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The horizon problem is a problem with the standard cosmological model of the Big Bang which was identified in the late 1960s, primarily by Charles Misner. It points out that different regions of the universe have not "contacted" each other because of the great distances between them, but nevertheless they have the same temperature and other physical properties. This should not be possible, given that the transfer of information (or energy, heat, etc.) can occur, at most, at the speed of light. Two theories that attempt to solve the horizon problem are the theory of cosmic inflation and variable speed of light. ==Basic concept== When one looks out into the night sky, distances also correspond to time into the past. A galaxy measured at ten billion light years in distance appears to us as it was ten billion years ago, because the light has taken that long to travel to the viewer. If one were to look at a galaxy ten billion light years away in one direction, say "west", and another in the opposite direction, "east", the total distance between them is twenty billion light years. This means that the light from the first has not yet reached the second, because the 13.8 billion years that the universe has existed is not a long enough time to allow it to occur. In a more general sense, there are portions of the universe that are visible to us, but invisible to each other, outside each other's respective particle horizons. In standard physical theories, no information can travel faster than the speed of light. In this context, "information" means "any sort of physical interaction". For instance, heat will naturally flow from a hotter area to a cooler one, and in physics terms this is one example of information exchange. Given the example above, the two galaxies in question cannot have shared any sort of information; they are not in "causal contact". One would expect, then, that their physical properties would be different, and more generally, that the universe as a whole would have varying properties in different areas. Contrary to this expectation, the universe is observed to be very close to isotropic, which also implies homogeneity.〔http://ned.ipac.caltech.edu/level5/Peacock/Peacock3_1.html〕 The cosmic microwave background radiation (CMB), which fills the universe, is nearly the same temperature everywhere in the sky, about . The differences in temperature are so slight that it has only recently become possible to develop instruments capable of making the required measurements. This presents a serious problem; if the universe had started with even slightly different temperatures in different areas, then there would simply be no way it could have evened itself out to a common temperature by this point in time. According to the Big Bang model, as the density of the universe dropped (while it expanded) it eventually reached a point where photons in the "mix" of particles were no longer immediately impacting matter; they "decoupled" from the plasma and spread out into the universe as a burst of light. This is thought to have occurred about 300,000 years after the Big Bang. The volume of any possible information exchange at that time was 900,000 light years across, using the speed of light and the rate of expansion of space in the early universe. Instead, the entire sky has the same temperature, a volume 1088 times larger. 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「horizon problem」の詳細全文を読む スポンサード リンク
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