An illustration of our cosmic history, from the Big Bang until the present, within the context of the expanding Universe. The first Friedmann equation describes all of these epochs, from inflation to the Big Bang to the present and far into the future, perfectly accurately, even today. (NASA / WMAP science team)

The Most Important Equation In The Universe

Just one equation relates the expansion of space to all the matter and energy we have. If you know this, you can know the fate of the Universe.

Last week, Perimeter Institute ran a feature where they asked 14 scientists what their favorite equation was, and why. There were many great answers from many different areas of research, from thermodynamics to pure mathematics. Many people went with fundamental equations, like the law of gravity, Newton’s famous F = ma, or the Schrödinger equation, which governs quantum particles. I had the honor of being included in this list, and the answer I gave was none of these. Instead, the equation I picked was a very specific one: the first Friedmann equation, which is derived from Einstein’s General Relativity under a specific set of circumstances.

A photo of Ethan Siegel at the American Astronomical Society’s hyperwall in 2017, along with the first Friedmann equation at right.(Perimeter Institute / Harley Thronson)

When they asked why I picked that equation, here’s what I said:

“The first Friedmann equation describes how, based on what is in the universe, its expansion rate will change over time. If you want to know where the Universe came from and where it’s headed, all you need to measure…