The cosmic microwave background that we observe uniformly around us is usually explained by assuming that our universe is the surface of a four dimensional sphere. That way the uniformity makes sense since there is no center. My question is if this is true then what is the explanation that describes the fact that the farther we look into space, the further we look back in time. I can't perfectly picture this and see how it would coexist. Help me out.
5
-
$\begingroup$ The microwave background is uniform and there is no center in any of the four spatial geometries of the Friedmann-Robertson-Walker family of solutions. The sphere is just one of those. The universe is very close to flat as far as we can see, so there's no strong reason to believe it's a sphere (and it might now be an FRW on the very large scale). $\endgroup$– Stan LiouCommented Apr 16, 2014 at 0:47
-
$\begingroup$ (v1) Could you clarify: how would your question not be answered by "there is a speed limit in the universe, $c$, so any signal you get from anything far away has been emitted in the past"? This is true regardless of the shape of the universe, so I'm confused as to why you bring it up. $\endgroup$– Stan LiouCommented Apr 16, 2014 at 0:51
-
$\begingroup$ I am just trying to picture a our universe as the surface of a 4- D sphere while realizing that the deeper in space we look, the further we see in time. It doesn't seem to make sense because it would have to have layers of a surface in order for us to see ancient light in all 3-dimensions as we do. $\endgroup$– user3138766Commented Apr 16, 2014 at 0:53
-
$\begingroup$ This is true but it's difficult to see everything coexisting together. If our universe is the surface of a 4-D sphere, then the CMBR makes sense. I guess it's difficult to see how it would work that's all. I am tearing my hair out trying to visualize in my head our universe being the surface of a 4-D sphere. I guess you could see it as a liquified form of our universe wrapping around this sphere, while maintaining all three of its spacial dimensions. Then every property we see today can be mirrored with this surface. $\endgroup$– user3138766Commented Apr 16, 2014 at 1:01
-
1$\begingroup$ The universe can't be the surface of 4D sphere because it's four dimensional. It can't be the surface of a 5D sphere because it's pseudo-Riemannian. $\endgroup$– John RennieCommented Apr 16, 2014 at 9:18
Add a comment
|
1 Answer
$\begingroup$
$\endgroup$
Just because space is curved, it does not mean it is a subspace of a higher dimension. All space is curved, even euclidean space. Flatness is a relation between a curvature of some subspace and the space it is part of.
There is the wonderful image of space expanding equally as one might blow up a balloon, but this image might be as easily explained by "things shrinking".
Mind you, when they say light travels in a straight (ie flat) line, it travels on geodesics of the space, and not some eclidean chord. In any case, an expanding space would give rise to the same as one sees of an ant on an expanding baloon. The speed in inches might be constant, but there are less degrees to the inch, so it should slow down in angle measure.
