B Can we identify the centre of the Universe?

[Drakkith]

Oh. That's actually pretty cool, but pretty weird as well.

Indeed it is! The universe constantly surprises us!

 

[ScottVal]

As I mentioned in an earlier post, the question of the universe's center (which it does not have), may also lead to the question of whether or not the universe has (or could have) an "edge." I believe this is related to the balloon analogy, that you could in theory move in any direction (or look in any direction) and never see an "edge," and yet the universe would not really go on forever, because perhaps it is bounded like the surface of the balloon. Perhaps if you moved many billions of light years in one "direction" you would eventually end up where you started.
-Scott V.

 

[Ssnow]

I believe there is no center in our universe. We don't know the form and at the origin ($t=0$) the mathematical models fails. In don't know if in a weak sense it is possible to speak of a ''center of mass'', but also in this case it is harder because we don't know much on the antimatter and it's properties ... Pascal said ''The universe is a sphere with infinity ray and center everywhere'', this tell all and nothing ...

 

[Neandethal00]

The universe has no center.

OP was not asking for a center of a circle or center of a sphere.
Any spatial region must have a central 'region'. The region may
shift or constantly shift, but there must be region that we can call
central region of the universe.

Just because it is beyond the knowledge of 'current science', declaring
that universe has no center, in my opinion, is kind of escapism.

 

[Bandersnatch]

Any spatial region must have a central 'region'.

The two dimensional surface of a sphere, or of a torus, are both spaces (or, 'spatial regions'). So is an infinite plane or an infinite 3D volume, or the 3D 'surface' of a hypersphre. Unless you can name the 'central region' of each of those spaces, your assertion would appear invalid, as are any conclusions you may draw based on it.

 

[rootone]

... Any spatial region must have a central 'region' ...

The surface of Earth is a finite 2 dimensional spatial region, (disregarding irrelevances such as mountains).
Where is the centre of Earth's surface?

 

[Neandethal00]

Bander and root:
The center is located off the surface.
Any shape has a area, on or off the shape, we can identify as center. Otherwise, the term 'center of curvature' is meaningless.

 

[Bandersnatch]

What if it's a flat, infinite plane (or 3D space)? Where's the centre then?

 

[jbriggs444]

Bander and root:
The center is located off the surface.

It is not required that a curved space be embedded in a higher dimensional space in which the supposed center would exist. Curvature can exist and can be measured without requiring a center or a center of curvature.

 

[Neandethal00]

What if it's a flat, infinite plane (or 3D space)? Where's the centre then?

Look around you, do you see anything "infinite"? A guppy in the middle of Atlantic will conclude after all measurements Atlantic is infinite.

Curvature can exist and can be measured without requiring a center or a center of curvature.

A curvature can have many "centers of curvature" when it is divided into sections. Saying a geometrical shape has no center is "practically incorrect".

 

[jbriggs444]

Saying a geometrical shape has no center is "practically incorrect".

That is utterly wrong.

 

[Flatland]

The universe has no center.

OP was not asking for a center of a circle or center of a sphere.
Any spatial region must have a central 'region'. The region may
shift or constantly shift, but there must be region that we can call
central region of the universe.

Just because it is beyond the knowledge of 'current science', declaring
that universe has no center, in my opinion, is kind of escapism.

If the Universe is infinitely large than it has no center. If the Universe is finite and unbounded it just means that if you travel in any direction far enough (theoretically speaking) you will end back at your starting point thus no center.

 

[Drakkith]

A curvature can have many "centers of curvature" when it is divided into sections. Saying a geometrical shape has no center is "practically incorrect".

It can only have multiple centers of curvature if the curvature of the surface itself varies. It can also have intrinsic curvature, which does not require that it exists embedded within a higher dimensional space, precluding the existence of a center of curvature.

 

[John Rampton]

The Universe has no centre from which it expanded. The notion of space and hence any coordinate system based on what we call space only came into existence at the moment of creation. Space is still being created but it is only referenced (the expansion that is) to every other bit of space out there

 

[John Rampton]

Locally bound galaxies within a group may be traveling towards each other. Where the gravitational attraction force exceeds the expansion. Expansion is related to the distance one goes so eventually expansion exceeds any gravitational attraction, hence Red Shift

 

[russ_watters]

The center is located off the surface.

You agree that the 2d surface has no center on the surface, so therefore you must agree that the 3d universe has no center in its 3d space.

 

[Orien Rigney]

What happens if in the near future we find to our dismay, there is a center to the universe?

 

[jbriggs444]

What happens if in the near future we find to our dismay, there is a center to the universe?

The relevant feature of the universe is that it is homogeneous and isotropic. That is, it is the same everywhere and in every direction. In order for there to be a center, there must be a violation of homogeneity. If such a violation is observed and verified then we would need to come up with an explanation. That explanation might or might not involve a center.

In the absence of any such evidence, this is pure speculation and not fit subject matter here.

 

[Drakkith]

What happens if in the near future we find to our dismay, there is a center to the universe?

To bounce off of Jbriggs post, in the very unlikely event that we do identify a center we will simply modify our theories accordingly or develop new ones if our current ones can't be modified. That's how science works, after all.

 

[Ryan Rankin]

I always like the balloon analogy.
The 2-D rubber represents our 3 dimensional space and as you blow the balloon up, the universe expands. That doesn't mean objects can't interact and collide with one another on the space that's expanding. It also highlights how it doesn't make sense (for the closed universe) to speak of a center, any more than asking someone standing on Earth which lat and long represent the center of the earth. You CAN however, define a CENTER-OF-MASS frame, which is as close as physically makes sense to a center of the universe, this corresponds to the frame in which the microwave background is isotropic (more properly called co-moving coordinates).

