ANTIMATTER GALAXIES
Has the impossible existence of these objects just become possible?
You shouldn’t exist. That you do suggests something is very wrong with our best theories in physics. Yet there is a way to both explain your existence and save theorists’ blushes: if bizarre entities called antimatter galaxies also exist.
In the beginning there was nothing, only energy. Our universe burst into existence 13.8 billion years ago in the Big Bang, and soon some of the energy was converted into particles. The kind of particles that are the building blocks of atoms, which in turn are the building blocks of you. We call this stuff matter. Except that’s only half the story. The process of converting energy into stuff is called pair production. Even the name hints at a deeper reality. It creates pairs of particles. Every particle of new matter is joined by another of antimatter.
Antimatter sounds futuristic, and you’ll often see it deployed in science fiction. It may sound made
“Our universe came to contain so much more matter than antimatter”
Dan Hooper
up for dramatic effect, but we know for certain that it exists. A banana, for example, contains traces of radioactive potassium. As the potassium decays it spits out a particle called a positron every 75 minutes. A positron is the antimatter partner of the more familiar electron. It has identical properties, save for the opposite electric charge. It’s different from a proton – which also has the opposite charge to an electron – but is considerably heavier.
Your fruit bowl isn’t the only source of antimatter on Earth. The Fermi Space Telescope has spotted beams of antimatter launched by thunderstorms acting as giant particle accelerators. We also make antimatter in our own particle accelerators, such as the Large Hadron Collider (LHC) and the Fermilab Tevatron. Yet making a single, usable antiproton requires 1 million ordinary protons and at least 26 million times the energy of the antiproton itself. It won’t come as a surprise that the total amount of antimatter we’ve created in particle accelerators runs to just a few tens of nanograms – 1 nanogram is a billionth of a gram, about the same mass as a single human cell.
But really there shouldn’t be any matter or antimatter at all. The opposite process to pair production – where a particle of matter meets its antimatter equivalent and they recombine
– is called annihilation. The most common annihilation is when an electron meets a positron and they convert back into energy in the guise of gamma rays, the most energetic form of light.
If matter and antimatter were created in equal quantities after the Big Bang, then there has been more than enough time since for it all to annihilate. There should be no stars, no planets and definitely no radioactive bananas.
We can explain away this discrepancy if pair production is an asymmetrical process, but that goes against the known laws of physics, which are inherently symmetrical. Imagine that for every billion antimatter particles created after the Big Bang, a billion and one matter particles appeared. Over time, all of the antimatter would annihilate with almost all of the matter, leaving a very tiny residue of matter left over. It’s that residue that forms everything in the vast and sprawling universe we see today, including you.
There’s evidence of this asymmetry from various experiments with subatomic particles. Back in the 1960s, we noticed something odd was going on with particles called kaons. “Physicists expected that kaons could transform into antikaons and vice versa, but were surprised to learn that these processes don’t happen in both directions at the same rate,” says Dan Hooper from the University of Chicago. Nature seems to prefer kaons.
It’s not the whole answer, however. “[It’s] not enough to explain how our universe came to contain so much more matter than antimatter,” Hooper says. “Some other unknown process(es) must exist that made this possible.”
Another clue came in April 2021 when the team behind the T2K detector in Japan – which deals with subatomic particles called neutrinos – released its latest results. Neutrinos and antineutrinos each come in three different varieties, or ‘flavours’, and routinely switch between them. The team found that these transitions happen at a higher rate for neutrinos compared with antineutrinos. More evidence that matter and antimatter are not as symmetrical as we’d like to think.
Then, in June 2021, physicists using data from the LHC discovered something new about another kind of subatomic particle called the charm meson. They found that it can spontaneously switch between its matter and antimatter versions. Now they can experiment further to see if, like the kaon and the neutrino, the switch happens more often one way than the other. Doing so would provide yet more valuable hints as to where all the antimatter went.
Part of the problem with understanding antimatter is that its existence is so fleeting. Once created in particle accelerators, it quickly decays into something else or annihilates with matter.
Yet slowly but surely, we’re starting to understand more. In March 2021, a team led by Christopher Baker at Swansea University revealed it’d invented a new device that creates, confines and cools antihydrogen using lasers. It gives researchers more time to study the properties of antimatter before it disappears. “Our experiments could be a significant step in solving the mystery of the missing antimatter in our universe,” Baker’s colleague Niels Madsen wrote in a recent piece.
There’s another option that would maintain the sacred symmetry of our current best theories. It’s possible that matter and antimatter were created equally, but then separated somehow before they were ever able to recombine. Maybe we live in the matter-dominated part of the universe and there’s another distant part dominated by antimatter. “If this were true, then there should also exist other galaxies made of antimatter,” says Hooper. “In almost every respect these antimatter galaxies would look and behave just like ordinary galaxies – containing stars, gas, dust and planets, all made of antimatter.” There could even be antibananas spitting out electrons instead of positrons.
The big problem with this idea is that it’s likely the antimatter would come into contact with matter at some stage. An antimatter galaxy
“Antimatter galaxies would look and behave just like ordinary galaxies”
Dan hooper