What were the relative energy densities in the early Universe? (below, DE means dark energy)

(a) They remain roughly equal: �rad ∼ �mat ∼ �DE
(b) �DE > �mat > �rad
(c) �rad > �mat > �DE
(d) �mat > �rad > �DE

To determine the relative energy densities in the early Universe, we need to understand the different components that contribute to the total energy density. The main components are radiation (rad), matter (mat), and dark energy (DE).

In the early Universe, the dominant energy component was radiation. This radiation was mainly in the form of high-energy photons and particles, such as photons from the cosmic microwave background radiation and relativistic particles.

As the Universe expands and cools, the energy density of radiation decreases faster than the energy density of matter. This is because the energy of photons decreases as the Universe expands, and relativistic particles become non-relativistic as they lose energy. On the other hand, the energy density of matter, which includes both ordinary matter (such as protons and neutrons) and non-relativistic particles, decreases at a slower rate.

Dark energy, which is an unknown form of energy responsible for the accelerated expansion of the Universe, has a nearly constant energy density throughout spacetime. It does not change significantly as the Universe expands.

Based on this understanding, we can deduce the relative energy densities in the early Universe:

The correct answer is (c) �rad > �mat > �DE.

In the early Universe, radiation (rad) dominates, and its energy density is higher than both matter (mat) and dark energy (DE). Matter has a lower energy density compared to radiation, but it is still higher than that of dark energy. Dark energy has the lowest energy density among the three components.