# Center of the Universe?

Allday
I don't know much about cosmology, but I just went to a talk that presented the current knowledge about the big bang and I had a question that I didn't get to ask because of the time constraints, so here it is.

The Hubble red shift data was presented as evidence that the universe is expanding in every direction with a velocity that increases the farther away you look. This implies that the universe was denser and hotter in the past. My question is, can you use the rate at which the velocity changes in different directions to find an area that it is all expanding away from?

In other words wouldn't the velocity be increasing quicker (as a function of distance) along a direction pointing away from the center than it would along the direction pointing towards the center?

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Gabriel

Staff Emeritus
Gold Member
Welcome to Physics Forums, Allday!

Originally posted by Allday
I don't know much about cosmology...

More than most, it seems!

My question is, can you use the rate at which the velocity changes in different directions to find an area that it is all expanding away from?

In other words wouldn't the velocity be increasing quicker (as a function of distance) along a direction pointing away from the center than it would along the direction pointing towards the center?

You may expect that if the universe had a center in 3D space...but no such center exists.

The expansion of space is essentially the same throughout. But the farther away something is from you, the more expanding space there is between you and that thing...so the faster it seems to be moving away.

Consider 3 galaxies in a line with equal distances.
A - B - C

The whole line is expanding. Here's the next step...
A - - B - - C

If you're on Galaxy A, it looks like Galaxy B is now 2 units away. It also looks like Galaxy C is 4 units away. So it seems that C has moved away from you twice as fast as B. But that is just from your perspective.

To observer B, both have moved away from him only 2 units.

Observer C has the opposite view of you...he thinks he is at rest and you (A) have moved away 4 units (twice as fast as B).

Now (simply) apply that same concept to infinite 3D space. From our perspective, other galaxies seem to be moving away in all directions. And more distant galaxies seem to be moving away at a faster rate (we can measure that by the redshift of the light from those galaxies). But those other galaxies see the same thing about us...moving away from them.

There's no center to 3D space and accordingly, we don't see skewed velocities pointing back to a central point. The whole universe is inflating...the trouble for our human brain, which is seeped in Earthly experiences where things have centers and edges, is to imagine this expanding infinity. But it may help to consider the 2D plane of the surface of the Earth. Where is that center? Where is the edge? There isn't one. Same idea for Space...only 3D.

Allday
Thanks for the welcome and the well written response Phobos that clears some things up. Here's a follow up,

Lets start with the 2-D model for ease of visualization and bring in one other concept that was explained in the lecture, the curvature of the universe. Now, the 2-D surface of the Earth is curved in 3-D space and hence the expansion (I'm picturing the Earth becoming larger and the surface area growing) has a center in 3-D space. It isn't accesible to any 2-D creatures on the surface (I suppose they couldn't even point to it yet alone reach it or have any contact with it for that matter), but mathematically it is there.
So 2-D curved space -> mathematical center

An infinite flat expanding plane, still no center.
So 2-D flat space -> no center

Is the difference between the curved and flat 2-D planes the difference between an open and closed universe?

Now to 3-D cases. I've read a couple of places that one of the wacky things about the universe is that the 3-D space is so close to flat that its uncanny because any small initial deviation would be amplified hugely by now. I assume when they say flat they mean on the grandest scale and that local regions are allowed to have huge curvatures if they are so inclined (around super massive bodies), so that an infinite flat 3-D universe has no definable center. Well and good, yet if there was any curvature at all would there be a center that could be defined in 4-D space? (good-bye powers of visualization) And then could one point (granted a point that only exists in the world of imagination) be labeled the center of the universe?

Thanks again,
Gabriel

Allday
Just saw meteor's post. Does a finite universe without boundry imply curvature? And hence a mathematical center in 4D-Space (Not the smae thing I think as the 4-Space of relativity but an extra mathematical spatial dimension)?

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Staff Emeritus
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My 2D analogy helps visualize the situation, but gives the incorrect impression of a center in a higher dimension (i.e., the center of the 2D surface is to be found in 3D space). So, someone's first thought would be that the center of 3D space is to be found in 4D space. But as far as can be shown, there are only 3 dimensions of space and 1 of time (a 4 dimensional universe of "Spacetime").

Yes, "flatness" is at the biggest scale. The cosmology lingo is that the universe is either "open", "closed", or "flat". If you shoot two parallel rays of light across a flat universe, they stay parallel (they eventually diverge in an open universe and converge in a closed universe). Open and flat universes are infinite (or at least "boundless"). Closed universes are finite (although you would never reach an edge because space would curve back on itself).

As meteor suggests, there is still a big debate about the finiteness/infiniteness of the universe. Try flipping through some of our archives or start a new topic for more discussions on this.

In the beginning, all the points of the universe, or at least the "visible universe", were closer together (with some mysterious singularity state at Time = 0). Now they're farther apart thanks to expansion. The Big Bang was the start of the expansion and, in effect, the Big Bang happened at every point in the universe (i.e., it was not an outward explosion of stuff...it was all of existence as we know it suddenly getting farther apart).

There's much more of your questions to be answered. I'm sure some other brilliant PF members will be along shortly to chime in.

dcl
I don't suppose they have noticed if we as a solar system or galaxy are moving 'closer' to any other major body in space?

Gold Member
Dearly Missed
http://archive.ncsa.uiuc.edu/Cyberia/Cosmos/HierarchUni.html

The Milky Way and Andromeda galaxies, along with about fifteen or sixteen smaller galaxies, form what's known as the Local Group of galaxies. The Local Group sits near the outer edge of an enormous supercluster, the Virgo cluster. What's more, the Milky Way and Andromeda are moving toward each other, the Local Group is falling into the middle of the Virgo cluster, and the entire Virgo cluster itself, along with a second supercluster, is speeding toward some unfathomable mass known only as "The Great Attractor."
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no need to worry yet

Staff Emeritus
Gold Member
Well said, wolram.
And yet, it's worth noting, even the Great Attractor is not the center of the universe.

Welcome to Physics Forums, dcl!
The position of our solar system with respect to other objects is an interesting topic (the Milky Way galaxy does not rotate as a rigid body...the stars move about at different velocities and different orbital inclinations)
...but is separate from the question of the center of the universe. Perhaps you could start a new topic on that.

WyldFyr
Hi, just read the post about 4D spaces, and I think I might actually be able to add something.
When trying to imagine a true 4D+ space, I like to think of the kind of scenario you might see on the twilight zone, where some people find themselves stuck in a furnished house that has a snow globe with a identical house within it. They discover much to their horror that the house in the globe and the house they are in are one and the same. Putting the details aside (like what happens when they walk outside), let's imagine some scientists with hitech laser modulating and recording equipment get stuck inside. They use their tools to verify what I just said by sending modulated pulses through the "snow globe" in a way that makes the path both linear and circular at the same time. Since the paths can be made at right angles (yes, there's a skylight!) but never cross, I conclude there must be more than 3 spatial dimensions. I think the scientists might conclude something different, however. Maybe a very curved, closed 3D space. I'm really not sure if I'm correct, this is just how I've visulized it.
Could this be applied some how to the way we see our 'flat universe'?
Or has that discussion come and gone?

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dcl
Thanks Phobos :)

Also thanks wolfram for the link. Found it to be a good read.