Flatness problem
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The flatness problem is a cosmological fine-tuning problem within the Big Bang model of the universe. Measurements find the current universe close to perfectly flat and expansion of the universe incre
The local geometry of the universe is determined by whether the relative density Ω is less than, equal to or greater than 1. From top to bottom: a spherical universe with greater than critical density (Ω>1, k>0); a hyperbolic, underdense universe (Ω<1, k<0); and a flat universe with exactly the critical density (Ω=1, k=0). The spacetime of the universe is, unlike the diagrams, four-dimensional.
General relativity
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General relativity, also known as the general theory of relativity, and as Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in May 1916 and is the acce
Einstein cross: four images of the same astronomical object, produced by a gravitational lens
Artist's impression of the space-borne gravitational wave detector LISA
This blue horseshoe is a distant galaxy that has been magnified and warped into a nearly complete ring by the strong gravitational pull of the massive foreground luminous red galaxy.
Projection of a Calabi–Yau manifold, one of the ways of compactifying the extra dimensions posited by string theory