# Big bang location

by hartlw
Tags: bang, location
 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.
 Mentor P: 21,652 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.
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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).
P: 2,193
 Quote by hartlw 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).
The balloon analogy should easily clear this up for you.
Mentor
P: 21,652
 Quote by hartlw 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).
It isn't bounded except insofar as it is a finite age and we can only see what has gotten here in that time at the speed of light - which makes us the center of the "observable" universe.
P: 15,294
 Quote by hartlw 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).
As Nabeshin points out, the balloon analogy solves this well.

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.
 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.
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 Quote by Denton 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.
You misunderstand the analogy. In the balloon analogy, our universe is mapped onto the 2D surface of the balloon. It is basically a way of envisioning our universe, stepped one dimension down because we obviously cannot picture a 3-space embedded on the surface of an object in 4-space (if indeed this even were the correct picture of the universe).

The point here is this: If we consider our universe to be the 2-D 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.
 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.
 PF Patron Sci Advisor Emeritus P: 8,837 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 dt(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 dt(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.
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 Quote by hartlw 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.
You might want to make an effort to understand the concept of a theory of physics. The only way that humans can learn anything about the universe is to find a theory, i.e. a set of statements that predicts the results of experiments, and then do experiments to find out how accurate those predictions are. If the best theory describes a 4-dimensional spacetime, then it certainly makes a lot more sense to say that we live in a 4-dimensional spacetime than to say that we live in a 3-dimensional space. Of course, if we want to be completely accurate, we should only use statements of the form "experiment E agrees with prediction P of theory T to an accuracy A", and never say anything about how things really are. It would however get pretty weird to use that kind of language, so scientists choose to be a bit sloppy. They say that things "are" as described by the theory, even though we can't ever really know what things are like. It certainly makes a lot more sense to do that than to say that things are the way we intuitively feel they are, which is what you're doing when you're dismissing the extremely useful and successful concept of 4-dimensional spacetime.
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 Quote by hartlw 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.
Yes but don't confuse the 4th physical dimension with time. The 4D universe model does assume a 4th physical dimension.
 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?
 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 sub-spaces in n dimensional space, but you are talking pure mathematics. Step 1 What is the definition of x,y,z,t?
 P: 72 And if we're describing the universe, where does mass enter into x,y,z,t?
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 Quote by hartlw How about the 5D universe model which assumes a fifth dimension.
What 5 dimensional universe? You need to specify what you mean by this.

 What is the definition of x,y,z,t?
I think your confusion is arising because you are still in the mind set that spatial dimensions are real and physical, and that temporal dimensions is somehow different. This is the exact opposite of what relativity proposes: relativity puts spatial and temporal dimensions on an equal footing. Space and time are combined into space-time, and a set of four coordinates (t,x,y,z), say, label our position in space-time.
P: 72
 Quote by cristo I think your confusion is arising because you are still in the mind set that spatial dimensions are real and physical, and that temporal dimensions is somehow different. This is the exact opposite of what relativity proposes: relativity puts spatial and temporal dimensions on an equal footing. Space and time are combined into space-time, and a set of four coordinates (t,x,y,z), say, label our position in space-time.
Sounds like mysticism to me. You haven't defined x,y,z,t other than as mathematical coordinates, and you purport to describe the universe without even considering mass as a variable.

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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