The Key to Dark Matter May Be Hidden Light

With a giant mirror, the FUNK experiment plans to capture dark matter of a different sort.

Oct 18 2014, 1:00pm

A team of German astrophysicists is at work repurposing a large metallic mirror, originally constructed as a cosmic ray detector prototype, for use in the hunt for dark matter. Compared to the exotic supercooled xenon reservoirs currently at work at various laboratories deep underground, a regular-looking sort of mirror might not seem terribly exciting, but it's after a very different sort of dark matter prey: hidden photons.

Most dark matter detection experiments have a certain kind of dark matter in mind: WIMPs, or weakly interacting massive particles. These are particles that would have originated in the very early universe when everything existed in a state of thermal equilibrium, e.g. everything was about the same temperature and all particles had the same limited properties. Cooling brought the universe definition and differentiation.

WIMPs got shorted in the whole cosmic cooling process, however, and while the universe's "normal" matter wound up with a whole suite suite of different sorts of interactions—electromagnetism, the strong and weak forces, and gravity—these dark matter particles only feel the weak force and gravity. Without the strong force or electromagnetism, they can't form into nuclei and atoms (via the strong force), nor can they repel/attract each other via the electromagnetic force.

We see the effects of dark matter gravity in abundance, and we can make estimations based on those effects as to how much dark matter there actually is out there: somewhere around 85 percent of all mass in the universe. Far from being exotic, dark matter is what holds galaxies together, allowing things like solar systems and life-harboring planets to form.

WIMPs aren't something astrophysicists just dreamed up. There's an extraordinary correspondence between the observed strength of the weak force, which governs radioactive decay, and the observed amount of dark matter in the universe.

Image: Ralph Engle/

This is known as the "WIMP miracle." "From a particle physics perspective, the early universe was a high energy place where energy and mass could switch from one form to the other freely as enshrined in Einstein's E = mc2," writes Stacy McGaugh, a University of Maryland astrophysicist and reluctant dark matter naysayer. "Pairs of particles and their antiparticles could come and go. However, as the universe expands, it cools. As it cools, it loses the energy necessary to create particle pairs."

"When this happens for a particular particle depends on the mass of the particle," McGaugh continues, "the more mass, the more energy is required, and the earlier that particle-antiparticle pair 'freeze out.' After freeze-out, the remaining particle-antiparticle pairs can mutually annihilate, leaving only energy. To avoid this fate, there must either be some asymmetry or the 'cross section'—the probability for interacting—must be so low that particles and their antiparticles go their separate ways without meeting often enough to annihilate completely."

The WIMP particles that don't meet their antiparticles and, thus, aren't annihilated continue on, leaving what's known as a "relic density." The cross-section McGaugh mentions must be roughly equal to the probability of particle interaction according to the weak force (the weak force's cross-section) to result in the dark matter distributions we observe in the universe. A powerful coincidence.

It takes more than a coincidence to prove the existence of WIMPs, however: We also need to actually see them. So far, after a quarter-century of hunting, we haven't registered a single WIMP. Perhaps then we should look elsewhere for dark matter possibilities as well, which is where the German's mirror scheme, aka the FUNK experiment, comes in.

The mirror, which is based at the Karlsruhe Institute of Technology, is being retrofitted to hunt for a different theorized form of dark matter particle known as WISPs, a form of which is hidden photons. These are photons similar to those that we experience everyday as carriers of the electromagnetic force (so: light, electricity, heat), but they interact via this force only very weakly. A WISP might interact with an electron just like a normal photon, but only the tiniest bit. It would be very easy to miss.

WISPs have some strange properties (or would have some strange properties), one of which is the possibility of suddenly changing into a regular old photon in the presence of a strong magnetic field. In the German group's dish-mirror scheme, the idea is that WISPy hidden photons will smack the mirror, exciting the electrons within it just enough such that they will emit regular photons. These regular photons will be be fired off as the tiniest bits of light at right angles to the incoming WISPs.

These emitted photons would then be concentrated toward a central detector, which itself would be tuned such that background light/photons would be filtered out. 

"To detect photons induced by this process, the advantage of using a spherical mirror is imminent," the German team writes in a paper posted to the arVix preprint server. "Photons from far away background sources impinging on the mirror will be focused in the focal point ... whilst the Dark-Matter-induced photons will propagate to the center of the 'mirror sphere.' There, a detector can be mounted."

Image: University of Leicester

The group's WISP-focused effort should gain a new weight given the release last week of a paper describing mysterious x-ray signals apparently traceable back to the Sun. It's possible that these x-rays are WISPy particles known as axions being converted to regular photons upon meeting the strong magnetic field of Earth.

"It appears plausible that axions—dark matter particle candidates—are indeed produced in the core of the sun and do indeed convert to x-rays in the magnetic field of the Earth," George Fraser, the new x-ray paper's senior author, concluded.

Is that the dark matter answer then? Hardly. The new x-ray results will take years more analysis, while the new mirror detection scheme is only the second of its kind, with the other being the United States' ADMX experiment. At the very least, it's always nice to see contrarian science get a leg up: What's more exciting than being wrong?