In the present theory of the Big Bang,what significant event occurred at about 500,000 years after the universe started expanding?
A) The primordial helium in the universe was produced.
B) The universe became transparent to neutrinos.
C) The temperature of the cosmic background radiation had cooled to its present level of about 3 K.
D) The universe became transparent to photons of radiation.

Question

Quizplus · Accepted Answer

At about 500,000 years after the universe started expanding, the temperature of the universe had cooled enough for electrons to combine with nuclei and form neutral atoms, allowing photons to travel through the universe unimpeded. This event is known as recombination and resulted in the universe becoming transparent to photons of radiation. This radiation can still be seen today as the cosmic microwave background radiation.

In the Present Theory of the Big Bang,what Significant Event