A screenshot from a simulation exhibits two merging supermassive black holes. Credit score: NASA’s Goddard Area Flight Middle/Scott Noble; simulation information, d’Ascoli et al. 2018
We see proof for merging supermassive black holes in every single place we glance. One downside: We’re not precisely certain how they handle to do it. Just lately, a group of astronomers has proposed {that a} explicit type of dark matter often is the key to unlocking this cosmic thriller.
Supermassive black holes are the biggest black holes within the universe. They weigh wherever from a couple of hundred thousand instances to some hundred billion instances the mass of the Solar. Nearly each galaxy within the universe hosts such a large black hole in its core.
Astronomers consider these black holes grew to their huge proportions over the course of billions of years, from a mix feeding on any materials that occurs to wander too shut and from merging with different big black holes. Final yr, a group used an observational approach often called a pulsar timing array to select up a “background hum” of gravitational waves per a big inhabitants of merging supermassive black holes. These gravitational waves are emitted because the black holes spiral in towards one another throughout the merger occasion.
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However in between these two scales, astronomers don’t know the way the black holes lose power. When they’re separated by roughly a parsec — 3.26 light-years — there isn’t sufficient materials to function a reservoir for power, and the gravitational waves aren’t sturdy sufficient to do it on their very own. Therefore the issue.
Nevertheless, astronomers aren’t precisely certain how the merger course of unfolds. What’s notably difficult is a matter often called the ultimate parsec downside. To get two black holes to merge, they’ve to come back shut to one another. And to come back shut to one another, they need to lose orbital power. At massive distances, the black holes can lose orbital power by quite a lot of means, reminiscent of interacting with the fuel or stars in a galaxy. By the point the black holes are a brief distance aside, only a handful of light-years, all the fabric has been cleared away, however by then the emission of gravitational waves can pull power out of the system, permitting the black holes to collide.
Self-interacting dark matter
A brand new paper revealed July 9 in Physical Review Letters gives a possible answer to this downside. The authors’ answer includes dark matter, the invisible matter that seems to dominate the mass of galaxies. Whereas there’s a substantial quantity of circumstantial proof for the existence of dark matter, such because the rotation charge of galaxies and the expansion of enormous buildings within the universe, we don’t but know the identification of the dark matter particle. All we all know is that dark matter doesn’t work together with mild. It solely interacts with the remainder of the universe by gravity.
The only mannequin for dark matter assumes that it’s collisionless, which signifies that dark matter doesn’t work together with itself. However there are some fashions of dark matter the place it could weakly work together with itself. These fashions are identified, appropriately sufficient, as self-interacting dark matter. Astronomers proposed this mannequin of dark matter a long time in the past as a solution to resolve among the deficiencies with the collisionless dark matter thought. Most significantly, collisionless dark matter has problem producing the noticed variety of dwarf satellite galaxies and matching the noticed densities within the cores of galaxies.
“The chance that dark matter particles work together with one another is an assumption that we made, an additional ingredient that not all dark matter fashions comprise,” mentioned research co-author Gonzalo Alonso-Álvarez, a postdoctoral fellow within the Division of Physics on the College of Toronto and the Division of Physics and Trottier Area Institute at McGill College, in a press release. “Our argument is that solely fashions with that ingredient can resolve the ultimate parsec downside.”
The astronomers behind the research discovered that once they switched out collisionless dark matter for self-interacting dark matter of their fashions, the ultimate parsec downside wasn’t an issue anymore. Supermassive black holes are thought to merge because the final stage within the merger of two galaxies. Because the supermassive black holes enter the core of the ensuing single galaxy, they encounter better densities of dark matter. The black holes work together with the dark matter by gravity — simply as they do with fuel and stars when they’re farther aside — which siphons power from the momentum of the black hole into the dark matter particles. With collisionless dark matter, the dark matter particles decide up this further power after which merely go away. However with self-interacting dark matter, the additional power added to the particles simply goes into extra interactions.
This enables the dark matter to behave as a reservoir that may soak up the kinetic power of the black holes as they arrive nearer collectively. With this further reservoir accessible, the supermassive black holes rapidly shut the ultimate parsec and meet of their ultimate gravitational embrace.
On the lookout for affirmation
The researchers predict that in such a state of affairs, the ensuing gravitational waves needs to be shifted barely from the anticipated case with collisionless dark matter. And that is precisely what astronomers have seen with pulsar timing arrays.
“A prediction of our proposal is that the spectrum of gravitational waves noticed by pulsar timing arrays needs to be softened [lower power] at low frequencies,” mentioned James Cline, research co-author and Professor at McGill College and the CERN Theoretical Physics Division in Switzerland. “The present information already trace at this habits, and new information might be able to affirm it within the subsequent few years.”
Whereas it’s not a confirmed detection of dark matter or a confirmed answer to the ultimate parsec downside, this research does present that dark matter could also be extra difficult than we naively assume, and it could play a major position within the evolution of black holes.
And, in flip, black holes could assist us reveal the true nature of dark matter.
“Our work is a brand new approach to assist us perceive the particle nature of dark matter,” mentioned Alonso-Álvarez. “We discovered that the evolution of black hole orbits could be very delicate to the microphysics of dark matter and which means we will use observations of supermassive black hole mergers to raised perceive these particles.”
