ajb Posted August 9, 2015 Share Posted August 9, 2015 Are you confusing several things here? 1. Light travels at finite speed and so when we look at distant objects they are as they were in the past and not now. So yes, we do 'look back in time'. Using photons we can only get as far back as the CMBR. 2. As the Universe is of finite age, we cannot see everything. We can only see our Observable Universe. 3. The expansion of the Universe does not imply a centre. Look the balloon analogy again. Partially inflate the balloon. Where is the centre? Blow up the balloon more, and ask the same question. 4. The expansion look the same for all observers. Meaning, that our alien friends would derive Hubble's law also. Nothing implies a meaningful, privileged and special place called the 'Centre of the Universe'. The modern picture is that the centre is not a meaningful notion. Link to comment Share on other sites More sharing options...
Scotty99 Posted August 9, 2015 Author Share Posted August 9, 2015 The expansion of the Universe does not imply a centre. Look the balloon analogy again. Partially inflate the balloon. Where is the centre? Blow up the balloon more, and ask the same question. This is exactly what im trying to get at. For a moment, no matter how small of time, there is a centre in the balloon analogy! (which is how science looks at the universe NOW!). Sure, its an ever evolving center in this view, but there IS a centre. Link to comment Share on other sites More sharing options...
ajb Posted August 9, 2015 Share Posted August 9, 2015 This is exactly what im trying to get at. For a moment, no matter how small of time, there is a centre in the balloon analogy! (which is how science looks at the universe NOW!). Sure, its an ever evolving center in this view, but there IS a centre. Go get a ball, any ball and a pen. The surface of this ball is a 2-sphere. The 2-sphere is the surface we should look at, just as an analogy. Please mark on that ball with your pen the centre of the 2-sphere. That is mark the special point on the surface of this ball that deserves to be called the 'centre'. You should see that there is no centre. The analogy between the blowing up a balloon and the expansion of the Universe is reasonable in this respect. Link to comment Share on other sites More sharing options...
Strange Posted August 9, 2015 Share Posted August 9, 2015 Ok think about what you are saying for a second people. Right. Of course, we haven't done that. Relativity says there are no special places in the universe, meaning there is no CHANCE of there being a center. Relativity doesn't say anything about there being a centre or not. However, cosmological models (based on GR) do not have a centre. This would imply you ALL think the universe is absolutely infinite and there is no possibilty of a center in any case. It doesn't have to be infinite for there to be no centre. It can be "finite but unbounded" (like the surface of the Earth). I dont presume this, science does. Go look, we predict there is a "wall" of sorts out there. I am not aware of any model that suggests that. 2.I cannot, in any sense, grasp what you mean by a sphere not having a center. The SURFACE of a sphere doesn't have a centre. Link to comment Share on other sites More sharing options...
Scotty99 Posted August 9, 2015 Author Share Posted August 9, 2015 I am not aware of any model that suggests that. ? This is big bang stuff. We currently believe if you go out far enough and fast enough you WILL reach a point where you can visibly observe the creation on the universe. If you are having trouble wrapping your head around this its easy, just imagine the big bang AS an actual balloon and the skin of the balloon is the beginning of space time. This is literally how science currently views the universe. The SURFACE of a sphere doesn't have a cent What an abstract thought. How many people out of 100 would think of a sphere in regards to its surfrace rather than a circle? The universe is a sphere according to all out research (read above, an ever expanding sphere) yet you deny it has a centre at any given point in time? Link to comment Share on other sites More sharing options...
ajb Posted August 9, 2015 Share Posted August 9, 2015 (edited) Science assumes the universe is ever expanding but NOT infinite. How then, at any point in time (as the universe continues to expand) is there not a centre to the universe? Sure its for a very brief milisecond, but there is a centre. Have a look at the FWR cosmologies again. You see that we can describe space-time as being decomposed into time and a 3-manifold. Upon rescaling the 3-manifolds allowed are i) elliptical space ii) Euclidean space iii) hyperbolic space We think based on observations that ii) is the closest to reality. The metric used on space-time is such that the metric on the 3-manifolds is scaled as a function of time. So at each time these 3-manifolds 'get bigger'. But still, there is no proper notion of a centre here and there is no need for such a notion. ...just imagine the big bang AS an actual balloon and the skin of the balloon is the beginning of space time. No! All of space is the surface of the balloon and the volume grows in time as you blow it up. We have to remove the initial singularity from this analogy as we do with the actual physics. This is literally how science currently views the universe. No! This is an analogy and one you have misunderstood. What an abstract thought. How many people out of 100 would think of a sphere in regards to its surfrace rather than a circle? Depends if you ask mathematically educated people or not! The universe is a sphere according to all out research (read above, an ever expanding sphere) No! Our models suggest that space is flat (on the scales of cosmology) and so modelled by ii) in my earlier list. This only really describes the local situation and we do not have much of a handle on the global topology of the entire Universe. ...yet you deny it has a centre at any given point in time? Hopefully you are no starting to see your misunderstandings. Edited August 9, 2015 by ajb Link to comment Share on other sites More sharing options...
