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Probably Dumb Questions
I hope you esteemed scientists will not mind a lay person like me
asking some basic questions. I'm no scientist, and don't normally read this newsgroup, but I can't find the answers anywhere else. My questions are stimulated by recent news of HUDF detecting a galaxy 13 million plus light years away and the oldest object detected. Do we know the direction of the center of the universe, the point where the big bang happened, the point from which everything is supposedly racing away in all directions? [[Mod. note -- The big didn't happen "at a point", but rather "everywhere at once". See the following web pages for explanations: Ned Wright's Cosmology tutorial http://www.astro.ucla.edu/~wright/cosmolog.htm and Cosmology FAQ http://www.astro.ucla.edu/~wright/cosmology_faq.html The Physics FAQ, see in particular the question "Where is the centre of the universe?" in the "General Relativity and Cosmology" section http://math.ucr.edu/home/baez/physics/index.html The Astronomy FAQ http://sciastro.astronomy.net/ has a good section on "Cosmology" -- jt]] Is the red shift of this 13-million-year-distant furthest galaxy corrected for our velocity relative to the center of the universe? That is, if we are on one side of the center of the universe, and this farthest galaxy is on the opposite side (or any other place for that matter), wouldn't that galaxies apparent motion be the vector sum of our true velocity relative to the stationary center of the universe, and it's true velocity relative to a stationary center? If that is true, then is it also true that the most likely rewarding direction to look for the oldest detectable object be further outward along the line of our movement away from the big bang center, since an object on this line, of fixed brightness, would have the highest apparent brightness, since it would be closer to us than any other object at the edge of the universe? Is that where HUDF is looking, or am I making a mess of the logic by my obvious ignorance of some basics of cosmology/astronomy? If this isn't the best news group forum for asking these questions, can you suggest which news group is best? John Pierce [[Mod. note -- There is also the unmoderated sci.astro group, although it's signal-to-noise ratio may be low. -- jt]] |
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Probably Dumb Questions
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Probably Dumb Questions
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Probably Dumb Questions
"APS" == Alf P Steinbach writes:
APS * (Gordon D. Pusch) APS schriebt: _EVERY_ observer sees _EVERY_ object "racing away" with a mean velocity given to a first approximation by Hubble's Law. APS Ah, there's exactly the question I have, not answered by the FAQs APS or any article or book I've read on the subject. APS Namely, the definition of object in the Big Bang hypothesis, APS which I'll put more succinctly: APS * At what level do structures not expand? A structure will not expand if its density is significantly above the mean density of the Universe, which is about 1E-29 g/cm^3. There are two ways of looking at this. The popular way is to say that, if the object is too dense, then its internal gravitational attraction overwhelms the Universal expansion. I prefer to think of it in terms of uniformity. A basic assumption in obtaining the equations describing the expansion of the Universe is that the Universe is of uniform density. On small scales, where that assumption clearly breaks down, then the expansion of the Universe must not be occurring. What are the relevant size scales? Peebles et al. (1991, Nature, 352, 769) state that on scales of about 40 Mpc (~ 120 million light years) the Universe becomes relatively uniform. Thus, objects smaller than about 40 Mpc in size (people, stars, galaxies, clusters of galaxies) should not be expanding. -- Lt. Lazio, HTML police | e-mail: No means no, stop rape. | http://patriot.net/%7Ejlazio/ sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html |
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Probably Dumb Questions
* Joseph Lazio schriebt:
* "APS" == Alf P Steinbach writes: * (Gordon D. Pusch) schriebt: _EVERY_ observer sees _EVERY_ object "racing away" with a mean velocity given to a first approximation by Hubble's Law. Ah, there's exactly the question I have, not answered by the FAQs or any article or book I've read on the subject. Namely, the definition of object in the Big Bang hypothesis, which I'll put more succinctly: * At what level do structures not expand? A structure will not expand if its density is significantly above the mean density of the Universe, which is about 1E-29 g/cm^3. ... objects smaller than about 40 Mpc in size (people, stars, galaxies, clusters of galaxies) should not be expanding. Thanks for that prompt answer. But I do not understand this notion of non-expansion. As a thought experiment, lets use an unbreakably strong (!) monofilament string to connect two objects residing in solar systems in galaxies in galaxy clusters in galaxy superclusters -- and