Age of universe-speed of light

neh4pres
we have seen galaxies 13 billion lightyears away... this means that 13 billion years ago those galagies were 13 billion light years away.. however the light we see from them was emited about 700 million years after the big bang... so if we and those galaxies were at that same point(big bang) and even if we were spewed in opposite directions .. y did it take 13 billion years for the light to get to us??when by math we should have been 1400 million lightyears apart at most... and stranger yet the galaxies that are 13 billion light years away were that far from us 13 billion years ago.. they must be farther from us in lightyears now than the predictided age of the universe

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First of all, there's nothing in GR that says that two galaxies can't be moving away from each other at speeds greater than c. They can, in an expanding universe, if they are far from each other. What GR says about the "speed limit" is (roughly) that one object can't move past another at one point in space at speeds greater than c. Galaxies that are moving away from each other at extremely high speeds aren't really moving fast locally. It's the space between them that expands.

I don't have time to answer the rest right now. I hope someone else will help you out.

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we have seen galaxies 13 billion lightyears away... this means that 13 billion years ago those galagies were 13 billion light years away
No it does not. 13 billion years ago this galaxy was quite close.

Consider: If you were running at 11.1mph to catch a bus that was moving at 11mph, you might have to run hundreds of yards to catch it - even though when you started you were only 10 yards behind the bus.

i.e. at the start, you were only 10 yards from the bus (compare to two galaxies). Yet, you had to run several hundred yards (13 billion years) to catch it.

neh4pres
Thank you that is fully understood. That part is poorly worded, but does show that we are moving from the universal center (big gang) at speeds near the speed of light.. Also i would think that those galaxies were thrown the opposite way during the big bang. Using them couldn't we figure out the place where the big bang took place?

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Gold Member
Thank you that is fully understood. That part is poorly worded, but does show that we are moving from the universal center (big gang) at speeds near the speed of light.. Also i would think that those galaxies were thrown the opposite way during the big bang. Using them couldn't we figure out the place where the big bang took place?
The Big Bang took place everywhere; there is no centre. Or more accurately, everywhere is centre.

I'll use the venerable balloon analogy: A bunch of ants are standing shoulder-to-shoulder on an uninflated balloon. The balloon is then inflated. The ants find themselves receding from each other. What does each ant see? Each ant sees all the other ants receding from it, as if it is the centre of the universe. So, which one is actually at the centre? Answer: They all are.

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peter0302
Isn't that assuming the universe is closed in space? That is to say if you travel far enough in a straight line you'll end up where you started? But is that known?

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No, what Dave said is valid even in the other cases. (I don't think any of the three options has been ruled out).

Consider the flat case, by imagining a grid of evenly spaced lines in three dimensions extending to infinity in all directions, with the distance between the lines increasing with time. The lines represent locations in space that have constant spatial coordinates (in this particular coordinate system). Now imagine watching this backwards in time. The lines are getting closer together the further we go back. The big bang is just a name for the limit they're heading towards, where the distance between the lines are zero. This clearly can't be a point in space.

So depending on taste, there is no center, or everywhere is the center. The big bang isn't an event in space-time, but every part of space goes through exactly the same thing in the limit t$\rightarrow$0.

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peter0302
Well, if the universe is not closed in space, then it has a clear center of mass. It would seem to me that that would be the closest to what we would call the "location" of the Big Bang. I mean, I agree and understand what you're saying that it was spacetime itself that has inflated, and that if everything were a singularity, then there is no center. But, at the same time, at t = planck time, the universe has volume. We can certainly then find the center of that volume - unless the universe is closed, in which case the ballon analogy works.

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Well, if the universe is not closed in space, then it has a clear center of mass.
No, there would be matter everywhere.

at t = planck time, the universe has volume. We can certainly then find the center of that volume - unless the universe is closed, in which case the ballon analogy works.
No, in the flat and open cases, that volume is still infinite. That's what I was trying to explain, but it's hard to get the point across in a few sentences. If you pick any two points in space "now", their distance from each other as a function of cosmological time T is increasing as T increases, and goes to 0 as T goes to 0. But at any time T>0, the universe is still infinite (except in the closed case).

neh4pres
if we have seen galaxies that are 700 million years after the big bang. is it safe to say that if we spun the hubble around and looked the opposite direction that we would not see any galaxies nearly that far from us?

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