A problem with Time and Space

thnkrx · Dec 28, 2008

Not exactly my area of expertise....but something I noticed years back and wonder about now and again ever since: If I remember right, the Universe is supposed to be something on the order of 14 billion years old, give or take a few hundred million (its been ages since I looked this up).  Assuming that it was expanding in all directions with a maximum speed of just under C (speed of light), that would give us a maximum diameter of the unverse of about 28 billion light years, give or take a billion or so.  Because the universe is only 14 billion years old, give or take, that would mean that light should not have had time to reach us from more than about 14 billion light years off (again, give or take).   Yet, every now and again, I come across stories about galaxies detected at well past this 14 billion light year limit, some times way past that limit, and while its been a while, I think I've seen a few detections of galaxies further off than the theoretical diameter of the universe.  So...assuming I'm remembering this stuff right (and maybe I'm not), but either: 1) the theoretical maximum age of the universe is much further back than what current theory allows for (which might mean rewriting some of the laws of physics from the basement up), or2) at some point in the past (or present?) a lot of things in the unverse (like galaxies) decided to ignore the relativistic speed limit, or3) there is something that allows for our observations of these distant objects to somehow bypass C. Which is it?  Or am I misremembering things worse than I thought?

derekmcd · Dec 28, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>Not exactly my area of expertise....but something I noticed years back and wonder about now and again ever since: If I remember right, the Universe is supposed to be something on the order of 14 billion years old, give or take a few hundred million (its been ages since I looked this up).  Assuming that it was expanding in all directions with a maximum speed of just under C (speed of light), that would give us a maximum diameter of the unverse of about 28 billion light years, give or take a billion or so.  Because the universe is only 14 billion years old, give or take, that would mean that light should not have had time to reach us from more than about 14 billion light years off (again, give or take).   Yet, every now and again, I come across stories about galaxies detected at well past this 14 billion light year limit, some times way past that limit, and while its been a while, I think I've seen a few detections of galaxies further off than the theoretical diameter of the universe.  So...assuming I'm remembering this stuff right (and maybe I'm not), but either: 1) the theoretical maximum age of the universe is much further back than what current theory allows for (which might mean rewriting some of the laws of physics from the basement up), or2) at some point in the past (or present?) a lot of things in the unverse (like galaxies) decided to ignore the relativistic speed limit, or3) there is something that allows for our observations of these distant objects to somehow bypass C. Which is it?  Or am I misremembering things worse than I thought?    Posted by thnkrx</DIV>In astronomy, you often see distances measured by redshift denoted as 'z'.  An object at redshift z=2 has a light travel time of 10.3 billion years (Gyr).  We see that object as it was 10.3 Gyr ago.  As fast as light may travel, the universe is pretty darned big and light, with its finite speed, still takes time to reach us.  When that object first emitted its light 10.8 Gyr ago, the universe was only 3.3 Gyr old.  The objects distance from us was only 5.7 Gly distance from us.  The reason it took 10.8 Gyr to reach us is due to the metric expansion of space and that object is now 17.1 Gly distance from us.   Despite the observable universe being 13.7 gyr old, it is said to have a radius of 46 Gly or 92 Gly in diameter.  (I used Ned Wright's calculator to get these figures). Look at it this way.  Galaxies A,B,C,D all start out one meter apart.  Galaxy B is 1 meter away from A and Galaxy D is 3 meters away. A-B-C-DAfter 1 second, they expand 1 meter.A--B--C--DNow, Galaxy B is 2 meters away having travelled at 1 meter per second.  However Galaxy D is now 6 meters away having travelled 3 meters per second.  Each galaxy is doubling its distance every second (this is only an example).  The next second would be:A----B----C----DNow, galaxy B is 4 meters away while D is now 12 meters away after only 3 seconds.  On cosmological scales, this adds up pretty quick.  The farther away the galaxy, the faster it's recessional velocity. Galaxies at a redshift of about z=1.4 have recessional velocities at the speed of light.  This is not a violation of the Theory of Special Relativity which states that no object with a non-zero rest mass can be accelerated to the speed of light.  The reason is that the galaxies are not travelling through space, they are travelling with space.   <div class="Discussion_UserSignature"> <div> </div> <div>"If something's hard to do, then it's not worth doing." - Homer Simpson</div> </div>

thnkrx · Dec 28, 2008

So you are saying then that space ('reality') is expanding (moving) faster than light?

