How big is our Galaxy.

OK sorry, back onto Jar's topic.The point I am trying to stick hard onto is that observable light never travels faster than c. Expansion may indicate that theoretical light traveling away from is thereby receding, so that it can be imagined as moving faster than c (relative) to us, but we cannot observe it, it is not moving towards our eyes. The light that is actually capable of reaching us is traveling towards us, the distance may increase but the light itself travels at its proper speed. Thus it appears to reach us more slowly, never more quickly, than it should have at c.Therefore the light we see from the edge of the universe, which was only say 2 billion light years away when it set out, took 13 billion years to reach us. The only way that it could be the other way round, with the light reaching us faster than the original distance, would be if the universe were contracting. Let's say for example that a universe 13 billion light years in radius were created 6 thousand years ago and has shrunk down now to only a thousand light years across, that kind of thing would do the trick.Everything would be blue-shifted though, and very very hot, quite different from the local universe we observe.

OK again, that's the Hubble Distance, originally set at 10 and currently running at about 15 billion light-years, good chance it will increase rather than decrease. The actual observed light is say 13 billion years old. The difference between the two, the lack of observable light older than that -- the "outer darkness" if you will, the reason the universe is not absolutely white with light -- is the radius of the "pure" space (nontime) created by inflation. Everything since then has been expanding spacetime.15-2=13If you like 13.7 and keep 15, then inflation amounts to only 1.3 or else if you expand Hubble to say 18 then inflation amounts to 4.3 and so on.Keep in mind that these distances only represent observability, assuming we live in an infinite (open) universe then by implication when everything we can now observe fit into the molecule in the teaspoon in the teacup, there was super-compressed tea not just there but several inches, miles, even light-years away. Inflation could have affected much more than just our little section of that, we just can't ever know it.

Relativistic effects are only caused by relative motion within space. That's why the expansion of space has no relativistic effect. It's why when observing distant galaxies the period of objects like pulsars and Cepheid variables does not have to be adjusted for recession velocity, otherwise Hubble would not have been able to calculate his constant. Red shifting can occur because of relative motion, or it can occur because of the expansion of space. The reason why distant galaxies do not have observable time dilation, the reason why their clocks beat at roughly the same rate as our own, is because their recession is due to the expansion of space, and not due to actual relativistic recession velocities.Though there's no such thing as a fixed reference frame, this analogy still might help. Think of space like a rubber band that is being stretched longer and longer. Imagine two ants running away from each other at a speed relative to the rubber band of 1 inch/second, which means they are receding from each other at a rate of 2 inches/seconds. But as the rubber band is stretched longer and longer and they become further apart their recession velocity becomes greater and greater. However, their recession velocity measured against the rubber band is still 2 inches/second.It's the same way with distant galaxies. Though they might be receding from us at a great apparent velocity, their actual relative velocity to us as measured against some common reference frame is actually quite small, maybe around 50 km/sec, and is not necessarily away from us but in any direction.--PercyThis message has been edited by Percy, 01-15-2006 11:09 AM

Hi Percy... first off, thanks for the detail in your post. However... You might say Special Relativistic effects are only caused by relative motion within space... but of course we are talking General Relativity here as we are dealing with a cosmological model. Until you consider GR that is oh, and observation Pulsars and Cepheids are observed in proximities that show virtually no expansion effects. The Virgo Cluster (the furthest Cepheid measurements of which I am aware) is on our doorstep and is still dominated by peculiar motion. The recessional red-shift effect is tiny compared to the many other errors. True, and also from local gravitational effects. One of the big breakthroughs in understanding relativity comes when you realise that these three are all the same thing. So what causes the time dilation between two observers sitting at different heights in a grav well? You would say that there is no motion at all here.What do you think red-shifting is? It is the very time dilation we are discussing whatever the source of the red-shift, be it relativistic motion, cosmological expansion, or gravitational potential. Imagine an atomic clock on a receding galaxy and think of observations of that clock from here... keep thinking... think of the resonant frequency of the clock... keep thinking... think of observations of that resonant frequency... keep thinking... now, do you believe me yet?This is a major observational difference between recessional red-shift and tired-light. Tired-light will have no associated time-dilation. Percy, as a member of the Cambridge Relativity Group I used to help teach Cambridge graduates their GR... it may be a long time ago now but my pride can still only take so much of this Yes, this is quite true. All objects will have peculiar motions wrt their own comoving frame. But this is irrelevent to the point that cosmological expansion produces time dilation.Oh, almost forgot. I did mention that is has been observed didn't I? Well, how about time dilation of Type 1A SN signatures at lowish z. And the initial stuff was all on time dilation of gamma ray burster signatures as a way of proving their cosmological origin.I hope this helps...This message has been edited by cavediver, 01-15-2006 11:14 AM

Well, if I've arrived at a misconception it won't be the first time. My source is Sylas, but I could have misinterpreted him or he could be wrong. My reading of my exchange with Sylas about this (see What's the Fabric of space made out of? starting at Message 211) is that the time dilation effect you're observing for distant galaxies is actually due to simple recession due to expansion of space, and not to relativistic effects. See the first paragraph of Sylas's post in Message 226.--PercyThis message has been edited by Percy, 01-15-2006 11:37 AM

Thanks, I'll check the reference out a little later. For now, the easiest way to understand all of this is to do as Einstein did and form simple Gedanken experiments. Hopefully my "atomic clock on receding galaxy" should be enough to convince yourself of what is going on.

re-read Sylas's post Message 226 in response to your message Message 225.It could be my reply above re-worded. Scary.So yes, I would say that you have misinterpreted him.

