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Brady Yoon
2005-Feb-23, 04:43 AM
I have a question about the Big Bang that has been bugging me for a long time. I asked something like it before, but I didn't really understand the explanations, or my mind wasn't working. :)

A book I had said that the solution to Olbers' Paradox is that starlight beyond a certain area hasn't had time to reach us yet, and whether the universe is infinite or finite in size is irrelevant. How could some of the light have not reached us yet? Why do they use the term visible universe?

Inferno
2005-Feb-23, 05:04 AM
Some light may not have reached us because it has very far to travel and hasn't had enough time to reach Earth yet. In this way there could whole chunks of the universe that we know nothing about because we can't see it. ie, it is not part of the "visible universe".

In a sort of reverse of this idea, if it turns out to be correct that the expansion of the universe is speading up, then in the far future more and more stars will fall over the "visibility line". They will be too far away from us and the light from them will never reach us.

Brady Yoon
2005-Feb-23, 05:08 AM
Some light may not have reached us because it has very far to travel and hasn't had enough time to reach Earth yet. In this way there could whole chunks of the universe that we know nothing about because we can't see it. ie, it is not part of the "visible universe".

Does this mean that the universe has expanded faster than the speed of light before? I think this is the only reason that this could work. If it receded at only 0.9c, then that means that light would always overtake the recession velocity.

Sam5
2005-Feb-23, 05:27 AM
Some light may not have reached us because it has very far to travel and hasn't had enough time to reach Earth yet. In this way there could whole chunks of the universe that we know nothing about because we can't see it. ie, it is not part of the "visible universe".

Does this mean that the universe has expanded faster than the speed of light before? I think this is the only reason that this could work. If it receded at only 0.9c, then that means that light would always overtake the recession velocity.

Yes, if the redshifts do indicate radial velocity, the most distant visible galaxies are receeding from us at faster than the speed of light.

Brady Yoon
2005-Feb-23, 05:44 AM
Thanks Sam. Am I right in saying that it doesn't violate relativity because it's talking about the actual expansion of space itself?

Sam5
2005-Feb-23, 05:52 AM
Thanks Sam. Am I right in saying that it doesn't violate relativity because it's talking about the actual expansion of space itself?

I don’t like the term “expanding space”. I think it is a nonsense term. The galaxies are clearly “moving” through space, just like our galaxy is, and M31 and all other galaxies.

The 1905 SR paper did not consider an expanding universe, and its Lorentzian speed limit rule (invented by H.A. Lorentz in 1895) does not apply in an expanding universe. It would apply in a static or non-expanding universe, and, in fact, it does apply within our solar system and within our own galaxy and local group.

The Lorentz “speed limit” of c for the maximum speed of moving masses seems to only apply within strong local fields such as gravity fields. So, we can’t speed things up on earth to faster than c. But in deep inter-galactic space, the most distant galaxies are not moving through strong fields and they seem to have no speed limit relative to the earth, although they do have local Lorentzian speed limits relative to each other’s local fields.

Light from the superluminal galaxies eventually reaches us by traveling through areas of space and local gravity fields that are moving slower and slower relative to us, the closer they are to us. Thus, the light gradually speeds up relative to us and we eventually receive it at c at the earth’s surface. See the 2000 Davis-Lineweaver paper for an explanation of how this happens. It’s a very simple process.

Sam5
2005-Feb-23, 06:13 AM
We can’t apply late 19th and early 20th Century theories, in which the authors thought the universe was “static” and not expanding, to modern times when the astronomers believe the universe is expanding. In 1905, with no expanding universe, c would have been the maximum speed limit of masses moving anywhere in space. As Lorentz explained in his book in 1895, there were electrodynamics reasons for such a speed limit, and Einstein accepted those reasons and agreed with them. In fact, the Lorentz transformation equations are designed to set “c” as a universal maximum speed limit for masses moving through strong fields.

However, with an expanding universe, the galaxies can move faster and faster the further away they are from other galaxies. So, the most distant galaxies can move at faster than c relative to our galaxy, but they can’t move faster to c relative to other galaxies they are closer to. This does not violate the Lorentz/Einstein speed limit law. But just keep in mind, they didn’t realize the universe was expanding. If they were alive today, and accepted the expansion as a fact, they would modify their original speed limit rules to take into account the expansion. The old rule would apply in local areas of space, but would not apply at great distances from our own or other galaxies.