MeteorWayne · Dec 28, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>So you are saying then that space ('reality') is expanding (moving) faster than light? Posted by thnkrx</DIV> More or less, that is correct. the combination of the expansion of space, plus the velocities of the masses within the spce is greater. <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

kg · Dec 28, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>.... Galaxies at a redshift of about z=1.4 have recessional velocities at the speed of light.  This is not a violation of the Theory of Special Relativity which states that no object with a non-zero rest mass can be accelerated to the speed of light.  The reason is that the galaxies are not travelling through space, they are travelling with space.   Posted by derekmcd</DIV> Would we still be influenced by the gravity of an object with such a redshift?  Not that it would have much influence anyway.

DrRocket · Dec 28, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>Would we still be influenced by the gravity of an object with such a redshift?  Not that it would have much influence anyway. Posted by kg</DIV>Gravity is a force that extends to infinity, basically an inverse square law.  According to general relativity it also propagates at the speed of lght.  So the gravitational effect of an object is consistent with what we "see" of that object.  As you observed the effect of an object that far away would be quite small. <div class="Discussion_UserSignature"> </div>

derekmcd · Dec 28, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>So you are saying then that space ('reality') is expanding (moving) faster than light? Posted by thnkrx</DIV>Not really.  Space is not a physical thing.  Space, in this context, is defined as a metric and the universe is described as a manifold.  Or more precisely, a 4 dimensional Lorentzian manifold.  Basically, a manifold is a surface.  The surface of a sphere is a 2-manifold.  The space we exist in has 3 dimensions.   Einstein tells us that space and time are intrisic to each other and this is modeled as a 4 dimensional surface or a manifold. What metrics do is define measurements on these manifolds.  It is the metric by which we measure distances that is expanding.  We can apply a set of coordinates to our local area here in the universe and, likewise, to a distant galaxy.  The coordinates don't change, but the distance still increases.  In other words, the galaxies are not moving through space... they are moving with space.The "metric expansion of space" is a bit of a misleading phrase.  It really should be more like "the increase measure of the metric tensor of the Lorentzian 4-manifold of spacetime"... but that's a mouthful.  <div class="Discussion_UserSignature"> <div> </div> <div>"If something's hard to do, then it's not worth doing." - Homer Simpson</div> </div>

thnkrx · Dec 28, 2008

Might be interesting if there were some way to tinker with the rate of space time expansion for a very small area.

trumptor · Dec 29, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>Gravity is a force that extends to infinity, basically an inverse square law.  According to general relativity it also propagates at the speed of lght.  So the gravitational effect of an object is consistent with what we "see" of that object.  As you observed the effect of an object that far away would be quite small. Posted by DrRocket</DIV> I always wondered if the sudden expansion of the universe may have happened in an area where matter had previously resided and this expansion blew most of this pre-inflation matter out of the bounds of our observable universe. Now, could this native matter be pulling on the matter in our universe from outside, pulling strongest on the outermost galaxies giving us the perceived acceleration of expansion? <div class="Discussion_UserSignature"> ______________Caution, I may not know what I'm talking about. </div>

jim48 · Dec 29, 2008

As the only real, genuine, bonafide scientist out here, let me assuage your doubts by informing you that the Universe is 16 billion years old and counting. In the future, if you have any questions at all, please feel free to ask me, the only real, genuine, bonafide scientist out here. My work continues... <div class="Discussion_UserSignature"> </div>

SpeedFreek · Dec 29, 2008

Jim48, I respectfully request that you keep your "comedy" out of threads that are asking serious questions and if you cannot resist the temptation then at least make it absolutely clear that you are joking. We have a lot of readers for whom English is not their first language and so your post above could be very misleading. Also, the idea of bold type is to put emphasis in a particular place, so it seems you want to emphasise your whole post above everyone else. You are not helping here. The general concensus is that the universe is around 13.7 billion years old.  <div class="Discussion_UserSignature"> _______________________________________________ SpeedFreek </div>

