The photons in the CMB were emitted when the universe was about 1/1100 of its current size. What was the temperature of the universe at that time? (The present day temperature of the CMB is 2.7 K.)
Question
The photons in the CMB were emitted when the universe was about 1/1100 of its current size. What was the temperature of the universe at that time? (The present day temperature of the CMB is 2.7 K.)
Solution
The temperature of the universe at the time the Cosmic Microwave Background (CMB) photons were emitted can be estimated using the fact that the universe was about 1/1100 of its current size.
The temperature of the universe is inversely proportional to its size due to the adiabatic expansion of the universe. This means that as the universe expands, its temperature decreases.
Given that the current temperature of the CMB is 2.7 K, we can estimate the temperature of the universe at the time the CMB photons were emitted by multiplying the current temperature by the factor by which the universe was smaller at that time.
So, the temperature at that time would be approximately 2.7 K * 1100 = 2970 K.
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