# What is the shape of the universe?

#### bwana

is the cmb radiation redshifted the same amount in ALL directions? as i understand the data, redshift is a function of distance between 2 points only(scalar quantity). is there a an angular component as well (vector quantity?)

does the fact that there is a 'dipole' mean that the universe is expanding more along a particular axis? in other words, the universe is shaped like a weiner (would Einstein laugh?) The ends of the weiner are further apart so light coming from the ends would be redshifted more than light from any other direction.

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#### phinds

Gold Member
is the cmb radiation redshifted the same amount in ALL directions?
yes

What is the shape of the universe?
unknown

does the fact that there is a 'dipole' mean that the universe is expanding more along a particular axis?
it is not; it is expanding the same everywhere except inside bound objects (galactic clusters and smaller) where it is not expanding at all.

All of this is cosmology 101 (or earlier)

Mentor

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#### phinds

Gold Member
Again, (as in the posts that have been deleted), you are talking about the structures in the universe, which is very interesting stuff but has nothing to do with the OP's question which is what is the shape of the universe. Do you understand the difference?

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#### rizheng1

Well, we can't verify the shape of our universe, but we can make good guesses. And in Ralph Rotten's defense, looking at structures inside the universe does gives us some clues to the shape of a larger system, with limitations ofc.

For example, from looking at our solar system and planets within it, we observed the interaction between gravity and angular momentum, or spin. From these observations and calculated approximations of mass and density of our galaxy both towards the galaxy center and away, we made a good guess of the shape of the Milky Way. We've never seen the entirety of our galaxy ever, but we can be pretty confident about our guess when there is a plethora of other galaxies to verify our models against and everything operates in 4 neat dimensions.

That being said, the Universe may or may not operate on the same principles as the things within it. For example, if the Cosmological Principle holds true and we use the same logic as before, we should be able to say that the universe is relatively close in size in each of its 3 spatial dimensions and that the Universe is not spinning, because matter is relatively distributed and it should not be the case if the Universe is spinning.
But if you think about it for another few minutes, you would realize this idea breaks in quite a few places. I'll leave finding those places to you as I found it quite fun to think about.

However, there are still pieces we can gleam from our observations that are likely to be useful.
What our current theories on the shape of the universe stem from stuff in Einstein's GR, the space-time curvature models portion of it to be more precise.

The overall answer is contingent on us getting a bunch of other things about our universe right, such as dimensionality, rate of change in the rate of expansion, etc.

"What is the shape of the universe?"

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