Scotty99 Posted August 9, 2015 Author Share Posted August 9, 2015 (edited) No! Our models suggest that space is flat (on the scales of cosmology) and so modelled by ii) in my earlier list. This, i am willing to admit i dont understand. But lets put that aside for just a second, lets assume the entire universe is on a flat scale rather than a sphere. Even then how do you explain a centre in an ever expanding universe? Yes it will only be for a speckle of time, but for that speckle, there is a centre. Albeit in this view the centre is more of a mid point than an actual centre, there is still a preferred point in space (as you suggest the universe is a flat line, then surely there is a mid point to THAT line, no?) Edited August 9, 2015 by Scotty99 Link to comment Share on other sites More sharing options...
ajb Posted August 9, 2015 Share Posted August 9, 2015 Even then how do you explain a centre in an ever expanding universe? Picking any one of the 3 universes does not change the fact there is no centre. Yes it will only be for a speckle of time, but for that speckle, there is a centre. What special point have you picked? Albeit in this view the centre is more of a mid point than an actual centre, there is still a preferred point in space Midpoint with respect to what? Just imagine an infinite piece of paper. How would you mark the centre of that? It looks the same in all directions from all points. What does the centre mean in this context? The best you could do is pick a point and define that as 'the centre'. Now, if you like coordinate geometry on the plane then that is exactly what you do. You pick some point and call it (0,0) and set up some coordinate system from there. But you can pick any other point and do the same (as well as picking different ways of setting up the coordinates). (as you suggest the universe is a flat line, then surely there is a mid point to THAT line, no?) Not a line, but we can use a line as an analogy. What is the centre of a line drawn on the plane? There is noway to make much sense of that. What I think you are confusing now is the notion of the real line. We have picked the origin (0) by 'hand' and set up a coordinate system on the line that corresponds to the real line. We make a choice of what point is the zero. 1 Link to comment Share on other sites More sharing options...
Strange Posted August 9, 2015 Share Posted August 9, 2015 ? This is big bang stuff. We currently believe if you go out far enough and fast enough you WILL reach a point where you can visibly observe the creation on the universe. If you are having trouble wrapping your head around this its easy, just imagine the big bang AS an actual balloon and the skin of the balloon is the beginning of space time. This is literally how science currently views the universe. I have no idea where you get that idea from. Can you provide a reference that says that? What an abstract thought. How many people out of 100 would think of a sphere in regards to its surfrace rather than a circle? How is that relevant? Mathematics is not a popularity contest. Even then how do you explain a centre in an ever expanding universe? There is nothing to explain: there is no centre. You seem to be visualizing the big bang model as if it were an explosion. Link to comment Share on other sites More sharing options...
Klaynos Posted August 9, 2015 Share Posted August 9, 2015 Relativity says there are no special places in the universe, meaning there is no CHANCE of there being a center. This would imply you ALL think the universe is absolutely infinite and there is no possibilty of a center in any case. Something can easily be finite and bit have a centre. Consider a sphere, a soccer back say, please tell me where the centre of the surface of the ball is? Link to comment Share on other sites More sharing options...
Thorham Posted August 9, 2015 Share Posted August 9, 2015 There is nothing to explain: there is no centre. There is if the universe is finite. It's just that this center is of no significance, and the galaxy we're in could be in the center by pure chance. Link to comment Share on other sites More sharing options...
Strange Posted August 9, 2015 Share Posted August 9, 2015 There is if the universe is finite. Not necessarily. Link to comment Share on other sites More sharing options...
Scotty99 Posted August 9, 2015 Author Share Posted August 9, 2015 (edited) What special point have you picked? How can this be anything except the exact center of mass in the universe? Im not picking a special place here, if the universe is finite (likely) you add up all the mass in the cosmos there IS a center of mass. Thinking of a top is probably the best way to visualize it, if you took your hand and spun the entire universe there is a mid point to which it would all spin around. Geocentrists suggest earth is exactly in this spot. (CMB data also appears to back this up) Not necessarily. How could there not be a center of mass in a finite volume? Edited August 9, 2015 by Scotty99 Link to comment Share on other sites More sharing options...
Delta1212 Posted August 9, 2015 Share Posted August 9, 2015 How can this be anything except the exact center of mass in the universe? Im not picking a special place here, if the universe is finite (likely) you add up all the mass in the cosmos there IS a center of mass. Thinking of a top is probably the best way to visualize it, if you took your hand and spun the entire universe there is a mid point to which it would all spin around. Geocentrists suggest earth is exactly in this spot. (CMB data also appears to back this up) How could there not be a center of mass in a finite volume? If the universe has positive curvature, it could be finite with no center. It would be the 3-dimensional equivalent of the 2-D surface of a sphere or torus. Link to comment Share on other sites More sharing options...
Scotty99 Posted August 9, 2015 Author Share Posted August 9, 2015 If the universe has positive curvature, it could be finite with no center. It would be the 3-dimensional equivalent of the 2-D surface of a sphere or torus. People seem to get confused on a geocentrists take on a center. They dont mean a place in terms of a visual center (dont know a better way to put that), they mean the center of MASS. Even an obscure shape like a torus would have a center of mass, correct? (given that the shape is finite, of course) Link to comment Share on other sites More sharing options...
