When cosmologist Daniel Holtz took off from Hong Kong on August 17, 2017, his head swirled with the ideas he’d spent the last week lecturing on, including his hope that vibrations in space would someday settle an ongoing debate regarding the size and age of the universe. But he knew it would take time. Time for two of the densest objects in existence to smack together and shake the cosmos hard enough for us to feel the rumble here on Earth, time to locate the disturbance, and time to swing our telescopes toward the collision before the accompanying burst of light faded back into darkness.

Optimistically, such paired observations of both gravitational waves and light from these neutron star collisions were about ten years off, he had told the audience in his last lecture the previous day. The Laser Interferometer Gravitational-Wave Observatory (LIGO) collaboration had already detected black hole mergers and the Virgo interferometer had just come online in Italy two weeks earlier, so the enterprise was developing smoothly. But when Holtz, who works at the University of Chicago, returned to Illinois, he learned the future had arrived early. The gravitational shockwaves set off by the collision of two nearby titans had rippled through his plane—and the entire planet—while he was in the air, and observatories around the world were scrambling for follow-up optical observations.

“We landed, and my phone exploded. I immediately connected and just sat there on my laptop starting to work,” Holtz recalls. “That was the most amazing experience of my life.” Twelve hours after touchdown he had a back-of-the-envelope calculation for the most contentious number in cosmology: the speed of the expanding universe. With just one data point he couldn’t get the decisive measurement he’s dreamed of for thirteen years, but he finally knew the project was possible. Now, after doing some more math, he’s back with a new prediction: the LIGO collaboration may be able to settle the decades-long debate within five years, according to his recent letter in Nature.

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