# Question about Expansion and the Center of the Universe

1. Aug 16, 2009

### Salbris

I've always wondered, have scientists been able to find the center of the Universe? I don't mean of just the observable universe. Since scientists says the universe is expanding doesn't that mean they must have found at least the direction the center could be?

Pretty straight forward I suppose.

2. Aug 16, 2009

### nicksauce

There is no centre of the universe. The usual analogy is to imagine drawing points (ie. galaxies) on a balloon, then blowing up the balloon. Each point will get further away from the other points, just like in our universe. However, the surface of the balloon clearly has no centre.

3. Aug 17, 2009

### Salbris

Oh, so it's a generally accepted fact the Universe is more like the surface of a balloon but in 3D? I guess the next question is, how do they know that then?

Are you also then saying that galaxies don't "move" but the space around them expands?

I always thought the universe was like the interior of a balloon.

4. Aug 17, 2009

### sylas

You can use various analogies in different ways to try and get a feel for cosmology without going into all the maths. The balloon analogy is always used with the skin of the balloon representing the universe.

This analogy can help give a notion of several ideas in cosmology... such as curvature, and the idea of an expansion without any boundary. The surface of a balloon has no edge; any point on the surface is similar to any other point. In the balloon analogy, galaxies are drawn onto the surface of the balloon, not inside it, and every galaxy is surround by more galaxies in all directions.

You can actually think of the inside of the balloon as the universe as well... but only if you consider the balloon to have no edge. The universe doesn't have a skin or edge, as far as we can tell. It just keeps going on and on forever. What does expansion mean in this case? It means that the air inside the balloon is getting less dense. All molecules are dispersing from all other molecules... just like galaxies are dispersing away from each other.

Also, there's no way to identify a center if there's no bound or skin or edge as a distinguished reference point. Every part of the interior looks like every other part. Without the edge, or skin, there's no sensible way to speak of a center anymore. That's like the universe as well. We don't have any sign of an edge or boundary to the universe; it just keeps going on and on. There's a boundary in time, when everything was jammed up together with density diverging without bound within a finite time. This is the "singularity" in the past, or the origin of the universe as we know it. But there's no center in space, because there's no edge.

Cheers -- sylas

5. Aug 17, 2009

### nicksauce

The cosmological principle is that the universe looks the same in every direction, and is the same everywhere in space. Apart from its aesthetic appeal, this highly idealized principle seems to work well on large scales. We can see this through the uniformity of the temperature of the cosmic microwave background, and also through the way that galaxies and clusters of galaxies are distributed throughout space.

If we accept the Cosmological Principle, there are 3 possible topologies for the universe. Flat, like an infinite 3 dimensional plane, Positive curvature, which is like the balloon in 3 dimensions, and Negative curvature, which looks something like a saddle. Current measurements show that the universe is very close to being flat, with possibly a bit of positive curvature. (I find the balloon analogy is the most helpful though).

However, we know that the Cosmological Principle can not be totally true. There are obviously regions of overly high density (galaxies and galaxy clusters) and regions of overly low density (voids). These realities are accommodated by perturbing the equations that come from the Cosmological Principle. Apart from high and low density regions, we also get hot and cold regions (which can be observed as fluctuations in the CMB temperature), and peculiar velocity fields (this is the recession velocity of a galaxy that can't be explained just through the expansion of space).

It is a common misconception to think that the big bang was at the centre of a balloon and that space is expanding away from that point. But really, the big bang did not occur at a single point in space; it is better to think of it occurring everywhere in space.

6. Aug 17, 2009

### Salbris

Thank you, that helps puts things in to perspective!

I'm just wondering then, if scientists detect red-shift and therefore they believe the universe is expanding and also believe that all matter and energy was once contained in at one point, then where was that point? Don't we even know where the big bang occurred?

Last edited: Aug 17, 2009
7. Aug 17, 2009

### QuantumKitty

The point, or singularity, didn't exist somewhere in space or the universe, it was the universe.

The big bang didn't occur at a specific location in space-time, it happened to space-time. There was no where before the big bang since it created space. There is also no before the big bang either since it created time along with space. So to answer your question to the best of my knowledge, the big bang occured to all of space-time at the same instant.

8. Aug 17, 2009

### Salbris

Okay, but how do scientists know this? As far I know they have never discovered things that create space-time, why does it make sense to think the Universe didn't exist before the big bang?

9. Aug 17, 2009

### QuantumKitty

I'm not saying the universe didn't exist before the big bang, nor that some thing created space-time. It's my understanding that the universe did exist prior to the big bang, as a singularity. Did the universe exist before that? I don't know, but I'm inclined to think that the universe has always existed in some form or another.

The singularity that became the universe we know may not have even existed in a real physical and tangible state. It just signifies a point at which all current models and mathematics break down and don't apply. Once we, or perhaps just me, better understand what a singularity is, a better model of the universe will emerge.

10. Aug 17, 2009

### DrChinese

There are a lot of reasons, you are sort of starting at the end of the story.

In the 20th century, Hubble discovered the red shift phenomena and Penzias & Wilson discovered the Cosmic Microwave Background Radiation (CMBR). So that sets the stage. You have Einstein with General Relativity and recent analyses of the CMBR indicate that the universe's expansion is increasing. So it is not random guesses. With the advent of Quantum Field Theory, it appears that the universe may represent a vacuum fluctuation.

As to the existence of the universe before the big bang: this is considered a metaphysical question because by definition, events before that time and outside of this region are inaccessible to us. Otherwise they would be part of our universe. There could be other universes, no one is really saying this is the only one.

11. Aug 17, 2009

### Salbris

Thanks for the info, but I'm not really sure that answered my question. I mean, how do scientists know the big bang was the creation of space-time as we know it? Couldn't space-time have already existed?

12. Aug 17, 2009

### DrChinese

Possibly something else existed, but it would be outside of our universe and inaccessible to us. No way to demonstrate that one way or another. You are probably familiar with the Big Bounce, which is another expression of that idea. The relevant point is that all "memory" of conditions prior to the Big Bang have been erased. The CMBR is the oldest remnant.

So you can speculate about other universes and there being some kind of "spacetime" fabric into which our universe was born. But the spacetime we see evolves as part of our universe.