The Hubble Constant from that event came in at a very rough 70, smack between the traditional two, but with uncertainty engulfing even the most extreme supernovae and CMB estimates. Settling the conflict calls for shrinking that possible error to 2 or 3 percent, which will take between 30 and 50 collisions of the type observed last year, Holz calculates in his recent paper. Based on LIGO's increasing sensitivity and the assumed rate of neutron star mergers, he now expects to have enough data to decide between the two Hubble Constant contenders within five years. Riess, who wasn't involved with the work, agrees that gravitational waves offer a plausible and exciting path forward, but points out that it's tough to guess how often we'll find mergers. "Maybe they'll accrue faster," Riess says, "but if they accrue 10 times slower I don't want to wait 50 years."