Airbrush Posted October 27, 2016 Share Posted October 27, 2016 (edited) There is no "outside the boundaries of the finite universe". Because there are no boundaries. Maybe I just keep using imprecise words, I'm not an expert on this. You say there are no boundaries to a universe or big bang that is finite in size. Then how can a universe or big bang be finite in size? Finite in size implies that the edges of the big bang do NOT extend to infinity, right? What do the edges of a FINITE big bang extend to? What am I missing? I am not saying the universe must be finite in size, I am asking HOW can a big bang be FINITE in size? Can you think of another analogy beside the balloon analogy? Edited October 27, 2016 by Airbrush Link to comment Share on other sites More sharing options...
Strange Posted October 27, 2016 Share Posted October 27, 2016 In current models there is no edge (or centre). As a 2D analogy consider the surface of the Earth: there is no edge and no centre, but it is finite in size. And it could get larger. Link to comment Share on other sites More sharing options...
Airbrush Posted October 28, 2016 Share Posted October 28, 2016 Thanks for trying, but the 2D "Balloon Analogy" does not make sense to me. Link to comment Share on other sites More sharing options...
StringJunky Posted October 28, 2016 Share Posted October 28, 2016 Thanks for trying, but the 2D "Balloon Analogy" does not make sense to me. The balloon analogy illustrates that you can have finite but unbounded space or area; you have to imagine yourself as having width and length, i.e. 2D, but not height on the balloon's surface. Link to comment Share on other sites More sharing options...
Strange Posted October 28, 2016 Share Posted October 28, 2016 Thanks for trying, but the 2D "Balloon Analogy" does not make sense to me. Which aspect does not make sense: - the idea that the surface has a finite area but no boundary? - how that can be extended to 3D space? Link to comment Share on other sites More sharing options...
Airbrush Posted October 28, 2016 Share Posted October 28, 2016 (edited) How can that 2D analogy be extended into 3D space? Can you think up a 3D analogy for a finite-sized universe? Trying to understand the reality of 3D space is too great a jump from a 2D analogy for my feeble mind. For example, imagine some huge irregular shaped blob that is about twice the size of our observable universe. That huge irregular blob has a center of gravity, right? There you have it, center of the U. Edited October 28, 2016 by Airbrush Link to comment Share on other sites More sharing options...
Strange Posted October 28, 2016 Share Posted October 28, 2016 You have to think of it like an old skool video game screen where if you go off the top, you reappear at the bottom and if you go off to the left you reappear at the right. Basically, if you travel far enough in any direction you could end up back where you started. Which also means ... If the Universe is finite but unbounded, it is also possible that the Universe is smaller than the observable universe. In this case, what we take to be very distant galaxies may actually be duplicate images of nearby galaxies, formed by light that has circumnavigated the Universe. https://en.wikipedia.org/wiki/Observable_universe#The_Universe_versus_the_observable_universe Link to comment Share on other sites More sharing options...
StringJunky Posted October 28, 2016 Share Posted October 28, 2016 How can that 2D analogy be extended into 3D space? Can you think up a 3D analogy for a finite-sized universe? Trying to understand the reality of 3D space is too great a jump from a 2D analogy for my feeble mind. For example, imagine some huge irregular shaped blob that is about twice the size of our observable universe. That huge irregular blob has a center of gravity, right? There you have it, center of the U. I don't know if this will make it harder for you but straight lies only exist at our scale of existence and observation. i.e local. If you look at lines at BLYR scales - global - straight lines or possible paths/trajectories start curving and it's not possible to travel in a straight line, as we know it, This follows on from what Strange said in his last post about going off the screen one side and coming back on another. Link to comment Share on other sites More sharing options...
imatfaal Posted October 29, 2016 Share Posted October 29, 2016 I don't know if this will make it harder for you but straight lies only exist at our scale of existence and observation. i.e local. If you look at lines at BLYR scales - global - straight lines or possible paths/trajectories start curving and it's not possible to travel in a straight line, as we know it, This follows on from what Strange said in his last post about going off the screen one side and coming back on another. If we are talking about possible universes then yes - for our observable universe no. At present and with our best surveys so far (which is one of the Planck surveys IIRC) the universe is flat (angles of triangle add to 180)the bounds of the error inherent in the calculation and measurements. This is not to say that the universe is not curved at an even larger scale - but from looking at the scales we can measure then the most likely scenario is flat, it could be positively curved (angles add to greater than 180) but unlikely or on a huge scale, or negatively curved (angles add to less than 180) even less likely or on an even larger scale. On a local scale there is curvature due to mass energy etc - but on a global scale we cannot see any evidence yet Link to comment Share on other sites More sharing options...
