As astronomers look at more distant regions of space, they see the regions as they existed at earlier times, but their farthest views are blocked by a "wall" beyond which the universe is opaque. What event occurred at the time marked by this wall?
A) Quarks combined to form neutrons and protons.
B) Electrons and protons combined to form neutral hydrogen atoms.
C) Protons combined with neutrons to form helium nuclei.
D) Gravity froze out as a separate force.

Question

Quizplus · Accepted Answer

The "wall" refers to the surface of last scattering, which occurred when the universe cooled enough for electrons and protons to combine and form neutral hydrogen atoms. This event made the universe transparent to radiation, allowing light to travel freely through space.

As Astronomers Look at More Distant Regions of Space, They