Rolando Posted January 17, 2015 Author Share Posted January 17, 2015 Redshift has nothing to do with homogeneity its a change in volume from one homogeneous point in time to another.. So I'm not really sure what your implying. It has: if everything expands in proportion, which will be the case if the universe is completely homogeneous, there will be no observable redshift caused by the expansion. If there are expanding voids and coherent bodies that do not participate in the expansion, there will be such a redshift. Only in this case, the standard of comparison does not expand in proportion to expanding light waves. Also in response to Strange: My point is is very simple: In my understanding, the universe cannot ever have had a volume smaller than that of the regions that are gravitationally bound at present. If this is true, then the FLRW-approach, which is derived on the basis of homogeneity at all scales is dramatically inadequate. There cannot have been a big bang scenario. Speculations about "inflaltion" are, then, also irrelevant. In order to convince me that I am wrong, it won't do with arguments that presuppose any of the rejected things to begin with. Link to comment Share on other sites More sharing options...
Strange Posted January 17, 2015 Share Posted January 17, 2015 It has: if everything expands in proportion, which will be the case if the universe is completely homogeneous, there will be no observable redshift caused by the expansion. The red shift is predicted by the model which is based on the universe being homogeneous. My point is is very simple: In my understanding, the universe cannot ever have had a volume smaller than that of the regions that are gravitationally bound at present. I have no idea why you would think that. In order to convince me that I am wrong, it won't do with arguments that presuppose any of the rejected things to begin with. Huh? Link to comment Share on other sites More sharing options...
Mordred Posted January 17, 2015 Share Posted January 17, 2015 (edited) There is Three forms of redshift. Cosmological Gravitational Doppler Gravitational applies inside large scale structures. Example Sache wolf effect. Your limits of the universe volume being limitted by matter we see today is wrong. Please read chapter 3 of the article I posted. Second link It has: if everything expands in proportion, which will be the case if the universe is completely homogeneous, there will be no observable redshift caused by the expansion. If there are expanding voids and coherent bodies that do not participate in the expansion, there will be such a redshift. Only in this case, the standard of comparison does not expand in proportion to expanding light waves. The above statement is incorrect. This is not how cosmological redshift is described. Cosmological redshift [latex]1+Z=\frac{\lambda}{\lambda_o} or 1+Z=\frac{\lambda-\lambda_o}{\lambda_o}[/latex] Gravitational redshift [latex]\frac{\lambda}{\lambda_o}=\frac{1}{\sqrt{1 - \frac{2GM}{r c^2}}}[/latex] Doppler shift [latex]f=\frac{c+v_r}{c+v_s}f_o[/latex] Read http://cosmology101.wikidot.com/redshift-and-expansion also read http://arxiv.org/abs/astro-ph/?9905116 "Distance measures in cosmology" David W. Hogg Forgot a key relation for cosmological redshift [latex]1+z= \frac{at_0}{at_e}[/latex] a is the scale factor http://en.m.wikipedia.org/wiki/Scale_factor_(cosmology) Your description above sounds more like gravitational redshift Another relation [latex]d_2=-c^2dt^2+\frac{a^2dr^2}{1-kr^2}[/latex] K is the curvature constant. Edited January 17, 2015 by Mordred 1 Link to comment Share on other sites More sharing options...
elfmotat Posted January 17, 2015 Share Posted January 17, 2015 This approximation may be acceptable at present, when most of the volume of the universe consists of voids, but it becomes grossly inadequate when we go back in time, e.g., to z=10 (of the most distant galaxy that has been observed). The scale factor a(t) was then roughly 0.05 and the volume of the universe about a factor of 10^-4 smaller than it is now. There was not much room for expanding voids then. I don't think you understand what the word "homogeneous" means. The universe was still homogeneous when it was younger, it was just denser. It has nothing whatsoever to do with the scale factor. In order to convince me that I am wrong, it won't do with arguments that presuppose any of the rejected things to begin with. So you won't accept approximations because... you don't like approximations? Are you waiting around for an exact solution to the EFE's for our universe? Good luck with that. In the mean time, the rest of us will be making progress with our useful approximations. My point is is very simple: In my understanding, the universe cannot ever have had a volume smaller than that of the regions that are gravitationally bound at present. It's hard to respond to arguments that don't make sense. Why in the world would you think this? What led you to this thought? Link to comment Share on other sites More sharing options...
