Question

The Big Bang theory predicted that the ordinary matter in the Universe would be made up of 75% hydrogen and 25% helium. Through observation and measurements, scientists have confirmed that this ratio is correct. How does the Big Bang theory explain this relative composition?
A.
Shortly after the Big Bang, all the matter in the Universe was made of clouds of hydrogen gas. This gas pulled together to create stars and fused a small amount of hydrogen into helium.
B.
The Big Bang was an explosion that occurred in previously empty space. This explosion was caused mostly by hydrogen, an explosive element, and produced a small amount of helium.
C.
The Big Bang was an expansion of matter, which started as hydrogen. The process caused some of the hydrogen to expand into the next biggest element, helium.
D.
Shortly after the Big Bang, protons and neutrons formed. Most of these created the simplest nuclei, hydrogen, while a smaller portion combined to form helium.

Answer 1

D.

Shortly after the Big Bang the hot dense plasma of protons and neutrons cooled enough for nuclear reactions (Big Bang nucleosynthesis). Most baryons remained as single protons (hydrogen nuclei), while a fraction of neutrons combined with protons to form helium-4, producing the observed ≈25% helium by mass.