Martin Posted January 18, 2008 Share Posted January 18, 2008 Cosmology is based on General Relativity, which allows distances to expand and many times the speed of light. Do you believe this? Show your opinion on the poll. Link to comment Share on other sites More sharing options...
insane_alien Posted January 18, 2008 Share Posted January 18, 2008 i don't see anything with points in space expanding faster than the speed of light. i'm not an expert on it so it is only opinion. i'm sure that some cosmologist somewhere some when will perform experiments to prove it or disprove it if it hasn't already been done. Link to comment Share on other sites More sharing options...
YT2095 Posted January 18, 2008 Share Posted January 18, 2008 although it was never stated how fast space itself can travel, I`v been taught that Nothing travels faster than c. Link to comment Share on other sites More sharing options...
gcol Posted January 18, 2008 Share Posted January 18, 2008 I voted yes because I considered the word "can" to be a variant of the word "could" in Martin's other question. I suspect that only a small tweak to relativity might allow it. Beware the Known unknowns and unknown unknowns. Link to comment Share on other sites More sharing options...
Klaynos Posted January 18, 2008 Share Posted January 18, 2008 You must remember that it's not objects moving faster than c we're talking about but space itself expanding.... Link to comment Share on other sites More sharing options...
gcol Posted January 18, 2008 Share Posted January 18, 2008 ...... And can we envisage anything more massless than empty space.....? Link to comment Share on other sites More sharing options...
Royston Posted January 18, 2008 Share Posted January 18, 2008 ...... And can we envisage anything more massless than empty space.....? But as Klaynos said, it's the rate of expansion that exceeds C, this isn't to do with the mass of space, being akin to massless particles (photons, gluons et.c) and so having a limit C. Without even going through the math, some of the neighbouring galaxies are receeding (some are approaching due to gravity taking hold...but anyway) just below C, the further away a galaxy is, the faster the recession speed, also given the fact the expansion has been accelerating, then it stands to reason the expansion is FTL. Link to comment Share on other sites More sharing options...
iNow Posted January 18, 2008 Share Posted January 18, 2008 I keep asking myself... "Relative to what?" Link to comment Share on other sites More sharing options...
Royston Posted January 18, 2008 Share Posted January 18, 2008 I keep asking myself... "Relative to what?" Relative to any point you care to pick...the universe is isotropic. Link to comment Share on other sites More sharing options...
Realitycheck Posted January 18, 2008 Share Posted January 18, 2008 Without even considering inflation, if the universe is expanding at the speed of light in all directions, then relative to one end, the other end is growing at twice the speed of light. Link to comment Share on other sites More sharing options...
YT2095 Posted January 18, 2008 Share Posted January 18, 2008 it Can`t have one end and another end tho can it, if it DID it would have a shape, and if it Had a shape it would have a Center point! and since it doesn`t.... well I dunno, I`ll let you guys work that one out! Link to comment Share on other sites More sharing options...
iNow Posted January 18, 2008 Share Posted January 18, 2008 If the universe is expanding at (or faster than) the speed of light, don't we need a point outside of the universe (I don't think there are any, but) by which to compare that speed of expansion? Agentchange - There is no edge of the universe. If you think I'm wrong, prove it. Link to comment Share on other sites More sharing options...
YT2095 Posted January 18, 2008 Share Posted January 18, 2008 Agentchange - There is no edge of the universe. If you think I'm wrong, prove it. that`s probably one of the most Logically Lame Circular args available. since to be outside the Universe to prove it, you would be part of the universe by default *Yawn* AKA, no matter where you go, there you are! Link to comment Share on other sites More sharing options...
swansont Posted January 18, 2008 Share Posted January 18, 2008 although it was never stated how fast space itself can travel, I`v been taught that Nothing travels faster than c. That's locally sorta true, but "nothing travels faster than c" is a journalistic/popular distillation of relativity, and is simplified to the point that it's wrong. c is the limit in motion through space that's described by special relativity (for events that are causally linked). But the expansion of space breaks that — the points in space can move apart faster than that limit Link to comment Share on other sites More sharing options...