SpeedFreek · Dec 29, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>I always wondered if the sudden expansion of the universe may have happened in an area where matter had previously resided and this expansion blew most of this pre-inflation matter out of the bounds of our observable universe. Now, could this native matter be pulling on the matter in our universe from outside, pulling strongest on the outermost galaxies giving us the perceived acceleration of expansion? Posted by trumptor</DIV>The problem with that scenario is that our observable universe is defined by the time that light takes to travel, and therefore the further we look, the more out of date our view is. When we are looking at the most distant galaxies we are seeing them as they were before the expansion started to accelerate (around 6 billion years ago is when the acceleration is thought to have begun), so our picture of the observable universe is one where the closer objects, whose light has been travelling for less than 6 billion years, are the objects whose recession speed is affected by the acceleration.The closer we look, the more an object is more distant than it would be if the rate of expansion was still decelerating. The implication is that the acceleration is happening across the whole universe, rather than just at the edges of our observable part of it.You might be confusing the accelerating expansion with the observation that, the further you look, the faster an object recedes, but that effect is purely due to the nature of metric expansion and even if the rate of expansion was constant we would still see the more distant objects receding faster than the closer ones. See Dereks post (the second post in this thread) for a more thorough explanation. <div class="Discussion_UserSignature"> _______________________________________________ SpeedFreek </div>

MeteorWayne · Dec 29, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>Jim48, I respectfully request that you keep your "comedy" out of threads that are asking serious questions and if you cannot resist the temptation then at least make it absolutely clear that you are joking. We have a lot of readers for whom English is not their first language and so your post above could be very misleading. Also, the idea of bold type is to put emphasis in a particular place, so it seems you want to emphasise your whole post above everyone else. You are not helping here. The general concensus is that the universe is around 13.7 billion years old.  Posted by SpeedFreek</DIV> MOD HAT ON***That seems like sound advice. <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

DrRocket · Dec 29, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>I always wondered if the sudden expansion of the universe may have happened in an area where matter had previously resided and this expansion blew most of this pre-inflation matter out of the bounds of our observable universe. Now, could this native matter be pulling on the matter in our universe from outside, pulling strongest on the outermost galaxies giving us the perceived acceleration of expansion? Posted by trumptor</DIV>The thing that you need to get your mind around is that the Big Bang did not happen "in an area where matter had previously resided..."What is being described is an expansion of all of space-time.  There was no other place, and there was no "before".  Space-time is the whole enchilada -- all matter, all energy, all space, all time.  What is described is an expanion of the whole thing, and not an explosion of matter within pre-existing space and time.  Space and time begain at the Big Bang.  There is no "prior" and no "elsewhere". If there were matter pulling on the matter in our universe, then because that matter is affecting our universe it would, by definition, be in our universe.  the Big Bang describes a theory for the formation of the entire universe, not just the observable universe. <div class="Discussion_UserSignature"> </div>

trumptor · Dec 29, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>The thing that you need to get your mind around is that the Big Bang did not happen "in an area where matter had previously resided..."What is being described is an expansion of all of space-time.  There was no other place, and there was no "before".  Space-time is the whole enchilada -- all matter, all energy, all space, all time.  What is described is an expanion of the whole thing, and not an explosion of matter within pre-existing space and time.  Space and time begain at the Big Bang.  There is no "prior" and no "elsewhere". If there were matter pulling on the matter in our universe, then because that matter is affecting our universe it would, by definition, be in our universe.  the Big Bang describes a theory for the formation of the entire universe, not just the observable universe. Posted by DrRocket</DIV> I understand the basic concept that our universe is everything that has inflated from a tremendously dense point infinitely small, but it just seems so counterintuitive. It seems counterintuitive, to me anyways, to think that there is no other side to the edge of our universe because space-time is curved into a sphere like a soccerball and an ant looking for the edge of the "soccerball universe" can keep on walking forever and never find an end. Just as we may see the ant walking around on the soccerball and knowing that there really is more than the surface of the soccerball, can there be a much more complex system that is more than the four dimensions we can observe? Also, I never knew that it was believed that time began when the singularity began to inflate. I thought there were theories that the big bang could be cyclical with an expansion and a big crunch going on for an infinite amount of time. Also, why would it not be possible that space is infinite and these singularities populate this space like stars in our galaxy do and something sets them off where they explode as our universe has, in all directions, like a supernova? I know we're not supposed to think of the inflation as an explosion because we're not exploding into anything and space itself is expanding, but how do we know this? I'm sure there are rational explanations, and I may have heard them before in an astronomy class or two, but may have been too busy doing crossword puzzles to grasp a firm understanding.<img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-cry.gif" border="0" alt="Cry" title="Cry" /> <div class="Discussion_UserSignature"> ______________Caution, I may not know what I'm talking about. </div>

