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The term ''cosmic variance'' is the statistical uncertainty inherent in observations of the universe at extreme distances. It has three different but closely related meanings: * It is sometimes used, incorrectly, to mean sample variance - the difference between different finite samples of the same parent population. Such differences follow a Poissonian distribution, and in this case the term sample variance should be used instead. * It is sometimes used, mainly by cosmologists, to mean the uncertainty because we can only observe one realization of all the possible observable universes. For example, we can only observe one Cosmic Microwave Background, so the measured positions of the peaks in the Cosmic Microwave Background spectrum, integrated over the visible sky, are limited by the fact that only one spectrum is observable from Earth. The observable universe viewed from another Galaxy will have the peaks in slightly different places, while remaining consistent with the same physical laws, inflation, etc. This second meaning may be regarded as a special case of the third meaning. * The most widespread use, to which the rest of this article refers, reflects the fact that measurements are affected by cosmic large-scale structure, so a measurement of any region of sky (viewed from Earth) may differ from a measurement of a different region of sky (also viewed from Earth) by an amount that may be much greater than the sample variance. This most widespread use of the term is based on the idea that it is only possible to observe part of the universe at one particular time, so it is difficult to make statistical statements about cosmology on the scale of the entire universe, as the number of observations (sample size) must be too small. == Background == The standard big bang model is usually supplemented with cosmic inflation. In inflationary models, the observer only sees a tiny fraction of the whole universe, much less than a billionth (1/109) of the volume of the universe postulated in inflation. So the observable universe (the so-called particle horizon of the universe) is the result of processes that follow some general physical laws, including quantum mechanics and general relativity. Some of these processes are random: for example, the distribution of galaxies throughout the universe can only be described statistically and cannot be derived from first principles. 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「Cosmic variance」の詳細全文を読む スポンサード リンク
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