The Tolman test is an observational test of general relativity and the expanding universe model, proposed by physicist Richard C. Tolman. It examines how the surface brightness of distant galaxies changes with redshift—the stretching of light due to cosmic expansion.
In an expanding universe governed by general relativity, light from distant galaxies is redshifted and spread over a larger area of the sky due to both the stretching of space and time dilation. As a result, the surface brightness (brightness per unit area) of those galaxies should decrease proportionally to the fourth power of (1 + redshift).
The Tolman test checks whether this expected dimming matches what is actually observed. If it does, it supports both cosmic expansion and the relativistic description of how light behaves over large distances. If not, it could imply alternative cosmological models or new physics.
Modern observations using deep-field galaxy surveys have shown consistency with the Tolman test, providing further confirmation of general relativity and the expanding nature of our universe.