At the heart of most galaxies lies a supermassive black hole (SMBH), a behemoth of unimaginable gravity. These giants weren’t always so massive; they are believed to have grown through repeated mergers with other black holes. Scientists use complex simulations to model this intricate dance of merging black holes, but until now, there’s been a stumbling block: the ‘final parsec problem’.
As two massive black holes spiral towards each other, they lose energy and accelerate due to interactions with surrounding gas and stars. However, when the black holes reach a separation of about a parsec (3.26 light-years), there isn’t enough surrounding material to drain their energy, preventing them from merging. This has been a major conundrum for astrophysicists, as it contradicts the observed gravitational wave background, which suggests countless black hole mergers are happening in the universe.
Now, a new study published in Physical Review Letters proposes a potential solution: self-interacting dark matter. This theory suggests that dark matter particles interact with each other, unlike the usual assumption that they are collisionless.
The gravitational pull of SMBHs attracts dark matter, creating a dense concentration called a ‘spike’. Previous models with ordinary dark matter failed to explain the energy dissipation needed for merger, as the dark matter spike couldn’t absorb the heat generated by the orbiting black holes. However, when the researchers incorporated self-interacting dark matter into their simulations, the spike absorbed the energy efficiently without being disrupted, allowing the black holes to continue spiraling inward and merge.
This new model solves the ‘final parsec problem’ and offers a timescale for mergers consistent with the observed gravitational wave background. Additionally, it provides a potential explanation for the observed ‘softening’ of the gravitational wave spectrum, a phenomenon that current models with ordinary dark matter cannot account for.
If future measurements from pulsar timing arrays confirm the softening of the gravitational wave spectrum, it would lend significant support to the self-interacting dark matter theory. This could be a game-changer in our understanding of dark matter, one of the universe’s most enigmatic components, by observing the behavior of some of its largest objects.
