Einstein’s revolution enters second century

Today's dark mysteries
Just as in the early 1900s, physicists today know their theories don't add up. In fact, they have an even finer appreciation of how much they don't know than Einstein's contemporaries ever did. But physicists are also hoping that ultra-powerful telescopes and ultra-high-energy particle accelerators — tools that were unavailable to Einstein — could shed light on those mysteries.

One such mystery is the nature of dark matter, the invisible stuff that makes up about a quarter of the universe and helps keep galaxies gravitationally bound. Physicists are coming around to the view that at least some dark matter consists of exotic, weakly interacting particles that don't register on current detectors.

That poses an experimental problem rather than a theoretical problem, Dyson says: "You want to find out what these damn particles are, whatever they are. What we need are better underground detectors."

NASA A sky map based on readings from the Wilkinson Microwave Anisotropy Probe shows the "afterglow" of the Big Bang.

Dark energy is an entirely different matter: In the past decade, physicists have gotten a great deal of high-quality data about the expansion of the universe from observations of faraway supernovas and the "afterglow" of the Big Bang itself. But instead of showing that the expansion of the universe was slowing down, as they expected, the observations revealed that the universe has been expanding faster and faster.

This weird accelerating effect was initially referred to as a kind of "antigravity," but physicists settled on the term "dark energy" as a parallel to dark matter. When you try to balance the accounts of the universe's matter-energy content, it turns out that dark energy is responsible for 70 percent of the total.

"It's not stuff that collects into one region or another," Carroll explains. "It's an energy density that as far as we know is the same everywhere in space, and also the same everywhere in time."

Einstein's theories go back to the future
When physicists started casting around for an explanation, the first place they looked was Einstein's own theories. At one time, Einstein had built a "cosmological constant" into his equations for general relativity, to account for the obvious fact that gravity wasn't causing the universe to crash in on itself. The cosmological constant represented a repulsive quality of empty space that would counteract the attractive force of gravity.

When later observations indicated that the universe was expanding due to a primordial Big Bang, Einstein discarded the cosmological constant, calling it the biggest mistake of his career. But dark energy has brought the idea back into vogue — and has also reinvigorated the Einsteinian quest to develop theories that explain general relativity as well as quantum mechanics.

"The really good news about dark energy is that it almost inevitably involves both gravity and quantum mechanics," Carroll says. "It's a feature of empty space, a feature of the quantum vacuum, and it's causing a gravitational effect. So therefore, the people who want to explain quantum gravity need to face up to the existence of dark energy. And in fact they are."

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