Rolando Posted January 17, 2015 Author Share Posted January 17, 2015 (edited) The red shift is predicted by the model which is based on the universe being homogeneous. This is only true if the wavelenght (λ in Mordred's 1st equation) is compared with a standard (λ_0) that has remained unaffected by the expansion while the light was on its way. If the universe is completely homogeneous, which is presupposed in deriving the model from GR, there exists no such standard within the universe. Everything expands in proportion, so that any measured λ/ λ_0 remains =1. In this case, there is only a calculated λ/ λ_0 > 1, where λ_0 is the wavelength of a standard that has not been affected by the expansion. In reality, the redshift becomes measurable since coherent objects and standards of comparison do not expand. (In the models, this possibility is first excluded by pretending the expansion to be all-pervasive - and then nevertheless presupposed as a matter of course.) No idea why I think that the universe cannot ever have had a volume smaller than that of the regions that are gravitationally bound at present? I think this is clear enough as long as none of the gravitationally bound objects (remember the coins embedded in a rubber sheet) fuse. If they fuse, it requires calculating the volumes enclosed by their zero-velocity envelopes before and after fusion. Added: I have done this on paper and found that the non-expanding volume is proportional to the enclosed mass. In this case, it does not matter whether the objects fuse or disrupt or whatever. Huh? In order to convince you that you are wrong, it won't do with arguments that presuppose any of the things you reject (or put into question) to begin with. In my case, I was thinking of arguments based on the FLRW-models, whose adequacy I have put into question, but mainly of just teaching standard cosmology, as Mordred does. Mordred, your equations are correct, and at least the three first ones remain valid also if I am right in my criticism, but they tell nothing about the problem under discussion. I was unaware of the fourth one, although I had seen the handy summary by David W. Hogg before. It is mentioned there. elfmotat: I think my responses here may clarify even the points you comment. The universe can never have been smaller than the total volume of the gravitationally bound regions within it, and, to my understanding, this volume cannot have been smaller in the past than it is now. It's hard to respond to arguments that don't make sense. Why in the world would you think this? What led you to this thought? I will try to say it even more simply: The universe can never have been smaller than the present volume of those regions that have never expanded (because of their being gravitationally bound). Edited January 17, 2015 by Rolando Link to comment Share on other sites More sharing options...
Strange Posted January 17, 2015 Share Posted January 17, 2015 The universe can never have been smaller than the present volume of those regions that have never expanded (because of their being gravitationally bound). That is just repeating the same point, not explaining why you think it. What makes you think those regions have never expanded? Link to comment Share on other sites More sharing options...
Rolando Posted January 17, 2015 Author Share Posted January 17, 2015 I am surprised about your comment, since Galaxies, etc. do not expand because they are held together by galaxy [gravity, I suppose]. In the absence of any such force, objects will tend to move apart. (For exactly the same reason, by the way, that objects with mass will tend to get closer together - aka gravity). Link to comment Share on other sites More sharing options...
Strange Posted January 17, 2015 Share Posted January 17, 2015 I am surprised about your comment, since They may be gravitationally bound now (because they collapsed due to gravity) but that doesn't mean they always were - any more than anywhere else: after all the universe was more homogeneous early on - the large scales structures are thought to have been seeded by quantum (i.e. very small) fluctuations. And anyway, gravity might stop them expanding but it wouldn't stop them collapsing. Link to comment Share on other sites More sharing options...
Rolando Posted January 18, 2015 Author Share Posted January 18, 2015 They may be gravitationally bound now (because they collapsed due to gravity) but that doesn't mean they always were Collapse due to gravity is only possible within regions that are already gravitationally bound. Link to comment Share on other sites More sharing options...
Mordred Posted January 18, 2015 Share Posted January 18, 2015 Collapse due to gravity is only possible within regions that are already gravitationally bound. The universe can never have been smaller than the present volume of those regions that have never expanded (because of their being gravitationally bound). Umm am I missing something here. First you say a region cannot be smaller as it is gravitationally bound. Then you state only gravitationally bound regions can collapse. Which is the exact opposite. Link to comment Share on other sites More sharing options...
Rolando Posted January 18, 2015 Author Share Posted January 18, 2015 (edited) Umm am I missing something here. First you say a region cannot be smaller as it is gravitationally bound. Then you state only gravitationally bound regions can collapse. Which is the exact opposite. The volume that is gravitationally bound remains unchanged when the matter within this region collapses by gravitation. The gravitationally bound volume can, however, increase by accidental mergers, which may happen occasionally. I see that his needs to be taken into account. Edited January 18, 2015 by Rolando Link to comment Share on other sites More sharing options...
