De Sitter space

In mathematics and physics, n-dimensional de Sitter space, denoted , is the Lorentzian analog of an n-sphere (with its canonical Riemannian metric). It is a maximally symmetric,  Lorentzian manifold with constant positive curvature, and is simply-connected for n at least 3. In the language of general relativity, de Sitter space is the maximally symmetric, vacuum solution of Einstein's field equation with a positive cosmological constant . When n = 4, it is also a cosmological model for the physical universe; see de Sitter universe.De Sitter space was discovered by  Willem de Sitter, and independently by Tullio Levi-Civita (1917).
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