How to understand the Universe’s the most important equation | by Ethan Siegel | Starts With A Bang! | Jun, 2025
If you want to understand the Universe, cosmologically, you just can’t do it without the Friedmann equation. With it, the cosmos is yours.
15 hours ago
Cosmology is the study of the Universe: from the smallest scales to the largest. If we want to understand this Universe we inhabit, we have no choice but to consider everything all together, from the subatomic particles that compose our reality to the largest-scale structures that they form. This requires understanding not only the various quanta that bind together and make up all that we can observe and interact with, but the proverbial stage upon which they move: the stage of spacetime itself. The Universe doesn’t simply exist, but rather evolves with both space and time as our cosmic story unfolds.
It was barely a century ago — back in 1915 — that Einstein first unveiled the General theory of Relativity, which detailed how space-and-time affected the matter and energy within it, and how, conversely, the matter and energy within it determined the shape and evolution of spacetime. Remarkably, it would be just seven years later, a full 103 years ago at present, that the single most important equation in cosmology was first derived: the first Friedmann equation.
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