The Redshift of Extragalactic Nebulae
Galaxies in a cluster swarm too fast for their stars to hold them — most of the mass is unseen.
A cluster of galaxies is spinning apart so fast that the galaxies we can see have nowhere near enough gravity to hold it together — yet somehow it holds.
The big idea
In 1933 Fritz Zwicky weighed a swarm of galaxies in two completely different ways. He added up the light of all the galaxies to estimate how much matter they contained. Then he watched how fast those galaxies were buzzing around inside the cluster, and used gravity to work out how much mass it would take to stop them flinging themselves into space.
The two answers should have matched. Instead the gravity answer was hundreds of times bigger. Something enormous and invisible was supplying the missing pull. Zwicky called it dunkle Materie — dark matter.
How it came about
Zwicky was a brilliant, abrasive Swiss-American physicist at Caltech, famous for both his ideas and his feuds. Studying the Coma cluster — a great city of galaxies — he noticed the galaxies' speeds were wildly spread out, around a thousand kilometres per second apart.
A loose swarm moving that fast should have evaporated long ago, like a jar of gnats with the lid off. For it to survive, far more gravity was needed than the visible stars could provide. Zwicky's conclusion was decades ahead of its time, and the astronomy community mostly shrugged. Only in the 1970s, when Vera Rubin measured stars whipping around the edges of spiral galaxies far too quickly, did the same invisible mass become impossible to ignore.
Why it mattered
Zwicky had stumbled onto the fact that the glowing stars and galaxies we can see are a minority of what the universe is made of. Most of the matter is some unknown stuff that neither shines nor blocks light — we know it only by its gravity. Everything from how galaxies form to the shape of the whole cosmos depends on it, and after ninety years we still don't know what it is. It is one of the biggest unanswered questions in all of science.
A way to picture it
Imagine watching dry leaves whirl around in what looks like still air. The leaves are flying so fast they should scatter — yet they keep circling, as if trapped in an invisible whirlwind. You can't see the wind, but you can infer it from how the leaves move. Zwicky read the galaxies the same way: their speeds betrayed a vast unseen mass, the wind you cannot see.
Where it sits
Zwicky's clusters and Vera Rubin's spinning galaxies are two windows onto the same dark matter. It sits beside the other great cosmic puzzles in this Library — Hubble's expanding universe, whose galaxies dark matter helped assemble, and the dark energy found in 1998 that drives that expansion ever faster. Together, ordinary matter, dark matter, and dark energy make up the universe's full inventory — and we understand only the smallest slice.
Falls sich dies bewahrheiten sollte, würde sich also das überraschende Resultat ergeben, dass dunkle Materie in sehr viel grösserer Dichte vorhanden ist als leuchtende Materie.