
#1
Feb209, 01:28 PM

P: 72

Would the average density of visible distant objects in different directions in the night sky give any indication that we are near the center of the "big bang?"
If we were out toward the edge, I would think it would be significantly different. 



#2
Feb209, 04:00 PM

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The Big Bang was not an explosion the way we normally think of explosions. It was the start of space (as opposed to matter exploding into an already formed void) and therefore happened everywhere. The result we would expect (and that we see) is that the density of space is relatively consistent everywhere. There is no center and no edge.




#3
Feb209, 04:14 PM

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Listen to this.




#4
Feb209, 07:50 PM

P: 72

Big bang location
If the universe is uniformly dense and bounded, then unless we are at or near the center, the average density of starlight that we see should vary (more stars in one direction than another).




#6
Feb209, 08:27 PM

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#7
Feb209, 08:56 PM

P: 15,325

An ant standing on the surface of a balloon experiences a "universe" that is finite yet unbounded. And it is consistent in all directions. And it has no centre. 



#8
Feb309, 12:15 AM

P: 114

That doesnt make sense, there is a clear center of a balloon. The balloon has a particular radius that expands from the centre. Right bang in the middle of 3 dimensions.




#9
Feb309, 01:25 AM

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The point here is this: If we consider our universe to be the 2D surface of the balloon, as the balloon expands (i.e the universe expands), every single point on the surface of the balloon sees the other points rush away with a velocity proportional to its distance. And, assuming that the universe is homogeneous (which is, more or less, a good assumption), this effect is observed identically everywhere. Any point, by your logic, would claim to be at the center of this expansion, but using this analogy we clearly see that this is not the case. 



#10
Feb309, 06:31 AM

P: 72

It's like saying the universe is a four dimensional space, (x,y,z,t), which is nonsense because time is a mathematical dimension, not a physical space dimension. You don't see time. You might as well call it a five dimensional universe, (x,y,z,t,c) where c is color.
Mathematical space is not physical space. 



#11
Feb309, 07:00 AM

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The balloon analogy represents one of the three simplest solutions of Einstein's equation that include a big bang. It's the only one of the three that describes a space that's finite in size. One of the other solutions can be imagined as an infinite plane that's expanding. An infinite plane doesn't have a center either. The distance between two points A and B on this plane is a function of time, so let's call that distance d_{t}(A,B). The big bang isn't a point in this plane, i.e. it's not an event in spacetime. It's just a name for the limit t→0. In that limit we have d_{t}(A,B)→0, for all points A and B in space A. (That's the reason why that limit is called "the big bang"). The time t=0 and all times t<0 are not defined by this solution. Note that even though all the distances go to zero in the big bang limit, the plane is still infinite for all t>0.




#12
Feb309, 07:18 AM

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#13
Feb309, 07:25 AM

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#14
Feb309, 07:40 AM

P: 72

Frederik,
Are you saying that there are areas of physical space not accessible to matter? Frankly, I can access any point of my living room. If you include time as a "dimension," then I can't "access" a point that occurred three days ago. The problem is still that you are confusing mathematical space with physical space. x,y,z,t are independent variables. A sequence of events may occurr for which the position of an object is given by x,y,z as a function of time . You can arbitrarily specify the functions (mathematics) or invoke some physical law (phyusics). To do anythiing other than pure mathematics, x,y,z,t must have meaningful definitions. The first step is defining x,y,z and t. Without that, you can still do all the mathematics you want, and talk about MATHEMATICAL space, but it still doesn't mean anything. Assuming the variables in Einsteins Equatiion are x,y,z,t, what is the definition of x,y,z,t and what is the physical basis for the formulation of the equation? 



#15
Feb309, 07:52 AM

P: 72

How about the 5D universe model which assumes a fifth dimension.
But again, without a physical definition of the mathematical dimensions the mathematical model is physically meaningless. You can talk about subspaces in n dimensional space, but you are talking pure mathematics. Step 1 What is the definition of x,y,z,t? 



#16
Feb309, 08:12 AM

P: 72

And if we're describing the universe, where does mass enter into x,y,z,t?




#17
Feb309, 08:18 AM

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#18
Feb309, 08:30 AM

P: 72

True, there is the axiomatic approach. Assume undefined variables, call them what you will, and assume a mathematical relation between them. Then associate the result with a physical reality. Depending on your mathematical creativity, you could explain almost any specific physical event that way and then claim the general truth of your "theory.". 


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