Shape of the universe

I brought this up in another thread, but no-one bit. Has anyone bothered to map the furthest object in all directions and thus get an outline of the shape of the universe?
Im guessing that the big bang would make a sphere, but wouln’t there be a mega void towards the middle? and what about hawkings big beer theory?
You’d think that theory would have been hatched after midnight is a college bar. “ish nont a bang, ishmor like a … hey whersh my beer? I lefn it rine heere ness to Steve.”

Well, there is no edge to the universe, so it is a bit hard describe a shape in that sense. Depending on the mass of the universe, there are different 4D “shapes” that space-time can take.

The pretty one, which used to be the most widely believed one, is the closed universe geometry. The closed universe geometry would be akin to a 4D sphere. This shape occurs if the universe has sufficient mass to make the curvature of space-time curl back on itself. In this geometry, parallel lines eventually converge. This universe would eventually contract and collapse in a “big crunch”.

The wacky one, which is now the most widely believed one, is a 4D analog to a hyperboloid of one sheet. In this universe, space is infinite in reach, however it is not infinite in volume. In this geometry, parallel lines eventually diverge. This universe will expand forever. Most cosmologists agree at this point that the evidence at hand strongly suggests this geometry one is the true one.
There is no “middle” to the universe. The analogy that generally gets this point across is the balloon analogy (which unfortuately describes the closed universe, but close enough). The universe starts as a point, but as it expands, it gets to be like the surface of an expanding balloon. As the surface expands, things start to get farther and farther away from each other, but nothing is being blown away from one place to another. Space itself is becoming larger, like the balloon. No one on the surface of the balloon is at any sort of middle point. It’s likewise for the universe.

we know where the most distant detectable object in the universe is, or was 10billion years ago. We can
A) assume that this is the edge of the mass regeon in that direction.
b) sit around and make stuff up. There might be stuff out there so young the light hast got here yet, maybe not. Doesnt matter for now. We dont have time to sit around a postulate everything in the univers that might be. So for this discussion lets assume anything beyond the known mass region is irrelevent.
Hyperbolic topology theories asside, lets not worry about time just yet. We can look through a telescope and get a good idea that distance of an object. If divide the sky into quadrants, and find the most distanct object in that quadrant this will describe a 3 dimentional shape. Factoring in the speed and distance we could even SWAG where the object currently is today. If this hyperwazit theory is true, that we should apear to be in the center. (Welcome back to the 1400’s)

Although my brain has trouble imagining 3 dimensions as a sheet, if this were true, we should be able to look in different directions and see the same object.

Ayup. And we do. :slight_smile:

Minor local variations aside, the density of “stuff” we see in the universe is the same in all directions. That observation has been a critical piece of evidence to support the Friedmann models of the universe which include the two models I described. The theory behind the Friedmann models requires that the universe be totally smooth, with variations kept to the minimal level that is required by the uncertainty principle.

'cept that generally speaking, light would need to travel for longer than the universe has existed for this to happen. There is one case in which we do, in a sense, see light from the same thing in different directions. We can detect what is believed to be the leftovers of the big bang. This “background” radiation has weakened to the point of being found in the microwave range. This radiation seems to come from all directions, which makes sense, since everything was present at the big bang.

Ok, then how does this happen?

Ok so is Time the only other dimention in this manifold, or are thier other mystery dimentions?

Re: Galactic collisions

On the local level, gravitation can overwhelm the effects of the expansion of the universe. Universal expansion doesn’t pull us away from the Sun for example. Galaxies often collect in clusters that are essentially galxies orbiting each other. The mutual gravitation of these clusters is strong enough to keep them together despite the expansion of the universe.

Re dimensions

General relativity only describes 4 dimensions (space + time). Any consideration of other dimensions will hafta be left for newer theories, such as the wacky, mebbe-there-will-actually-be-some-evidence-for-it-someday superstrings theory.

There is absolutely nothing to be gained from comparing the big bang to a conventional explosion. The BB resulted in an expansion of space, it did not throw matter outwards from a central location.

All points in the observable universe were at the same place before the bang, and all points in the universe are at the center of the original “explosion”.


(For those that didn’t look at the link, it shows two colliding galaxies). Because the hubble constant is about 65 km/s/megaparsec, which for objects close to each other, is not a large value. It’s not overly hard for local gravitational effects to attract to nearby galaxies to each other - it happens all the time.

I think that most current cosmological theories agree that we can only observe a small fraction of the total universe. In fact, if I understand them right, we’re not even causally connected to the whole thing.

Not only do WE appear to be in the center, but every point in the universe appears to be in the center as well, so it doesn’t really put us in a special place like the earth-centric theories of the 1400’s did.

You might check out Ned Wrights cosmology tutorial, which is here: . It has a lot of answers to stuff like this.

peas on earth

As Douglas Adams said (paraphrased):

“The universe isn’t curved; it’s totally bent.”

Ok so the crossection of the spacetime at the present leaves only 3 dimentions so it cant possibly be bent baloonwise or saddlewise.

this also does not jive with the unbent space of the present.

I’m a little unclear on how this works too. I know that the 4D examples are designed merely for concept and not for accuracy. The dimension of time is not the same as space dimensions. It is not truly independent of the spacial dimensions.

The reality of space-time curvature is represented in the effects of gravitation. In general relativity, the bent course of light around a star is the direct path in curved space. In general relativity, gravitation is not represented as a force at all, but rather the tendency for mass to cause space-time curvature. The state of freefall (including orbits) is considered to be a non-accelerated (i.e. inertial) reference frame in curved space.