Baryon asymmetry

The world is made of stuff.

Most stuff is matter. Some stuff is called antimatter. They are almost the same. But we have more matter. This is a big mystery!

Scientists think the Big Bang made both. They should have been equal. But they were not. Matter won the race.

One idea says they are far apart. They might live in different spots. But we have not seen this.

Another idea is a mirror world. It would flow backward in time. This is still a mystery. We want to know why matter won.

The universe is mostly made of matter. Matter is the stuff that makes up the Earth and people. There is also something called antimatter. Antimatter is almost the same as matter. They act in nearly identical ways. Scientists think the Big Bang made equal amounts of both. But the universe is not equal. Matter makes up the vast majority of everything we see. This is a great mystery in physics.

In 1967, a scientist named Andrei Sakharov shared three ideas. He said certain rules must be met to make more matter than antimatter. First, there must be a way to change the number of particles. Second, there must be a way to break symmetry. Symmetry means things are the same on both sides. This is called CP violation. It means a process happens at a different rate for antimatter. Third, things must be out of thermal equilibrium. This means the universe must expand very fast. This prevents particles from meeting and canceling each other out.

One idea is a mirror anti-universe. This model says the Big Bang made a pair of universes. Our universe flows forward in time. The other one flows backward. This could explain why we see so much matter.

The universe is filled with matter, which is the stuff that makes up everything we see. This includes the Earth, the stars, and even people. There is also something called antimatter. Antimatter is almost exactly like matter, but it acts as its opposite. Scientists expected the Big Bang to create equal amounts of both. However, matter makes up the vast majority of our universe. This imbalance is known as the baryon asymmetry problem. It is one of the great mysteries in physics today.

To explain this, we must look at how matter might have won the race. In 1967, a scientist named Andrei Sakharov proposed three necessary conditions. First, there must be a way to violate the baryon number. This means the total number of particles can change. Second, there must be CP violation. This means certain processes happen at different rates for matter and antimatter. Third, the universe must be out of thermal equilibrium. This happens when the universe expands so fast that particles cannot meet and cancel each other out.

Scientists have worked for a long time to find these rules in action. In 1964, the Fitch–Cronin experiment verified CP violation using particles called neutral kaons. This important work led to a Nobel Prize in 1980. More recently, the LHCb collaboration at the Large Hadron Collider found new clues. They studied the decays of particles called bottom Lambdas starting in March 2010. They saw an asymmetry of up to 20% in certain measurements. This suggests that new sources of CP violation might exist beyond what we already know.

There are other ideas about where the antimatter went. Some thought there might be huge regions of antimatter far away. These could be antimatter galaxies in deep space. However, if matter and antimatter touched, they would create bright gamma radiation. Scientists have searched for these bright zones for 30 years. They have not found any, so they think antimatter regions are unlikely. Another idea is the mirror anti-universe model. This theory suggests the Big Bang created two universes.

In the mirror model, our universe flows forward in time. The mirror universe would flow backward in time. This model was created by physicists at the Perimeter Institute in Canada. It uses the idea of quantum uncertainty to explain the difference. This model might even explain dark matter through superheavy neutrinos. We can also measure the imbalance using an asymmetry parameter. This compares the number of baryons to the number of photons. Today, we know there are about 411 photons for every cubic centimeter of space.

The baryon asymmetry problem is a major mystery in physical cosmology. It refers to the observed imbalance between baryonic matter and antibaryonic matter in our universe. Matter and antimatter behave in nearly identical ways. Because of this, scientists expected the Big Bang to create them in equal amounts. In reality, matter makes up almost everything in the observable universe. This includes all the stars, planets, and even human beings. Neither the Standard Model of particle physics nor general relativity explains this imbalance. It is a natural assumption that the universe should be neutral regarding conserved charges. Since it is not, some physical laws must have acted differently for matter and antimatter.

In 1967, Andrei Sakharov proposed three necessary conditions to explain this phenomenon. These are known as the Sakharov conditions. They describe how a baryon-generating interaction could produce matter at a higher rate than antimatter. These ideas were inspired by discoveries like the Cosmic microwave background and CP violation. The first condition is baryon number violation. This means the total number of baryons can change during particle interactions. The second condition is the violation of C-symmetry and CP-symmetry. C-symmetry refers to charge conjugation, while CP-symmetry combines charge and parity. Without these violations, interactions would simply balance each other out. The third condition is that interactions must occur out of thermal equilibrium. If the universe stayed in equilibrium, CPT symmetry would ensure that processes increasing and decreasing baryon numbers would cancel out.

Baryon number violation is essential to create an excess of matter. Currently, there is no experimental evidence that particle interactions break this number in a simple way. In the Standard Model, the baryon number appears to be conserved. However, the Standard Model does contain an Adler-Bell-Jackiw anomaly. This anomaly involves a triangle Feynman diagram with the weak interaction gauge group. To explain how this violation actually happens, scientists look to Grand Unification Theories (GUTs). These theories and supersymmetric (SUSY) models suggest that massive bosons, like the X boson, could allow for such events. These events could even include the decay of a proton.

CP-symmetry violation is the second requirement for baryogenesis. This means a process must happen at a different rate for matter than for its antimatter counterpart. The 1964 Fitch–Cronin experiment verified this using neutral kaons. This discovery was so important that it led to the 1980 Nobel Prize in Physics. In the Standard Model, CP violation appears as a complex phase in the quark mixing matrix. However, this amount of violation is not enough to explain the observed baryon asymmetry of the universe. Scientists are now looking for new sources of CP violation. For example, the LHCb collaboration at the Large Hadron Collider found a possible new source. During the first three years of LHC operations starting in March 2010, they studied the decay of the bottom Lambda particle. They found an asymmetry of up to 20% in certain quantities.

Some scientists have explored if antimatter is simply hidden in distant regions. They wondered if entire galaxies made of antimatter might exist in deep intergalactic space. If matter and antimatter regions touched, they would create detectable gamma radiation through annihilation. Scientists have spent 30 years searching for these bright zones. They have found no such boundaries in the observable universe. Because of this, it is now considered unlikely that large antimatter-dominated regions exist.

A different idea is the mirror anti-universe model. Physicists at the Perimeter Institute in Canada proposed that the Big Bang created a universe-antiuniverse pair. In this model, our universe flows forward in time. The mirror anti-universe would flow backward in time. This model suggests the Big Bang was a double-sided event. It uses quantum uncertainty to explain why the two universes are not perfect mirror images. This theory also provides a possible explanation for dark matter. It suggests that the pair would produce many superheavy neutrinos, also called sterile neutrinos. These neutrinos might also cause observed bursts of high-energy cosmic rays.

To understand the scale of this problem, scientists use the asymmetry parameter, η. This value relates the number density of baryons to the number density of photons. In the current universe, there are approximately 411 photons per cubic centimeter. Because the photon density changes as space expands, scientists prefer using entropy density, s. The entropy density of the universe has remained relatively constant throughout its evolution. By studying these densities and the way matter decoupled from radiation, physicists hope to finally solve this great mystery of the cosmos.