"Scientists have measured space to be flat to high accuracy"

[sevensages]

TL;DR

Space is three dimensional. So how can space be flat?

In Max Tegmark's book Our Mathematical Universe, Tegmark wrote the following: "As we saw in the last chapter, we've measured our space to be flat to high accuracy" (99). Since space is three dimensional, I am totally baffled by Tegmark's statement that scientists have measured space to be flat to high accuracy. Since space is three dimensional, how can space be flat?

 

[Ibix]

Flat in this context means that it obeys the rules of Euclidean geometry - for example, if you draw a triangle its internal angles will add to 180°. If we lived in a "closed" or "open" universe (the other two possibilities aside from flat) the angles in sufficiently large triangles would add to more than or less than 180°.

 

[sevensages]

Flat in this context means that it obeys the rules of Euclidean geometry - for example, if you draw a triangle its internal angles will add to 180°. If we lived in a "closed" or "open" universe (the other two possibilities aside from flat) the angles in sufficiently large triangles would add to more than or less than 180°.

That definitely makes sense. Thank you for your answer. I would have never figured it out on my own.

 

[sevensages]

Flat in this context means that it obeys the rules of Euclidean geometry - for example, if you draw a triangle its internal angles will add to 180°. If we lived in a "closed" or "open" universe (the other two possibilities aside from flat) the angles in sufficiently large triangles would add to more than or less than 180°.

If the angles of a large triangle added up to more than 180 degrees or less than 180 degrees, meaning that we live in a close or an open universe, would that mean that the universe does not have three dimensions?

 

[Ibix]

If the angles of a large triangle added up to more than 180 degrees or less than 180 degrees, meaning that we live in a close or an open universe, would that mean that the universe does not have three dimensions?

No. You can measure curvature without reference to any higher-dimensional space that the curved surface might be embedded in - so the fact you can measure curvature by measuring angles in triangles doesn't necessarily mean that the surface is embedded in a higher dimensional space.

Spacetime is a four dimensional structure (a "manifold"). Space at one instant of time is a three dimensional "slice" through that four dimensional manifold. There is a lot of flexibility in how you choose to do the slicing, so "space" is a lot more subjective than spacetime. However, specifically in the kinds of spacetimes used in cosmology, there's only one sensible way to do the slicing and the geometry of the slices you get (flat, closed, or open) turns out to depend on the densities of matter and other stuff like radiation and dark energy.

Space, therefore, is a 3d manifold embedded in a 4d manifold. Spacetime is a 4d manifold that isn't embedded in anything as far as we know. Both can be curved, but it turns out that spacetime is curved and space (at least in cosmological models and cosmological conventions) isn't.

So to answer your question, depending on whether you meant space or spacetime when you wrote "universe", the universe is either three or four dimensional. If you meant space, yes it's part of a 4d structure, but if you meant spacetime then no it's not embedded in any higher dimensional space. Either way, the curvature or lack thereof doesn't imply anything about higher dimensions.

Sorry that's such a delightful mess of a yes-and-no.

(Note: there are a lot of theories that do require more than four dimensions, but we don't know which of them, if any, is correct. My answers are all based on standard GR cosmology - anybody telling you about 5+ dimensions is talking about much more speculative theories, however confident they sound.)

 

[sevensages]

No. You can measure curvature without reference to any higher-dimensional space that the curved surface might be embedded in - so the fact you can measure curvature by measuring angles in triangles doesn't necessarily mean that the surface is embedded in a higher dimensional space.

Spacetime is a four dimensional structure (a "manifold"). Space at one instant of time is a three dimensional "slice" through that four dimensional manifold. There is a lot of flexibility in how you choose to do the slicing, so "space" is a lot more subjective than spacetime. However, specifically in the kinds of spacetimes used in cosmology, there's only one sensible way to do the slicing and the geometry of the slices you get (flat, closed, or open) turns out to depend on the densities of matter and other stuff like radiation and dark energy.

Space, therefore, is a 3d manifold embedded in a 4d manifold. Spacetime is a 4d manifold that isn't embedded in anything as far as we know. Both can be curved, but it turns out that spacetime is curved and space (at least in cosmological models and cosmological conventions) isn't.

So to answer your question, depending on whether you meant space or spacetime when you wrote "universe", the universe is either three or four dimensional. If you meant space, yes it's part of a 4d structure, but if you meant spacetime then no it's not embedded in any higher dimensional space. Either way, the curvature or lack thereof doesn't imply anything about higher dimensions.

Sorry that's such a delightful mess of a yes-and-no.

(Note: there are a lot of theories that do require more than four dimensions, but we don't know which of them, if any, is correct. My answers are all based on standard GR cosmology - anybody telling you about 5+ dimensions is talking about much more speculative theories, however confident they sound.)

That is mighty interesting and informative.

What do the letters GR mean in "standard GR cosmology"?

 

[Jaime Rudas]

What do the letters GR mean in "standard GR cosmology"?

The most widely accepted cosmological model today is based on General Relativity (GR) and the cosmological principle that assumes that, at very large scales, the universe is homogeneous and isotropic.

 

[Hornbein]

That definitely makes sense. Thank you for your answer. I would have never figured it out on my own.

It might have been better to call it "straight space", because if not influenced by anything light goes in a straight line. Or maybe "parallel space", because two such beams of light that start out parallel remain parallel and the same distance apart. Or Euclidean space, because this is what was postulated by Euclid way back when.

 

[bob012345]

A mental model one can use is this, imagine you are standing on a smooth desert plain. It looks flat, very flat. But you are actually standing on a very large sphere, the Earth. The ground deviates from flat by about 8 cm per Km. If you walked a quarter way around the Earth, turned 90 degrees, walked the same distance, turned 90 degrees and walked again, you would close a triangle but the sum of the angles would be 270 degrees, not 180 degrees. A shorter walk of say 1 Km would make a triangle deviate from 180 degrees by a tiny fraction of a degree.