The ionization energy of a hydrogen atom, that is the energy required to remove the electron from the atom completely, is 13.6 electron volts. Suppose it were 25 electron volts instead. What difference would this make to cosmology?
A)None. Individual atoms are much too small to have any effect on the evolution of the universe.
B)The cosmic microwave background radiation would be hotter than 2.7 kelvins.
C)The cosmic microwave background radiation would be cooler than 2.7 kelvins.
D)There would be no cosmic microwave background radiation.

Question

Quizplus · Accepted Answer

The ionization energy of hydrogen affects the temperature at which hydrogen atoms can form in the early universe. If the ionization energy were higher, it would take more energy (or a higher temperature) for electrons to escape hydrogen atoms, meaning the universe would have to cool down more before atoms could form. This would delay the release of photons that create the cosmic microwave background radiation, leading to a hotter cosmic microwave background than the observed 2.7 kelvins.

The Ionization Energy of a Hydrogen Atom, That Is the Energy