Final answer:
The expanding universe being extremely hot provides explanations for the temperature of the Cosmic Microwave Background (CMB) and the density of the universe.
Step-by-step explanation:
The expanding universe was extremely hot in the beginning, and this provides an explanation for several observations in astronomy. The hot, uniform state of the universe explains why the Cosmic Microwave Background (CMB) has the same temperature in all directions. The CMB is the leftover radiation from the Big Bang explosion, and its temperature of 2.725 K is evidence of the universe's creation in a gigantic explosion. The notion of an extremely hot early universe also helps explain why the density of the universe is so close to the critical density.