Delta1212 Posted August 9, 2015 Share Posted August 9, 2015 People seem to get confused on a geocentrists take on a center. They dont mean a place in terms of a visual center (dont know a better way to put that), they mean the center of MASS. Even an obscure shape like a torus would have a center of mass, correct? (given that the shape is finite, of course) Not it's surface. You're getting the three dimensional object confused with the two dimensional surface that is being used as an analogy for the harder-to-conceptualize three dimensional curvature of space. (Besides which, even if we were talking about the shape, a toroid's center of mass is outside the torus) Link to comment Share on other sites More sharing options...
Scotty99 Posted August 9, 2015 Author Share Posted August 9, 2015 Not it's surface. You're getting the three dimensional object confused with the two dimensional surface that is being used as an analogy for the harder-to-conceptualize three dimensional curvature of space. (Besides which, even if we were talking about the shape, a toroid's center of mass is outside the torus) Why is there not a center of mass on a surface? No one knows the exact curvature of space of course, but lets pretend its a perfectly straight line. If you took all of the mass on that line and did the math to find the center, why is that not a feasible concept as well? Your comment of a toroid's (props on that word, first time ive ever seen that lol) center of mass being outside itself went straight over my head. Link to comment Share on other sites More sharing options...
Klaynos Posted August 9, 2015 Share Posted August 9, 2015 Why is there not a center of mass on a surface? No one knows the exact curvature of space of course, but lets pretend its a perfectly straight line. If you took all of the mass on that line and did the math to find the center, why is that not a feasible concept as well? Your comment of a toroid's (props on that word, first time ive ever seen that lol) center of mass being outside itself went straight over my head. The point is the centre of mass doesn't have to be on the surface. It memory serves, for a 3d curved "surface" which space might be the centre of mass might be uniform so everywhere. Again no special point. Link to comment Share on other sites More sharing options...
swansont Posted August 9, 2015 Share Posted August 9, 2015 Geocentrists suggest earth is exactly in this spot. (CMB data also appears to back this up) Since when does the CMB have position information? Link to comment Share on other sites More sharing options...
Strange Posted August 9, 2015 Share Posted August 9, 2015 People seem to get confused on a geocentrists take on a center. They dont mean a place in terms of a visual center (dont know a better way to put that), they mean the center of MASS. Even an obscure shape like a torus would have a center of mass, correct? (given that the shape is finite, of course) What is the center of mass of the surface of a billiard ball? Why is there not a center of mass on a surface? No one knows the exact curvature of space of course, but lets pretend its a perfectly straight line. If you took all of the mass on that line and did the math to find the center, why is that not a feasible concept as well? Where is the center of mass of a line around the surface of a billiard ball? Geocentrists suggest earth is exactly in this spot. (CMB data also appears to back this up) How does the CMB back this up? Please take into account our motion relative to the CMB in your answer. Link to comment Share on other sites More sharing options...
Delta1212 Posted August 9, 2015 Share Posted August 9, 2015 (edited) Why is there not a center of mass on a surface? No one knows the exact curvature of space of course, but lets pretend its a perfectly straight line. If you took all of the mass on that line and did the math to find the center, why is that not a feasible concept as well? Your comment of a toroid's (props on that word, first time ive ever seen that lol) center of mass being outside itself went straight over my head. The center of mass of a donut is located in the hole. It's not actually within the donut itself. Do if you were limited to traveling only within the confines of the donut, you'd never be able to locate its center of mass. That said, again, no one is saying that the universe is shaped like a donut. But it may be shaped like a three dimensional analog of the two dimensional surface of a donut. Edited August 9, 2015 by Delta1212 1 Link to comment Share on other sites More sharing options...
Strange Posted August 9, 2015 Share Posted August 9, 2015 The center of mass of a donut is located in the hole. It's not actually within the donut itself. But remember, in these lower-dimensional analogies we are talking about the surface of these shapes. So where would the centre of mass of the surface of a torus be? Link to comment Share on other sites More sharing options...
Delta1212 Posted August 9, 2015 Share Posted August 9, 2015 But remember, in these lower-dimensional analogies we are talking about the surface of these shapes. So where would the centre of mass of the surface of a torus be? See my edit. Link to comment Share on other sites More sharing options...
Scotty99 Posted August 9, 2015 Author Share Posted August 9, 2015 Since when does the CMB have position information? Sorry, i just meant the readings are seemingly pointing into our direction. You are right that does not give location information. What is the center of mass of the surface of a billiard ball? Got me on this one lol. That said, none of the various models suggested in the articles have read about infinite vs finite universe proposed a shape like this so im not sure the point of the exercise i guess. Link to comment Share on other sites More sharing options...
pzkpfw Posted August 10, 2015 Share Posted August 10, 2015 Sorry, i just meant the readings are seemingly pointing into our direction. ... Pick some Galaxy other than our own. Imagine an alien on some planet orbiting some star in that Galaxy. What would that alien read for the direction "pointed" by the CMB? Link to comment Share on other sites More sharing options...
Recommended Posts