so on up to whatever level is needed -- significantly more than 40 Mpc apart. Under the assumption of local non-expansion either that string will experience so much tension that it breaks, or, and let's assume that it doesn't break, the two objects will start to accelerate relative to their local neighborhoods. So -- barring a major fault in the short & simple logic above -- under the assumption of local non-expansion the expansion of the universe equates to a force, equivalent in size to the force experienced by those two objects. The size of that force as a function of distance is readily derived but isn't my point here. The point is rather, that that force can't disappear altoghether at distances less that 40 Mpc, or can it? If not, then in effect the assumption of local non-expansion says that the force that that assumption equates to is somehow _exactly cancelled_ at all local levels, by gravitation, by nuclear forces, etc.? I don't understand this. -- A: Because it messes up the order in which people normally read text. Q: Why is top-posting such a bad thing? A: Top-posting. Q: What is the most annoying thing on usenet and in e-mail? |
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Probably Dumb Questions
Joseph Lazio wrote:
"APS" == Alf P Steinbach writes: APS * At what level do structures not expand? A structure will not expand if its density is significantly above the mean density of the Universe, which is about 1E-29 g/cm^3. There are two ways of looking at this. The popular way is to say that, if the object is too dense, then its internal gravitational attraction overwhelms the Universal expansion. I prefer to think of it in terms of uniformity. A basic assumption in obtaining the equations describing the expansion of the Universe is that the Universe is of uniform density. On small scales, where that assumption clearly breaks down, then the expansion of the Universe must not be occurring. What are the relevant size scales? Peebles et al. (1991, Nature, 352, 769) state that on scales of about 40 Mpc (~ 120 million light years) the Universe becomes relatively uniform. Thus, objects smaller than about 40 Mpc in size (people, stars, galaxies, clusters of galaxies) should not be expanding. As you have been told before, but for some reason have chosen to ignore; setting a 40 Mpc scale limit for the cosmic expansion is grossly at odds with observations. In fact observations show that the linear Hubble law extends at least down to 1.5 Mpc in the Local Group of galaxies. Besides there is no clear indication that the cosmic expansion is disturbed at the scale of the Local Group. So the claim that the clumpiness of the Universe should be crucial for determining the scale where the cosmic expansion should be observed is clearly falsified by observations. Reference: T. Ekholm et al., A&A 368, L17-L20 (2001) (astro-ph/0103090) and references therein. |
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Probably Dumb Questions
Alf P. Steinbach wrote:
[...] As a thought experiment, lets use an unbreakably strong (!) monofilament string to connect two objects residing in solar systems in galaxies in galaxy clusters in galaxy superclusters -- and so on up to whatever level is needed -- significantly more than 40 Mpc apart. You might look at http://arxiv.org/abs/astro-ph/0104349, ``Solutions to the tethered galaxy problem in an expanding universe...'', which analyzes almost exactly this problem. To quote the abstract: We use the dynamics of a galaxy, set up initially at a constant proper distance from an observer, to derive and illustrate two counter-intuitive general relativistic results. Although the galaxy does gradually join the expansion of the universe (Hubble flow), it does not necessarily recede from us. In particular, in the currently favored cosmological model, which includes a cosmological constant, the galaxy recedes from the observer as it joins the Hubble flow, but in the previously favored cold dark matter model, the galaxy approaches, passes through the observer, and joins the Hubble flow on the opposite side of the sky. We show that this behavior is consistent with the general relativistic idea that space is expanding and is determined by the acceleration of the expansion of the universe -- not a force or drag associated with the expansion itself. We also show that objects at a constant proper distance will have a nonzero redshift; receding galaxies can be blueshifted and approaching galaxies can be redshifted. Steve Carlip |
#9
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Probably Dumb Questions
In article ,
