How science can solve the mystery of why we exist at all

How science can solve the mystery of why we exist at all

The laws of physics state that you can’t create or destroy matter without also creating or destroying an equal amount of antimatter. So how are we here?

We can only make or destroy matter by creating or destroying an equal amount of antimatter in every experiment ever performed, both on Earth and in the wider Universe. Despite this, our Universe is predominantly composed of matter, with only a minuscule fraction of a percent consisting of antimatter. Although the baryogenesis problem remains one of physics’ greatest unresolved riddles, we may be on the verge of finally understanding why.

Human beings appeared for the first time after more than 13 billion years of cosmic evolution, on a planet orbiting a very unremarkable star in a basic, everyday galaxy. We began to unravel many of the Universe’s mysteries hundreds of millions of years later. On cosmic scales, we’ve learned that the Universe is filled with planets, stars, galaxies, and other objects, stretching only to the boundaries established by the speed of light and the Big Bang. On a lesser scale, we’ve discovered the Universe’s building blocks, such as molecules, atoms, and the subatomic particles that make up our reality.

Yet, everywhere we turn, we see the same thing: matter is the material that makes up every planet, star, galaxy, and cloud of gas or dust. This is significant because, as far as we know, the only method to make or destroy matter is to also produce or destroy an equivalent amount of antimatter. The Universe appears to be in a matter-dominated cosmic imbalance. The baryogenesis puzzle, or why there is a matter-antimatter asymmetry, is one of the great unanswered problems of existence. Although we haven’t found the solution to this riddle, we do know a lot about it. Here’s where science is right now.

The main image depicts our galaxy’s antimatter jets creating ‘Fermi bubbles’ in the halo of gas that surrounds it. Actual Fermi data, shown in the small inset picture, demonstrates the gamma-ray emissions produced by this activity. These “bubbles” are created by the energy released by electron-positron annihilation, which is an example of matter and antimatter interacting and being turned into pure energy via E = mc2. (Photos by David Aguilar (main); NASA/GSFC/Fermi (inset))

What is the Universe made of?

The first step in resolving any problem is to ensure that it is indeed the problem you believe it to be. When matter and antimatter collide in the Universe, they annihilate, and the resulting signal is incredibly particular. When a particle of matter collides with its antimatter counterpart, two photons with equal energy and opposite momenta are often produced (in the collision’s center-of-momentum frame of reference). An electron colliding with a positron, for example, creates two photons with exactly 511,000 electron-Volts of energy each: the energy equivalent of the mass of the colliding particles, according to Einstein’s E = mc².

We may observe these annihilation signals anywhere in space, allowing us to determine where matter and antimatter interact. If there were:

  • planets,
  • stars,
  • galaxies,
  • galaxies clusters,
  • or even intergalactic space regions,

We’d detect evidence of those high-energy photons from annihilation at the interface where some were matter and others were antimatter. The fact that we observe such photons, although infrequently and only in specific locations (mainly corresponding with emissions from active black holes), indicates that the entire observable Universe is composed primarily of matter rather than antimatter.

We can constrain the presence of antimatter from the radiation at their interfaces by studying colliding galaxy clusters. In all cases, the amount of antimatter in these galaxies is less than one part in 100,000, which is compatible with their formation from supermassive black holes and other high-energy sources. There is no indication that antimatter is abundant in the universe. (Image courtesy of G. Steigman, JCAP, 2008)

Another piece of evidence we can use to figure out what the Universe is made of is the particular ratios of the lightest elements and their isotopes in the Universe. In the early stages of the hot Big Bang, before any stars formed, the Universe was smaller, denser, more uniform, and filled with higher-energy radiation. There is radiation left over from the hot Big Bang in the Universe today, but it is diffuse and low in energy; we can measure it and assign it a temperature, and we find that it is just 2.725 K now.

Because the wavelength of each light wave defines its energy, and the Universe expands through time, the wavelength of each photon gets stretched as times goes. However, if we extrapolate backwards, we find that the wavelength of each photon was shorter — more compressed — in the past, implying that the further back in time we look, the hotter the Universe was in its early phases. At some point in time, the Universe became so hot that neutral atoms could not form because there were not enough photons with enough energy to prohibit electrons from stably binding to the atomic nuclei that were present. But if we want, we can go back even more.

Photons scatter off of electrons and have enough energy to knock any atom back into an ionized state in the early stages (left). Once the Universe cools enough and is devoid of such high-energy photons (right), they are unable to interact with neutral atoms and instead merely free-stream, because they have the wrong wavelength to excite these atoms to a higher energy level. If there is an early version of dark energy, the early expansion history, and hence the scale at which we experience acoustic peaks, will shift significantly. (Photo by E. Siegel/Beyond the Galaxy)

We can travel back in time to a time when the Universe was so hot that even atomic nuclei couldn’t join together. Every time they tried, a photon would blast the individual protons and neutrons, preventing them from combining to form heavier atoms. Only once the Universe has cooled below a certain critical level — approximately 3 to 4 minutes after the start of the hot Big Bang — can we begin building atomic nuclei heavier than a single, simple proton.

