Exploring the Expanding Universe: Can We Determine the Center of the Universe?

[Reclamist]

Hello,

I have just started to read Stephen Hawking's The Illustrated A Brief History Of Time, and I'm finding it very stimulating. It's stirring up questions in my mind.. I note that Edwin Hubble has observed, and that it is apparently common knowledge now among physicists, that all observable galaxies appear to be moving away from one another. This seems to indicate both that the universe is expanding, and that matter once did occupy a common point (of stated "infinite" density).

I recall a particular model which describes this situation well: A point, representing the galaxy that you and I reside within, is drawn onto a balloon along with many other points, which each represent other galaxies and objects in the universe. Imagine that the balloon were being inflated. Upon inflation, all points drawn onto the balloon's surface would appear to be moving away from one another. Thus, we would appear to have a model of the expanding universe. (I wonder if there are problems with this model?)

My questions are as follows: If it is observed that all(?) large-scale objects (such as galaxies) appear to be moving away from each other, as per the above model, could one potentially extrapolate the point from which all matter is moving away from, as if one were deflating the balloon? Thus, could one discover the approximate "centre" of our apparently infinite universe? Some follow-up questions would be: Supposing that this point did exist, would it be of any relevance or significance? Would there be any matter near this point at all, or would all matter have long-since moved away from it? Alternatively, from it, would matter be constantly spewing forth like a geyser? In occupying this point, would one witness all matter in the universe moving away from oneself more uniformly than what one observes from our present stand-point (which is, I think, the witnessing of neighbouring galaxies moving away in an "angular" fashion from a common point)?

Thank you.

 

[WannabeNewton]

There is no absolute center for the current model of the universe we use; there can't be. The current model not only requires homogeneity but also isotropy of space. An isotropic universe must look the same in all directions at any point that one can claim to be the "center" but again any center is as good as the next so there is no real center, just what an observer picks for reference. If there was an absolute center there would only be that one point with a same- looking universe in all directions; isotropy would be violated.

 

[marcus]

...

as per the above model, could one potentially extrapolate the point from which all matter is moving away from, as if one were deflating the balloon? Thus, could one discover the approximate "centre" of our apparently infinite universe? ...
...

The way to understand the balloon analogy is that in the analogy space and all existence is 2D. All existence is concetrated on the 2D surface. I have to go. Newton is right.

 

[Reclamist]

Oh, I see. So, no matter where one stands, the universe should look the same in all directions? Is this something like being a person who is standing on a giant, smooth sphere, with the ability to see in all directions and around the curvature of the planet—and no matter where this person stood, he or she would be able to see the whole of the planet (by rotating), plus the back of his or her own head, and thus, there is no "centre"? Is this to do with the fact that we, as humans, cannot naturally perceive more than three spatial directions? Are we like that person standing on the giant globe, without the ability to look up and off of it?

 

[marcus]

Oh, I see. So, no matter where one stands, the universe should look the same in all directions? Is this something like being a person who is standing on a giant, smooth sphere, with the ability to see in all directions and around the curvature of the planet—and no matter where this person stood, he or she would be able to see the whole of the planet (by rotating), plus the back of his or her own head, and thus, there is no "centre"? Is this to do with the fact that we, as humans, cannot naturally perceive more than three spatial directions? Are we like that person standing on the giant globe, without the ability to look up and off of it?

Exactly! I think that is a perfect way to describe the analogy. We can think of ourselves as like that person, except in 3D instead of 2D.

In the generalized geometry of GR, that cosmology runs on, lightrays follow geodesics. Analogous to great circles on a 2D spherical surface.

You have answered the question so I don't need to say anything but I will throw in some extra comment anyway. Cosmology has a handle on the issue of finiteness but more observation is needed before we can say with 95% certainty whether the U is spatially finite or not. If it is spatially finite then the balloon analogy could be really good!