Alf P. Steinbach wrote: As a thought experiment, lets use an unbreakably strong (!) monofilament string to connect two objects residing in solar systems in galaxies in galaxy clusters in galaxy superclusters -- and so on up to whatever level is needed -- significantly more than 40 Mpc apart. Under the assumption of local non-expansion either that string will experience so much tension that it breaks, or, and let's assume that it doesn't break, the two objects will start to accelerate relative to their local neighborhoods. So -- barring a major fault in the short & simple logic above -- under the assumption of local non-expansion the expansion of the universe equates to a force, equivalent in size to the force experienced by those two objects. The size of that force as a function of distance is readily derived but isn't my point here. The point is rather, that that force can't disappear altoghether at distances less that 40 Mpc, or can it? This is a variant of what's sometimes called the "tethered galaxy problem." One detailed exposition of it is at http://arxiv.org/abs/astro-ph/0104349 The main point is that there is no force associated with the expansion of the Universe. Temporarily forget about all this stuff about local inhomogeneities and the fact that the expansion is only happening on large scales and all that. Suppose for simpliicity we lived in a perfectly uniformly expanding spacetime (a Friedmann-Robertson-Walker spacetime, as the pros call it). Also, suppose temporarily that there's no cosmological constant or dark energy or other mysterious stuff making the expansion of the Universe speed up. (I'll say a little later why I mention this at all.) Suppose you observe a distant galaxy moving away from you. You ask yourself why that galaxy is moving away from you. Is there some mysterious force pushing it away? The answer is no! That galaxy is moving away from you today simply because it was moving away from you yesterday. Just as Galileo et al. figured out way back when, once something has started moving, there's no need for a force to keep that something moving. (The question of why that something started moving in the first place is a very legitimate one, and one that current cosmological theory doesn't really answer. But it's a separate question from what has to happen in order to keep something moving, given that it got started.) What I just wrote is a bit imprecise and heuristic. The correct language for describing the recession of distant galaxies is general relativity, and you have to go to a fair amount of trouble to turn the vague statement that "there's no force making the galaxy recede" into something precise enough to be meaningful in general relativity. But one way to do it is to imagine a tethered-galaxy-style thought experiment of the sort you're proposing. Suppose somehow a rope has been strung between the two galaxies (ours and the one we observed), and the rope is just now being stretched taut. If the rope doesn't break or stretch, then the two galaxies have to stop receding from each other. That requires a force (that is, a set of little stress gauges along the rope would read nonzero values). But the force only has to be exerted while the galaxies are coming to rest. After the rope has been stretched taut, and the galaxies are no longer receding from one another, no additional force is required to keep the distant galaxy from "resuming the expansion." The tension in the rope drops to zero. The two galaxies remain at rest with respect to each other -- actually, in realistic models, they start approaching each other due to gravitational attraction -- with no help from the rope. All of this is true whether the galaxies are nearby or far apart. It remains true even if we drop the assumption of a perfectly homogeneous Universe. Switch to a more realistic picture in which there are inhomogeneities, including dense lumps that are not expanding with the Universe. Still, the tethered galaxies work out the same way: if at one time they've stopped receding from each other, there's no "expansion force" that tries to make them start receding again. There is one exception to this: I asked you to assume temporarily that there was no cosmological constant / dark energy. Something like a cosmological constant, which tends to accelerate the expansion, does act as a repulsive force. In a Universe with a cosmological constant (such as, apparently, ours), the two tethered galaxies would "try to recede" from each other, and the rope holding them together would have to maintain some nonzero tension to stop them. But please note that that tension doesn't come from the fact that the Universe is expanding, but rather from the fact that it's accelerating. So I don't think it changes the fact that there is no force associated with the *expansion* of the Universe. -Ted -- [E-mail me at , as opposed to .] |
#10
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Probably Dumb Questions
[mod. note: quoted text trimmed -- mjh]