Saiph · Dec 29, 2008

First, I'd like to point out two things:  That the expansion of space does not need to be FTL in order to achieve 'real' diameters larger than the observed universe.  Second: That space is the only thing that we know of that 'can' break the FTL barrier, as only matter is restricted to slower than light velocities. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

SpeedFreek · Dec 29, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>the expansion of space does not need to be FTL in order to achieve 'real' diameters larger than the observed universe. Posted by Saiph</DIV>Hi Saiph! Could you clarify what you mean by real diameters and the observed universe here, please? I am a little unclear on what you mean.  <div class="Discussion_UserSignature"> _______________________________________________ SpeedFreek </div>

DrRocket · Dec 30, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>I understand the basic concept that our universe is everything that has inflated from a tremendously dense point infinitely small, but it just seems so counterintuitive. It seems counterintuitive, to me anyways, to think that there is no other side to the edge of our universe because space-time is curved into a sphere like a soccerball and an ant looking for the edge of the "soccerball universe" can keep on walking forever and never find an end. Just as we may see the ant walking around on the soccerball and knowing that there really is more than the surface of the soccerball, can there be a much more complex system that is more than the four dimensions we can observe? Also, I never knew that it was believed that time began when the singularity began to inflate. I thought there were theories that the big bang could be cyclical with an expansion and a big crunch going on for an infinite amount of time. Also, why would it not be possible that space is infinite and these singularities populate this space like stars in our galaxy do and something sets them off where they explode as our universe has, in all directions, like a supernova? I know we're not supposed to think of the inflation as an explosion because we're not exploding into anything and space itself is expanding, but how do we know this? I'm sure there are rational explanations, and I may have heard them before in an astronomy class or two, but may have been too busy doing crossword puzzles to grasp a firm understanding. Posted by trumptor</DIV>There are a lot of speculations, and you can speculate on anything that you want.  But the Big Bang is based on general relativity, and that limits what can reasonably happen so long as general relativity holds.If you assume that the universe has some minimum amount of matter, consistent with what is observed, and if you assume that the universe is currently expanding, again consistent with what is observed, and if you assume that general relativity is correct you can apply the relevant mathematics to show that the universe originated in the past in a very compact form, no larger than a few centimeters in diameter.  The theory breaks down and predicts a singularity, which indicates that there is more to the problem than just general relativity, but general relativity seems to hold after about the first 10^-26 seconds or so.Now maybe general relativity is not as good as we think it is, or maybe an eventual theory that combines general relativity and quantum mechanics will yield some surprises, but the best theory that we have is that the universe is a 4-dimensional manifold that includes both space and time, twisted together, and that it originated about 13.7  billion years ago in a very compact state.  We have no idea of the actual moment of the Big Bang (time 0), or what, if anything caused it to occur.  In fact the very notion of "cause" makes no sense because cause involves a preceeding event which would require a preceeding time and there is no "before" the Big Bang.So one can speculate at will and speculate on anything, but there is no firm foundation for speculation outside of the Big Bang, since our understanding of physics is simply not up to that task.  There have in the past been other cosmologies considered, based on things other than general relativity, but most have been set aside since the observation of the expansion of the universe and the work of Hawking and Penrose that showed how the Big Bang follows from general relativity.  If you would like to read some of the thinking from the 1950's for historical purposes, the book Cosmology by Bondi is interesting, but ought to be read recognizing that our knowledge has progressed markedly since the book was written. <div class="Discussion_UserSignature"> </div>

