![]() They assign constant spatial coordinate values to observers who perceive the universe as isotropic. Comoving coordinates are an example of such a natural coordinate choice. ![]() This model of the universe includes dark energy which causes an accelerating expansion after a certain point in time, and results in an event horizon beyond which we can never see.Īlthough general relativity allows the formulation of the laws of physics using arbitrary coordinates, some coordinate choices are more natural or easier to work with. ![]() The x-axis is distance, in billions of light years the left-hand y-axis is time, in billions of years since the Big Bang the right-hand y-axis is the scale factor. At other times, the Universe's expansion results in the proper distance changing, while the comoving distance remains constant.Ĭomoving coordinates The evolution of the universe and its horizons in comoving distances. Comoving distance and proper distance are defined to be equal at the present time. Proper distance roughly corresponds to where a distant object would be at a specific moment of cosmological time, which can change over time due to the expansion of the universe. Comoving distance factors out the expansion of the universe, giving a distance that does not change in time due to the expansion of space (though this may change due to other, local factors, such as the motion of a galaxy within a cluster). In standard cosmology, comoving distance and proper distance (or physical distance) are two closely related distance measures used by cosmologists to define distances between objects.
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