The conditions in the early universe at the time matter became decoupled from radiation were (in terms of age t and equivalent temperature T):
A) t = 2 × 10¹¹ years , T = 3 K
B) t = 3 minutes, T = 10⁷ K
C) t = 380,000 years, T = 3000 K
D) The universe was never radiation-dominated since matter has always dominated by generating the gravitational field into which the universe has expanded.

Question

Quizplus · Accepted Answer

The time when matter became decoupled from radiation is also known as the cosmic microwave background (CMB) radiation era, which occurred about 380,000 years after the Big Bang. At this time, the temperature of the universe was around 3000 K, which allowed neutral atoms to form, and the universe became transparent to radiation. This is consistent with choice (C). Choices (A) and (D) are incorrect because they do not describe the period of decoupling. Choice (B) describes the time of Big Bang nucleosynthesis, when the temperature was around 10⁷ K, but this is not the same as the time of decoupling.

The Conditions in the Early Universe at the Time Matter