Large Hadron Collider
Inside the most famous particle accelerator
colliding with a matter galaxy, for example, would create an unmissable cacophony of gamma rays. Yet we’ve never seen such a distinctive signal.
“For this reason it doesn’t seem plausible that our universe contains equal amounts of matter and antimatter,” says Hooper.
That doesn’t rule out antimatter galaxies entirely. Instead it suggests that if they do exist, they must be rare. It would help if we knew if antistars exist. We’ve never seen one, but what would they look like if we could? In many ways an antimatter star would look no different to the stars that blaze in the night sky. Stars create light by turning hydrogen into helium in a process called nuclear fusion. In fact, the Sun converts 620 million tonnes of hydrogen into 616 million tonnes of helium each and every second. The missing 4 million tonnes is turned into sunshine. An antistar would create light by churning antihydrogen into antihelium. Crucially, the light produced would be identical in both cases. Light is made up of particles called photons, and the photon is its own antiparticle – there is no antiphoton.
Antistars would still spew antihydrogen and antihelium into space, just as the Sun emits normal hydrogen and helium through the solar wind. So imagine the excitement when the AMS02 experiment strapped to the International Space Station detected eight particles of antihelium back in 2017. “The easy way to produce them would be the existence of a nearby antistar,” says Simon Dupourqué from L’Institut de Recherche en Astrophysique et Planétologie in Toulouse, France.
But if the light created by antistars is identical to ordinary stars, how could we find the antistar that’s potentially creating this antihelium? By
the way it interacts with any ordinary matter surrounding it. Like with the colliding galaxies, the resulting annihilation would create a distinctive burst of gamma rays. Inspired by the discovery, Dupourqué searched through ten years of data from Fermi, looking for the right kind of gammaray signal. Of the 5,787 gamma-ray sources he looked at, only 14 fit the bill. Dupourqué says this means there are no more than 14 antistars within a 32,600 light year radius of Earth. “To be clear, they are probably not antistars,” he says. The gamma rays could also be coming from the active centres of distant galaxies or from rapidly rotating dead stars called pulsars. Future searches could rule out antistars by looking for these less exotic objects.
Still, Dupourqué’s work is important because it places an upper limit on the possible local antistar population. A maximum of 1 in 400,000 stars in our local neighbourhood could be antistars. Scaling that up to the whole Milky Way, there are no more than a million antistars scattered among its estimated 400 billion ordinary stars. It adds to the idea that antistars, and by inference antigalaxies, are very rare, and the universe really does have a strong preference for matter over antimatter.
For antigalaxies to exist, antistars would have to clump together into larger structures. One such structure is a globular cluster, a dense blob of old stars that typically orbits around galaxies. There are about 150 swirling around our home galaxy, the Milky Way. A team led by Maxim Khlopov from the Southern Federal University in Russia wondered what would happen if one of those globular clusters, Messier 4, was made entirely of antistars. In a paper released in November 2020, the team discusses three different ways antimatter from antistars in M4 could reach Earth.
The first, as we’ve seen, is through a solar wind. Other space weather events such as solar flares and coronal mass ejections could also fire antiparticles across the void. Yet to penetrate the magnetic field of the Milky Way, enter the galaxy and reach our planet, the antiparticles would likely need even more energy than those two processes can provide. The team concluded that supernovae explosions as the antistars reached the end of their lives are the most likely culprit. It’s possible that this is the source of the antihelium seen by AMS-02.
The universe keeps on dropping us tantalising clues, teasing us with strings to pull at in order to unravel one of the greatest mysteries in the cosmos. Finding out why nature prefers matter over antimatter, or whether there are vast reservoirs of antimatter huddled in distant galaxies won’t just improve our theories of physics, it’ll also tell us why we’re here to ask these questions in the first place.
“Our experiments could be a significant step in solving the mystery of the missing antimatter in our universe”
Niels Madsen