In the present universe, the "mass density" of radiation, using Einstein's equation E = mc² for the photons, is A) essentially zero since there is very little radiation left in the universe at the present time. B) much greater than the mass density of matter leading to a radiation-dominated universe. C) equal to the mass density of matter, since these parameters have remained balanced throughout the evolution of the universe as a consequence of the equipartition of energy. D) much less than the mass density of matter, leading to a matter-dominated universe.

Question

Quizplus · Accepted Answer

The mass density of radiation in the current universe is much less than the mass density of matter, which is why the universe is considered to be matter-dominated. This is because the universe has expanded and cooled since the Big Bang, significantly reducing the density of radiation compared to matter.

In the Present Universe, the "Mass Density" of Radiation, Using