Gravitational waves are ripples in the fabric of spacetime caused by cataclysmic events such as neutron stars colliding and black holes merging. The biggest of these events, and the easiest to see, are the collisions between supermassive black holes at the centre of galaxies. So an important question is how often these events occur.
Sean McWilliams and a couple of pals at Princeton University say that astrophysicists have severely underestimated the frequency of these upheavals. Their calculations suggest that galaxy mergers are an order of magnitude more frequent than had been thought. Consequently, collisions between supermassive black holes must be more common too. That has important implications for the ability of today’s gravitational wave observatories to see them. There is an intense multi-million dollar race to be first to spot gravitational waves but if McWilliams and pals are correct the evidence may already be in the data collected by the first observatories. The evidence that McWilliams and co rely on is various measurements of galaxy size and mass. This data shows that in the last 6 billion years, galaxies have roughly doubled in mass and quintupled in size.
Astrophysicists know that there has been very little star formation in that time so the only way for galaxies to grow is by merging, an idea borne out by various computer simulations of the way that galaxies must evolve. These simulations suggest that galaxy mergers must be far more common than astronomers had thought.
That raises an interesting prospect–that the supermassive black holes at the centre of these galaxies must be colliding more often too. McWilliams and co calculate that black hole mergers must be between 10 and 30 times more common than expected and that the gravitational wave signals from these events are between 3 and 5 times stronger.
That has important implications for astronomers’ ability to see these signals. Astrophysicists are intensely interested in these waves since they offer an entirely new way to study the cosmos.
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