into something else - what is happening is that the metric is expanding such that for each megaparsec of distance from any point objects appear to gain about 70km/s of recessional velocity. Because the universe is not expanding into something you cannot really point at an edge of it and say "this is how fast the universe as a whole is expanding".

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I think expansion of the metric in GR gives a good description of it. Modifications of space time due to mass/energy are well described by GR too - and the metric expansion is separate from that.

The Big Bang was not a blast and the model does not describe the start of the universe. It describes the evolution of the observable universe from a hot dense state to what we see today. We have no knowledge of an edge to the universe - it does not require one.

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Originally Posted by Shaula
It is expanding at about 70 km/s per megaparsec. So bits of it are receding from us at greater than the speed of light. It is hard to actually answer that question because there is no fixed rate the whole universe is expanding at into something else - what is happening is that the metric is expanding such that for each megaparsec of distance from any point objects appear to gain about 70km/s of recessional velocity. Because the universe is not expanding into something you cannot really point at an edge of it and say "this is how fast the universe as a whole is expanding".
My three day break holiday was welcomed because I have been managing to refresh my memory on some of these delicate subjects.

I know that the Inflationary Phase has left some parts of our universe expanding at different rates... some galaxies are moving at speeds very close to c relative to us.

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Interestingly, due to the Hubble Constant, our universe doubles in size about every 10billion years. Now that it is expanding faster than light, it seems this will cut this dramatically.

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The distance between any two distant enough points have always expanded faster than light. Velocity X per distance Y is always larger than lightspeed for a big enough Y if X is not exactly 0.

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Originally Posted by Aethelwulf
Interestingly, due to the Hubble Constant, our universe doubles in size about every 10billion years. Now that it is expanding faster than light, it seems this will cut this dramatically.
"Now" it is expanding faster than light? I am not sure what you mean by this. The Hubble constant describes how recessional velocity (attributed to metric expansion) increases with range. This 'constant' is actually variable and is increasing. But far enough away parts of the universe have been receding from us at the speed of light all along.

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I'm with Shaula on this one. The question is subject to interpretation, so no one answer is correct. It is simply defined as recessional velocity for a unit distance.

The 'universe' is not subject to expanding faster than light. Parts of it might appear faster than light with respect to each other, but this has no bearing on the universe as a whole. So you get to pick your point of view and answer that. That's not answering the original question though, because we don't know what the specific question is supposed to be.

I can say yes it is, because two points opposite each other on the Hubble sphere are currently receding at faster than light. I can also say no, because we can break up the visible universe into smaller areas such that no two neighboring areas see anything faster than light.

Then you have to add that two galaxies receding faster than light from each other can still see each other, eventually.
But far enough away parts of the universe have been receding from us at the speed of light all along.
Exactly! Yet we will be able to see them, if we haven't seen them already.

As the universe gets older, it expands. Conversely, as time goes by, we get to see more of the universe. You need to consider both points when talking about the 'universe' as a defined thing. Which is another way of saying that if expansion came to a complete stop(the universe went static), we would still have things past the Hubble sphere come into view in the future. So here we have faster than light galaxies in a (now) static universe.

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Originally Posted by Shaula
"Now" it is expanding faster than light? I am not sure what you mean by this. The Hubble constant describes how recessional velocity (attributed to metric expansion) increases with range. This 'constant' is actually variable and is increasing. But far enough away parts of the universe have been receding from us at the speed of light all along.
The Observable distance is receding faster than light - this is directly due to our universe now expanding faster than light.

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The so-called, accelerated expansion.

11. Originally Posted by Aethelwulf
I know that the Inflationary Phase has left some parts of our universe expanding at different rates... some galaxies are moving at speeds very close to c relative to us.
I'm not sure what you mean by this. The expansion rate appears to be uniform throughout the universe. It changes slightly according to the age of the universe, but at any one time, it is uniform throughout. And the expansion is very, very small. Viewed from any local region, it is so small as to be negligible. So no, the "universe" is not expanding faster than light, cjackson. But as you look deeper into the sky, billions of lightyears, the tiny expansion in all those small regions you're looking through adds up. There is a lot of space in our visible universe, so much that we can look far enough distant such that all those little rates of expansion can add up to the speed of light. But obviously the universe is not expanding at the speed of light here.

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Originally Posted by Cougar
I'm not sure what you mean by this. The expansion rate appears to be uniform throughout the universe. It changes slightly according to the age of the universe, but at any one time, it is uniform throughout. And the expansion is very, very small. Viewed from any local region, it is so small as to be negligible. So no, the "universe" is not expanding faster than light, cjackson. But as you look deeper into the sky, billions of lightyears, the tiny expansion in all those small regions you're looking through adds up. There is a lot of space in our visible universe, so much that we can look far enough distant such that all those little rates of expansion can add up to the speed of light. But obviously the universe is not expanding at the speed of light here.
I know this because Steven Hawking said it in his popularized book, A Brief History of Time. In it, he explains how the Inflationary Phase left different parts of the universe moving at different speeds.

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Originally Posted by Aethelwulf
The Observable distance is receding faster than light - this is directly due to our universe now expanding faster than light.
So you are just using terms a little sloppily. When you say universe you mean all parts of the universe that could in the future be observed.

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Originally Posted by Shaula
So you are just using terms a little sloppily. When you say universe you mean all parts of the universe that could in the future be observed.
You knew what I meant. And if you didn't, then I apologize for being sloppy. But most knew what I was talking about.

15. Originally Posted by Aethelwulf
The Observable distance is receding faster than light - this is directly due to our universe now expanding faster than light.
Let's be careful and keep the units correct. There are objects whose distance from us is increasing faster than c. That is not the same as the units for rate of expansion however, which is speed per unit distance.

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Originally Posted by Aethelwulf
You knew what I meant. And if you didn't, then I apologize for being sloppy. But most knew what I was talking about.
I managed to work out what you meant - but your statements taken as is were inaccurate and potentially misleading. Which is why I had to ask to get clarification of what you were trying to say.

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