Knocking on Heaven's Door - Lisa Randall [178]
We have further evidence for dark matter’s existence from the cosmic microwave background discussed earlier. Unlike lensing, the radiation measurements don’t tell us anything about the distribution of dark matter. Instead they tell us the net energy content carried by dark matter—how big a piece of the cosmic pie is constituted by the energy it carries.
CMB measurements tell us a great deal about the early universe and give us detailed information about its properties. These measurements argue not only for dark matter. They also support the existence of dark energy. According to Einstein’s equations of general relativity, the universe could only be flat with just the right amount of energy. Matter, even accounting for dark matter, simply didn’t suffice to account for the flatness measured by WMAP and balloon-based detectors. Other energy had to exist. Dark energy is the only way to account for the universe’s flatness—with no measurable curvature of three-dimensional space and agree with all other measurements to date.
Dark energy, which carries the bulk of the universe’s energy—approximately 70 percent—is even more puzzling than dark matter. The evidence that convinced the physics community of dark energy’s existence was the discovery that the expansion of the universe is currently accelerating—much as it did during inflation earlier on but at a very much slower rate. In the late 1990s, two independent research teams, the Supernova Cosmology Project and the High-z Supernova Team, surprised the physics community when they discovered that the rate of expansion of the universe is no longer slowing down, but is actually increasing.
Before the supernova measurements, a few hints had pointed to the existence of missing energy, but the evidence had been weak. But careful measurements in the 1990s showed that distant supernova were dimmer than expected. Since this particular type of supernova has fairly uniform and predictable emission, this could only be explained by something new. And that something new seems to be an accelerated expansion of the universe—that is, it is expanding at an increasingly faster rate.
This acceleration would not arise from ordinary matter, whose gravitational attraction would slow the universe’s expansion. The only explanation could be a universe that acts like one that is inflating, but with far smaller energy than during the inflationary phase the universe had undergone much earlier on. This acceleration could be due only to something that acted like the cosmological constant that Einstein had introduced, or dark energy, as it has become known.
Unlike matter, dark energy exerts negative pressure on its environment. Ordinary positive pressure favors inward collapse, whereas negative pressure leads to accelerated expansion.68 The most obvious candidate for negative pressure—one that agrees with measurements so far—is Einstein’s cosmological constant, representing an energy and pressure that permeates the universe but is not carried by matter. Dark energy is the more general term we now use to allow for the possibility that the cosmological constant’s assumed relationship between energy and pressure isn’t precisely true but is only approximate.
Today dark energy is the dominant component of the universe’s energy. This is all the more remarkable because the amount of dark energy density turns out to be extraordinarily small. Dark energy has dominated only for the last few billion years. Earlier in the universe’s evolution, first radiation and then matter were dominant. But radiation and matter, which are shared over the volume of an ever-increasing universe, dilute. Dark energy density, on the other hand, remained constant, even when the universe grew. By the time the universe had lasted so long as it has, the energy density in radiation and matter had decreased so enormously