In the present theory of the Big Bang, what significant event occurred at about 400,000 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.

Question

Quizplus · Accepted Answer

At about 400,000 years after the Big Bang, the universe cooled enough for electrons to combine with protons and form neutral hydrogen atoms. This process is known as recombination. Before recombination, the universe was filled with a plasma of charged particles (electrons and protons) and photons (light particles). These photons were constantly scattered by the charged particles, making the universe opaque. Once neutral atoms formed, the photons could travel freely without being scattered, making the universe transparent to photons. This is why we can observe the Cosmic Microwave Background radiation today, as it is essentially the afterglow of the Big Bang, freed at the moment of recombination.

In the Present Theory of the Big Bang, What Significant