 

[Johninch]

I always like the balloon analogy.

Glad it helps. I don’t like any of the analogies – Ballon, Fruit Bread, Donuts are all shown with 3D boundaries. Big Bang analogy is thought to be an explosion. That’s why there are all these questions about center of the universe and boundary.

There is a lot of discussion about the observable universe, which I understand because that is all we can observe. But it seems to me that the original question relates to the total universe.

I suppose that the questions about an imagined center of the universe arise from the assumption that the high original density had a source. A source implies causality, so if time started with the BB, there was no cause and therefore no source.

Can somebody help me with the question: if we are saying that the original high density occurred everywhere, where does all this mass go when the density falls?

Is it assumed that dark energy has mass? With the observed expansion of the universe, which we assume also to be the case in the non-observable universe, are we saying that there is an increasing emptiness (not containing dark energy or anything else) which is reducing mass per cubic light year?

Or does dark energy maintain mass density? If not, where does the emptiness come from?

 

[Drakkith]

I suppose that the questions about an imagined center of the universe arise from the assumption that the high original density had a source.

Indeed. It is only natural to expect such a thing. It is, after all, extremely difficult for most people to wrap their head around a center-less expansion. We never encounter this in our everyday life.

A source implies causality, so if time started with the BB, there was no cause and therefore no source.

That assumes the big bang was a single event and that time started with it. All we know is that the universe gets denser the further back we look. Since we can only see so far, we have to extrapolate past backwards beyond a certain point in time, which is where the idea of a singularity comes from. If the universe continues to contract as you go back in time, then our extrapolation shows a singularity. Who knows what actually occurred...

Can somebody help me with the question: if we are saying that the original high density occurred everywhere, where does all this mass go when the density falls?

The mass of matter and dark matter simply spreads out along with those types of matter. The mass of the emitted light and radiation is, well, lost. Kind of. Sort of. (It's actually very complicated)

Is it assumed that dark energy has mass?

It is not. It has the reverse effect that mass has.

With the observed expansion of the universe, which we assume also to be the case in the non-observable universe, are we saying that there is an increasing emptiness (not containing dark energy or anything else) which is reducing mass per cubic light year?

Dark energy is believed to remain at the same density while the density of normal and dark matter falls. This is what leads to the accelerating expansion.

 

[Johninch]

Dark energy is believed to remain at the same density while the density of normal and dark matter falls.

How can dark energy remain at the same density when the space between the (super) clusters is increasing? I can think of three reasons:

• Dark energy is increasing in volume terms.
• Dark energy is decreasing, which takes up more space because it is negative.
  
• Emptiness is increasing.
  
  Which do you think is more likely?
  
  Has emptiness been fully discredited (by Dirac) or does the universe contain emptiness?
  
  I have a problem with expansion without available emptiness.
  

 

[Drakkith]

How can dark energy remain at the same density when the space between the (super) clusters is increasing?

One of the prevailing ideas is that dark energy is just the cosmological constant, a built-in property of spacetime that causes a change in the overall geometry (the origin of the accelerating expansion) when the density of matter falls below some level. If so, then dark energy has no choice but to "remain constant", as it isn't something that fills space at all.

Has emptiness been fully discredited (by Dirac) or does the universe contain emptiness?

Unknown. I think that depends entirely upon what you consider "emptiness". If you count fields as "something", then the universe is never empty. But whether or not there is "true emptiness" really has no bearing on expansion. It's known to occur, regardless of the labels you and I place on things.

 

[Johninch]

Uh-uh, it isn't going to work

Just forget about the raisin-bread, which has a boundary.

The Universe is everything, without a boundary, and the main constituent parts are seen to be moving away from each other with increasing velocity.

In the beginning they were compactly close together, without any space between, but still everything.

You can’t have a center of everything.

We have the phenomenon of a universe which was highly dense everywhere, without boundaries and which is now expanding exponentially, but not into anything, because it is everything. So we go from very high density to very low density with increasingly lots of empty space beween the constituent parts.

It’s no use talking about raisin-bread. That’s ridiculous.

What do you conclude. I am very interested in your opinion.

 

[phinds]

It’s no use talking about raisin-bread. That’s ridiculous.

No, it is not the least bit ridiculous, it is an analogy and analogies are by their very nature, flawed to one degree or another. The trick is to understand what part of the analogy is flawed and what part is helpful, but none of it is ridiculous. Similarly with the balloon analogy, as I explain in the link in my signature.

 

[Enquerencia]

So expansion only exists in empty space, between galaxies and clusters? Why would this be?

 

[phinds]

So expansion only exists in empty space, between galaxies and clusters? Why would this be?

Because at non-cosmological levels, expansion is SO incredibly tiny that it cannot even begin to overcome the forces of gravity, let alone forces like the strong force. This is discussed some in the link in my signature.

 

[mfb]

Some posts that derailed the thread, and replies to those posts, have been deleted.

 

[Matthew Smith]

There is no centre of the universe! According to the standard theories of cosmology, the universe started with a "Big Bang" about 14 thousand million years ago and has been expanding ever since. Yet there is no centre to the expansion; it is the same everywhere. The Big Bang should not be visualised as an ordinary explosion. The universe is not expanding out from a centre into space; rather, the whole universe is expanding and it is doing so equally at all places, as far as we can tell.

In 1929 Edwin Hubble announced that he had measured the speed of galaxies at different distances from us, and had discovered that the farther they were, the faster they were receding. This might suggest that we are at the centre of the expanding universe, but in fact if the universe is expanding uniformly according to Hubble's law, then it will appear to do so from any vantage point.

http://math.ucr.edu/home/baez/physics/Relativity/GR/centre.html