Center of the universe

superdave

So every physics/astro student is told there is no center of the universe, imagine it like a balloon and every point expands equally from every point.

Except, there is a center of a balloon. Right in the middle where the air is. The universe isn't the flat surface of a balloon, it is 3 dimensions and expands in 3 dimensions.

Now, if we are measuring from Earth, and pick a direction, there must be a galaxy out there that is the farthest from Earth. Unless space curves completely back in on itself. There is some debate about that. Now, this galaxy is likely too far for us to actually ever see, but it exists.

And if we measure in every direction, there should be similar 'farthest galaxies'. The distances to these farthest galaxies won't necessarily be the same. But there should be a point in space where the distance to these 'farthest galaxies' is roughly the same in all directions. That point would then be the center of the universe.

I'm sure this has been thought of and holes poked through it, but I'd like to hear others' thoughts.

mgb_phys

The balloon analogy is for a 2d universe.
Imagine you are a 2d flat creature on the surface of the balloon there is no centre.

xxChrisxx

It all depends on definitions really. There is a centre of the universe with regard to the fact that it's the point where it expanded from.

The centre doesn't exist in any of the three dimensions we can comprehend, so it's all a bit moot.

WhoWee

If a center could be identified, wouldn't it need to be plotted over time?

Molydood

I have always struggled with this one
I am not arrogant enough to challenge the statement that there is no centre, as I have heard it stated before, and I am sure by much brainier people than me, but in the interests of discussion and provoking the right answers via my own stupidity, here is my (incorrect) thought process on it anyway:

At t=0s, all matter is in the centre
At t=1s all matter is heading off in different directions
At t=1s some matter is at one side of the universe, and some matter is on another side of the universe
At t=1s the universe now has a finite size with edges/boundaries

Therefore, why is there not a point within the universe, perhaps occupied by nothing, that is no more towards one side than the other? And the same in all dimensions thus a true 3 dimensional centre point?

xxChrisxx

But its 3D spacetime that is expanding. You could set off in 1 direction, and so long as you went straight you'd end up back where you started.

Just like blowing up the balloon with the 2D universe, for the 2D creatures there is no centre in the 2 dimensions they are aware of.

Lenth, width.

To be aware of the fact the centre of the baloon lies in a 3rd dimension, you need to be aware of depth.


Moving this concept to a 3D universe. The centre lies in an nth spatial dimension.

Length, width, height, sometihng else.


It acutally can't have centre defined by the 3 dimensions.

Schlofster

there are certain parts of the analogy that are not important - I discussed it here:
http://physicsforums.com/showpost.ph...79&postcount=6

Nebozilla

I think its because our perception of what the big bang was is whats confusing. It wasnt an explosion with a center but it was the expansion of space itself. Also you couldnt even find a center since there is no definite point of complete rest. Its all relative.

bannerman100

Wasn't the three dimensional expansion of the universe discovered by observing the red shift, then logically reckoning that by working backwards, everything must have come from some definite point in space ?
Big Bang matter and light radiated outward in all directions, so there must be a very big dark hole left behind. (Just kidding. Or am I ? ;)

Light emittted from the Big Bang must still be expanding the universe by continually creating more space (at the speed of light), and that light is continually creating the ever expanding "edge" of the universe. Physical matter (unable to keep up with C) is surely being left far behind.

May I take the original question a little further ?

We know gravity can bend light, but can it slow it down ? Yes, because black holes stop it entirely. So the gravitic pull of the mass of the universe must be slowing down its expansion by decelerating the light from the Big Bang. (Imagine gravity's effect if you travelled from the Earth's centre to it's surface.)
So the further you are from the centre of the universe, yet still within the sphere of expanding matter, the slower light will travel in the outward direction. (Until it goes beyond the mattersphere.)(Did I just invent a new word ?)(I bet someone got there first.)
But does this gravity also speed up light going toward the centre of the universe ?

If so, could we construct an instrument that would compare the two speeds, and thereby indicate our distance from the centre ?

If it's not actually heresy, can I ask:
Has anyone figured out which direction from our solar system is "inward", back toward the origin of the Big Bang ?
Or does that question naively ignore the extra dimensions ?

Dmitry67

no, working backwards, all objects were closer and closer, and everyhting was more and more dense
BUT
there is no 'definite point in space'

So the rest of your post is wrong.

mgb_phys

No - this is a fundemental (and common) misunderstanding of the big bang.

There wasn't an empty black space where everything exploded out into.
The big bang created the space.
It was the actual universe (ie space-time and everything) being created and expanding

bannerman100

So where is the universe expanding from ? The concept of expansion requires a centre as a starting point.

Dmitry67

No it does not.

Imagine INFINITE line (or surface).
Now you expand (or contract it, if we "go backwards")
You divide all distances by 1000, and now all objects are 1000 times closer. But still it is infinite.
Go go backwards even deeper, dividing all by 10000000000000000000. Now it is extremely dense and still it is infinite and there is no 'center'.

bannerman100

But that would mean there was no Big Bang, locatable in time, and everything just expanded, and always has been expanding, from an infinitely small space ?

That would mean time would have no beginning.

Please patiently correct me if I have misunderstood you.

Dmitry67

Why?
Big Bang has definite location in time, but not in space.

If Universe is Infinite now (which is very likely) then it was ALWAYS infinite.

here si a mathematical model
At t=1 we have an infinite line (all real numbers) from -infinity to +infinity
Going back in time, say, t=0.01 we have all distances 100 times smaller, but our line is infinite.
It is also infinite at t=0.00000001 or t=0.00000000000000000001
So it was always infinite at any t>0
However, the location in time of Big Bang is clear, it is at t=0.

Hoku

Saying that "the big bang created space" is a little presumptious. I know that many people are using that as an *assumption* based on relativity, but the idea is only an interpretation - and it may not be right. This is one of the things that the Gravity Probe B is out to answer.

James Overduin, currently assistant professor at Towson University in Maryland, is keeping in close contact with the project and has published the statement that, "spacetime behaves relationally but exists absolutely", based on information that is coming in from the probe.

On another note, I don't understand why people make the statement that time existed before space did. Why do we constantly forget that spacetime is a singularity? You cannot isolate one from the other. (This is another problem with Verlinde's idea. He based it on the assumption that time existed before space).

Chronos

Eternal time is another confounding fallacy, time is irrelevant prior to the big bang. As Einstein noted, time is what clocks measure. No clocks, no time.