It's quite hard to imagine how it could be spatially infinite, at least for many people it is, including myself. Because then it would ALWAYS have been infinite in extent, even at the outset of expansion (say e.g. at a very high density bounce) and that's hard to picture at least for me. But it has not yet been ruled out by observation! So we can't pretend to know which.

The observational handle is a measurement of largescale average curvature that has been getting more and more precise over the past 10 years. It is one of the reasons for sending up better space instruments---to narrow down the errorbar on the curvature.

The 95% errorbar includes the zero curvature case (where space would extend infinitely) but it also is slightly lopsided in favor of the positive overall curvature case (which would be more analogous to a 2D sphere balloon picture).
The lopsidedness cannot be taken as suggesting anything, one has to stay cool objective and not indulge wishful thinking: the errorbar is the errorbar--- representing real ignorance.
All one can say is that the overall average curvature is very nearly zero, within less than one percent, with high certainty. If the U were a 3D analog of a sphere then if that sphere were immersed in a hypothetical higher dimension space so it would have a radius, the radius could not be less than 100 billion lightyears. The circumference could not be less than 2pi times that, i.e. 628 billion lightyears.
Or 600 billion not to seem to put too fine a point on it.

 

[Reclamist]

Ah, thank you both!

 

[DavidMcC]

Why, you might ask, is the balloon analogy correct?
IMO, it comes from the validity of loop quantum cosmology - the centre of the "balloon" is in the hyperspace of LQC. This is, of course, not the standard model (which ignores space altogether, pretending that cosmology is particle physics and ONLY particle physics).

 

[phinds]

You ask about the "problem" (if any) of the balloon analogy. The one issue that stands out is that WE can see the center of the balloon, which, not being on the surface in which they exist, cannot be seen by the 2D inhabitants of the balloon surface. But we know where it is so the whole thing makes sense.

BUT ... in OUR 3D surface, there's no "center" (or analog) that we can perceive ... some people find this to be a problem with the balloon analogy but that's because they take it too far. As long as you confine EVERYTHING in your thought process to just the surface of the balloon, then the analogy is a good one.

 

[bcrowell]

Another common source of confusion about the balloon analogy is that if you imagine marking the galaxies on the balloon with a sharpie, you'd expect the galaxies to get bigger as the balloon expands. In reality, a strongly bound system like a galaxy does not expand by any measurable amount due to cosmological expansion.

 

[DavidMcC]

Another common source of confusion about the balloon analogy is that if you imagine marking the galaxies on the balloon with a sharpie, you'd expect the galaxies to get bigger as the balloon expands.

Umm, don't you mean smaller, as it shrivels up and falls on the floor?

 

[Reclamist]

No no, that's just when it pops! What about before that?

Thank you again, everyone. I now consider the balloon analogy differently: We are merely like its surface dwellers, unable to see off of it... How interesting! A real centre may be there, but we would be/are completely oblivious to it. This is fun to imagine!

 

[Shenstar]

What if the solar system is the centre of the universe? I watched a YouTube video of Lawrence krauss lecture about a 'universe from nothing'.

It made me think, what if the expansion and accelaration is only observable to us and not any other point in the universe. I know this is against the Copernican principle, but Krauss also is quoted as saying that the CMB shows elliptical alignment.
But when you look at CMB map, you also see that the structure that is observed, is in fact, in a weird way, correlated with the plane of the Earth around the sun. Is this Copernicus coming back to haunt us? That's crazy. We're looking out at the whole universe. There's no way there should be a correlation of structure with our motion of the Earth around the sun — the plane of the Earth around the sun — the ecliptic. That would say we are truly the center of the universe.
-Lawrence Krauss

Is the CMB data correct?

 

[WannabeNewton]

CMB radiation comes at us pretty much equally from all sides on large distance scales. It obeys isotropy and homogeneity.

 

[Shenstar]

Is there any correlation in the data to show an elliptical alignment and is this anything to do with the so called 'axis of evil'?