Astronomy clicker question: whence cosmic background radiation?

Astronomy 10, Spring Semester 2007
Cuesta College, San Luis Obispo, CA

Astronomy 10 learning goal Q12.4

Students were asked the following clicker question (Classroom Performance System, einstruction.com) at the start of their learning cycle:

[0.3 points.] What is the cosmic background radiation?
(A) Photons that have always existed, even before the start of the big bang.
(B) Photons from the very start of the big bang.
(C) Photons from about 400,000 years after the start of the big bang.
(D) Photons from the end of gravitational deceleration (start of dark energy acceleration).

Correct answer: (C).

It was too hot for protons and electrons to form atoms in the early universe, until approximately 400,000 years after the start of the big bang. Since atoms scatter light less efficiently than separate protons and electrons, when the universe made this "recombination" or "decoupling" transition from an opaque to a much more transparent state, photons were effectively freed from this time forward. However, any photons from earlier, back to the start of the big bang, were effectively lost.

Student responses
Section 5076
(A) : 5 students
(B) : 7 students
(C) : 1 student
(D) : 6 students