Slipher reported the velocities for 15 spiral nebulae spread across the entire celestial sphere, all but three having observable "positive" (that is recessional) velocities. Subsequently, Edwin Hubble discovered an approximate relationship between the redshifts of such "nebulae" and the distances to them with the formulation of his eponymous Hubble's law. Milton Humason worked on these observations with Hubble. These observations corroborated Alexander Friedmann 's 1922 work, in which he derived the Friedmann–Lemaître equations. In the present day they are considered strong evidence for an expanding universe and the Big Bang theory.