You know that dream where you’re about to take a final exam, only to realize that you have neglected to study and, moreover, to put on pants? I imagine that’s what astronomers must have felt like in 1998, when they found out that most of the cosmos had somehow escaped their notice.
Astronomers knew that the universe had been expanding since the Big Bang, but they assumed that the gravitational pull of all the stuff inside it was gradually slowing that expansion. So they were caught off guard when supernova observations showed that in fact the expansion was speeding up, thanks to a mysterious phenomenon they dubbed “dark energy.”
X-ray image of the remnant of SN 1572, a supernova of the type used to measure the accelerating expansion of the universe. Credit: NASA/CXC/Rutgers/J.Warren & J.Hughes et al.
Nearly 16 years and a trio of Nobel Prizes later, the initial shock has worn off, but the sense of chagrin lingers. “About 70% of the universe is dark energy, so it’s embarrassing not to know what it is,” says Valeria Pettorino, a physicist and cosmologist at the University of Geneva in Switzerland and the University of Heidelberg in Germany.
That uncertainty ...