Eddington number

In astrophysics, the Eddington number, N_Edd, is the number of protons in the observable universe. Eddington originally calculated it as about 1.57×10⁷⁹; current estimates make it approximately 10⁸⁰.

The term is named for British astrophysicist Arthur Eddington, who in 1940 was the first to propose a value of N_Edd and to explain why this number might be important for physical cosmology and the foundations of physics.