Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Burt Johnson on 12/01/2010 11:30:02
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Burt Johnson asked the Naked Scientists:
Thanks for your new show (http://www.thenakedscientists.comastronomy) -- sounds great so far!
I have one burning question that has bothered me for a long time about astronomy...
If I stand on the North Pole and look up, I can see (assuming I had the Hubble for eyes) over galaxies over 14 Billion light years away. If I then go the South Pole and look "down" I can see over 14 Billion light years in that direction also.
That seems to me as though we can see light from early galaxies that are 28 Billion years apart. It seems highly unlikely that we are in the centre of the universe. If we are offset from the centre, that would imply the universe is larger than 28 Billion light years across, which in turn implies that it is more than 28 Billion years old.
Yet we always hear that the universe is 14.5 Billion years old (or thereabouts -- I have heard different numbers, but they are all in that basic range).
I have never heard anyone claim that space is bending around so that the "North Pole" and the "South Pole" extreme light is really the same, and my feeble understanding of Einstein's bending of space does not allow for it to bend that extreme an amount (other than a wormhole, which would be a localized phenomenon).
The more I try to figure this out, the bigger my headache becomes...
What do you think?
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In a way you are correct, in another your flippin out :)
It's easiest to understand this way, the light that reach us now, that is the oldest is 14 billion years old. That light started to travel at the birth of our universe. But we also have an 'inflationary period' just after the big bang where everything moved faster than light, so there is, as far as I know, nothing stopping the universe to be 'bigger' than those 14 billion years of light travel. But for those places light to reach us now, they would have needed to exist before the Big Bang happened to be able to send their light 'in time' for it to reach us now.
We can only see to the Big Bangs start, and that seems to have been around 14 billion years ago. Hope it made sense? Well, except for the 'inflation' of course :)
14 B ly (http://www.cfa.harvard.edu/seuforum/howfar/see.html)
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Also you have to remember that the things we see by that light have had 14 billion years on it to 'move' further away under this time. But it's the BB that limit lights 'travel'.
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I understand that light takes XX time to reach us. That is because of the distance though, not because of time itself. That is, light travels at a finite speed, so takes 8 minutes (roughly) for our sun's light to reach us. We can think of it as "seeing 8 minutes into the past", but it is just as fair to say it is "91 million miles (minimum) from earth."
If I stand at the North Pole and look up, I will not see the sun. It is off to the side. Instead, I will see the North Star (Polaris, about 430 light years from earth). I am thus seeing the light that left Polaris 430 years ago, or "430 years into the past."
Now I move to the South Pole and look up. Again, the sun is off to the side, and I see the South Star (Sigmas Octantis, about 270 light years away).
In order for me to see "430 years into the past" looking north, there must be a star 430 light years away (or at least there must have been one there 430 years ago). Similarly, the South Star indicates that there was a star there 270 years ago. That means that the two stars were 700 light years apart at the time the light was first emitted (forgive ignoring the 430 vs 270 year starting point for the moment).
So, we are back to my initial question. On the North Pole, I can see past the North Star out to a distance of 14+ Billion light years. On the South Pole, I can see past the South Star out to a distance of 14+ Billion light years. They are in opposing directions. That means that 14+ billion years ago there was a galaxy 14+ Billion light years to the North and another 14+ Billion light years to the South.
Why does that not imply that they were 28+ Billion light years apart (in distance, not time, since the light from both left 14 Billion years ago to reach us now)? If we assume that the current theory of Big Bang is correct, that would mean the universe must be old enough to scatter matter over 28+ Billion light years by a time 14+ Billion years in the past. (when I was in school in the 60's, Big Bang and Solid State theories were both battling it out, but it appears that the BB crowd has won)
Just because we can only see 14 Billion years "into the past" does not seem to me to be a sufficient reason to assume that is the "edge of time" and that the universe is thus only that old. Why could it not be 100 Billion years old (or pick any other number larger than 14 out of a hat)?
hmmm... Not sure if that last one is a new question or really a continuation of the first... :-)
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When you see light that has taken ~14 billion years to reach Earth (the age of the Universe is currently reckoned to be between 13.3 and 13.9 billion years) it doesn't mean that the galaxy that emitted the light was ~14 billion light years away when the light was emitted. This is because the Universe appears to be expanding (and has been expanding since the Big Bang) so during the time it has taken the light to reach us the distance between the Earth and the galaxy where the light was emitted has increased.
Imagine that you and a friend are standing 10 metres apart, facing away from each other, and your dog starts running away from you and towards your friend at a speed of two metres per second. As long as both you and your friend stay still it will take your dog 5 seconds to reach your friend.
Now though, instead of you and your friend standing still, you both start to walk away from each other at one metre per second relative to the ground i.e. you and your friend are increasing the distance between you at two metres per second.
This time, when your dog has run for 5 seconds it is still five metres away from your friend because he has moved five metres further away in the time it took your dog to get to where he was. And instead of now being ten metres apart, you and your friend are now separated by 20 metres (and your dog is now 15 metres from you instead of ten metres).