The “expanding space” business is somewhat misleading. The universe is expanding, and, thus, the space between the galaxies is growing, but that’s because the galaxies are moving through it. Just as if you had a line of cars moving through space, with the fastest one being up front and the slowest one being in the back. You could say the distance between the cars is expanding, and the space in between the cars is expanding, but that’s because the cars are moving apart through space.

The most important thing that happens when the galaxies move apart, is that their fields grow weaker and weaker at greater distances. This changes the electrodynamics rules of the Lorentz theory, and this is what allows the most distant galaxies to move fast relative to our galaxy and local group, while they must maintain the old speed limit rule relative to other galaxies they are closer to, since they are within the stronger field areas of those galaxies, while they are in very very weak field area of our galaxies in this part of the universe. So, our weak fields at their great distances do not impose the original Lorentz electrodynamics “speed limit” upon them.

ngc3314
2005-Feb-23, 01:38 PM
Thanks Sam. Am I right in saying that it doesn't violate relativity because it's talking about the actual expansion of space itself?

I don’t like the term “expanding space”. I think it is a nonsense term. The galaxies are clearly “moving” through space, just like our galaxy is, and M31 and all other galaxies.



Hold on a minute. There is a difference, which can be shown by considering the behavior of light emitted in some short time interval. If the galaxies are flying apart through space, that pulse of light wil follow an inverse-square law with distance as it propagates. If the space is expanding, the intensity of light falls off faster, being spread through more space than predicted for the Euclidean case. It is this which distingsuishes the cosmological-observable equations derived from GR, and these are the ones that fit distant objects' fluxes at all. (The debate over distant SN Ia involves effects much smaller than this, and which show up only after correcting for the cosmological issues).

Brady Yoon
2005-Feb-23, 05:14 PM
Some light may not have reached us because it has very far to travel and hasn't had enough time to reach Earth yet. In this way there could whole chunks of the universe that we know nothing about because we can't see it. ie, it is not part of the "visible universe".

In a sort of reverse of this idea, if it turns out to be correct that the expansion of the universe is speading up, then in the far future more and more stars will fall over the "visibility line". They will be too far away from us and the light from them will never reach us.

But these objects have been around pretty much since the universe formed, so they have been emitting light for billions of years. How can there be objects too far to be seen?


Light from the superluminal galaxies eventually reaches us by traveling through areas of space and local gravity fields that are moving slower and slower relative to us, the closer they are to us. Thus, the light gradually speeds up relative to us and we eventually receive it at c at the earth’s surface. See the 2000 Davis-Lineweaver paper for an explanation of how this happens. It’s a very simple process

It doesn't seem too simple to me. Can someone elaborate?

Thumper
2005-Feb-23, 07:54 PM
I'll throw this out and let it get torn apart, Brady. Suppose two galaxies form about a billion light years apart. It will now take a billion years for the light from each to reach the other. However, they are receding from each other such that after a billion years, they are now much further apart. If they are say 1.5 billion light years apart, the light has traveled a billion light years and has half a billion to go. But the galaxies are still traveling away from each other. So that it will take much longer for the light from one to actually reach the other.

But suppose that after a billion years, the galaxies are 3 billion light years apart. If they continue to recede from each other at an accelerated rate, it's possible that light from one will never reach the other. At least that's how I take it. There's some neat graphics and pictures trying to show this that the experts here linked to in a previous discussion about this topic that was helpful to me.

edit: here's one of the links (http://www.anzwers.org/free/universe/redshift.html) from a previous thread (http://www.badastronomy.com/phpBB/viewtopic.php?t=11638&postdays=0&postorder=asc&hig hlight=comoving+space&start=0)

Grey
2005-Feb-23, 08:08 PM
But these objects have been around pretty much since the universe formed, so they have been emitting light for billions of years. How can there be objects too far to be seen?
They're more than billions of light years away. It's really just that simple. :) And since it looks like the expansion is accelerating, objects will indeed "fall off the edge". That is, there are objects which we can see now which will eventually be far enough away that light from them will never reach us again, so they'll leave our observable universe forever. To me, that actually seems like a sad parting of ways. :(


It doesn't seem too simple to me. Can someone elaborate?
I'll let Sam5 explain his own ideas, but I'll interject to say that most cosmologists would concur with your first statement, that it's possible for things to recede at effectively superluminal velocities because of the expansion of space. The speed of light limit is a local one, and there are no global reference frames under general relativity. If you're interested, I'd actually suggest you go read Davis and Lineweaver's paper (http://arxiv.org/abs/astro-ph/0011070) yourself.

