|
Knowledge of Earth's location in the Universe has been shaped by 400 years of telescopic observations, and has expanded radically in the last century. Initially, Earth was believed to be the center of the Universe, which consisted only of those planets visible with the naked eye and an outlying sphere of fixed stars. After the acceptance of the heliocentric model in the 17th century, observations by William Herschel and others showed that the Sun lay within a vast, disc-shaped galaxy of stars.〔(【引用サイトリンク】url=http://cosmology.carnegiescience.edu/timeline/1781 )〕 By the 20th century, observations of spiral nebulae by Edwin Hubble revealed that our galaxy was one of billions in an expanding universe,〔(【引用サイトリンク】publisher=Eberly College of Science )〕〔(【引用サイトリンク】publisher=Carnegie Institution for Science )〕 grouped into clusters and superclusters. By the end of the 20th century, the overall structure of the visible universe was becoming clearer, with superclusters forming into a vast web of filaments and voids.〔(【引用サイトリンク】publisher=Carnegie Institution for Science )〕 Superclusters, filaments and voids are the largest coherent structures in the Universe that we can observe. At still larger scales (over 1000 megaparsecs) the Universe becomes homogeneous meaning that all its parts have on average the same density, composition and structure. Since there is believed to be no "center" or "edge" of the Universe, there is no particular reference point with which to plot the overall location of the Earth in the universe.〔. P. 55.〕 Because the observable universe is defined as that region of the Universe visible to terrestrial observers, Earth is, by definition, the center of the observable universe. Reference can be made to the Earth's position with respect to specific structures, which exist at various scales. It is still undetermined whether the Universe is infinite. There have been numerous hypotheses that our universe may be only one such example within a higher multiverse; however, no direct evidence of any sort of multiverse has ever been observed, and some have argued that the hypothesis is not falsifiable. ==See also== *Orders of magnitude (length) *''Cosmic View'' *''Cosmic Zoom'' *''Pale Blue Dot'' *''Powers of Ten (film)'' *''Galaxy Song'' 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「Earth's location in the Universe」の詳細全文を読む スポンサード リンク
|