The First Limits on Dark Matter Decaying Into Gravitons Came From the Magnetic Threads Between Galaxies

The graviton is the most confidently predicted particle nobody has ever seen. If gravity works like the other three forces, it should be carried by a particle — and that particle should be so weakly coupled to everything else that detecting a single one is generally considered hopeless. David Dunsky of New York University has now published the first observational limits on a process involving gravitons anyway, in Physical Review D, by using the universe itself as the apparatus.

The idea rests on a chain of three things that are individually well established. First, dark matter makes up about 85% of the matter in the universe and is known only through its gravitational pull; some theories hold that dark matter particles slowly decay, and among the products they could decay into are gravitons. Second, the space between galaxies is not empty: it is threaded by filaments, vast structures stretching millions of light-years, laced with weak magnetic fields. Third, there is the Gertsenshtein effect — a prediction dating to 1962 that a graviton passing through a magnetic field can convert into a photon.

Put those together and a dark matter particle decaying somewhere in the cosmic web produces a graviton, that graviton crosses a magnetized filament, and some fraction of the time it emerges as light. Specifically, it emerges as a gamma ray. That is something an existing instrument can look for.

Dunsky compared the gamma-ray signal that this chain predicts against the diffuse gamma-ray background measured by the Fermi Large Area Telescope, which has been mapping the high-energy sky since 2008. The result was a null: no unexplained excess. But a null in this context is not nothing. Because the predicted signal scales with how fast dark matter decays, the absence of an excess sets a ceiling on that rate — the first such limit ever placed, across a broad range of possible dark matter masses.

This is the ordinary logic of exclusion physics, and it is how most of the dark matter search has actually progressed. Direct-detection experiments buried in mines have spent two decades not finding weakly interacting massive particles, and in the process have carved away most of the parameter space those particles could occupy. What is unusual here is the detector. Instead of a tank of liquid xenon underground, the conversion volume is a magnetic filament spanning millions of light-years, and the only reason it works is that the filament is enormous enough to compensate for how astronomically improbable graviton-to-photon conversion is per unit length.

The constraint could tighten considerably. The proposed Advanced Particle-astrophysics Telescope would improve sensitivity to the diffuse gamma-ray background by roughly a factor of 10. If dark matter really does decay into gravitons, scanning cosmic filaments could turn out to be the single most effective way to find it — a detection strategy in which the target, the detector and the shielding are all the same object.

Two caveats belong with any result of this kind. It constrains a specific decay channel, not dark matter in general, and it depends on models of intergalactic magnetic field strength that remain uncertain to within roughly an order of magnitude. But the paper establishes something that did not exist before: a real number, from real data, bounding how quickly the universe's dominant matter component could be turning into the quanta of gravity.