Nick
2005-Feb-23, 08:54 PM
The thing I can't my head around is the 'space', or void if you like, that is here even before the big bang.

Remove all the matter, stars, planets... and there is this infinite 'space'.

Without matter there in no time (time is a measurement between events - no events, no time) and no 'space warp' due to gravity...

What is the space!!?!

Nick

Doodler
2005-Feb-23, 09:01 PM
The thing I can't my head around is the 'space', or void if you like, that is here even before the big bang.

Remove all the matter, stars, planets... and there is this infinite 'space'.

Without matter there in no time (time is a measurement between events - no events, no time) and no 'space warp' due to gravity...

What is the space!!?!

Nick

Simple answer, we don't know. No accepted model of physics I've seen discussed here or elsewhere can define it. The empty depth width and breadth of which you speak is an inherent part of the universe in the BB model, without the universe, it doesn't exist. The universe is more than the matter and mass that is contained within it, but the volume within which that matter exists. Beyond that, its anyone's guess.

Sam5
2005-Feb-23, 10:53 PM
I'll throw this out and let it get torn apart, Brady. Suppose two galaxies form about a billion light years apart. It will now take a billion years for the light from each to reach the other. However, they are receding from each other such that after a billion years, they are now much further apart. If they are say 1.5 billion light years apart, the light has traveled a billion light years and has half a billion to go. But the galaxies are still traveling away from each other. So that it will take much longer for the light from one to actually reach the other.



Ok, I’ll agree with what you are saying, and I’ll add my 2 cents worth.

What I think we all have to face up to, is the acceptance of a variable body-relative speed of light and three separate relative light speeds inside an expanding universe. 1) the local and distant speed relative to the emitting galaxy, 2) the speed relative to the local area of space through which each photon is traveling at the moment, and 3) the local and distant speed relative to the galaxy and observer inside the galaxy that will eventually observe the light.

This does not “contradict” the Lorentz/Einstein “speed limit” or the “constancy postulate”, since they were base on the belief by Lorentz and Einstein in a “static” universe that was not expanding or contracting. So we can’t compare what we observe today to what they were talking about in 1895 and 1905, because today we assume we are dealing with an “expanding” universe, while back in 1895 and 1905 they thought they were dealing with a “static” universe.

Now that we think we’ve got an “expanding” universe, their “speed limit” and “constancy” rules simply do not apply on a large universal scale, although they do seem to apply on a small local scale, relative to the space or fields through which the photons are presently traveling.

So, what I get out of the Davis-Lineweaver paper is that the light from a distant high-z galaxy moves at “c” relative to that galaxy while the photons are still inside that galaxy, as measured by atomic clocks located inside that galaxy, but the photons are initially moving away from the earth at first, right after they are emitted, since the galaxy is moving away from the earth at faster than the speed of light, relative to the earth.

So, if that’s true, then we’ve got, at first:
1) the emitter-relative speed of the photons = c
2) the local space-relative speed where the photons are first emitted = c
3) the initial earth-relative speed of the photons when first emitted = -1 c or maybe –2 c (depending on how fast the galaxy is moving away from us).

As the photons move into areas of space and fields (gravity, electric, and magnetic) that are not moving away from us as rapidly as the galaxy that initially emitted the light, then the photons speed up relative to the earth. This earth-relative speed up of the photons can occur while their speed remains at about “c” relative to the local space and fields through which they travel. So we’ve really got 3 different relative speeds for the very same group of photons: 1) the emitter-galaxy relative speed; 2) the local space through which they are traveling relative speed; and 3) the earth-relative speed. According to Davis and Lineweaver, what always remains a constant “c” is #2, the space-relative speed of the photons, relative to the local space through which they are currently traveling. However, the #1 and #3 galaxy-relative speeds constantly change. That’s ok, since the original Lorentz/Einstein models didn’t have any “moving galaxies” or “expanding universe”.

