Hi guys, first time I am posting. I just had a question in my head right now, and would like to spit it out so to be able to sleep well:) The universe is expanding or we can say stretching. The space is expanding/stretching. If the space is made of something (right now I do not remember what the string theory was saying about the "particles/strings" that the universe was made of, and I know-still theory), but for how much can the universe stretch? Is there limit for the stretching? Thanks guys
As far as we know there is no material that stretches or expands, it is simply that distances between remote galaxies increase by a certain percentage each year. Galaxies close together can be held in a cluster by gravity, orbiting each other as a group. Only the very large distances take part in this pattern of increase. It is a small percentage. Amounting only about 1/140 of a percent every million years. The pattern of increasing distances is called Hubble Law. There is no limit on the amount distances can increase, according to that law. The balloon analogy is mean to help people learn to imagine the pattern of increasing distances. It is not a physical or material model. Empty space is not like rubber or anything. check out the animations of the balloon analogy to get in mind how distances between stationary galaxies increase, and how this affects the distances that photons of light can travel over time (they are shown as wrigglers in the computer animation).
Thanks for the answer.:) But that is clear for me, and I am familiar with both baloon-analogy and the rest, including the Hubble law. But as I understood it, the Hubble law only gives us what happens in the universe, nothing is bounded by it as is the case with Newtons law. It is "just" an observation relation. But my question goes more in the direction of the connection between the expansion (on large scales) and the fabric of the space. This idea of the last one I got from the books on string theory. It talks about strings which the space itself is made of (it is not matter, not radiation, just strings - as I recall it) If that is the case, than you have something finite, which is to be "streched" to infinite. I just hoped that maybe somebody here had an idea of answer to my question or could show me to some article on the subject. Thanks:)
The first part about the strings and fabric of the cosmos? Wrong? I'll try to look it up (long time ago I read it), but is that not the idea? Or the second part about the finite/infinite? This one is just in my head;) This is what I am trying to find out, if that has anything to do with theory/reality.
I think you are mistaken about that, Bosanac. The Hubble law is both observation and a mathematical relation derived from theory. The theory it derives from ultimately is General Relativity, which has been checked in a lot of different ways. So Hubble law rests on a solid theoretical foundation. ======================= Given the accelerating expansion we see around us, General Relativity does not predict any limit to the universe expansion. If the observed expansion were weak and rapidly slowing then we would expect things to eventually stop and begin to collapse but that is not how it is. The only well-tested theory we have that covers this kind of question says no limit to expansion of distances. ======================= You were asking about some Superstring/M issue. As far as I know there is no string theorist who has ever proposed a limit to the extent that the universe can expand. I think I would have heard about it if any had, because it would have caused a furor. Young untested theories are supposed to agree with Gen Rel on largescale matters where it is Gen Rel turf. People would be talking about it and I certainly would have heard.
Thanks for the answers marcus. When I said what I have said about the Hubble law, I meant in the way that it does do nothing physical in a way, let's say, gravitation does. That is what I meant with "just an observation". Or I am mistaken here, is there something more in GR? (should actualy go deeper and try to learn/remember more about GR - long time since my last course in physics). The last part, about the strings, that is exactly what I was wandering about.
We both see that Hubble Law is just one of many results based General Relativity. (our universe model is a solution of the basic GR equation that fits observations) So if I understand you, you are asking is General Relativity "just an observation"? Depends on what you mean of course, and people would differ. I suppose that you could say that! It depends somewhat on what one expects theories to look like and to do for you. I suppose one could say that Quantum Field Theory and the Standard Model are "just an observation" because they break down at a certain energy. They are effective theories, not fundamental, because they have a limited domain of applicability. They don't cover what happens at very high energy or in curved geometry. They don't cover spacetime geometry or gravity. So QFT is "just an observation" in that sense. General Relativity also has big gaps. It doesn't tell us what matter is! It is not a quantum theory. It has a limited range of applicability----also failing at very high density and curvature. It does not describe geometry at very small scale (one suspects.) So you could say that GR is "just an observation." I think there may be better ways to say it, though. GR and QFT are both very successful theories in the sense that they make precise predictions, that are right. That is basically what one asks of a theory. They both have limited domains of applicability. They work well up to a point and then they fail. Both GR and QFT teach us essential lessons that will probably carry over to the next, more fundamental, theory. This will probably be a theory BOTH of spacetime geometry and of matter. It will tell us the degrees of freedom underlying both matter and geometry. It will tell us how matter and geometry interact at a microscopic level. How matter is able to shape geometry and is guided by it. Any theory that can do this will probably have absorbed essential lessons from both GR and QFT. And hopefully it will have a broader domain of applicability---will be vaiid to higher energies, and in extreme density/curvature situations like the supposed big bang bouncepoint and black hole pit. But every theory I can imagine is also, in a certain sense, "just an observation". Because it is a human construct intended to fit empirical observation and to predict future observation correctly. Do you have a different idea of theories?
No, I don't have a different idea of theories. I was thinking more in the direction of what "power" it has to shape things. But that thought was too shallow :) I am familiar with the your description of theories, and I am aware of their range of aplication. But anyway, it is good to read your easy and short explenation of the subject. Cheers
By the way, if I was too strengthen my knowlede in GR, what book is there to recomend? I've had some GR during my education, but this is relatively long time ago. Now I do not have the time to attend a course.
Maybe it was my thought that was shallow, in this case. Perhaps I simply didn't understand what you were saying. I think that there is a sense in which one can say that GR does not go deep enough. A deeper theory, with more power, would say why matter interacts with geometry. It would describe matter and geometry on the same basis, so they are seen as the same thing, and so it appears natural for them to interact. Maybe this is what you were saying. GR is unsatisfactory---the Einstein equation has an ad hoc character? I can only watch what other people do, in their efforts to bring GR and QM together and find something more fundamental. Here at Physicsforum there is a Relativity forum. There they like to give suggestions about things to read in GR. they would inquire what you have read already and they would say things like the MTW book or the Wald book. You can post a thread asking about it. Sean Carroll's book is free online so it saves money to use it, it you find the book helpful. Rather than suggest GR books for you, I would like to suggest ways for you to find out what is currently going on in QG (quantizing GR and applications to cosmology). I will get some links for you about that. You can try it or not, as you please. This is not what you asked for, it is what I am interested in. I will get some links in a few hours (can't do it now.) Before I go: one interesting window on current research is the September 2008 workshop at Sussex (and the 2-day school that preceded it). There are links at Renate Loll's website. Google Renate Loll, and go to "events", where the conferences are listed---there will be links to the programmes of the school and the workshop. I think the organizers were especially good at picking some of the most promising research to have talks on. When I get back I will find specific links, in case you want.