Mordred Posted January 18, 2015 Share Posted January 18, 2015 (edited) Ok I have to ask you these questions. What is the difference between a supernova becoming a black hole and the situation I will describe in a later questions? Why does a super nova become a black hole in terms of pressure.? Now if you remove all voids between large scale structures thereby increasing the average mass density of the universe. What is preventing gravity from doing the same thing to all the resultant galaxy mergers? Keep in mind the current critical mass density is 10^-29 grams per cubic meter. Or if you prefer 6.0 *10^-10 joules per m^3. It will be far higher without the voids. Without the voids the universe will be matter dominant as the cosmological constant is extremely weak per cubic meter. So what will keep gravity from attracting all that matter? Keep in mind you already allowed for galaxy mergers Lol don't forget to change all the dark matter to the same scale. The reason for the above questions is this statement makes zero sense. The volume that is gravitationally bound remains unchanged when the matter within this region collapses by gravitation. The gravitationally bound volume can, however, increase by accidental mergers, which may happen occasionally. I see that his needs to be taken into account. If you place all the gravitationally bound objects side by side in the universe. You can bet in any metric not just the FLRW metric that universe will collapse without an opposing force to counter the gravity Edited January 18, 2015 by Mordred Link to comment Share on other sites More sharing options...
michel123456 Posted January 18, 2015 Share Posted January 18, 2015 Ok I have to ask you these questions. What is the difference between a supernova becoming a black hole and the situation I will describe in a later questions? Why does a super nova become a black hole in terms of pressure.? Now if you remove all voids between large scale structures thereby increasing the average mass density of the universe. What is preventing gravity from doing the same thing to all the resultant galaxy mergers? Keep in mind the current critical mass density is 10^-29 grams per cubic meter. Or if you prefer 6.0 *10^-10 joules per m^3. It will be far higher without the voids. Without the voids the universe will be matter dominant as the cosmological constant is extremely weak per cubic meter. So what will keep gravity from attracting all that matter? Keep in mind you already allowed for galaxy mergers Lol don't forget to change all the dark matter to the same scale. The reason for the above questions is this statement makes zero sense. If you place all the gravitationally bound objects side by side in the universe. You can bet in any metric not just the FLRW metric that universe will collapse without an opposing force to counter the gravity IIRC that is exactly where inflation comes on the basis of a spooky repulsive gravity thing. Link to comment Share on other sites More sharing options...
Strange Posted January 18, 2015 Share Posted January 18, 2015 The volume that is gravitationally bound remains unchanged when the matter within this region collapses by gravitation. That is so obviously not true you need to provide some justification rather than making these assertions. Collapse due to gravity is only possible within regions that are already gravitationally bound. Gravity was not a significant force when the universe consisted of a hot dense quark-gluon plasma. Link to comment Share on other sites More sharing options...
Mordred Posted January 18, 2015 Share Posted January 18, 2015 (edited) IIRC that is exactly where inflation comes on the basis of a spooky repulsive gravity thing.What spooky repulsive gravity thing? Gravity isn't repulsive. No one knows which of the 70+ good fits to observation inflation models is correct. Some models are thermodynamic phase transitions. Some involve the Higgs field. Others involved the inflaton. While others the curvaton. My personal bet is on Higgs related thermodynamic phase change via the seesaw mechanism Don't mean I'm right. Edited January 18, 2015 by Mordred Link to comment Share on other sites More sharing options...
Rolando Posted January 18, 2015 Author Share Posted January 18, 2015 (edited) In response to Rolando saying "The volume that is gravitationally bound remains unchanged when the matter within this region collapses by gravitation", That is so obviously not true you need to provide some justification rather than making these assertions. According to the simplest definition, gravitational binding extends to the distance r where the velocity due to expansion, v_exp=rH, begins to exceed the radial velocity of escape v_esc=(2GM/r)^1/2. This gives us r_gb=(2GM/H^2)^1/3. By ”the volume that is gravitationally bound”, I mean the volume that is enclosed within a sphere with radius r_gb. As you can see, this radius is given by M (and some constants). It remains the same when M (the total mass within this volume) is present in form of a low density gas or in form of a neutron star or a black hole or whatever. If you place all the gravitationally bound objects side by side in the universe. You can bet in any metric not just the FLRW metric that universe will collapse without an opposing force to counter the gravity This is certainly so in the Schwarzschild metric, but the FLRW metric allows for expansion as well as for contraction. This remains a mystery to me. Edited January 18, 2015 by Rolando Link to comment Share on other sites More sharing options...