Martin Posted January 18, 2008 Author Share Posted January 18, 2008 I agree with what Swansont just said. So far I think the only question asked has been from iNow. Everybody else AFAICS has been making assertions. iNow asked what recession speed is relative to I keep asking myself... "Relative to what?" Snail gave what I think is a good answer. Relative to any point you care to pick...the universe is isotropic. I just want to back Snail up and clarify a little. Recession speeds are just the rate that distances from us (or any other basepoint) are expanding. According to GR, distances between stationary points change---they expand or contract according to the EFE (einst. field eqn.). That is what it means for spacetime to be curved. Standard cosmology is just one particular solution to the EFE, that happens to be on average expanding. Relativity requires that much of the expansion be at rates which far exceed c, the speed of light. the expansion of distance does not represent anything moving thru space. it is simply the expansion of distance Snail is quite right that you can pick any point as basepoint. For example you can pick the milkyway galaxy and measure all the distances outwards from that. Local shortrange distances have all sorts of random variation so we are only talking about longrange. Beyond where our own gravity has any significant influence. Longrange distances measured outwards from Milky as basepoint are increasing at a rate of one percent every 140 million years. So if a distance is long enough it can be increasing faster than c. iNow, does what Snail said provide a sufficient answer to your question? If not please keep asking. Without even considering inflation, if the universe is expanding at the speed of light in all directions, then relative to one end, the other end is growing at twice the speed of light. Agent, I don't understand the point of this remark. What are you trying to say? It would help if you would give complete arguments including your assumptions, if you are trying to argue for something. Or if you are trying to ask a question, it would help if you would put it as a question. Link to comment Share on other sites More sharing options...
Realitycheck Posted January 18, 2008 Share Posted January 18, 2008 Agent, I don't understand the point of this remark. What are you trying to say? It would help if you would give complete arguments including your assumptions, if you are trying to argue for something. You have a star. Photons travel out one side of the star at c. Photons travel out the other side at c. If you are a lucky quark that somehow got to hitch a ride on one of these photons, surfing through the universe, then the photons travelling in the other direction would be travelling at double c relative to you. As swanson pointed out, they would not be travelling faster than the speed of light, except via special relativity. Link to comment Share on other sites More sharing options...
Martin Posted January 18, 2008 Author Share Posted January 18, 2008 Without even considering inflation, if the universe is expanding at the speed of light in all directions, then relative to one end, the other end is growing at twice the speed of light. You have a star. Photons travel out one side of the star at c. Photons travel out the other side at c. If you are a lucky quark that somehow got to hitch a ride on one of these photons, surfing through the universe, then the photons travelling in the other direction would be travelling at double c relative to you. As swanson pointed out, they would not be travelling faster than the speed of light, except via special relativity. Do you mean to make a logical connection between these two statements, Agent? In this thread we are talking about the universe expanding, not about photons going in two directions from a star. So you must have some assumption in mind that connects these two things. Please complete the argument. What are you trying to say? Link to comment Share on other sites More sharing options...
swansont Posted January 18, 2008 Share Posted January 18, 2008 You have a star. Photons travel out one side of the star at c. Photons travel out the other side at c. If you are a lucky quark that somehow got to hitch a ride on one of these photons, surfing through the universe, then the photons travelling in the other direction would be travelling at double c relative to you. As swanson pointed out, they would not be travelling faster than the speed of light, except via special relativity. Quarks don't hitch rides on photons, and I think you have misunderstood what I said. In the example you give, nothing with mass would be seen to exceed c — it's a local example. You have a patch of space, which is locally flat. No gravity, no noticable expansion. Here, special relativity works just fine — no two objects are going to move relative to each other at a speed exceeding c. However, if you go far enough away, you find that space is expanding. At that scale, SR no longer applies — two distant objects recede from each other and can do so at speeds exceeding c, even though the local behavior does not violate SR. That's because the intervening space is expanding. Link to comment Share on other sites More sharing options...
