Inhomogeneous cosmology
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An inhomogeneous cosmology is a physical cosmological theory (an astronomical model of the physical universe's origin and evolution) which, unlike the dominant cosmological concordance model, postulates that inhomogeneities in the distribution of matter across the universe affect local gravitational forces (i.e., at the galactic level) enough to skew our view of the Universe. When the universe began, matter was distributed homogeneously, but over billions of years, galaxies, clusters of galaxies, and superclusters coalesced. Einstein's theory of general relativity states that they warp the space-time around them.
While the concordance model acknowledges this fact, it assumes that such inhomogeneities are not sufficient to affect large-scale averages of gravity observations. Two studies claimed in 1998-1999 that high redshift supernovae were further away than the distance predicted by calculations. It was suggested that the expansion of the universe was accelerating, and dark energy, a repulsive energy inherent in space, was proposed as an explanation. Dark energy became widely accepted, but remains unexplained. Inhomogeneous cosmology falls into the class of models that might not require dark energy.
Inhomogeneous cosmologies assume that the backreactions of denser structures and those of empty voids on space-time are significant. When not neglected, they distort understanding of time and observations of distant objects. Burchert's equations in 1997 and 2000 derive from general relativity, but allow for the inclusion of local gravitational variations. Alternative models were proposed under which the acceleration of the universe was a misinterpretation of astronomical observations and in which dark energy is unnecessary. For example, in 2007, David Wiltshire proposed a model (timescape cosmology) in which backreactions caused time to run more slowly or, in voids, more quickly, thus leading supernovae observed in 1998 to be thought to be further away than they were. Timescape cosmology may also imply that the expansion of the universe is in fact slowing.