How Ancient Light Reveals the Universe’s Contents

A photograph of the infant cosmos reveals the precise amounts of dark matter and dark energy in the universe, leaving precious little room for argument.

Charlie Wood | NAUTILUS

 Imagine Earth at the center of the sphere. Cosmic microwaves coming to us from every direction in the sky provide a snapshot of the young universe, which was filled with a clotted fluid made of dark and visible matter. Credit: Dillon Berger

In early 2003, Chuck Bennett learned the precise contents of the cosmos.

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% of the cosmos, while unseen dark matter comprises 24%. The remaining 71.4% 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.

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