We can measure the temperature of the CMB photons that are hitting the Earth on one side and compare that to the temperature of the photons hitting the Earth from the other side. And what we find is a little surprising: the temperature is the same (about 2.73 K) no matter which direction you look! It seems unlikely that we are standing at the exact center of a microwave-reheated universe, so we can only conclude from our measurement that the entire universe is at the same even temperature. That is, the universe is more like a warm bath that’s been sitting there invitingly for a while rather
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