“If you see an antimatter version of yourself running towards you, think twice before embracing.” -J. Richard Gott III

It may not occur to you that it's a special thing that the Earth and everything found on it is made of matter; it seems intuitive that it couldn't be any other way. And yet, the very laws of nature themselves haven't yet told us why or how the Universe is this way! For this week's Ask Ethan (and leave your own questions or suggestions here), our regular reader Michael Fisher wants us to get at the heart of this, asking:

Is it true that in the early universe matter & antimatter were created in equal amounts? And if not do we know why there was a difference? If they were created in equal amounts how is it we are left with one particle per billion of matter with very little [or no?] antimatter remaining? In other words ~ do we have a mechanism to explain the dominance of matter over antimatter in our observable universe?

Think about this for a minute, if you will.

This is (a little bit of) our Universe. Hundreds of billions of stars and star systems within our galaxy alone; hundreds of billions of galaxies within our observable Universe. Out of all the possibilities, we've only directly explored our own star system, which it turns out -- to no one's surprise -- is made of matter and not antimatter.

Image credit: U.S. Air Force.

But, as far as we can tell, being made of matter and not antimatter is something that's true of everything in the Universe! More specifically, there's stuff everywhere in the Universe, and if there was a part of the observable Universe that was made of antimatter, we would see catastrophic results where the matter and antimatter meet!

For example, the interstellar medium -- the space between stars in galaxies -- is full of material, even if there aren't any stars in many regions. Space is vast, of course, and the density of matter is sparse, you might be wondering if you threw a single antimatter particle (say, an anti-proton) into the mix, how long it would last before running into a particle of matter an annihilating, on average. In our own galaxy's interstellar medium, the mean lifetime would be on the order of about 300 years, which is tiny compared to the age of our galaxy! This huge constraint tells us that, at least by us, the amount of antimatter that's allowed to be mixed in with the matter we observe is at most 1 part in 1015!

On larger scales, we've now mapped out galaxies and galaxy clusters to huge distances, and taken detailed looks in many different wavelengths, including visible light, infrared, microwave, radio, ultraviolet, X-ray and gamma-ray wavelengths. In particular, X-ray and gamma-ray observations are hugely important, because when matter and antimatter meet each other and annihilate, they emit characteristic high-energy radiation that would be detectable to our great observatories.

Image credit: Karen Teramura, UHIfA / NASA.

With 55 galactic clusters well-measured under our belt, from just a few million light-years to over three billion light-years away, we've seen that even on large, cosmological scales, 99.999%+ of what exists in our Universe is matter and not antimatter.

The overwhelming conclusion is that everything we see in our Universe is made predominantly out of matter, not antimatter.

And yet, in many ways, this is a huge surprise! You may know that E = mc2, and that tells you that not only does mass, inherently, have a certain amount of energy to it, but that you can create a particle with a given mass if you have enough energy to do it, and that energy is given by Einstein's most famous equation. But there's a little more to the story than that.

Image credit: CSIRO; Australia's version of the NSF.

You see, as far as we've been able to produce reactions between fundamental particles in laboratory-and-observatory conditions here on Earth, the only way to create matter is by taking twice the amount of energy that E=mc2 would require, and creating equal amount of both matter and antimatter. And conversely, the only way we've ever found to destroy matter is by colliding it with its antimatter counterpart, producing pure energy as a result. All the laws of physics -- all the reactions and interaction ever discovered -- indicate that this is the case at all energies and at all time.

And yet, then there's our Universe.

Image credit: ESA and the Planck collaboration; minor edits by me.

If we started off our Universe with a hot Big Bang, after the end of inflation, with all the right initial conditions and all the known laws of physics and nothing else, we'd have an early state that was perfectly in-line with our expectations:

The Universe would be hot, dense, expanding, and full of radiation and equal parts matter-and-antimatter.

Matter and antimatter would sometimes collide, annihilating into radiation, while high-energy particles would also sometimes collide, spontaneously creating new particles of matter-and-antimatter, in equal amounts, with that excess energy.

The Universe would expand and cool, and as it did, energies and densities would drop.

Those last two parts -- energies and densities dropping -- leads to a race.

Image credit: Addison-Wesley, retrieved from J. Imamura / U. of Oregon.

As energies drop, it gets harder and rarer for high-energy particles to produce new matter/antimatter pairs (b), decreasing the ratio of matter-(and-antimatter)-to-radiation in the Universe. But as densities drop, it gets harder and harder for matter/antimatter pairs to find one another (a), which means that the ratio will never drop all the way to zero; there will always be some matter (and antimatter) left in these scenarios.

And that's where it gets odd. Because if we just stick to the known laws of physics and the reactions we've seen, we'd expect that there'd be about 1020 particles of radiation for every one particle of matter (or antimatter). But in our Universe, there are "only" about a billion -- or 109 -- particles of radiation for every one particle of matter (something we can tell by measuring the photon-to-baryon ratio indirectly, via the CMB), and the amount of antimatter is much lower.

Image credit: NASA and the WMAP science team.

So where did all this extra matter come from? Why was it extra matter that was created and not extra antimatter? And when did it happen, and how did it happen?

Michael (and everyone), I'm going to be honest with you: this is one of the biggest unsolved puzzles in all of physics today. But not knowing everything doesn't mean we don't know some really important clues. It's been known since the 1960s, for instance, that so long as you fulfill these three famous criteria:

Out-of-equilibrium conditions, Baryon-number-violating interactions, and C- and CP-violation,

you not only can create more matter than antimatter (or vice versa), but an asymmetry is inevitable. And, as luck would have it, two of these criteria are very easy to satisfy.

