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In physical cosmology, fractal cosmology is a set of minority cosmological theories which state that the distribution of matter in the Universe, or the structure of the universe itself, is a fractal across a wide range of scales (see also: multifractal system). More generally, it relates to the usage or appearance of fractals in the study of the universe and matter. A central issue in this field is the fractal dimension of the universe or of matter distribution within it, when measured at very large or very small scales. ==Fractals in observational cosmology== The first attempt to model the distribution of galaxies with a fractal pattern was made by Luciano Pietronero and his team in 1987, and a more detailed view of the universe’s large-scale structure emerged over the following decade, as the number of cataloged galaxies grew larger. Pietronero argues that the universe shows a definite fractal aspect over a fairly wide range of scale, with a fractal dimension of about 2. The fractal dimension of a homogeneous 3D object would be 3, and 2 for a homogeneous surface, whilst the fractal dimension for a fractal surface is between 2 and 3. The ultimate significance of this result is not immediately apparent, but it seems to indicate that both randomness and hierarchal structuring are at work on the scale of galaxy clusters and larger. The universe has been observed to be homogeneous and isotropic (i.e. is smoothly distributed) at very large scales, as is expected in a standard Big Bang or FLRW cosmology, and in most interpretations of the Lambda-Cold Dark Matter model. The scientific consensus interpretation is that the Sloan Digital Sky Survey (SDSS) suggests that things do indeed smooth out above 100 Megaparsecs. One study of the SDSS data in 2004 found "The power spectrum is not well-characterized by a single power law but unambiguously shows curvature...thereby driving yet another nail into the coffin of the fractal universe hypothesis and any other models predicting a power-law power spectrum". Another analysis of luminous red galaxies (LRGs) in the SDSS data calculated the fractal dimension of galaxy distribution (on a scales from 70 to 100 Mpc/h) at 3, consistent with homogeneity; but that the fractal dimension is 2 "out to roughly 20 Mpc/h". In a paper of 2008 entitled "Parabolic drift towards homogeneity in large-scale structures of galaxies" (Physica, A, 387: 3641-3646) D. Queiros-Conde showed that large-scale structure of galaxies are much better described by a scale-dependent fractal dimension drifting from zero to three at a scale around 55 Mpc/h in the context of a "scale-entropy diffusion equation". It gives a way to estimate the number of galaxies in the universe in agreement with Hubble measurements. Moreover, fractal dimension is varying linearly with scale-logarithm. This means that the geometry of galaxy distribution is a "parabolic fractal". Two years later, the same author with M. Feidt showed that the fractal dimension 2 found in numerous studies can be explained as being a problem of measurement. In 2012, Scrimgeour et al. definitively showed that large-scale structure of galaxies was homogeneous beyond a scale around 70 Mpc/h, close to the value found by D. Queiros-Conde. In 2013, astronomers discovered a large quasar group (LQG) that is 1.6 billion light-years in diameter, far larger than allowed by the cosmological principle, which asserts that the universe should be homogeneous at scales this large. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「fractal cosmology」の詳細全文を読む スポンサード リンク
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