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Starts With A Bang

What was it like when matter defeated antimatter?

In the earliest stages of the hot Big Bang, equal amounts of matter and antimatter should have existed. Why aren't they equal today?
A diagram showing the difference between matter and antimatter.
At the high temperatures achieved in the very young Universe, not only can particles and photons be spontaneously created, given enough energy, but also antiparticles and unstable particles as well, resulting in a primordial particle-and-antiparticle soup. Although the laws of physics are largely symmetric between matter and antimatter, it's very clear that today's Universe is filled with matter and nearly completely devoid of antimatter. Any asymmetry must have been generated in the very early Universe, shortly following the hot Big Bang.
Credit: zombiu26 / Adobe Stock
Key Takeaways
  • As far as we can tell, nearly all the laws of physics are identical between matter and antimatter, and all observed reactions create or destroy them only in equal amounts.
  • But today, we are certain we inhabit a Universe that's predominantly made of matter, as though for every ~1 billion antimatter particles early on, there were ~1 billion and 1 matter particles for them.
  • If the Universe began in a matter-antimatter symmetric state, how did it come to create more matter than antimatter from it? It's one of the greatest cosmic mysteries still remaining to be solved.

13.8 billion years ago, at the moment of the Big Bang, the Universe was the hottest it’s ever been in history. Every single known particle existed in great abundance, along with equal amounts of their antiparticle counterparts, all smashing rapidly and repeatedly into everything around them. They spontaneously create themselves from pure energy, and annihilate away into pure energy whenever particle-antiparticle pairs meet up.

Additionally, anything else that can exist at these energies — new fields, new particles, or even dark matter — will spontaneously create itself under these conditions, too. But the Universe cannot sustain these hot, symmetric conditions. Immediately, it not only expands, but cools. In a fraction of a second, these unstable particles and antiparticles vanish, leaving a Universe favoring matter over antimatter. Here’s how it happens.

A 3d image of a red, green, and blue sphere.
The early Universe was full of matter and radiation, and was so hot and dense that it prevented all composite particles, like protons and neutrons from stably forming for the first fraction-of-a-second. There was only a quark-gluon plasma, as well as other particles (such as charged leptons, neutrons, and other bosons) zipping around at nearly the speed of light. This primordial soup consisted of particles, antiparticles, and radiation, and although was in a lower entropy state than our modern Universe, there was still plenty of entropy.
Credit: Models and Data Analysis Initiative/Duke University

At the moment of the Big Bang, the Universe is filled with everything that can be created up to its maximum total energy. There are only two barriers that exist:

  1. You have to have enough energy in the collision to create the particle (or antiparticle) in question, as given by E = mc2.
  2. You have to conserve all the quantum numbers that need to be conserved in every interaction that takes place.

That’s it. In the early Universe, energies and temperatures are so high that you not only make all of the Standard Model particles and antiparticles, you can create anything else that energy allows. This could include:

So long as you have enough energy and your interactions obey the quantum rules governing all of reality, any species of particle (or antiparticle) that can be created must be created in the early stages of the hot Big Bang.

grand unified theory
The particle content of the hypothetical grand unified group SU(5), which contains the entirety of the Standard Model plus additional particles. In particular, there are a series of (necessarily superheavy) bosons, labeled “X” in this diagram, that contain both properties of quarks and leptons, together, and would cause the proton to be fundamentally unstable. Their absence, and the proton’s observed stability, provide strong evidence against the validity of this theory in a scientific sense.
Credit: Cjean42/Wikimedia Commons

It must be stated that it isn’t certain that any of these particles that take us beyond the Standard Model in some way can actually exist in our Universe.

They’re theoretically allowed, in the sense that there’s nothing obvious that strictly forbids their existence, but that doesn’t mean they physically must or even can exist. In order to prove it, from an experimental/observational standpoint, we’ll have to actually achieve the energies necessary to create them and measure them.