As time goes by, the photons speed up more and more relative to the earth and we finally receive them at “c”, as judged by our earth based atomic clocks. By then, those very same photons are traveling at 2 or 3 c relative to the galaxy that first emitted them. That’s ok, because when we finally receive them, their local speed is no longer being influenced by the local space or fields of the distant galaxy that emitted them, and their local speed (relative to the earth) is being influenced by the space and fields inside our own galaxy. All during the trip, the photons never went faster or slower than “c” relative to the local space and fields through which they traveled, thus preserving some of the original Lorentz/Einstein “constancy” and “speed limit” ideas. But we can’t preserve all of the original Lorentz/Einstein “constancy” and “speed limit” ideas, since they were base on a “static” and non-expanding universe and non-expanding space.

Hey Grey, what do you think of this hypothesis?

Grey
2005-Feb-24, 02:23 AM
Hey Grey, what do you think of this hypothesis?
Actually, I think you're pretty close. The thing I'd disagree with is that the fields of the galaxies or other objects, whether electromagnetic or gravitational, don't really have anything to do with it. But I would agree that photons move at c relative to the local space they're in at any given moment, and that if you add in the effects of the expansion of space, that can mean that a photon is moving relative to* the emitter or receiver at some other velocity overall.

*I almost hesitate to use the phrase "relative to" in this second case, since as I pointed out above, there's no such thing as a global frame of reference, only local ones. So the "relative to" in this latter case is used in a different sense than when we talk about photons or any other objects moving relative to some local frame of reference, but hopefully the distinction is clear.

crosscountry
2005-Feb-24, 02:43 AM
[quote=Inferno]Some light may not have reached us because it has very far to travel and hasn't had enough time to reach Earth yet. In this way there could whole chunks of the universe that we know nothing about because we can't see it. ie, it is not part of the "visible universe".


If I may,

They were at one time close enough to us for us to see them. That light has passed by Earth and is beyond us in the opposite direction. Now that everything is further apart we cannot see them because we are moving ,relatively to them, faster than the speed of light.

Nothing is breaking the speed of light, we are just moving in opposite directions with a total difference of >c.

Please correct me if I'm wrong.

Sam5
2005-Feb-24, 03:10 AM
Hey Grey, what do you think of this hypothesis?
Actually, I think you're pretty close. The thing I'd disagree with is that the fields of the galaxies or other objects, whether electromagnetic or gravitational, don't really have anything to do with it. But I would agree that photons move at c relative to the local space they're in at any given moment, and that if you add in the effects of the expansion of space, that can mean that a photon is moving relative to* the emitter or receiver at some other velocity overall.

Yes, ok, very good. I think we agree on that. I think that’s what D&L are trying to say in their paper, but they seem to be very hesitant to come right out and state it in the simple and fundamental way we both have just stated it. There are some old traditions that, in my opinion, made them quite hesitant to state it so simply. I think that’s why they said in their paper, “When άχγ > c the distance between us and the photon increases.”

What this means is, (in my opinion), when άχγ > c, when the photon is first emitted, the photon is aimed in our direction but it is moving away from us, being carried away by the expansion of the universe. Only later does the photon start moving toward us. That’s why it takes so long for us to receive a photon from a galaxy that was much closer to us when the photon was originally emitted. That’s why it takes 13 billion years for us to receive a photon that was emitted when the galaxy that emitted it was just 2 or 3 billion light years away from us.



*I almost hesitate to use the phrase "relative to" in this second case, since as I pointed out above, there's no such thing as a global frame of reference, only local ones. So the "relative to" in this latter case is used in a different sense than when we talk about photons or any other objects moving relative to some local frame of reference, but hopefully the distinction is clear.

I understand.

