Space-time ripples record black hole crashes

DID monster black holes pull the first galaxies together, or were they born inside those galaxies? It's a long-standing mystery. Now a new analysis of the gravitational ripples from colliding black holes could reveal the answer by helping astronomers reconstruct a crash rather than just surveying its aftermath.

Most large galaxies we see have supermassive black holes at their centres. When these galaxies collide, their black holes merge into one even more massive beast, according to theory. Observations of the final black hole yield no information about the original black holes, however.

So astronomers have been trying to look for gravitational waves. General relativity predicts that colliding black holes should emit such ripples in the fabric of space-time, including a wave called the ringdown that contains information about the final black hole's mass and spin.

Now computer simulations led by Ioannis Kamaretsos of the University of Cardiff, UK, show that the ringdown can also tell us the masses and spins of the two original black holes (arxiv.org/abs/1207.0399).

Some theories say the gravity of nascent black holes pulled matter together to form the first galaxies, but it remains unclear if the earliest black holes were massive enough to do this. By applying the new analysis to observations of the early universe, we might be able to settle that question at last.

Although nobody has detected gravitational waves as yet, next-generation observatories may spot them within a few years.

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