This is a daunting task, as the energies achieved in the earliest stages of the Universe are approximately a factor of a trillion (1012) higher than the maximum energies achieved in particle collisions at the Large Hadron Collider at CERN. The most powerful thing we’ve ever created in all of human history absolutely pales in comparison to the energies, densities, and temperatures achieved in the crucible of the early Universe following the start of the hot Big Bang.

particle tracks cern lhc
The particle tracks emanating from a high energy collision at the LHC in 2012 show the creation of many new particles. By building a sophisticated detector around the collision point of relativistic particles, the properties of what occurred and was created at the collision point can be reconstructed, but what’s created is limited by the available energy from Einstein’s E = mc². The maximum LHC energies are about a factor of a trillion (10^12) lower than the energies created at the start of the hot Big Bang.
Credit: Panos Charitos/Wikimedia Commons user PCharito

Once the hot Big Bang commences, then immediately thereafter the Universe continues to expand, and as it does, it not only gets less dense, but cools. The one factor that determines the energy of any quantum of radiation is its wavelength: short wavelength means higher energy, while long wavelength means lower energy. When the Universe is at its hottest and densest, the wavelength of light is at its shortest. But as the fabric of space expands, the wavelengths of the radiation within it stretches and lengthens.

This means that everything within the Universe cools, and anything you can describe as having a wavelength — which applies both to massless particles like photons as well as massive particles, which possess a de Broglie wavelength — will see that wavelength stretch, lengthen, and hence the quantum loses kinetic energy.

balloon expanding Universe
As a balloon inflates, any coins glued to its surface will appear to recede away from one another, with ‘more distant’ coins receding more rapidly than the less distant ones. Any light will redshift, as its wavelength ‘stretches’ to longer values as the balloon’s fabric expands. This visualization solidly explains cosmological redshift within the context of the expanding Universe. If the Universe is expanding today, that means it was smaller, hotter, and denser in the past: leading to the picture of the hot Big Bang. It also explains why all quanta lose kinetic energy as the Universe expands.
Credit: E. Siegel/Beyond the Galaxy

Consequently, in very short order, the expanding Universe cools tremendously. With lower energies available, it becomes harder and harder to create particles of a given mass. E = mc2 works both ways:

  • particle-antiparticle pairs can annihilate into radiation,
  • but collisions can also spontaneously create particle-antiparticle pairs.

If there are new particles (and/or antiparticles) beyond what’s in the Standard Model, they’re created at ultra-high energies, but then cease being created when the Universe falls below a certain threshold temperature.

What happens to the particles-and/or-antiparticles that are left over from that time? There are three possibilities:

  1. They annihilate away, like particle-antiparticle pairs are supposed to, until their densities are low enough that they can no longer find one another to collide with.
  2. They decay, like all unstable particles, into whatever decay products are allowable by the laws of physics.
  3. They happen to be stable, and remain until the present day, where they influence the Universe and can be detected.
kaon decay
When the neutral kaon decays, it typically results in the production of either two or three pions. Supercomputer simulations are required to understand whether the level of CP-violation, first observed in these decays, agrees or disagrees with the Standard Model’s predictions. With the exception of only a few particles and particle combinations, almost every set of particles in the Universe are unstable, and if they don’t annihilate away, they will decay in short order.
Credit: Brookhaven National Laboratory

The first possibility happens for everything imaginable, but always leaves some relic particles behind. If what’s left over is stable, it makes an excellent dark matter candidate. Right-handed neutrinos and the lightest supersymmetric particle make excellent dark matter candidates in exactly this vein. They:

  • are massive,
  • are created in great numbers,
  • then some of them annihilate away,
  • leaving the rest to persist until the present day,
  • where they no longer interact substantially with any of the particles in today’s Universe.

That’s a perfect recipe for dark matter. But if what’s left over isn’t stable, like hypothetical superheavy boson particles that arise in grand unification scenarios, they create a perfect recipe for creating a Universe with more matter than antimatter.

Universe without matter antimatter asymmetry
As the Universe expands and cools, unstable particles and antiparticles decay, while matter-antimatter pairs annihilate and photons can no longer collide at high enough energies to create new particles. Antiprotons will collide with an equivalent number of protons, annihilating them away, as will antineutrons with neutrons. But antineutrinos and positrons can remain interconverting with neutrinos and electrons to create-and-destroy matter/antimatter pairs until the Universe is between 1 and 3 seconds old.
Credit: E. Siegel/Beyond the Galaxy

Let’s illustrate how this works with an example. In the Standard Model, we have two types of fermions: quarks, which make up atomic nuclei, and leptons, like the electron or neutrino. Quarks contain a quantum number known as baryon number. It takes three quarks to make one baryon (like a proton or neutron), so each quark has a baryon number of +⅓. Each lepton is its own entity, so every electron or neutrino has a lepton number of +1. Antiquarks (-⅓) and antileptons (-1) have correspondingly negative values for lepton and baryon numbers.

