# Big bang location

1. Feb 2, 2009

### hartlw

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. Feb 2, 2009

### Staff: Mentor

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. Feb 2, 2009

### chemisttree

Listen to http://www.astronomycast.com/astronomy/ep-77-where-is-the-centre-of-the-universe/" [Broken]

Last edited by a moderator: May 4, 2017
4. Feb 2, 2009

### 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).

5. Feb 2, 2009

### Nabeshin

The balloon analogy should easily clear this up for you.

6. Feb 2, 2009

### Staff: Mentor

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.

7. Feb 2, 2009

### DaveC426913

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.

8. Feb 3, 2009

### 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.

9. Feb 3, 2009

### Nabeshin

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.

10. Feb 3, 2009

### 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.

11. Feb 3, 2009

### Fredrik

Staff Emeritus
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.

12. Feb 3, 2009

### Fredrik

Staff Emeritus
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.

13. Feb 3, 2009

### DaveC426913

Yes but don't confuse the 4th physical dimension with time. The 4D universe model does assume a 4th physical dimension.

14. Feb 3, 2009

### hartlw

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. Feb 3, 2009

### hartlw

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?

16. Feb 3, 2009

### hartlw

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

17. Feb 3, 2009

### cristo

Staff Emeritus
What 5 dimensional universe? You need to specify what you mean by this.

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.

18. Feb 3, 2009

### hartlw

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.".

19. Feb 3, 2009

### cristo

Staff Emeritus
But that's because that's what they are. One can't write down a theory in mathematics, and expect not to use mathematical coordinates! The theory of relativity says that we live in a four dimensional spacetime. This theory has agreed with experiment to an outstanding accuracy, and is currently our best theory. I don't think that your argument of "mysticism" holds up, because it is a personal opinion. Just because you don't understand something, or you don't think it intuitive doesn't automatically void all experimental evidence, or make the theory incorrect. Finally, I don't see anywhere in my post that I went on to describe the universe at all, and I didn't discuss anything to do with mass. Let's not put words into the mouths of others, please.

20. Feb 3, 2009

### hartlw

Relativity says something about the speed of light without defining either distance or time. With that kind of flexibility, you can explain anything you want.

Classical physics and science has always started with very specific definitions of space and time. The problem with classical physics was that it exposed the theories to intelligent criticism.

I have a 5D model of the universe. I call the dimensions spatial location x,y,z , time, and color, but do not define them. They satisfy hartl's equation.

I would explain it to you but you would have to be well versed in modern algebra, euclidean and affine geometry, algebraic geometry, covariant and contravariant tensors in n-dimensional space, n-dimensional and infinite dimensional vector space, and advanced calculus.

My equation explains all physical phenomena.

21. Feb 3, 2009

### hartlw

And of course, since my equation explains space, time and the physical universe, I am responsible for all material and scientific progress in the twentieth century.

Transistor? me
DNA? me
Space Travel and Satelite Communications? me
Atomic Energy? me
and so on

Last edited: Feb 3, 2009
22. Feb 3, 2009

### marcus

Please don't leave us in suspense. What is Hartl's equation? Please write it down for us.

23. Feb 3, 2009

### Fredrik

Staff Emeritus
No.

Now you're talking about spacetime, not space, and what you can access in your future is irrelevant to what we're talking about.

No, it isn't. (And I don't). The problem is that you believe that your intuitive ideas about space and time are somehow "better" than any theory. You don't seem to reallize that your intuitive ideas (which are the same as everyone else's intuitive ideas) is just another theory about the real world, which by the way has been thoroughly disproved by experiments.

I can't teach you general relativity here, but I can give you a brief outline of some of the basic ideas. Spacetime is a 4-dimensional manifold M. A coordinate system is a function $x:U\rightarrow\mathbb R^4$, i.e. the coordinate system is the function that assigns coordinates to events:

$$x(p)=(x^0(p),x^1(p),x^2(p),x^3(p))$$

If you'd like, you can use the notation (t,x,y,z) for the thing on the right. These are however not the variables in Einstein's equation. The variables are the components of the metric tensor, which contains all the information about the geometric properties of spacetime. Einstein's equation describes the relationship between the metric tensor and the stress-energy tensor, which represents the properties of matter. You asked specifically about mass. Mass enters the equation through the equivalence between mass and energy (E=mc2) because one of the ten independent components of the stress-energy tensor is energy density.

The relevant solutions of this equation are found by first assuming that spacetime can be "sliced" into a one-parameter family of spacelike hypersurfaces $\Sigma_t[/tex] (we can think of each [itex]\Sigma_t[/tex] as "space, at time t"), such that each [itex]\Sigma_t[/tex] is homogeneous and isotropic (according to a precise mathematical definition of those terms). There are only three solutions of Einstein's equation that are consistent with that assumption. (Wikipedia link). These three solutions describe space as a 3-dimensional version of a sphere, a plane and a hyperboloid respectively. Spheres are finite in size. Planes and hyperboloids are not. It's convenient, but not necessary, to define a coordinate system x that assigns time t to all the points in [itex]\Sigma_t[/tex]. (I.e. [itex]x^0(p)=t$ when $p\in\Sigma_t$). If we do, we find that t can't be defined for all real t. There exists a t0 such that t is only defined for t>t0. It's convenient to choose t0=0.

The fact that each $\Sigma_t$ looks like a sphere, a plane or a hyperboloid means that there's also a very natural way to assign the spatial coordinates to points on $\Sigma_t$. This gives us a way to identify a point on $\Sigma_t$ with a point on $\Sigma_s$ when $t\neq s$, and this allows us to define the distance between any two points in space as a function of time. It can be shown that this distance goes to zero as t goes to zero. That's why the limit t→0 is called "the big bang".

Last edited: Feb 3, 2009
24. Feb 3, 2009

### DaveC426913

A point of order if I may.

Your original post was asking where the centre of the universe is. You don't have to delve very far into cosmology to dispose of this misconception. We've shown you that the centre of the universe is everywhere. This was shown in the BB model - without resorting to extra dimensions.

Aside from that, you've asked how a volume can be finite yet unbounded, which we're showing you using the notion of extra dimensions. This is what we are now discussing.

Let's just back up.

Are you satisfied that your idea of a centre of the universe was naive?

I am a bit confused as to how you could have - in the same breath - thought there was a centre to the universe and yet argue with confidence that you know about higher-order dimensional space.

Can you clarify you level of understanding of this science field?

25. Feb 3, 2009

### hartlw

Since you haven't identified (defined) what you are talking about, I don't understand a word you are saying. You are simply taking symbols, manipulating them mathematically, and calling the result a proof of something.

How can you say your equation says something about the real world when you don't even define the terms of your equation other than giving them a name, which anybody can do. Let's see, space is really time and time is really space, so your results are wrong.