Saiph · Dec 31, 2008

Observed diameter:  Light travel time since BB (or distance to CMBR) ~13.7 bly'Real' Diamter: Taking into account expansion, motion, inflation etc, the furthest a galaxy is believed to be beyond the observable region...which I don't recall. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

derekmcd · Dec 31, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>I understand the basic concept that our universe is everything that has inflated from a tremendously dense point infinitely small, but it just seems so counterintuitive. It seems counterintuitive, to me anyways, to think that there is no other side to the edge of our universe because space-time is curved into a sphere like a soccerball and an ant looking for the edge of the "soccerball universe" can keep on walking forever and never find an end. Just as we may see the ant walking around on the soccerball and knowing that there really is more than the surface of the soccerball, can there be a much more complex system that is more than the four dimensions we can observe? Also, I never knew that it was believed that time began when the singularity began to inflate. I thought there were theories that the big bang could be cyclical with an expansion and a big crunch going on for an infinite amount of time. Also, why would it not be possible that space is infinite and these singularities populate this space like stars in our galaxy do and something sets them off where they explode as our universe has, in all directions, like a supernova? I know we're not supposed to think of the inflation as an explosion because we're not exploding into anything and space itself is expanding, but how do we know this? I'm sure there are rational explanations, and I may have heard them before in an astronomy class or two, but may have been too busy doing crossword puzzles to grasp a firm understanding. Posted by trumptor</DIV>I'd like to point out that inflation is different than expansion in the context of the Big Bang theory.  Expansion is explained via the Hubble constant using redshift measurements and Inflation is a hypothesis put forth to have happened in the very early expansion of the universe to explain things such as the flatness of space, causality, the homogeneous nature of the universe, etc.You mention the counterintuitiveness of these thing and that's a good thing you recognize that.  There are a whole host of successful theories that are counterintuitive.  If we simply relied on common sense, science would never progress.  If you ever read some of the comments on the SDC articles when they are discussing their interpretation of such things as dark energy and dark matter... Most of the commenters are trying to use common sense approaches and, well, they just sound silly.Your notion that space-time is curved like a sphere is likely not accurate.  Most recent measurement place some pretty high constraints on the topological curvature.  If 1=flat space, the WMAPs most recent measure is 1.02 +/- .02.  This is essentially say that the universe if flat, or very nearly flat.  If there is a curvature, you are correct that it might be a closed universe and curved like a sphere, but it is not something we could ever physically experience.  Imagine your ant on the surface of an expanding sphere say the size of the Milky Way.  For him, any curvature of that sphere would be physically useless for him to try to experience.  Given proper technology, it might be measured, but to physically experience it is just not gonna happen. <div class="Discussion_UserSignature"> <div> </div> <div>"If something's hard to do, then it's not worth doing." - Homer Simpson</div> </div>

SpeedFreek · Dec 31, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>Observed diameter:  Light travel time since BB (or distance to CMBR) ~13.7 bly'Real' Diamter: Taking into account expansion, motion, inflation etc, the furthest a galaxy is believed to be beyond the observable region...which I don't recall. Posted by Saiph</DIV>Ahh ok, but I think your statement that "the expansion of space does not need to be FTL in order to achieve 'real' diameters larger than the observed universe" is misleading or needs clarification. In terms of light travel time, the most distant galaxy we have seen is around 12.8 billion light-years away. That galaxy was only 3.5 billion ly away (angular-diameter distance) when the light we see was emitted and is believed to now be nearly 29 billion light-years away (co-moving distance) as we receive that light and therefore has apparently receded faster than the speed of light. The particle horizon (the coordinate distance from which the CMBR we detect was emitted) is thought to be 46.5 billion light years away, but was only 42 million light years away, 13.7 billion years ago, so that horizon has receded faster than the speed of light too. If we take inflation into account and assume at that time the inflaton field had a value of at least 10^30 times that of Einsteins cosmological constant (which is the generally accepted minimum value for inflation although some estimates put that figure up at around 10^100) then the whole universe is many magnitudes larger than our observable part of it. The implication is that the Hubble distance, the distance where a coordinate is receding at c, was down near the planck length during inflation.. so all of space was essentially expanding at the speed of light over the smallest distance possible - the edges of the observable universe were moving apart much faster than c.So what were you referring to when you said the expansion of space does not need to be FTL? The surface of last scattering was receding at over 58 times the speed of light when the CMBR was emtted, and that particle horizon is still receding at around 3 times the speed of light today. With inflation, the whole universe pretty much blew up superluminally. This is why I am confused, but I am probably misunderstanding you. <div class="Discussion_UserSignature"> _______________________________________________ SpeedFreek </div>