If grand unification is true, then there ought to be new, super-heavy particles, which we’ll call and Y. There also ought to be their antimatter counterparts: anti-X and anti-Y. Instead of baryon or lepton numbers, however, these new XY, anti-X and anti-Y particles only have a combined B – L number, or baryon number minus lepton number. According to the simplest [SU(5)] model of grand unification, X bosons will have a B – L number of +⅔ (and anti-Xs will have B – L of -⅔), while Y bosons will have a B – L number of -⅔ (and anti-Ys will have B – L of +⅔). Their electric charges will be different than this, with Xs having +4/3 and Ys having -1/3, and anti-Xs having -4/3 while anti-Ys have +1/3.

existence of X and Y bosons and their antimatter counterparts in the early primordial plasma soup
In addition to the other particles in the Universe, if the idea of a Grand Unified Theory applies to our Universe, there will be additional super-heavy bosons, X and Y particles, along with their antiparticles, shown with their appropriate charges amidst the hot sea of other particles in the early Universe.
Credit: E. Siegel/Beyond the Galaxy

At high energies, plenty of these new particles and antiparticles are created. Once the Universe expands and cools, however, they’ll either annihilate away or decay, without the energetic possibilities of making new ones. There’s a powerful theorem in physics that dictates how these particles can decay. Any decay that the X or Y particle exhibits, the anti-X or anti-Y particle needs to have the corresponding antiparticle decay pathway. That symmetry must exist.

But what doesn’t need to be symmetric is known as the decay branching ratios: which decay pathway the particles or antiparticles prefer. We’ve already seen these ratios differ in the Standard Model, and if they differ for these hypothetical new particles, we can spontaneously wind up with a Universe that prefers matter over antimatter. Let’s take a look at one specific scenario that shows this.

matter antimatter asymmetry
If you create new particles (such as the X and Y here) with antiparticle counterparts, they must conserve CPT, but not necessarily C, P, T, or CP by themselves. If CP is violated, the decay pathways — or the percentage of particles decaying one way versus another — can be different for particles compared to antiparticles, resulting in a net production of matter over antimatter if the conditions are right.
Credit: E. Siegel/Beyond the Galaxy

Say your X-particle has two pathways: decaying into two up quarks or an anti-down quark and a positron. The anti-X must have the corresponding pathways: two anti-up quarks or a down quark and an electron. In both cases, the X has – of +⅔, while the anti-X has -⅔. For the Y/anti-Y particles, the situation is similar, but flipped, with the Y having B – L of -⅔ and the anti-Y having +⅔. So far, everything is still symmetric between matter and antimatter.

But it doesn’t necessarily remain that way. Here’s how you make a Universe with more matter than antimatter: the X could be more likely to decay into two up quarks than the anti-X is to decay into two anti-up quarks, while the anti-X could be more likely to decay into a down quark and an electron than the X is to decay into an anti-down quark and a positron.

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If you have enough X/anti-X and Y/anti-Y pairs, and they decay in this allowed fashion, you’ll get an excess of baryons over antibaryons (and leptons over anti-leptons) where there was none previously.

anitmatter annihilation
After quark/antiquark pairs annihilate away, the remaining matter particles bind themselves into protons and neutrons, amidst a background of neutrinos, antineutrinos, photons, and electron/positron pairs. There will be an excess of electrons over positrons to exactly match the number of protons in the Universe, keeping it electrically neutral.
Credit: E. Siegel/Beyond the Galaxy

This is only one of very few known, viable scenarios (GUT baryogenesis) that could lead to the matter-rich Universe we inhabit today, with the other three involving new lepton/neutrino physics (leptogenesis), supersymmetric physics (Affleck-Dine baryogenesis), or new physics at the electroweak scale (electroweak baryogenesis), respectively.

Yet in all cases, it’s the out-of-equilibrium nature of the early Universe, which creates everything allowable at high energies and then cools to an unstable state, which enables the creation of more matter than antimatter. We can start with a completely symmetric Universe in an extremely hot state, and just by cooling and expanding, wind up with one that becomes matter-dominated. The Universe didn’t need to be born with an excess of matter over antimatter; the Big Bang can spontaneously make one from nothing. The only open question, exactly, is how.


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