The 4% Universe: Dark Matter, Dark Energy, and the Race to Discover the Rest of Reality

by Richard Panek

Peebles would be performing what scientists call an N-body simulation. Take a number—N—of points, program them to interact according to whatever properties you want, and see how the action unfolds. In this case, Peebles would be taking 300 points and treating each as if it were a galaxy in one particular part of the universe—the Coma Cluster, the closest and most-studied galaxy cluster. He would assign each galaxy a position and velocity based on rough observations of real galaxies in the cluster, and he would teach the computer the law of universal gravitation. And then he’d let the model do whatever galaxies interacting gravitationally in an expanding universe do over billions of years. He’d already been thinking about how clusters develop, and he’d done some cluster calculations during his time at Caltech. Now he took that initial research and converted it into a computer program. Then he punched the holes in the 7%-by-3%-inch computer cards himself, stacked them in the metal feeder, and ran a simulation. At the end of the simulation the points had moved a bit. He transferred the image to a frame of 35-millimeter film, and then he ran the next simulation, using the galaxies’ positions at the end of the previous simulation as a starting point. Again, over a period equivalent to millions of years, the galaxies shifted slightly. When Peebles had enough frames, he ran them together, loaded the film in a projector, and sat back. The universe swirled to life. Galaxies moved o...