Roughly every 100,000 years, a binary-star system wanders too close to the black hole at our galaxy's center and gets ripped apart. While one star falls into the black hole, the other star is flung out of the galaxy, in a classic demonstration of Newton's law of action-reaction. In 2009, the Hubble Space Telescope pinpointed one such hypervelocity star leaving the Milky Way.
Even far into the future our galaxy will occasionally eject a star from the core, like a baseball player hitting a home run out of the stadium. That runaway missile could yield transformational clues as big as the Copernican revolution.
Diligent astronomers will pick up the runaway star and be curious as to how far it's going into the inky black of the extragalactic abyss. They will be shocked to see the star speed up the farther it got from our galaxy. This would be due to the effect of dark energy, which continues stretching space apart.
To their amazement, they would even see it disappear over an "event horizon" where information traveling at the speed of light can no longer be received because of the rapid expansion of space. "These hypervelocity stars will allow residents to learn about the cosmic expansion and reconstruct the past," writes Loeb.