Could there be a cluster of antimatter stars orbiting our galaxy?

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of Ask a Spaceman and Space Radio, and author of How to Die in Space. He contributed this article to Space.com's Expert Voices: Opinions and Insights.

We don't know why the universe is dominated by matter over antimatter, but there could be entire stars, and maybe even galaxies, in the universe made of antimatter. 

The anti-stars would continuously shed their antimatter components out into the cosmos, and could even be detectable as a small percentage of the high-energy particles hitting Earth.

The universe: Big Bang to now in 10 easy steps

Unbalanced birth

Antimatter is just like normal matter, except not. Every single particle has an anti-particle twin, with the exact same mass, exact same spin and exact same everything. The only thing different is the charge. For example, the anti-particle of the electron, called the positron, is exactly like the electron except that it has positive electric charge.

Our theories of fundamental physics point to a special kind of symmetry between matter and antimatter — they mirror each other almost perfectly. For every particle of matter in the universe, there ought to be a particle of antimatter. But when we look around, we don't see any antimatter. Earth is made of normal matter, the solar system is made of normal matter, the dust between galaxies is made of normal matter; it looks like the whole universe is entirely composed of normal matter.

There are only two places where antimatter exists. One is inside our ultra-powerful particle colliders: When we turn them on and blow up some subatomic stuff, jets of both normal and antimatter pop out. The other place is in cosmic rays. Cosmic rays aren't really rays but rather are streams of high-energy particles streaking in from across the cosmos and hitting our atmosphere. Those particles come from ultra-powerful processes in the universe, like supernovae and colliding stars, and so the same physics applies.

But why is antimatter so rare? If matter and antimatter are so perfectly balanced, what happened to all the anti-stuff? The answer lies somewhere in the early universe.

The universe's dark secret: Where did all the antimatter go?

The anti-galaxy

We're not exactly sure what did it, but something went off balance in the young cosmos. Presumably in the good old days (and I'm talking when the universe was less than a second old here), matter and antimatter were produced in equal amounts. But then something happened; something caused more matter to be produced than antimatter. It wouldn't take much, just a one part per billion imbalance, but it would be enough for normal matter to come to dominate essentially the entire universe, eventually forming stars and galaxies and even you and me.

But whatever that process was — and I should mention that the detailed physics of that antimatter-killing mechanism in the early universe are currently beyond known physics, so there's a lot up in the air here — it may not have been entirely perfect. It's totally possible that the early universe may have left large clumps of antimatter alone, floating here and there throughout the universe.

Those clumps, if they survived long enough, would grow up in relative isolation. Sure, when matter and antimatter collide, they annihilate each other in a flash of energy, and that would've caused some headaches in the early universe, but if the antimatter clumps made it through that trial, they would've been home free.

Over the course of billions of years, those clumps of antimatter could have assembled together and grown larger. Remember that the only difference between antimatter and matter is their charge — all other operations of physics remain exactly the same. So you can form anti-hydrogen, anti-helium, and anti-all-the-other-elements. You can have anti-dust, anti-stars fueled by anti-fusion, anti-planets with anti-people drinking refreshing anti-glasses of anti-water, the works.

Counting backward

Astronomers don't suspect that there are entire anti-galaxies floating around out there, because their interactions with normal matter (say, when two galaxies collide) would release quite a bit of energy — enough for us to notice by now. But smaller clumps could be possible. Smaller clumps like globular clusters.

Globular clusters are small, dense clumps of fewer than a million stars orbiting larger galaxies. They are thought to be incredibly old, as they are not forming new stars in the present epoch, and are instead filled with small, red, aged populations. They are also relatively free of gas and dust — all the fuel you need to make new stars. They just sort of hang around, orbiting lamely around their larger, more active cousins, remnants of a bygone and largely forgotten era. The Milky Way itself has a retinue of about 150 of them.

And some of them may be made of anti-stars. 

A team of theoretical astrophysicists calculated what would happen if one of the globular clusters orbiting the Milky Way was actually an anti-cluster, as reported in a paper recently appearing in the preprint journal arXiv. They asked a simple question: what would happen?

Unless the globular cluster plunged right through the disk of the Milky Way, it wouldn't really blow up. Since the anti-cluster would just be made of stars, and stars don't take up a lot of volume, there aren't a lot of opportunities for big booms. Instead, the anti-stars in the anti-cluster would go about their normal lives, doing normal star-like things.

Things like emitting a constant stream of particles. Or having huge flare and coronal mass ejection events. Or colliding with each other. Or dying in fantastic supernova explosions.

All those processes would release tons of antiparticles, sending them flowing out of the anti-cluster and into the nearby volume of the universe, including the Milky Way. Including our solar system, where those antiparticles would appear as just another part of the cosmic ray gang.

So could some of the anti-particles hitting our atmosphere every single day have been launched by an anti-star millions of years ago? Right now it's too difficult to tell. There are certainly anti-particles mixed in as a part of the total cosmic ray population, but because our galaxy's magnetic field alters the paths of charged particles (normal and anti alike), it's hard to tell exactly where a particular cosmic ray actually came from.

But if astronomers are able to pinpoint a globular cluster as a particularly strong source of anti-particles, it would be like opening a time capsule, giving us a window into the physics that dominated the universe when it was only a second old.

We also couldn't ever visit the anti-cluster, because as soon as we did we would blow up.

Read more: "Antihelium flux from antimatter globular cluster"

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