StringJunky Posted October 29, 2016 Share Posted October 29, 2016 If we are talking about possible universes then yes - for our observable universe no. At present and with our best surveys so far (which is one of the Planck surveys IIRC) the universe is flat (angles of triangle add to 180)the bounds of the error inherent in the calculation and measurements. This is not to say that the universe is not curved at an even larger scale - but from looking at the scales we can measure then the most likely scenario is flat, it could be positively curved (angles add to greater than 180) but unlikely or on a huge scale, or negatively curved (angles add to less than 180) even less likely or on an even larger scale. On a local scale there is curvature due to mass energy etc - but on a global scale we cannot see any evidence yet I accept what you are saying and I know that's the latest conclusion but I was trying to get Airbrush to picture a kind of overlying geometry where you can't just travel outwards, straight to the edge of the "whole" universe, regardless of whether it's flat. torus or saddle; you are ultimately forced to follow that 'shape'. if that makes sense? Link to comment Share on other sites More sharing options...
substitutematerials Posted October 29, 2016 Author Share Posted October 29, 2016 Are there any theories right now as to why the observable universe is so close to flat, and consequently mass-energy is so close to the critical density, other than inflation? Or are the only mainstream options a.) we are in a tiny portion of the entire universe that has been stretched flat by inflation or b.) density is randomly fine-tuned? Link to comment Share on other sites More sharing options...
MigL Posted October 30, 2016 Share Posted October 30, 2016 No explanation as to WHY it is so flat. Assuming near-linear expansion, it would have to have been immeasurably flatter as we reverse towards the Big Bang event. The flatness can however, be accounted for by an exponential inflation shortly after the Big Bang event ( see A. Guth ). Inflation would have tended to 'smooth out' any deviation from flatness, and also account for many other unexplained observations. And so inflation has become an 'accepted' cosmological model. Link to comment Share on other sites More sharing options...
geordief Posted October 30, 2016 Share Posted October 30, 2016 (edited) No explanation as to WHY it is so flat. Assuming near-linear expansion, it would have to have been immeasurably flatter as we reverse towards the Big Bang event. The flatness can however, be accounted for by an exponential inflation shortly after the Big Bang event ( see A. Guth ). Inflation would have tended to 'smooth out' any deviation from flatness, and also account for many other unexplained observations. And so inflation has become an 'accepted' cosmological model. Does the concept of flatness vs curvedness apply inside or near to Black Holes ? Is the Black Hole circumstance similar to what pertained at times near (whatever "near" means) to the Big Bang? I accept what you are saying and I know that's the latest conclusion but I was trying to get Airbrush to picture a kind of overlying geometry where you can't just travel outwards, straight to the edge of the "whole" universe, regardless of whether it's flat. torus or saddle; you are ultimately forced to follow that 'shape'. if that makes sense? Does it make any sense to contemplate that journey where you end up where you came from? Do you not just (as a thought experiment) keep creating "new universe" if the universe is flat at large distances? Is it not the (non existent) curvature that would cause you to return to the starting point? I realise my understanding and imagination are probably critically lacking in this area. Edited October 30, 2016 by geordief Link to comment Share on other sites More sharing options...
Delta1212 Posted October 30, 2016 Share Posted October 30, 2016 It is the curvature that would cause that. We don't know that the curvature is non-existent, however. We can just put a bound on how much curvature there could possibly be that means that, if the universe does have intrinsic curvature, it curved on a much large scale than we are capable of measuring (possibly ever). The universe being infinite is also another possibility. We're currently a bit like a person stuck in a room with no windows trying to figure out whether the Earth is curved or flat by trying to measure the curvature of the floor. Link to comment Share on other sites More sharing options...
Airbrush Posted October 31, 2016 Share Posted October 31, 2016 (edited) Is there a way to explain how a 3D volume can be "curved"? The only curvatures I know about are 2D, such as the surface of a balloon or the surface of the Earth, or the 3D space curvature near the gravity of some massive objects like a planet, star, or black hole. To me the universe, which is mostly space with only a tiny sprinkling or matter, seems entirely different from all those examples of curvature. Edited October 31, 2016 by Airbrush Link to comment Share on other sites More sharing options...
geordief Posted October 31, 2016 Share Posted October 31, 2016 (edited) Is there a way to explain how a 3D volume can be "curved"? The only curvatures I know about are 2D, such as the surface of a balloon or the surface of the Earth, or the 3D space curvature near the gravity of some massive objects like a planet, star, or black hole. To me the universe, which is mostly space with only a tiny sprinkling or matter, seems entirely different from all those examples of curvature. I am not sure (and I am trying to learn) but if you look at a piece of wood (a 3D volume) there are areas (eg knots) where density is higher than elsewhere.(so a bullet would not pass through in a straight line as seen from the outside) Is there a curvature in that 3D volume based on the density parameter and in a corresponding 3D volume of space (a vacuum) is the curvature caused by density and distribution of mass-energy sources? Edited October 31, 2016 by geordief Link to comment Share on other sites More sharing options...