Saiph · Jan 1, 2009

Or I could be making a rare and extremely noteworthy gaff...a.k.a the 'brain fart'

Glad you've got such confidence in me not to just call BS the moment you see it. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

thnkrx · Jan 1, 2009

So what were you referring to when you said the expansion of space does not need to be FTL? The surface of last scattering was receding at over 58 times the speed of light when the CMBR was emtted, and that particle horizon is still receding at around 3 times the speed of light today. With inflation, the whole universe pretty much blew up superluminally.

And that is the core of my OP.  It appears that the universe (or 'space') decided to ignore the relativistic speed limit, and the rest of us sort of went along for the ride.I also find myself wondering: this purportedly happended naturally, on a universal scale.  It might be interesting to see if it could somehow be duplicated with a much much smaller slice of reality.  Given that Relativity and Quantum Mechanics don't seem to connect that well, there might be room in there for a solution where something like this is feasible.

SpeedFreek · Jan 2, 2009

Replying to: <DIV CLASS='Discussion_PostQuote'>

Glad you've got such confidence in me not to just call BS the moment you see it. Posted by Saiph</DIV>Well, the reason I didn't call it immediately is because there is a context in which your statement that "the expansion of space does not need to be FTL in order to achieve 'real' diameters larger than the observed universe" might make sense, but it all depends on your definition of "real" diameters and the "observed" universe! This is why I asked what you meant by those terms.<img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-smile.gif" border="0" alt="Smile" title="Smile" /> We are back to my old favorite, the angular-diameter distance. One interpretation is to take the angular-diameter distance of an object, the distance that object was from this coordinate when the light we see was emitted, and use only those distances to define the "observed" universe. This differentiates the "observed" from the observable.The most distant galaxies we see, as defined by their angular-diameter distance, are the galaxies we see at the edge of our Hubble sphere, where objects were receding at the speed of light when they emitted the light we now detect. These galaxies have a redshift of around z=1.4 and emitted their light only 9.1 billion years ago, but they were around 5.7 billion light-years away at that time.We have received no electromagnetic radiation that was originally emitted at a distance larger than 5.7 billion light-years away, so that can be considered to be the radius of our observed universe. We see objects with higher redshifts, but these were closer than 5.7 billion light-years to us when they emitted their light.So I like to think of the size of the universe that we have actually observed as being only 5.7 billion light-years in radius. Sure, those z=1.4 galaxies are thought to be a lot further away by now, but they have only been observed to be 5.7 billion light-years away. In that sense, this is a "real" radius for the observed universe, and so indeed is the maximum light-travel time of 13.7 billion light-years. The question is whether those two distance measures can be compared in any meaningful way. <div class="Discussion_UserSignature"> _______________________________________________ SpeedFreek </div>

SpeedFreek · Jan 2, 2009

Replying to: <DIV CLASS='Discussion_PostQuote'>It appears that the universe (or 'space') decided to ignore the relativistic speed limit, and the rest of us sort of went along for the ride. Posted by thnkrx</DIV>That's an interesting (but essentially correct!) way of putting it! You might like to have a look at the link below, it is a .pdf of an article originally published in Scientific American, which explains this stuff very well.Misconceptions about the Big-Bang.And if you are up to it, the original paper that the article was based on:Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe.Not that I am suggesting you are harbouring any misconceptions, by the way!  <div class="Discussion_UserSignature"> _______________________________________________ SpeedFreek </div>

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