Once that moment occurs, we can use nuclear physics to construct the Universe’s lightest elements.  Remarkably, the ratio of the light elements and their isotopes that we get out, including:

  • hydrogen (a single proton),
  • deuterium (a proton plus a neutron),
  • helium-3 (two protons plus a neutron),
  • helium-4 (two protons and two neutrons),
  • lithium-7 (four protons and three neutrons),

The ratio of photons to the total number of protons and neutrons combined is the sole metric that matters. Observations from the most pristine clouds of gas we can locate, as well as the imprint in the cosmic microwave background, provide the same result: there is approximately one proton or neutron for every 1.6 billion photons in the Universe. There was more matter than antimatter even in the very early moments of the hot Big Bang.

The lightest elements in the Universe were formed during the early stages of the hot Big Bang, when raw protons and neutrons fused to form hydrogen, helium, lithium, and beryllium isotopes. All of the beryllium was unstable, leaving the Universe with only the first three elements before star formation. The measured element ratios allow us to estimate the degree of matter-antimatter asymmetry in the Universe by comparing baryon density to photon number density. (Photos courtesy of E. Siegel/Beyond the Galaxy (L) and NASA/WMAP Science Team (R).)

Why the problem is so problematic

This is a serious quandary. On the one hand, this is a positive development. If the Universe contained equal amounts of matter and antimatter, practically all of it would have annihilated. At the moment, the Universe contains fewer than one particle of matter or antimatter every cubic kilometer.

However, the Universe is nearly a billion times denser than that, and virtually all of what remains is matter, not antimatter. However, the only technique we know of converting energy into mass or mass into energy always produces the same result: the number of matter particles minus the number of antimatter particles is always a constant.

Something other than what the Standard Model predicts has to be happening with the particles in the Universe to generate the Universe as we see it today. To approach the problem scientifically, we must extrapolate back to the earliest state of the hot Big Bang, where particles and antiparticles of all types could easily be created at the highest energies, and see what it would take for the Universe to create a matter-antimatter asymmetry where none previously existed.

The Big Bang creates matter, antimatter, and radiation, with slightly more matter created at some point, resulting in the Universe we know today. It’s unclear how such asymmetry developed or emerged from a lack of asymmetry to begin with. (Photo by E. Siegel/Beyond the Galaxy)

At all energies, every interaction between particles (and antiparticles) that we’ve ever observed has never generated or destroyed a single particle of matter without also creating or destroying an equal number of antimatter particles. This holds true for all quarks and antiquarks, all leptons and antileptons, all three lepton families (electron, mu, and tau), and all particle combinations. We don’t know how to generate matter without also producing antimatter, and we don’t know how to destroy antimatter without also destroying matter.

Nevertheless, it is theoretically feasible to establish a matter-antimatter asymmetry when none previously existed. Furthermore, Soviet physicist Andrei Sakharov demonstrated in 1967 that it is possible to do so without violating the laws of physics. All you have to do is meet the three conditions listed below:

  • The Universe must be out of thermal equilibrium.
  • The Universe must contain examples of both C-symmetry and CP-symmetry violation.
  • And the Universe must admit interactions that violate the conservation of baryon number.

These three components are now known as the Sakharov criteria. (And, yes, we will explain them in plain English!) If all three of these conditions are met, a matter-antimatter asymmetry is unavoidable. This is how it would work:

Given enough energy, the very young Universe’s high temperatures can produce not only particles and photons, but also antiparticles and unstable particles, resulting in a primordial particle-and-antiparticle soup. Even under these conditions, however, only a few distinct states or particles can emerge. (Photo courtesy of Brookhaven National Laboratory)

How to solve the puzzle, theoretically

Because you have so much energy at the start, you will abundantly and spontaneously produce all of the particles and antiparticles that energy allows you to create: once again via E = mc². You’ll make an equal amount of them, but as the Universe grows, they’ll cool.

This is when the first step, “out of thermal equilibrium,” comes into play. If you are in thermal equilibrium, the number of particles and antiparticles of each species you make will match the number of particles and antiparticles of that species destroyed with each time interval. However, if you’re not in thermal equilibrium, which occurs when the Universe cools below the energy threshold required to create that species via E = mc², you’re out of thermal equilibrium.

When you are out of thermal equilibrium, you make particles and antiparticles of each species at a slower rate than you destroy them via matter-antimatter annihilation. If the particles (and antiparticles) of that species are unstable, they will decay rather than annihilate when their densities fall low enough. And if they decay with the necessary qualities (that is, if they satisfy the other two Sakharov requirements), they have a potential to cause matter-antimatter asymmetry.