If your dog continues to run for another second it will cover another two metres but your friend will have also moved a further metre, so your dog is still 4 metres from your friend despite having now covered a distance of 12 metres (and you and your friend are now 22 metres apart, and your dog is now 18 metres from you).
Eventually, your dog reaches your friend after ten seconds of running (if I've done my maths correctly), instead of five, and has travelled 20 metres instead of ten. Meanwhile, you and your friend are now 30 metres apart, three times the distance you were when you all started off.
In practice, it's more complex than that because the way that the Universe is expanding means that as you and your friend get further away from each other you should both increase your speed, instead of walking at a constant speed (and in real life it's doubly more complex because instead of you and your friend walking across the stationary ground, you should both be standing still and it's actually the two patches of ground that are moving apart, with new ground appearing to fill in the space between you both, and across which your dog can run).
So the light that took ~14 billion years to reach us didn't start off ~14 billion light years away, and the place where it started off from is now further than ~14 billion light years away.
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Thanks for your explanation. I was all prepared to marshal an argument that the universe is not expanding fast enough to make the kind of differences you describe, when I ended up proving your point instead... [;D]
For a fascinating article titled "Is the universe expanding faster than the speed of light?", see (I hope it is OK to use external URLs here...):
http://curious.astro.cornell.edu/question.php?number=575
They show that it only takes a red shift of 1.4 to indicate a galaxy that is moving away from us at the speed of light. We can see many red shifts larger than that, and thus can see many galaxies moving away at faster than the speed of light. (Sounds bizarre at first, but they give an excellent explanation so I won't try to repeat it here.)
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"I understand that light takes XX time to reach us. That is because of the distance though, not because of time itself."
Ah, yes?
Distance is time my friend, without time it's meaningless to discuss anything.
And your question is all about a point centered in a circle.
You're the point, the circles edge is where the earliest light that can reach you comes from. What you're doing is to stare at one side of that circle and then on the opposite and from there draw the conclusion that you now have seen double the distance.
Well you haven't. Wherever you turn your eyes the same number 'pops up' 14. ~ billion ly old. Can you see what I'm saying? Put it as a 3D sphere with you in the middle and the same will be true. As for expansion, it will redshift light but it won't make it take 'longer time' to reach you, as far as I know?
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Or will it?
How much if so?
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The real reason why we can't say how big our universe is in the inflation. If that happened, and it seems plausible to me then there is no limit to any 'size' as we can't say how 'fast' that moved. The expansion we might count on from the redshift but as we don't have a original number for 'size' we can't limit SpaceTime. What we can do is discuss the 'shape' of it, but that's not the size.
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Thinking some more. If the expansion is in all points of space and lights 'speed' is directly bound to 'distance' then it has to 'slow' that light arrival. So that seem correct.
And then we have two limitations, but redshift should be 'countable' it seems to me?
(Presuming that the values you get from lights redshift should be valid for all other 'points' in space too)
Inflation isn't.
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I looked for it but I can't seem to find any numbers for the relation with redshift versus 'speed of expansion'. Can't see why there shouldn't be a correlation between them though?
Also I saw some drawing the conclusion that at some time/distance it means that objects will move away from us FTL.
Wrong, if space is expanding in every point that is like being on the expanding 2D surface of a balloon, getting inflated. Relative you a point further away might seem to move away faster than the point closer too you, but that is as it comes into existence, at all times, a lot more points the further you look from your 'center'. So even thought that points light never will reach you, it still doesn't 'move' over light speed. It's just the '3D floor' we stands on that's expanding preventing light to catch up with you, as it is a 'constant' with a limit of 'c' in a vacuum.
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In fact, considering that the 'expansion' is thought to be restricted to Space you could as easily state that no objects of matter moves at all as space grows like rings on the water uniformly, in all points, around every object of mass. Somehow reminding me of a fractal.
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Burt - The light we see red shifted 1.4 still pass us at 300,000 km/hr. The same speed as the light emitted by our own very close sun which is not red shifted at all. For whatever reason, the speed of light in a vacuum is always the same. The only thing that changes is the wave length (energy) of the light we get from far off places.
I can see no circumstances where a far off galaxy or object still in existence will cease emitting light in our direction regardless of its physical speed in space/time expansion. Accordingly, we will simply observe longer and longer wave lengths. Perhaps wave lengths light years long. Still, these photons will pass us at 300,000 km/s without fail.
In addition, I do not believe infinities of any sort exist in our universe. First, if it had a beginning it will always have an age. Second, if it is expanding from something the size of a plank unit or such, it will always have a volume. It can not have been at one time smaller then an atom, and at some later time become infinite in size without our noticing.
As for the increasing speed of expansion? For my entire adult life the received wisdom is it was slowing down. Then all of a sudden, well, never mind. We get these 'well never mind' moments on a regular, and at seemingly increasingly short time scale. We don't even know what the hell something like 90% of the entire universe is made of.
Well, anyway, thats the latest I can make of the whole thing. If past is any indicator it is almost certainly wrong.