De Sitter space
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In mathematics and physics, a de Sitter space is the analog in Minkowski space, or spacetime, of a sphere in ordinary Euclidean space. The n-dimensional de Sitter space, denoted dSn, is the Lorentzian manifold analog of an n-sphere (with its canonical Riemannian metric); it is maximally symmetric, has constant positive curvature, and is simply connected for n at least 3. De Sitter space and anti-de Sitter space are named after Willem de Sitter (1872–1934), professor of astronomy at Leiden University and director of the Leiden Observatory. Willem de Sitter and Albert Einstein worked closely together in Leiden in the 1920s on the spacetime structure of our universe.
In the language of general relativity, de Sitter space is the maximally symmetric vacuum solution of Einstein's field equations with a positive cosmological constant (corresponding to a positive vacuum energy density and negative pressure). When n = 4 (3 space dimensions plus time), it is a cosmological model for the physical universe; see de Sitter universe.
More recently it has been considered as the setting for special relativity rather than using Minkowski space, since a group contraction reduces the isometry group of de Sitter space to the Poincaré group, allowing a unification of the spacetime translation subgroup and Lorentz transformation subgroup of the Poincaré group into a simple group rather than a semi-simple group. This alternative formulation of special relativity is called de Sitter relativity.
De Sitter space is the submanifold described by the hyperboloid of one sheet
where is some nonzero constant with dimensions of length. The metric on de Sitter space is the metric induced from the ambient Minkowski metric. The induced metric is nondegenerate and has Lorentzian signature. (Note that if one replaces with in the above definition, one obtains a hyperboloid of two sheets. The induced metric in this case is positive-definite, and each sheet is a copy of hyperbolic n-space. For a detailed proof, see geometry of Minkowski space.)
The isometry group of de Sitter space is the Lorentz group O(1, n). The metric therefore then has n(n + 1)/2 independent Killing vector fields and is maximally symmetric. Every maximally symmetric space has constant curvature. The Riemann curvature tensor of de Sitter is given by
This means de Sitter space is a vacuum solution of Einstein's equation with cosmological constant given by
The scalar curvature of de Sitter space is given by
For the case n = 4, we have Λ = 3/α2 and R = 4Λ = 12/α2.
We can introduce static coordinates for de Sitter as follows:
where gives the standard embedding the (n − 2)-sphere in Rn−1. In these coordinates the de Sitter metric takes the form:
Note that there is a cosmological horizon at .
where . Then in the coordinates metric reads:
where is the flat metric on 's.
Setting , we obtain the conformally flat metric:
where forming a with the standard metric . Then the metric of the de Sitter space reads
is the standard hyperbolic metric.
where s describe a . Then the metric reads:
Changing the time variable to the conformal time via we obtain a metric conformally equivalent to Einstein static universe:
where s describe a . Then the metric reads:
is the metric of an dimensional de Sitter space with radius of curvature in open slicing coordinates. The hyperbolic metric is given by:
This is the analytic continuation of the open slicing coordinates under and also switching and because they change their timelike/spacelike nature.
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