According to a paper posted to the arXiv pre-print server last week, the difference between an everyday supermassive black hole and a space-time tunneling wormhole may be a lacing of dark matter. While it sounds like crank fodder of the sort that not infrequently winds up on arXiv, the idea may hold actual water.
The theory pertains to one particular proposed form of dark matter known as axionic dark matter. Axions, a hypothesized fundamental particle of matter relating to the strong nuclear force, aren't the only proposed candidate for dark matter, but as searches for WIMPs (weakly-interacting massive particles)—far and away the favored proposed particle comprising dark matter—come up empty, axionic dark matter has become a more and more plausible scenario. As theorized, dark matter axions would permeate the universe as an energetic condensate, interacting only very weakly via the electromagnetic force and existing as a kind of ghostly cosmic foam.
Crucially, while individual axions would be very light, they would together make up enough mass to account for the dark matter halos that form the gravitational scaffolding of galaxies. Axions are currently being hunted for via experiments involving giant Earth-based mirrors.
The wormhole idea has to do with how axions interact with electromagnetic fields. "Many authors have considered the electromagnetic interaction of axion particles," writes Konstantinos Dimopoulos of Lancaster University in the UK. "However, the effect of this interaction to the axionic condensate itself has been largely ignored, assuming that it is negligible." Maybe there's more to it, Dimopoulos surmises.
This something more has to do with axions' weak but still present relationship to the electromagnetic force. Put some axions in the presence of an extreme magnetic field and strange stuff should start to happen. Dimopoulos' paper finds that "if the magnetic field is strong enough, axionic dark matter is modified to lead to the violation of the weak and dominant energy conditions." In other words, axionic matter may be transformed into a state of negative energy.
And matter with negative mass-energy, otherwise known as exotic matter, is prescriptive of a wormhole, a "throat" connecting two mouths located at different, potentially very far apart locations in the universe.
So, where do we find magnetic fields strong enough to turn regular old axionic dark matter into space-time shredding exotic matter? Yep, next to the supermassive black hole of an active galactic nucleus (AGN). As a black hole sucks in its surroundings, those surroundings start colliding with each other as they fall through the event horizon. These collisions cause friction, which creates heat, and the result is in some cases completely epic light-speed jets of particles. These jets are accompanied by helix-shaped magnetic fields (as in the illustration above).
An important thing about axions (should axions exist) is that they naturally oscillate. Because there's nothing really to interfere with them, there's nothing to dampen this motion, which we can imagine as being leftover from the early, energetic days of the universe. As such, the universe is a vast expanse of these perpetual yo-yos, which are having a great time because there isn't a thing in the whole of existence that can fuck with them.
That is, until they meet a supermassive black hole. Here, the weak electromagnetic coupling of axionic matter is plenty sufficient to couple with the energetic photons of the magnetic helix racing outward from the black hole. A strange thing then happens. In a process somewhat analogous to the process of creating mass via the Higgs mechanism (think of the popular guy moving through the crowded party analogy), the axion winds up with a bit of extra mass, which is offset by a bit of negative energy. This is what the math shows, anyway.
And so we have the makings of a wormhole. "Is it possible to render the supermassive black holes in the centres of active galaxies into stable (and possibly traversable) wormholes?," the paper asks. "As Wheeler famously said, 'matter tells spacetime how to curve.' Thus, the presence of negative density matter and strong magnetic fields may force the appearance of wormholes in the AGN centres."
That would be cool, but not very useful in the usual science-fictional sense. It may tell us some new and very weird things about how galaxies form, and also about the nature of dark matter. Also: as Sabine Hossenfelder points out at Backreaction, the closest supermassive black hole to Earth is a cool 26,000 light-years away.
That said, Dimopoulos does finally just go for the sci-fi gold in his paper's conclusion: "As a corollary, if dark matter is axionic, one can imagine that an advanced civilisation may generate artificially a helical magnetic field, with the appropriate characteristics to alter the nature of the local dark matter and maybe to give rise to a wormhole. This might become a way to realise interstellar travel (and/or time travel)."
Let's get on that.