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Entropy of a system can be understood as "the number of states that are macroscopically indistinguishable from the state its currently in". (Sean Carroll, "The Big Picture")

Given that the early universe was such a tiny thing with all matter and energy compressed in it, why does it make sense to assume that the universe had extremely low entropy in the past?

How do we know that it was highly non-uniform at that time. I mean, intuitively speaking, how can we even imagine things in such a small space to be "in separate chunks"?

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marked as duplicate by Jon Custer, Community Sep 21 '18 at 7:57

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  • $\begingroup$ A key-word to look for observing the uniformity of the early universe is cosmic-microwave background radiation. Look it up and you'll find some great explanations. $\endgroup$ – Tausif Hossain Sep 20 '18 at 17:39
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Remember that the early uniform was highly uniform in its mass-energy distribution. Hence it had such a low entropy as there were very little variation and hence very low information and hence very low entropy. I'll try to answer your question for the evidence for this uniformity. This comes from the cosmic-microwave background radiation the so called "after-glow" of the universe which was produced after the splitting of the electromagnetic and weak nuclear force and as matter and anti-matter annihilation finally ceased starting to leave behind the glow of photons that we'd now come to know as cosmic-microwave background radiation.

Hence by studying the variations in this radiation that directly links to mass-energy distributions in the early universe we find that this radiation has variations of less than 0.003% from having perfect uniformity, hence proving our high uniformity in the early universe and hence, the very low entropy in the early universe.

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