At an age of 500,000 years,the temperature of the universe had fallen to 3000 K,and electrons could then combine with protons to produce neutral hydrogen gas.What major transition took place in the universe at this time?
A) The present laws of physics began to apply for the first time.
B) The universe suddenly lost its electrical charge and become neutral.
C) Nuclear fusion no longer occurred below this temperature.
D) The universe became transparent to light for the first time.

Question

Quizplus · Accepted Answer

At this temperature, the universe had cooled enough for electrons to combine with protons and form neutral hydrogen gas. This allowed photons to travel through the universe without being scattered by free electrons, making the universe transparent to light for the first time. This event is known as recombination, and it marks the point at which the universe became transparent to radiation. option A is incorrect, as the laws of physics would have been in effect long before this point. Option B is partially correct, as the universe did become neutral, but this occurred gradually over a period of time. Option C is incorrect, as nuclear fusion still occurs in stars, even at temperatures lower than 3000 K.

At an Age of 500,000 Years,the Temperature of the Universe