The detection of a gravitational wave was a historic event that heralded a new phase of astronomy. A numerical model of the Universe now allows researchers to tell the story of the black-hole system that caused the wave. See Letter p.512
The first gravitational-wave source was detected on 14 September 2015. What surprised some was that the signal came from the merger of two black holes, each about 30 times the mass of the Sun. Now, on page 512, Belczynski et al.1 not only show that such a system can arise naturally from our understanding of how the stars in binary systems interact, but also unlock the history of the black holes from their birth as two massive stars.
Other groups2,3,4 have sought to characterize the source of the gravitational wave (now known as GW 150914), but what makes Belczynski and colleagues' work stand out is that they have created a numerical model of the Universe that allows every phase of the evolution of binary stars to be followed, from the birth of the Universe to the present. This enabled them to search through the list of observable black-hole binaries to find those that match the parameters of the gravitational-wave source. They then tracked back each candidate source's evolution to estimate the relative probability that the source could have caused the event, and thus identify which was the most likely.
The authors conclude that the black holes probably started off as two stars that had masses 40 to 100 times that of the Sun and were born about 2 billion years after the Big Bang. These stars turned into black holes after a further 5 million years, and merged 10.3 billion years after that, emitting the gravitational-wave signal that was detected 1.2 billion years later (Fig. 1). Other scenarios are possible, but less likely.
The black holes were monsters, and the results show that their progenitor stars would have been some of the brightest and most massive in the Universe. If the proposed age of the stars' formation is correct, then they might have contributed to the reionization of the Universe — one of the key events in the Universe's evolution. It is also likely that the stars were relatively pure in composition: they consisted mostly of hydrogen and helium, and contained less than 10% of the heavy elements (such as carbon, oxygen and iron) that pollute our Sun. This indicates that the stars would have been in a small dwarf galaxy, rather than a large spiral galaxy, such as our own Milky Way.
This study is important for two reasons. First, GW 150914 provides an exciting test for stellar evolution theory. Previously, core-collapse supernovae represented the latest stage of a star's life that could be used to constrain the nature of the progenitor stars5. Belczynski et al. have gone beyond that to the final event that occurs within a stellar binary that has already survived two supernovae. Their work, therefore, places firm constraints on stellar evolution and on how stars die in supernovae. Second, it provides a new way to measure the accuracy of models of star formation and cosmic evolution throughout the history of the Universe.
There are, of course, caveats and assumptions that add uncertainty to Belczynski and colleagues' model. One uncertainty is how massive the black hole formed by a star can be; this is determined by how explosive the black-hole-forming supernovae are. The explosive nature of massive stars is a hot topic of research, with some evidence6,7 suggesting that black holes can form directly from stars without a supernova, which is what Belczynski and colleagues assume. But stars might also form black holes and explode. In binary systems, this would affect the nature of the final black-hole system, and the time taken for the black holes to merge.
Another uncertainty involves an intermediate phase of binary-star evolution. As stars in binaries evolve, their radii increase, sometimes growing to the size of their orbit so that they get in each other's way — this is called the common-envelope phase. Typically, the star that grows first loses its outer envelope of gas, leaving a small, hot core that eventually forms a black hole. The binary's orbit decreases in size during this process. During the merger of two black holes, the closer the two objects were when they formed, the sooner they will merge. But researchers do not know how much the orbit can shrink in the common-envelope phase despite decades of work dedicated to finding an answer8.
Future gravitational-wave signals may help astrophysicists to constrain both uncertainties, but for now, Belczynski et al. generate an 'optimistic' and a 'pessimistic' model Universe, to assess the highest and lowest possible rates of black-hole mergers. They demonstrate that systems that would form black-hole binaries of the sort that generated GW 150914 would form in both models, and that the rate of black-hole mergers in the Universe matches that inferred from the gravitational-wave detection. The authors also suggest that rotation of stars about their own axes is not required to explain most gravitational-wave sources, but it has been suggested that such rotation could increase the number of black-hole mergers9. Nevertheless, there is still more work to be undertaken, and more physics to include in the models.
Belczynski and colleagues' study is tremendously exciting because it examines the effects of a new constraint on how stars and the Universe evolve, identified by GW 150914. With each gravitational-wave signal detected we'll learn something new. And with rumours that more events will be announced soon, we may not have to wait too long for the next lesson.