What is the Tolman surface brightness test?
Quick Answer
The Tolman test is a way of checking whether the Universe is expanding by looking at how surface brightness changes with distance. In an expanding Universe, distant objects should appear much dimmer than a static-universe model predicts.
The Full Story
Named after Richard C. Tolman, who discussed the idea in the 1930s, the surface brightness test compares how bright extended objects appear at different distances. In an expanding Universe, light is redshifted, photon arrival rates slow down, and surface brightness declines more rapidly than in non-expanding models. Astronomers have used the test as one piece of evidence supporting cosmic expansion. It is difficult to apply cleanly because galaxies also evolve over time, so interpretation requires careful modeling.
Key Facts
YouTube Angle
Suggested video title for this topic:
"A clever way to tell whether the Universe is really expanding"