By then, most cosmologists had concluded that the universe contains much more than meets the eye. Observations of pinwheeling galaxies suggested that scaffolds of invisible matter held their stars together, while a repulsive form of energy drove galaxies apart. To learn more, Bennett and his Wilkinson Microwave Anisotropy Probe (WMAP) team had spent a year collecting microwaves coming from all directions in the sky—light rays that left their source long ago, when the universe was just 380,000 years old. By snapping this photograph of the young cosmos, the WMAP team could pin down its age and shape and determine exactly how much so-called dark matter and dark energy it contains.
“All of a sudden we had this list of numbers,†recalled Bennett, an astrophysicist at Johns Hopkins University.
The team announced their first results in February 2003. Their map of the “cosmic microwave background†(CMB), which they refined in subsequent years, indicated that the familiar matter of planets, gas and stars makes up just 4.6 percent of the cosmos, while unseen dark matter comprises 24 percent. The remaining 71.4 percent of the cosmic pie chart had to be dark energy, which is thought to infuse the fabric of space itself. The numbers changed only a little when WMAP’s successor, the Planck satellite, took an even sharper image of the CMB 10 years later. And whereas other evidence of dark energy and dark matter continues to be contested, their fingerprints in the CMB have gone virtually unquestioned.
The CMB is “definitely one of, if not the most important, pillar of modern cosmology,†said Yacine Ali-Haïmoud, an astrophysicist at New York University.
Here’s how the universe scrawled such a telling message in the cosmic microwave background, and how researchers learned to read it.
In the beginning, before the microwaves left their source, the universe was a nearly featureless fluid made of dark and visible matter. This primordial substance once shone white-hot, then cooled to a light orange over the universe’s first few hundred thousand years. Light rays couldn’t travel far before ricocheting off neighboring particles. This scattered light kept the fluid foggy and pressurized.
But the seeds of today’s stars and planets had already been sown. Nothing in nature is perfect, and the smooth primordial soup came ever so slightly clotted, with some regions about a thousandth of a percent denser than the surrounding fluid, and some that much thinner.
The fluid sloshed as gravity pulled matter together and light waves pushed it apart. This tug-of-war thinned dense spots as excess matter spilled outward and thickened thin spots as material rushed inward. When one area got too thin, particles would rush in again, and vice versa, so that each blob swung back and forth between high and low density. Fortunately, physicists have all the theoretical tools they need to analyze such undulations of a simple fluid at a reasonable temperature. “The physics is really pretty old,†Ali-Haïmoud said.
The CMB captures the sloshing fluid at a particular time. After expanding for about 380,000 years, the cosmos cooled enough for protons and electrons to pair off into hydrogen atoms, an event called recombination. With few charged particles to bang into, light beams suddenly became free, releasing the pressure and freezing the density blobs in place. Since then, the expanding universe has stretched the wavelengths of the liberated light rays into microwaves. By collecting them from all over the sky, the WMAP and Planck telescopes caught the early universe and its contents mid-slosh. Their maps reveal a blotchy pattern of denser spots and thinner spots, signified by microwaves that measure a fraction of a degree warmer or cooler (just as blue indicates a hotter flame than yellow).