The cosmic microwave background radiation was emitted when the universe had a size about 1/1000 of today's value. What was the temperature of the microwave background radiation when it was emitted?
A) 30 K
B) 300 K
C) 3000 K
D) 30,000 K
E) 300,000 K

Question

Quizplus · Accepted Answer

The temperature of the cosmic microwave background radiation at the time of its emission is directly related to the size of the universe at that time. Since the universe was about 1/1000 of today's size at that time, we can use the relationship between temperature and size known as Wien's law, which states that the temperature is inversely proportional to the size. Therefore, at 1/1000 of today's size, the temperature would have been approximately 1000 times higher, or around 3000 K.

The Cosmic Microwave Background Radiation Was Emitted When the Universe