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The measure problem in cosmology concerns how to compute fractions of universes of different types within a multiverse. It typically arises in the context of eternal inflation〔 but can apply even with non-eternal inflation and a finite universe. The problem arises because different approaches to calculating these fractions yield different results, and it's not clear which approach (if any) is correct.〔 Measures can be evaluated by whether they predict observed physical constants, as well as whether they avoid counterintuitive implications, such as the youngness paradox or Boltzmann brains. While dozens of measures have been proposed,〔 few physicists consider the problem to be solved.〔 ==The problem== Infinite multiverse theories are becoming increasingly popular, but because they involve infinitely many instances of different types of universes, it's unclear how to compute the fractions of each type of universe.〔 Alan Guth put it this way:〔 : In a single universe, cows born with two heads are rarer than cows born with one head. (in an infinitely branching multiverse ) there are an infinite number of one-headed cows and an infinite number of two-headed cows. What happens to the ratio? Sean M. Carroll offered another informal example: : Say there are an infinite number of universes in which George W. Bush became President in 2000, and also an infinite number in which Al Gore became President in 2000. To calculate the fraction N(Bush)/N(Gore), we need to have a measure — a way of taming those infinities. Usually this is done by “regularization.” We start with a small piece of universe where all the numbers are finite, calculate the fraction, and then let our piece get bigger, and calculate the limit that our fraction approaches. However, different procedures for computing the limit of this fraction may yield different answers.〔 This is important because in order to compare cosmological theories in an infinite multiverse, we need to know which types of universes theories predict to be more common than others.〔 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「Measure problem (cosmology)」の詳細全文を読む スポンサード リンク
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