One of the strongest indicators of this limit is the cosmic microwave background radiation, faint afterglow from the Big Bang. It was released when the Universe was just 3,80,000 years old, before which matter and energy were too tightly coupled for light to travel freely. Today it appears across the entire sky at a temperature of 2.7 degrees above absolute zero. Beyond this ‘surface of last scattering,’ no light can reach us, fixing the boundary of direct observation.
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