Mordred Posted January 18, 2015 Share Posted January 18, 2015 (edited) To understand why you need to look at the acceletion and deceleration equations, curvature constant and the equations of state for the various contributors. Radiation,matter and the cosmological constant. The FLRW has ideal gas pressure terms you need to look at. http://en.m.wikipedia.org/wiki/Friedmann_equations The acceleration equation is on this page http://en.m.wikipedia.org/wiki/Deceleration_parameter Edited January 18, 2015 by Mordred Link to comment Share on other sites More sharing options...
michel123456 Posted January 18, 2015 Share Posted January 18, 2015 What spooky repulsive gravity thing? Gravity isn't repulsive. No one knows which of the 70+ good fits to observation inflation models is correct. Some models are thermodynamic phase transitions. Some involve the Higgs field. Others involved the inflaton. While others the curvaton. My personal bet is on Higgs related thermodynamic phase change via the seesaw mechanism Don't mean I'm right. The miracle of physics, these are Alan Guth's own words at 1.59. i'd like you to follow till the end, it is only 10 min. to reach A. Guth's little marble. Link to comment Share on other sites More sharing options...
Mordred Posted January 18, 2015 Share Posted January 18, 2015 (edited) Short answer all particles exert pressure via interactions. Matter exerts effectively zero pressure. Then the acceleration equation correlates the positive pressure contributors and the negative pressure contributors. (Positive vs negative vacuum). Cosmological constant being positive. This relationship also determines the curvature constant. http://cosmology101.wikidot.com/universe-geometry Page 2 http://cosmology101.wikidot.com/geometry-flrw-metric/ These two articles will also help http://arxiv.org/pdf/hep-ph/0004188v1.pdf :"ASTROPHYSICS AND COSMOLOGY"- A compilation of cosmology by Juan Garcıa-Bellido http://arxiv.org/abs/astro-ph/0409426 An overview of Cosmology Julien Lesgourgues The miracle of physics, these are Alan Guth's own words at 1.59. i'd like you to follow till the end, it is only 10 min. to reach A. Guth's little marble. There is considerable different between pressure and gravity Though gravity does exert pressure via the stress energy tensor it isn't the only pressure contribution to consider in the acceleration equation. Edited January 18, 2015 by Mordred Link to comment Share on other sites More sharing options...
michel123456 Posted January 18, 2015 Share Posted January 18, 2015 Obviously you did'nt follow the small video. You should. Here a screenshot. Link to comment Share on other sites More sharing options...
Mordred Posted January 18, 2015 Share Posted January 18, 2015 I never trust pop media vids. I'll stick with the metric and textbooks Link to comment Share on other sites More sharing options...
michel123456 Posted January 18, 2015 Share Posted January 18, 2015 It's Alan Guth, not any pop media video. Also here: http://ned.ipac.caltech.edu/level5/Guth/Guth3.html Once a patch of the early Universe is in the false vacuum state, the repulsive gravitational effect drives the patch into an inflationary period of exponential expansion. Link to comment Share on other sites More sharing options...
Mordred Posted January 18, 2015 Share Posted January 18, 2015 Yeah I read his analogies before. Keep in mind he's simply stating similarities between gravitational pressure influence and other pressure influences. He is not stating gravity itself is repulsive. Nor is he claiming anti gravity. Just anti gravity like influence There is validity in the correlation. I prefer to relate to the dynamics in terms of pressure distributions Guth's false vacuum model for inflation is a higher energy region false vacuum tunneling to the true vacuum (lower energy region). Though he was involved in numerous inflation models trying to solve runaway inflation. Link to comment Share on other sites More sharing options...
michel123456 Posted January 18, 2015 Share Posted January 18, 2015 Yeah I read his analogies before. Keep in mind he's simply stating similarities between gravitational pressure influence and other pressure influences. He is not stating gravity itself is repulsive. Nor is he claiming anti gravity. Just anti gravity like influence There is validity in the correlation. I prefer to relate to the dynamics in terms of pressure distributions Well it seems to me that you close your ears when he makes statements that are against your convictions. Hear and look after 1.59 Link to comment Share on other sites More sharing options...
Mordred Posted January 18, 2015 Share Posted January 18, 2015 I'll see if I can dig up some of his earlier papers from my archives later on. No I heard the analogy before and studied what they meant by it. Link to comment Share on other sites More sharing options...
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