"Out-of-equilibrium conditions" would be something where, if something happens in one part of a system, there are other parts of the system that will be unaffected by what's happening, as that information can't reach it in a timely manner. The expanding Universe is a perfect example of a system that's out-of-equilibrium by definition, and the description I gave you above of matter/antimatter creation-and-annihilation as the Universe expands and cools is a perfect example of an out-of-equilibrium process.

So that one's easy.

We also know that there are many fundamental examples of how matter and antimatter can differ from one another, and how certain symmetries are broken. One of these ways is charge conjugation (C) symmetry, where you replace all the particles with antiparticles; if C-symmetry is conserved, the particle-and-antiparticle systems will behave the same way. Another of these ways is parity (P), or mirror-image symmetry; if P-symmetry is conserved, the real system and the mirror-image system will behave in the same way.

Image credit: James Schombert / U. of Oregon.

If you have an unstable particle -- like a spinning muon -- it's going to decay in a very particular way: by ejecting an electron in a certain direction relative to its spin. If you reflect that muon in a mirror (P), it's going to appear to eject the electron in the opposite direction relative to its spin, something that doesn't happen in real life. If, instead, you replace that spinning muon with a spinning anti-muon (C), it would eject a positron in that original direction, also something that doesn't happen in real life. But if you replace the spinning muon with a spinning mirror-image anti-muon (C and P together, or CP), you'd hope that this decay happened just as reliably as a muon would decay in the real (non-mirror) world. But it doesn't, and there are other examples of how C and CP are violated, including in Kaon and B-meson systems.

So all we need is for an interaction that violates baryon number in sufficient amounts, or, in other words, create baryons where there were none (but there were other things) before. Unfortunately, that requires some physics beyond the physics that's known, and a part of our Standard Model!

But, there are many known mechanisms that could allow us to make this happen, including:

From grand unified theories containing new GUT-scale particles,

From theories with new scalars that contain the Affleck-Dine mechanism,

From extensions of the standard model that include heavy, sterile neutrinos,

From a lepton excess in the early Universe (leptogenesis, for the experts), and

From new physics at the electroweak scale that could enhance the matter/antimatter asymmetry found in the Standard Model.

I'm going to walk you through one example (that I've detailed in the past and alluded to twice before) just for illustrative purposes.

Image credit: me, background by Christoph Schaefer.

Imagine the hot, dense, young Universe, and imagine that in addition to all the radiation and the matter-and-antimatter particles found in the Standard Model flying around, there's also a new particle (and antiparticle), the Q (and the anti-Q). The Q is massively heavy (much heavier than a proton), has a positive charge of +1 (the same as the proton charge), and gets created in great abundance in the early Universe, along with its antimatter counterpart the anti-Q, which has the same mass as the Q but the opposite charge.

Since they're both unstable, when the Universe cools enough, we're going to stop making new ones eventually, and while most Qs and anti-Qs will find each other and annihilate, the ones that remain after that will simply decay.

Image credit: me, background by Christoph Schaefer.

For every decay that can happen to a Q, the antimatter counterpart must happen for the anti-Q. If a Q decays into a proton and a neutrino, the anti-Q must decay into an antiproton and an antineutrino. If a Q decays into an antineutron and a positron, the anti-Q must decay into a neutron and an electron.

(These aren't real particles -- the Q and anti-Q are totally made up for illustration -- but there are examples of particles, such as the X-and-Y bosons in GUTs, or leptoquarks in some Standard Model extensions, that follow very, very similar rules.)

If there were no CP-violation, they'd decay in the exact same way as one another.

Image credit: me, background by Christoph Schaefer.

And this is very boring; it wouldn't create a matter excess, for one. But if you allow for CP-violation, one of the things that can be different between particles and antiparticles is what we call the branching ratio, or what percent of Qs decay into protons/neutrinos vs. what percent of anti-Qs decay into antiprotons/antineutrinos. So we could have something like the following, which is similar to what we see in Kaon/B-meson systems. Note the differences between the decays of the Qs and anti-Qs; you may have to look at it for a second to see the difference.

Image credit: me, background by Christoph Schaefer.

So let's assume we had a Universe full of matter and antimatter in equal parts and radiation, which we ignore. Let's also assume that there were a bunch of Qs and anti-Qs in equal numbers, which decayed according to the CP-violating rules above.

What would be left?

A sea of protons, neutrinos, antineutrons, positrons, antiprotons, antineutrinos, neutrons and electrons, for sure. But -- and here's the key -- there would be more protons-and-neutrinos than there would be antiprotons-and-antineutrinos, and there would be fewer antineutrons-and-positrons than there would be neutrons-and-electrons. If we ignore the leptons (neutrinos, electrons and their antimatter counterparts), this is what a sea of Qs and anti-Qs decaying would give us.

Image credit: me, background by Christoph Schaefer.

And after the matter-antimatter pairs all find one another, that's what we'd have left: an excess of matter over antimatter!

And some variation of that is almost definitely where the matter-antimatter asymmetry came from, and why every place in the Universe appears to have the same density of matter (and not antimatter) no matter where we look! Even though this is one of the greatest unsolved problems in all of physics, we still know an awful lot about it, and that's a story worth telling!

So congratulations, Michael, we owe you a prize (and you owe me your address), and if you'd like a chance to win one of three remaining prizes (or just have a question you'd like to see featured on Ask Ethan), ask us here!