Airbrush Posted October 31, 2016 Share Posted October 31, 2016 (edited) ... but if you look at a piece of wood (a 3D volume) there are areas (eg knots) where density is higher than elsewhere.(so a bullet would not pass through in a straight line as seen from the outside) If the universe has an average density of about one atom per cubic meter, it is hard to imagine how space itself would cause light to bend in any consistent direction. It will bend when it passes near a massive object, but such masses are randomly distributed, so the curvature would be random, right? Edited October 31, 2016 by Airbrush Link to comment Share on other sites More sharing options...
substitutematerials Posted October 31, 2016 Author Share Posted October 31, 2016 I see this as simply a limitation of our spatial minds. Mathematically, we can address any number of dimensions, but we must resort to lower dimensional metaphors to mentally picture it. I can picture a 3 dimensional cube, but I can't picture a 4 dimensional cube. I can however resort to a metaphor of a 2 dimensional square being extended to a 3 dimensional cube, and thus I can imagine a similar transformation of a cube into 4 dimensions. I can picture a curvature of a 2 dimensional surface, and extend that metaphorically into a 3 dimensional curvature in 4 dimensional space. That said, we don't see any observational evidence for global curvature in the universe, i.e. parallel lines remain parallel as far as we can see, apart from local effects of gravitational objects. This means we either have a profoundly precise density of mass-energy in the universe, or an exotic inflation process took place to smooth out our section of the whole universe, or maybe- curvature actually is not possible on global scale in our universe. The last possibility is not conventional science. Link to comment Share on other sites More sharing options...
geordief Posted October 31, 2016 Share Posted October 31, 2016 If the universe has an average density of about one atom per cubic meter, it is hard to imagine how space itself would cause light to bend in any consistent direction. It will bend when it passes near a massive object, but such masses are randomly distributed, so the curvature would be random, right? That is what I seem to be hearing. On the large (very large) scale curvature is almost zero . However ,as the scale increases beyond what has been observed all bets are off as ,practically by definition we cannot know what we cannot observe. I can picture a curvature of a 2 dimensional surface, and extend that metaphorically into a 3 dimensional curvature in 4 dimensional space. I think we may be talking about intrinsic curvature ....which does not need to be embedded in an extra dimension . Is spacetime curvature 3 or 4 dimensional?(I think it is 4 -dimensional) Link to comment Share on other sites More sharing options...
StringJunky Posted October 31, 2016 Share Posted October 31, 2016 Airbrush Is there a way to explain how a 3D volume can be "curved"? We don't know what gravity is but it is described classically by GR as though space was curved; it is a model. Scientists only have numbers to play with and from those numbers they see patterns and in them they see curvature. The next model, when it's finally sorted, will describe gravity as mediated by virtual particles. You can only go so far with visualising thisf and it becomes necessary to learn the mathematical descriptions. Link to comment Share on other sites More sharing options...
Strange Posted October 31, 2016 Share Posted October 31, 2016 Is there a way to explain how a 3D volume can be "curved"? No. Link to comment Share on other sites More sharing options...
imatfaal Posted October 31, 2016 Share Posted October 31, 2016 Is there a way to http://www.scienceforums.net/topic/99967-the-big-bang-happened-everywhere/page-3#entry952592explain how a 3D volume can be "curved"? .... No. Read Flatland whilst dropping some LSD...I presume someone has already posted the obligatory xkcd cartoon Link to comment Share on other sites More sharing options...
Mordred Posted October 31, 2016 Share Posted October 31, 2016 In this case the curve is in the expansion history. Following the cosmological principle. The mass density per time slice is uniform. The rate of expansion varies over time. This is what's curved is the history of expansion/contraction. Due to the expansion/contraction your light paths are also affected. Rough analogy light deflection due to moving medium. Link to comment Share on other sites More sharing options...
geordief Posted October 31, 2016 Share Posted October 31, 2016 In this case the curve is in the expansion history. Following the cosmological principle. The mass density per time slice is uniform. The rate of expansion varies over time. This is what's curved is the history of expansion/contraction. Due to the expansion/contraction your light paths are also affected. Rough analogy light deflection due to moving medium. In my post#44 I was getting the wrong end of the stick. This is not about the spacetime curvature that models gravity? You are talking about another kind of curvature? Link to comment Share on other sites More sharing options...
Mordred Posted October 31, 2016 Share Posted October 31, 2016 (edited) Correct in gravity case the curvature is a result of time dilation/length contraction. This is localized anistropy regions. In Cosmology its the density change due to expansion/contraction over time. More accurately the curvature constant is determined by the following formula [latex]\rho_{crit} = \frac{3c^2H^2}{8\pi G}[/latex] k = curvature constant as a dimensionless value, k = critical density. As far as we can determine the curvature constant stays constant throughout our universes history. Prior to inflation the limited volume makes the curvature constant negligable. After inflation is when k can be determined. The following article is one I wrote using Barbera Rydens "Introductory to Cosmology" as a reference. Her methodology to explain the FLRW metric in terms of curvature was one of the easiest to relate to for the math challenged lol. http://cosmology101.wikidot.com/universe-geometry Page 2 with the FLRW metric coverage is here. http://cosmology101.wikidot.com/geometry-flrw-metric/ Edited October 31, 2016 by Mordred Link to comment Share on other sites More sharing options...
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