Most non-scientists are undoubtedly familiar with the process of radioactive decay, in which an atomic nucleus can change the amount of protons and/or neutrons in its nucleus to generate a new element. During decays, “daughter” particles with certain energy and momentum properties in relation to the spin of the “parent” particle are emitted. Although particles and antiparticles share many features, these decays are sometimes seen to be asymmetric in specific ways. (Credit: Inductiveload/Wikimedia Commons)

Let’s pretend we have unstable particles and antiparticles that are decaying. Certain parallelism exists between all matter and antimatter particles. Matter and antimatter particles, for example, must all have the same:

  • rest mass,
  • magnitude of electric charge,
  • quantum spin,
  • allowable decay pathways,
  • and total mean lifetimes

as well as one another Certain features, however, are allowed to differ. Certain features of matter and antimatter particles of the same species have already been shown to differ under weak interactions.

Consider imagining a spinning particle and visualizing it by curling the fingers of your left hand towards your thumb. If your thumb is pointing upwards, your fingers will curl clockwise when you gaze “down” at your thumb. When that particle decays, it will spit a certain decay product out in the direction of your thumb a certain percent of the time.

If the Universe were perfectly symmetric between matter and antimatter, you’d expect that if you replaced that particle with an antiparticle, the matching decay product (the anti-version of whatever particle was spit out earlier) would always go in the same direction as your thumb.

Along with time-reversal and charge-conjugation symmetry, parity, or mirror-symmetry, is one of the three fundamental symmetries in the Universe. If particles spin in one direction and decay along a specific axis, flipping them in the mirror should result in them spinning in the opposite direction and decaying along the same axis. This was not observed to be the case for weak decays, which are the only known interactions to violate charge-conjugation (C) symmetry, parity (P) symmetry, and the combination (CP) of those two symmetries as well. (Credit: E. Siegel/Beyond the Galaxy)

That is “C” symmetry: replacing particles with antiparticles (and any antiparticles with their particle counterparts) to achieve the same outcome. If you don’t obtain the same result — if you detect a difference between particle and antiparticle decays — then the C-symmetry is broken.

Similarly, “P” symmetry allows you to reflect your particles (and/or antiparticles) in the mirror. If you replace particles with their mirror images and achieve the identical effects, you have maintained P-symmetry. If you don’t receive the same results, P-symmetry has been broken.

CP-symmetry is a combination of the two, which means that in order to break it, your particle system must act differently from an antiparticle system with the mirror-image configuration of your own system. It is violated in weak interactions for decaying particles such as Kaons (which contain weird quarks) and particles containing bottom and charm quarks.

The only thing that hasn’t been observed is the third Sakharov criterion, which is to see a response that deviates from the baryon number. (A quark has a baryon number of ⅓; an antiquark has a baryon number of -⅓.) There are no other known particles with a baryon number.) It suggests that there is a reaction, interaction, or new particle out there that has yet to be identified.

With the right GUT features, an equally-symmetric collection of matter and antimatter (of X and Y, as well as anti-X and anti-Y) bosons might give rise to the matter/antimatter asymmetry we see in our Universe today. However, we believe that the matter-antimatter asymmetry we see today has a physical, rather than a divine, explanation, but we don’t know for sure. (Photo by E. Siegel/Beyond the Galaxy)

We hope that precisely measuring particle properties, particularly at high energies, will reveal whatever that new interaction or particle is. In Grand Unified Theories, for example, as shown above, new particles known as X and Y bosons couple to both quarks and leptons. If these particles have different branching ratios than their antiparticle counterparts — that is, the particle version decays via two different pathways with a specific ratio, while the antiparticle version takes the same corresponding pathways but with a different specific ratio than the particle version — then all three Sakharov conditions are met, and matter-antimatter asymmetry is unavoidable.

New electroweak physics could also solve the baryogenesis problem. Another possibility for resolving the baryogenesis problem is the introduction of supersymmetry. Furthermore, it’s feasible that some fascinating lepton and/or neutrino physics emerges at extremely high energies, resulting in a lepton asymmetry that the Standard Model alone can later translate into a baryon asymmetry.

Due to the effect of sphaleron interactions working on a neutrino excess, when the electroweak symmetry breaks, the combination of CP-violation and baryon number violation can create a matter/antimatter asymmetry where none previously existed. (Credit: University of Heidelberg)

While we don’t know how the Universe got more matter than antimatter, we do know that it was an essential step in allowing our Universe, and the things and animals within it, to exist as they do. Many experiments from around the world are continually exploring matter and antimatter at subatomic scales, looking for evidence of baryon-number violation as well as additional C-symmetry and CP-symmetry violating interactions. We may not have discovered the “tree of life” that enabled our own existence, but based on what we know so far about physics, we may be confident we’re looking in the right forest.

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