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

by Richard Panek

“The radiation,” Peebles explained in one of his many papers during this period, “performs the great service of defining the epoch at which the galaxies can start to form.” That epoch occurred when the temperature of the primeval fireball fell below 4000 K. At that point the electrons and protons that had been ricocheting independently since the first instants of the universe recombined to form atoms of matter. This matter now took on a “life” of its own and decoupled from the radiation—the fossil radiation that survived today as the cosmic microwave background. And although that background seemed to be as uniform—as homogeneous—as theory had predicted, it couldn’t be entirely uniform. It had to contain the irregularities, or inhomogeneities, that identified the concentrations of matter that existed at the moment that matter and radiation decoupled and went their separate ways—the matter that through gravitational interactions would have grown into the large-scale “distributions of mass and size” that we see today: “galaxies, and clusters of galaxies, and the material within galaxies,” including us. The universe was simple. It just couldn’t be perfectly simple. Yet to the radio telescopes at the time, the microwave background was absolutely uniform; it lacked the inhomogeneities that had to be there in order for us to be here. Eventually, astronomers or physicists wanting to test the Big Bang theory would have to develop instruments sensitive enough to detect those subtle ...