NASA’s Voyager-1 spacecraft, now cruising through the local interstellar medium, has sent back pristine cosmic ray data allowing researchers to set new limits on the cosmos’ exotic dark matter.
Analysis of the 40-year-old spacecraft’s cosmic ray detections out beyond the heliopause — where the solar wind’s influence ends and the flux of low energy galactic cosmic rays begins — found no evidence of dark matter.
The findings were presented late last month at the CRISM 2018 (Cosmic Rays and the InterStellar Medium) Conference in Grenoble, France.
The researchers theorized that they would have seen a higher density of these lower energy cosmic rays — which are inaccessible inside our solar system’s heliopause — if there had been evidence for some sort of dark matter interactions.
The goal of the analysis was to measure an excess of cosmic rays in comparison to the interstellar cosmic ray background, as Mathieu Boudaud, a postdoctoral researcher at Le Laboratoire de Physique Théorique et Hautes Energies (LPTHE) in Paris, explained in his conference presentation.
“In my wildest dreams, I had no idea that this Cosmic Ray System (CRS) instrument on the Voyager would have anything to do with dark matter,” Caltech physicist Alan Cummings told me at the conference.
Cummings, who has been a part of the Voyager science team since 1973, some four years before its launch, says its cosmic ray detector was designed specifically to look for galactic cosmic rays. That is, the low-energy cosmic rays that can only be detected outside our solar system.
Galactic cosmic rays are trapped inside the Milky Way’s global magnetic field which in turn is turbulent and made up of magnetic knots. It’s these magnetic knots which are thought to diffuse the cosmic rays inside our galaxy. And although some cosmic rays may be related to dark matter, most are thought to originate within supernova remnants.
In fact, the term cosmic ray itself is a late 19th-century misnomer. Cosmic rays are not rays at all but rather charged elemental particles which sometimes move at velocities approaching that of light. What’s surprising is that they are helping researchers better understand the dark matter’s lower mass limits.
As Boudaud has written, since dark matter’s 1933 discovery in the Coma Galaxy cluster by the late Swiss astronomer Fritz Zwicky, its presence in galaxies and in galaxy clusters has largely been confirmed. The Standard Cosmological Model predicts that about 85% of matter in the universe is composed of dark matter, Boudaud writes.
The most talked about theoretical dark matter particle has long been the WIMP (or Weakly-Interacting Massive Particle), a particle that only weakly interacts with standard normal matter particles. Although Voyager’s cosmic ray data could have pointed to an indirect detection of wimps, thus far, it hasn’t found any evidence for them.
If dark matter is made of particles with masses lower than a Giga-electron Volt (GeV), then existing Voyager data is not compatible with the most popular dark matter models, Boudaud told me at the conference.
Dark matter particles are actively hunted in particle accelerators and in direct-detection experiments, Boudaud has written. But an alternative strategy is to look for signatures of the dark matter in the Milky Way through cosmic rays. The idea is that at least some dark matter particles present in the galaxy will annihilate into particle-antiparticle pairs. But antimatter is rare.
Alternatively, dark matter may be made of microscopic black holes. They would be everywhere, even inside the solar system at a theoretical spacing of one Earth-Sun distance. A microscopic black hole would be no bigger than a nucleus of the element Xenon nucleus, says Salati. But he says they would have the incredible gravitational pull of Mount Everest.
Hawking had this idea that quantum effects would cause black holes to continually evaporate, Pierre Salati, a physicist at France’s Laboratoire d’Annecy-le-Vieux de Physique théorique (LAPTh), told me at the conference. This evaporation, in theory, would convert all microscopic black holes into cosmic rays. So, the idea with this new Voyager data is to look for the evaporation of black holes emitting cosmic rays , electrons and positrons (the positively-charged, antimatter counterpart of an electron).
But if they are separated by only one Earth-Sun distance, then why haven’t we observed their effects?
Maybe because they don’t exist, says Salati. But the aim here is to set limits on their abundance. “The idea is that the black holes evaporate and that evaporation emits cosmic rays,” he said.
Cummings hopes that Voyager will continue sending such cosmic ray data for another five years, or until 2023.
As for a dedicated mission to measure such cosmic rays beyond the heliopause? Cummings says lots of researchers relish the idea. But to make it practical in our lifetimes, any new mission would need to hit a speed of 10 Earth-Sun distances per year instead of Voyager’s current rate of 3.6 Astronomical Units (or Earth-Sun distances) per year.
Voyager is now 142 Astronomical Units out, says Cummings. If propulsion tech could cut such a spacecraft’s flight 35-year journey to the ISM to 10 years, that would make a big difference.
Meanwhile, Boudaud says he will continue his dark matter search with ongoing new Voyager data as long as the spacecraft is able.