When the universe was a couple of hundred million years old, it resembled a giant kitchen where the first galaxies were being shaped from primordial 'dough'—hydrogen and helium. At their centers, black holes ignited like ovens of varying power, from light 'camp stoves' to heavy 'hearths.' But how quickly did they gain mass by swallowing surrounding gas? That determines how many pairs of such 'ovens' will collide and merge, emitting a ripple in space, gravitational waves, that we hope to catch billions of years later.
In the new study, astrophysicists modeled two growth scenarios. In the first, black holes fed modestly, within the Eddington limit (when light pressure prevents matter from falling faster). In the second, they 'gorged,' surpassing this barrier, so-called super-Eddington accretion. The authors then traced how pairs of black holes, left over after galaxy mergers, slowly spiral together due to dynamical friction, like spoons in thick honey, and calculated their mergers up to the present day. Future detectors (the space antenna LISA, ground-based facilities ET and LGWA) will listen to these events like sensitive microphones placed in different corners of an orchestra pit.
The results are mind-boggling. With modest feeding, LISA would catch about 32 mergers per year, and with gluttony, nearly 64. ET, on the other hand, loses 'customers': it drops from 64 signals per year to a measly 4. LGWA remains a stable middle-of-the-roader — roughly 21 and 12 events, respectively. In other words, how the first black holes ate directly determines which detector will become the chief storyteller of their history. And the mass ratios in pairs, along with how the merger rate changes with distance, provide a key to deciphering their origins.
Thus, gravitational astronomy is turning into archaeology: from the 'bones'—masses and orbits of merged objects: we reconstruct the menu and appetites of the universe's first inhabitants. And when the sensitive antennas come online, they might tell us whether those black holes were modest eaters or cosmic gourmands.
Super-Eddington accretion is like drinking a thick smoothie through a straw, but the smoothie gets sucked up faster than fluid resistance would allow. In black holes, the 'straw' is played by the magnetic field, which helps pull in matter against radiation pressure.
In Carl Sagan's novel 'Contact,' an alien signal is disguised as gravitational waves from a black hole merger, so our future detectors might hear not only natural cataclysms but also, perhaps, someone's distant call.