In this task you will be given a text passage from a trivia quiz. You should choose the category this question belongs to from these categories: History, Science, Social Science, Fine Arts, Literature. You can only choose one category, so if in doubt, please choose the closest match.

Erwin Finlay-Freundlich proposed one explanation for this phenomenon that claimed its strength should be proportional to the radiation density. A version of this phenomenon in the CMB is known as the Sachs-Wolfe effect, while the quantity that describes the strength of this effect has a value of about 1100 for the surface of last scattering. One version of this effect was measured using the Mossbauer effect in an experiment by Pound and Rebka, and the magnitude of this effect is usually denoted "z. " For 10 points, name this phenomenon, which can be caused by gravity or the Doppler effect, in which the wavelength of an object's light appears longer to an observer.
Science