Realitycheck Posted January 18, 2008 Share Posted January 18, 2008 Ok. I was using the premise that the universe grows by photons going where no photon has gone before, but I see where you are coming from. Link to comment Share on other sites More sharing options...
losfomot Posted January 18, 2008 Share Posted January 18, 2008 The problem I have is in distinguishing between expansion velocities and peculiar velocities. If there is no (physical, material thing that we can call..) space.. that is, if space is just distance between objects... then a galaxy moving away from us FTL is still a galaxy moving away from us FTL, regardless of why it is moving away from us that fast... Link to comment Share on other sites More sharing options...
Martin Posted January 18, 2008 Author Share Posted January 18, 2008 I agree entirely with Swansont here---he gives a clear explanation. I'll add an example to illustrate. Some of the best studied radiation by astronomers is the CMB. A lot of effort has gone into analyzing and mapping it and reconstructing the conditions which gave rise to it (when the expansion of the universe was about 370,000 years old and the temperature was around 3000 kelvin.) According to the standard model the CMB light which we are receiving now, with for example the WMAP satellite, came from matter which at the time was receding from us at about 60c. That is, sixty times the speed of light. When it emitted the light, this matter was about 41 million LY from us (or rather from the matter which eventually condensed to form the solar system). The light has taken approximately 13.7 billion years to get here. The matter which emitted the light which we currently receive as CMB is now receding from us much more slowly---at 3.3c. It is now at a distance from us of somewhat over 45 billion LY. Sometimes the figure quoted is 46 billion LY. This is roughly the radius of our observable universe as astronomers define it (encompassing the objects which we can see and study.) Is anybody still having problems with the idea that expansion speeds can be faster than light? Please ask questions, if you do, so we can understand what the trouble is. Here are some educational links http://www.scienceforums.net/forum/showthread.php?p=384716#post384716 especially useful are the sidebars to the Scientific American article by Lineweaver. I have included links to individual sidebars that are relevant here. Link to comment Share on other sites More sharing options...
uncool Posted January 19, 2008 Share Posted January 19, 2008 You have a star. Photons travel out one side of the star at c. Photons travel out the other side at c. If you are a lucky quark that somehow got to hitch a ride on one of these photons, surfing through the universe, then the photons travelling in the other direction would be travelling at double c relative to you. As swanson pointed out, they would not be travelling faster than the speed of light, except via special relativity. That's not even correct - if you are a quark traveling very close to the photon relative to the star (because nothing can travel with the photon, not even a viewpoint), the other photon would still seem to be traveling at c. This is because your system has different lengths and different time intervals than the original system - as shown by special relativity. This is actually the first thing you find in relativity - the second postulate: anything going at the speed of light in one inertial frame is going at the speed of light in any inertial frame. =Uncool- Link to comment Share on other sites More sharing options...
Realitycheck Posted January 19, 2008 Share Posted January 19, 2008 It is now at a distance from us of somewhat over 45 billion LY. Sometimes the figure quoted is 46 billion LY. This is roughly the radius of our observable universe as astronomers define it (encompassing the objects which we can see and study.) ... Link to comment Share on other sites More sharing options...
losfomot Posted January 21, 2008 Share Posted January 21, 2008 Just so I am clear about things... a galaxy that is moving away from us due to expansion is not subject to relativistic effects (as a result of said motion)... right? Link to comment Share on other sites More sharing options...
swansont Posted January 21, 2008 Share Posted January 21, 2008 Just so I am clear about things... a galaxy that is moving away from us due to expansion is not subject to relativistic effects (as a result of said motion)... right? It's not limited by relativity, because the motion is due to the expansion of the space, not the motion of the galaxy. Link to comment Share on other sites More sharing options...
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