Adam Platz writes,

“The equation is called the Friedmann equation and, simply put, governs the expansion of space in a homogenous universe such as our own. In the 1920s Alexander Friedmann, a Russian astrophysicist, sought to unite Einstein’s recently conceived theory of general relativity with a general model for our universe’s behavior. The Freidmann equation resulted. From its basic form, one can derive the density of the universe at a given time, the pressure, the mass, the age of the universe, and finally the rate of expansion of the universe (found in a term known as the Hubble constant). In 2008 during my senior year at Dartmouth, my senior seminar in astrophysics focused in part on this equation. I always found the equation to be elegant and beautiful. My own little god equation. Explaining where we came from and what we are made of. That year I thought up the idea of the tattoo and decided that if in 5 years I was still interested in the ink, I would get it. And so I did in 2013.”

The Wikipedia page for the Friedmann equation is suitably intense. The American Institute of Physics has a more accessible history of Friedmann and other early twentieth-century cosmologists.

You can see the rest of the Science Tattoo Emporium here or in my book, *Science Ink: Tattoos of the Science Obsessed.*