Alternative explanations posit these anomalously massive black holes grew and merged in throngs of stars called globular clusters, but that process can easily require more time than the current age of the universe. (scientificamerican.com)
The two black holes close together gradually over time as their orbits degrade as energy is lost generating gravitational waves, but the process can take longer than the current age of the universe. (sciencedaily.com)
A star that weighs about one - tenth as much as the sun, for example, has a projected lifetime of roughly 12 trillion years — more than 800 times the current age of the universe. (sciencenews.org)