Compared to the average density of matter in the present universe, the equivalent "mass density" of radiation, using Einstein's relation E = mc² for the photons, is: A)much greater, leading to a radiation-dominated universe. B)almost nonexistent, because there is very little radiation left in the universe at the present time. C)equal, because these parameters remain balanced throughout the evolution of the universe as a consequence of the equipartition of energy. D)much less, leading to a matter-dominated universe.

Question

Quizplus · Accepted Answer

The equivalent "mass density" of radiation in the present universe is much less than the average density of matter, leading to a matter-dominated universe. This is because, as the universe expands, the density of radiation decreases more rapidly than the density of matter. Radiation was more dominant in the early universe, but currently, matter (including dark matter) constitutes the majority of the universe's density.

Compared to the Average Density of Matter in the Present