The expansion of the universe will eventually render the CMB invisible because its photons will
A) have wavelengths that will exceed the size of the observable universe.
B) be swamped by the light from the ever-increasing star formation rates in the universe.
C) not have enough energy to escape the gravity of degenerate stellar remnants.
D) all combine and again produce pairs of massive particles.
E) not escape the strong gravity of supermassive black holes the size of galaxy clusters.

Question

Quizplus · Accepted Answer

As the universe expands, the wavelengths of the CMB photons will continue to stretch, eventually becoming so long that they will exceed the size of the observable universe, making them effectively invisible.

The Expansion of the Universe Will Eventually Render the CMB