Which of the following properties of the universe has decreased as the universe has expanded?
a. mass
b. energy
c. temperature
d. volume
c. temperature
The correct answer is c. temperature.
As the universe expands, the temperature decreases. This is because the expansion of the universe causes a decrease in the density of matter and radiation. As the density decreases, interactions between particles and radiation become less frequent, leading to a decrease in temperature.
To determine which of the given properties of the universe has decreased as it has expanded, let's analyze each option:
a. Mass: The total mass of the universe is believed to remain constant as the universe expands. This is due to the law of conservation of mass, which states that mass cannot be created or destroyed.
b. Energy: The total energy of the universe is also believed to remain constant as the universe expands. This is due to the law of conservation of energy, which states that energy cannot be created or destroyed.
c. Temperature: As the universe expands, the average temperature decreases. This is known as cosmic cooling. The reason behind this is that as the universe expands, it also spreads out the energy within it, leading to a decrease in the average temperature.
d. Volume: The volume of the universe increases as it expands. This is evident from the definition of expansion, where the universe is stretching out and becoming larger.
Therefore, the correct option is:
c. Temperature