Yes, I referred you to paragraph one of that post in my previous reply. Is that the part you're looking at? If so, I think we have a different interpretation of it.--Percy

I know, that's how I found it!

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Caution... if you speak about what is "seen", then actually you do see the second hand ticking more slowly. Suppose a photon leaves the watch at a certain instant. Another photon leaves when the second hand has ticked off another second. The second photon has further to travel than the first, and so arrives more than a second after the first. You see the clock ticking off seconds more slowly.

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I "think" I've found the issue... are you concerned that the "time-dilation" of the slow ticking watch is caused by just the obvious point that the photons have further to travel, and so isn't the "true" time-dilation that you are used to in Special Relativity?In relativity, we don't really make any distinction. All sources of time-dilation are down to geometry. Just because the geometry in this cosmological case seems more obvious makes no difference. None of these time-dilations are real as such. However, in each case, if you try to move from your frame to the other frame, you will get a very real time difference (say, respective elapsed times from the Big Bang).This comsological "trivial" time-dilation still gives rise to an infinte red-shift horizon as Iblis was describing.This message has been edited by cavediver, 01-15-2006 12:39 PM

Offhand, the messages seem to deserve better than a long series of "Re: Night of the Creationists".Adminnemooseus

Might it be a matter of perspective and approach whether one prefers to draw distinctions or not? Sylas was clearly drawing a distinction between relativistic versus "just getting further away" effects. In other words, he was identifying and distinguishing between the different contributions. I can tell that you prefer including them all in a single category, i.e., just measure the recession velocity and be done with it. This has obvious advantages, but understanding the nature of the fabric of space and what the effects of its expansion are is not one of them.One of the interesting questions that came up in that earlier thread, and that Sylas was unfortunately no longer around to address, was what would we would observe for a distant galaxy that was stationary relative to us. Such a galaxy would be moving at tremendous speeds relative to its neighbors, but relative to us it would be stationary. However, relative to us in space (and this was where Sylas's comment about long distances rendering these things questionable has me still scratching my head) it is moving toward us at a tremendous velocity. We would see no red shift because the expansion of intervening space cancels out the compression of wavelengths, but the relativistic effects from our tremendous relative velocities should cause its clocks to appear to be moving very slowly.I have a feeling I'm about to receive another yuk-yuk smilie, but I'll click the submit button anyway.--Percy

OK I think there are 2 points that you are missing. I have been stuck on the first one because I still don't really grok it, I've just been provisionally convinved that it is true. But the second is probably more important to help you get this.1) There is no simultaneity, it seems to hold out fine as long as we talk about 2 objects with one in motion relative to the other, but it breaks down as soon as we add a third inertial frame; therefore talking about what is happening "now" in distant galaxies is plain nonsense2) The time-dilation of distant galaxies due to expansion is the exact same time-dilation we experience when falling into a black hole, that's why the Big Bang is sometimes spoken of as a "naked" singularity or "white hole"; the effect is in fact exactly what we would experience if we were falling into a black hole but perceiving time as moving backward* and this is particularly interesting in relation to what I was saying earlier about the relative speeds of objects beyond the light horizon in that time stopped for them (from our viewpoint) at the point where they were traveling at the speed of light and yet "now" they would be/appear to be traveling much faster than light, so therefore presumably they would be experiencing time as going backwards relative to us; the same effect as we get at the event horizon of a black hole, the thing that makes a singularity singular, the dividing by 0 that occurs on the way toward -1

Agreed. You didn't quote what you're replying to, but if it's my example of the distant galaxy that appears stationary to us, I was only talking about observations from our own perspective. I can see you and Cavediver agree about relativistic and Doppler effects being the same, but there still seems to me an important distinction is being missed. For example, what do we observe with that distant stationary-relative-to-us galaxy I described, and why?--Percy

Sorry no, I meant the part where the distant galaxies not experiencing special relativity in relation to their own comoving frame has anything to do with their relativity to us; this is the part where I want to agree with you and say that if we could somehow teleport over there right "now" we would not pop up in the midst of objects moving faster than light, but rather into something like our normal space, i e the same "kind" of section of your rubber band, with everything else moving away from it; but I would be wrong because a) we couldn't, not ever, and also b) because if we could, I could go "immediately" to any of an infinite number of different "nows", depending on where I did my calculations We observe it as a stationary object without red-shift and aging normally; in its own "now" it isn't though, it's whizzing toward us at just under the speed of light and experiencing almost no aging relative to its comoving frame. The little aging it is experiencing though, accelerated by the blue-shift of the light it is traveling right along behind, causes us to see it as "normal"

Yes he was, and I'm in complete agreement with doing that. No, absolutely not. Remember, I said

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All sources of time-dilation are down to geometry

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but not down to recessional velocity. I say that the time-dilation of cosmological expansion is most definitely down to expansion of space and "nothing to do with recessional velocity" although I would caveat that by saying again that it is all highly related. Here's your problem. We have no "tremendous relative velocity". This is what Sylas was trying to explain. The two observers are in two different inertial frames. You cannot make these kinds of comparison. The simple world of Special Relativity does not apply. Just because the galaxy is moving rapidly wrt its [abe: remove "inertial", replace with "local comoving"] frame, it doesn't mean that it is wrt ours, whatever that means... you must be psychic This message has been edited by cavediver, 01-16-2006 04:22 AM