From Earth, the night sky looks fairly static. Sure, the stars rotate from evening to evening, and the planets move among them. But from a terrestrial perspective, the celestial sphere appears essentially unchanging.
Perception, though, is not reality: our eyeballs don’t hint that beyond nearby planets, stars and galaxies, everything is moving away from us. The universe is constantly expanding—at an ever faster rate.
“When we say that the universe is expanding, we mean something pretty literal,” says Dan Scolnic, an associate professor of physics at Duke University, who studies this cosmic growth. “I think it’s a little different than how people think of it. But we mean that the distance that objects are away from us—particularly other galaxies—is increasing.”
Scientists don’t currently know whether that expansion will continue indefinitely or, if so, whether it will keep accelerating ad infinitum. The universe’s ultimate end state—whether it will expand so rapidly that it will tear itself apart, continue to calmly enlarge and cool off or eventually reverse and contract in on itself—will be determined by the balance of dark matter, dark energy, and regular matter and energy in space. The two unknown, or dark, parts of that equation make up 95 percent of the universe, and their nature continues to elude scientists, who don’t know how the contributions of those components to the universe’s life story might change over time. [Continue reading…]