* schriebt: This is a variant of what's sometimes called the "tethered galaxy problem." One detailed exposition of it is at http://arxiv.org/abs/astro-ph/0104349 Thanks. The main point is that there is no force associated with the expansion of the Universe. In the article the authors consider two objects initially at constant proper distance, held that way by means of applied force if necessary. The authors calculate that when the objects are released they gain velocity towards or away from each other, or not at all, depending on the parameters of the universe's expansion. With non-accelerated expansion they find that the objects gain no relative velocity, that is, continue to be at constant distance. My 2 cents (just _very_ simple logic, I'm afraid): for accelerated expansion, either no force is required to hold the initial constant distance, in which case the conclusion about gaining relative velocity when released is necessarily incorrect, or else force is required to hold the initial constant distance, in which case the conclusion about no force is necessarily incorrect unless it's just empty wordplay meaning "no local acceleration". Temporarily forget about all this stuff about local inhomogeneities and the fact that the expansion is only happening on large scales and all that. Suppose for simpliicity we lived in a perfectly uniformly expanding spacetime (a Friedmann-Robertson-Walker spacetime, as the pros call it). Also, suppose temporarily that there's no cosmological constant or dark energy or other mysterious stuff making the expansion of the Universe speed up. (I'll say a little later why I mention this at all.) Suppose you observe a distant galaxy moving away from you. You ask yourself why that galaxy is moving away from you. Is there some mysterious force pushing it away? The answer is no! That galaxy is moving away from you today simply because it was moving away from you yesterday. Just as Galileo et al. figured out way back when, once something has started moving, there's no need for a force to keep that something moving. Well, this is the "simple" case of non-accelerated expansion, which is already far beyond my ken... As I understand it, the recession speed relative to us for a very distant galaxy is proportional to the distance D to that galaxy (roughly), with constant of proportionality H, giving speed v = H*D. If it always had that recession speed then H would have to vary with time, H = f(SomeVeryMuchMoreConstantH, t), to yield constant v. But this, as I understand it, is not how Hubble's constant is constant. Also, if the recession speed of any given object does not increase with time then, as best I can visualize it, the set of various recession speeds and object distances would point to some centre of the expansion -- right splat at the centre of Earth. And that is a horrible notion. So I don't really understand the idea of [galaxies having recession speeds because they always have had those very same recession speeds]. (The question of why that something started moving in the first place is a very legitimate one, and one that current cosmological theory doesn't really answer. But it's a separate question from what has to happen in order to keep something moving, given that it got started.) What I just wrote is a bit imprecise and heuristic. The correct language for describing the recession of distant galaxies is general relativity, and you have to go to a fair amount of trouble to turn the vague statement that "there's no force making the galaxy recede" into something precise enough to be meaningful in general relativity. But one way to do it is to imagine a tethered-galaxy-style thought experiment of the sort you're proposing. Suppose somehow a rope has been strung between the two galaxies (ours and the one we observed), and the rope is just now being stretched taut. If the rope doesn't break or stretch, then the two galaxies have to stop receding from each other. That requires a force (that is, a set of little stress gauges along the rope would read nonzero values). But the force only has to be exerted while the galaxies are coming to rest. After the rope has been stretched taut, and the galaxies are no longer receding from one another, no additional force is required to keep the distant galaxy from "resuming the expansion." The tension in the rope drops to zero. The two galaxies remain at rest with respect to each other -- actually, in realistic models, they start approaching each other due to gravitational attraction -- with no help from the rope. This seems to be based on the assumption of constant recession speed for any given far-away object, i.e. a time-varying Hubble constant? For if the recession speed for a very distant given object B _does_ vary and increase with time, as given by v = HD, then we could set object A somewhere in the other direction from us to be initially at constant distance from B. And without any forces applied, the above explanation that A and B once at relative rest will continue at relative rest, requires that A not only coasts towards us with the initial velocity but, to maintain constant distance to B which follows the Hubble law, that object A accelerates towards us. In other words any object that has a velocity towards us must accelerate towards us. But then we can shift our frame of reference so that the object has velocity away from us. And voila, it's now accelerating away from us. But is acceleration direction a relative thing? I thought it couldn't be. So I'm afraid I still don't understand this, but thanks for the explanation. -- A: Because it messes up the order in which people normally read text. Q: Why is top-posting such a bad thing? A: Top-posting. Q: What is the most annoying thing on usenet and in e-mail? |
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