John Kierein
2005-Feb-24, 03:14 PM
Well, if the red shift is doppler, then you must consider the fact that the light received is in the form of photons. Each photon's wavelength is observed to be stretched due to the relative motion betwen the source and the observer. But another, even more important effect for this discussion, occurs from the doppler effect. That is that the distance between photons also is observed to be stretched. So a distant source not only appears to be red shifted, but it appears dimmer because the number of photons received per unit time is less. This is in addition to an inverse square law loss of energy for an isotropically radiating source. A red shift of 4 for example, would not only cause the wavelength to be 5 times as great as an unshifted relatively non-moving source, but also 1/5 as bright. This loss of energy is what solves Olbers paradox for an expanding universe.

Olbers paradox can be stated in a somewhat more difficult manner. "If the sources is a constant number density, then the number of sources increases as the cube of the distance, but the energy received only falls off as the inverse square, so why isn't the energy being received in an observationally infinite universe infinite?" Part of the answer is the red shift. A red shifted photon has less energy than an unshifted one; but the main solution is the doppler dimming.

(Since most sources are bright in the visible, a visible light observer would eventually see an edge to an infinite expanding universe as all the light is shifted out of the visible range into the IR and longer wavelengths; but this alone does not solve the more difficult Olbers paradox.)

Of course if the red shift is not doppler but some other mechanism such as Compton, then this does not apply. The solution to Olbers paradox becomes more important for such a model (which is "against the mainstream", although I hold this view.)

Sam5
2005-Feb-24, 04:22 PM
Well, if the red shift is doppler, then you must consider the fact that the light received is in the form of photons. Each photon's wavelength is observed to be stretched due to the relative motion betwen the source and the observer. But another, even more important effect for this discussion, occurs from the doppler effect. That is that the distance between photons also is observed to be stretched. So a distant source not only appears to be red shifted, but it appears dimmer because the number of photons received per unit time is less. This is in addition to an inverse square law loss of energy for an isotropically radiating source. A red shift of 4 for example, would not only cause the wavelength to be 5 times as great as an unshifted relatively non-moving source, but also 1/5 as bright.

Doppler mentioned this phenomenon in his original 1842 paper. One reason he is given so much credit today as the originator of the “Doppler effect” is because he worked out many of the exact physical details of the various phenomena. I have a German version of his paper, which I’ve been slowly translating.

If you are interested in obtaining H.A. Lorentz’s 1907 book which contains reprints of 11 Doppler papers, you can find it listed as “Abhandlungen” by Christian Doppler. You might find it listed as “Abhandlungen” Hn. H.A. Lorentz. The “Hn.” means “edited by”. It costs from $30 to about $60.

Bathcat
2005-Feb-27, 08:09 PM
One difficulty with visualizing spacetime expansion after the big bang is that we poor humans nearly always have to picture something expanding into a pre-existing space.

After all, that's exactly what our savanna-ape experience leads us to believe is always the case.

But Stephen Hawking and James Hartle (among many others) note that spacetime as we know it is of the universe itself, and not external to it. So the universe is not expanding into some external space, the universe IS space!

No, I can't visualize that. I can't visualize spacetime curvature, either, except by pretending it's a rubber sheet. Darn that old universe anyways -- why can't it manifest its Reality in ways a simple hominid fresh off the plains of Africa can intuitively understand?

All joking aside, I agree with Grey: light has had some 13.7 billion years to travel, since that's the age of the universe, but the universe as a whole may be hundreds or thousands of billions of lightyears across. (It expanded very, very, very rapidly for a brief time near the beginning, and that's why it may be so immensely gargantuan now.) And so of course everything beyond 13.7 lightyears distance is hidden from us.

---

My three or four synapses are always boggled a bit by the observation that there is no universal reference frame. But I guess there is simply not much sense in talking about "right now" when speaking of the universe at large.

For example, "right now" a protoplasmic superbrain watching one of his robot probes fall into a black hole sees the robot, in ultra-long wavelength infrared light, virtually motionless at the edge of the black hole's event horizon. And in the "right now" of the robot itself, it is already past the event horizon and being torn apart by tidal gravity near the center of the black hole.

Both "right nows" are valid, and yet my tiny mind insists that they can't both be right.

*shrugs*

I suppose the error lies in thinking that there is an external, universal "right now" which must apply across all situations. There just isn't.