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Articles: Devotion | Quest for Infinity - 08
- Prof. venkata ramanamurty mallajosyula
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The Horizon Problem
The details are a bit complicated to explain here but in brief the problem may be stated as follows. Let us go back to a long long time ago, say one second after the Big Bang. Based on the currently observed rate of expansion of the Universe, the Baby Universe when it was about 10-35 second old must have been a ball of radius about 1 cm! Incredibly small is it not?! Let us consider light emitted from some point on the surface of this ball having a radius of 1 cm. In 10-35 s, this light can travel only a distance of 3 x 10-25 cm. That means, for light to cross the Baby Universe, it would take an awfully long time; but in that time, the Universe would have expanded a good deal more. If you reflect on all this carefully, you would soon find that because of the rapid rate of expansion of the Universe, radiation within would have a hard time filling it up UNIFORMLY. But experiments show that radiation is indeed filling the Universe uniformly, apart from some important fluctuations, which, however are very minor in size [something like one part in 1000 or even less]. So how come radiation is filling the Universe so very uniformly?
This figure, which please note is not to scale [!], shows schematically the Baby Universe when it was 10-35 s old. Thanks to expansion, the radius of the Baby sphere is 1 cm at this point of time. If we consider a typical point on the surface of the sphere like A, for example, light originating there at birth would have travelled only 3 x 10-25 cm.
This distance, shown by AB, is clearly much, much smaller than the radius. That means that the region shown shaded would be optically unconnected with the rest of the Universe. Thus, mixing of radiation across the Universe, essential for uniformity of CMB, is not possible.
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