Curved space

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[edit] Overview

Curved space often refers to a spatial geometry which is not “flat” where a flat space is described by Euclidean Geometry. Curved spaces can generally be described by Riemannian Geometry though some simple cases can be described in other ways. Curved spaces play an essential role in General Relativity where gravity is often visualized as curved space. The Friedmann-Lemaître-Robertson-Walker metric is a curved metric which forms the current foundation for the description of the expansion of space.

[edit] Simple 2D Example

A very familiar example of a curved space is the surface of a sphere. While to our familiar outlook the sphere looks three dimensional, if an object is constrained to lie on the surface, it only has two dimensions that it can move in. The sphere’s surface can be completely described by two dimensions.

[edit] Embedding

In a flat space, the sum of the squares of the side of a triangle is equal to the square of the hypotenuse. This relationship does not hold for curved spaces.
In a flat space, the sum of the squares of the side of a triangle is equal to the square of the hypotenuse. This relationship does not hold for curved spaces.

One of the defining characteristics of a curved space is its departure with the Pythagorean theorem. In a curved space dx^2 + dy^2 \neq dl^2.

The Pythagorean relationship can often be restored by describing the space with an extra dimension. Suppose we have a non-euclidean three dimensional space with coordinates (x',y',z'). Because it is not flat dx'^2 + dy'^2 + dz'^2 \ne dl'^2. But if we now describe the three dimensional space with four dimensions (x,y,z,w) we can choose coordinates such that dx2 + dy2 + dz2 + dw2 = dl2. Note that the coordinate x is not the same as the coordinate x'.

For the choice of the 4D coordinates to be valid descriptors of the original 3D space it must have the same number of degrees of freedom. Since four coordinates have four degrees of freedom it must have a constraint placed on it. We can choose a constraint such that Pythagorean theorem holds in the new 4D space. That is: x2 + y2 + z2 + w2 = constant. The constant can be positive or negative. For convenience we can choose the constant to be κ − 1R2 where R2 now is positive and \kappa \equiv \plusmn 1.

We can now use this constraint to eliminate the artificial fourth coordinate w. The differential of the constraining equation is xdx + ydy + zdz + wdw = 0 leading to dw = − w − 1(xdx + ydy + zdz).

Plugging dw into the original equation gives

dl^2 = dx^2 + dy^2 + dz^2 + \frac{(xdx+ydy+zdz)^2}{\kappa^{-1}R^2 - x^2 - y^2 - z^2}.

This form is usually not particularly appealing and so a coordinate transform is often applied: x = rsinθcosφ, y = rsinθsinφ, z = rcosθ. With this coordinate transformation

dl^2 = \frac{dr^2}{1-\kappa\frac{r^2}{R^2}} + r^2d\theta^2 + r^2\sin^2\theta d\phi^2.

[edit] Open,Flat,Closed

An isotropic and homogenous space can be described by the metric:

dl^2 = \frac{dr^2}{1-\kappa\frac{r^2}{R^2}} + r^2d\theta^2 + r^2\sin^2\theta d\phi^2.

In the limit that the constant of curvature (R) becomes infinitely large, a flat, Euclidean space is returned. It is essentially the same as setting κ to zero. If κ is not zero the space is not Euclidean. When κ = + 1 the space is said to be “closed” or elliptic. When κ = − 1 the space is said to be “open” or hyperbolic.

Triangles which lie on the surface of an open space will have a sum of angles which is greater than 180°. Triangles which lie on the surface of a closed space will have a sum of angles which is less than 180°. The surface area of a sphere in curved space is 4πr2. The volume, however, is not (4 / 3)πr3.

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