The Very Early Big Bang

Most of us are familiar to some extent with three forces: gravity, electricity, and magnetism. However, James Clerk Maxwell, in the late 19th century, showed that the latter two forces are actually just two manifestations of a single force we now call electromagnetism. This was the situation until studies of the physics of atomic nuclei in the early 20th century revealed the existence of two other forces, the strong and weak nuclear forces, the effects of which are only apparent on the subatomic level. In the following, we discuss the properties of these four forces of nature.

• Gravity: Contrary to common experience, gravity is actually the weakest force in nature by far. On a scale where the strength of electromagnetism is one, the strength of gravity is only about 10⁻³⁶! Our perception that gravity is strong results from two of its properties. First of all, gravity is always attractive so that the effects of the many, many atoms in a typical object add together. Second, gravity is a long-range force. It follows an inverse-square law and can act over very large distances such as those between neighboring galaxies.
• Electromagnetism: This force is also long range, and as mentioned it's incredibly stronger than gravity. However, electrical charges come in two 'flavors' (+/−) and most atoms in the universe are electrically "neutral," i.e., they have equal numbers of positive and negative charges. This means that the effects of electromagnetism are only apparent in cases where the charges are unbalanced, as for example in a lightning storm.
• The strong nuclear force: Studies of the atomic nucleus in the 1930s showed that there must be a force other than the very weak force of gravity to hold it together. This force clearly must be stronger than electromagnetism since all the protons in the nucleus repel each other and would otherwise completely disrupt it. In fact, its strength is about 100 times that of electromagnetism. The explanation of the fact that we don't directly perceive this very strong force is that it has a very short range, not much more than the radius of a proton or neutron, and doesn't act beyond that distance.
• The weak nuclear force: This force is responsible for the radioactive decay of atomic nuclei. Its strength is about 10⁻³ times that of electromagnetism so, despite its name, it's many orders of magnitude stronger than gravity. However, its range is much less than even the radius of a proton.

In his study of the nature of electromagnetism Richard Feynman showed that, on the deepest level, this force results from the exchange of photons between charged objects. He built on the " Uncertainty Principle," introduced by Werner Heisenberg in 1927, which is the foundation of quantum mechanics. In its energy–time formulation, this principle can be expressed in the form of a simple equation:

$$\begin{eqnarray*}&&{\rm{\Delta }}E\cdot {\rm{\Delta }}t=h/4{\rm{\pi }}.\end{eqnarray*}$$

Here h is Planck's constant and the two quantities on the left-hand side of the equation are as follows. ΔE is an uncertainty in the energy of a system and Δt is a time interval. One way to look at this is that there is a fundamental limit on the precision to which the energy of a system can be measured, and that the longer the time interval of the measurement, the smaller the uncertainty in energy will be since the product of these two quantities is a constant. (The equation above is actually written in the so-called quantum limit. Since it's always possible to make a poor measurement, the equality sign is generally replaced by "greater than or equal to.") Because h is very small, this fundamental limit is usually (but not always) far smaller than any realizable experimental precision, but the principle itself has been verified in many different situations. However, there is another way to look at this equation in the light of the law of conservation of energy, which states that energy may be changed from one form to another but the total amount of energy always remains constant. (Note: since special relativity implies the interchangeability of mass and energy, what is actually conserved is mass plus energy.) This is usually thought of as a fundamental law of nature, but the uncertainty principle provides a way around it. Specifically, the energy of a system may briefly increase by an amount ΔE so long as this extra energy disappears in a time Δt given by the above equation. One analogy is with a bank having a very unusual lending rule: the more money you borrow, the sooner you have to pay it back. In the context of the uncertainty principle, you can "borrow" any amount of energy from the universe as long as you pay it back it the appropriate amount of time. Returning now to Feynman's picture of electromagnetism, the force between two charged objects is transmitted by the exchange of photons between them. This is often illustrated by analogy to two people in boats. One carries a heavy ball, which he throws to his partner. The momentum of the ball causes her to move away from him after she catches it. Similarly, Newton's third law says that he will also recoil after releasing it. The net result is that the boaters appear to repel each other (Figure 10.1).

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Of course, electromagnetism generates attractive as well as repulsive forces so the analogy breaks down in detail. Nevertheless, it gives some feeling for the concepts involved. Note, however, that in Feynman's case the photon doesn't exist before it is exchanged. Since a photon always has some energy, its creation involves a violation of the law of conservation of energy so it must disappear in a time given by the above equation. Since every photon moves at the speed of light, the distance it can travel before it is absorbed by another charged object is directly determined by the amount of energy it contains...the more energetic the photon, the smaller its range. The net effect is that the force becomes weaker with distance, but its range can be arbitrarily large as the borrowed energy approaches zero. It turns out that this exactly generates an inverse-square-law force. Feynman called his theory quantum electrodynamics (QED), and it's among the most accurate theories in physics that have ever been constructed. Some of its predictions have been verified at the level of only a few parts per trillion!

It is now believed that all four forces are transmitted by the exchange of a force quantum. In the case of gravity, this particle is called the graviton. It's presumed to have zero rest mass just like the photon since that is required to generate an inverse-square-law force as discussed further below. However, due to the extreme weakness of gravity, the graviton has not yet been observed and remains a hypothetical particle. The quantum of the strong force is the gluon. Free gluons have also not been directly observed, but for the exact opposite of the case of the graviton. The strong force is so strong that free gluons cannot exist, as will also be discussed further below. In the present state of the universe, they must remain "bound" inside other particles. Gluons have another property that is crucial for the understanding of the force they generate. Unlike the photon and the graviton, they have a rest mass that is approximately 0.2 times the mass of a proton. What this means is that there is a minimum amount of energy (its rest-mass energy) that must be borrowed to create a gluon in the first place. This puts an upper limit on the range of the force, which can be calculated by assuming that the gluon travels at most at the speed of light. This distance turns out to be about 1 × 10⁻¹⁵ m, in agreement with the observed very short range of the strong force. Finally, the weak nuclear force is transmitted by three force-carrying particles discovered in 1983. These are the W^± and the Z⁰, where the superscripts indicate their electrical charge. The mass of each of these particles is about 100 times the proton mass and the associated range (~2 × 10⁻¹⁸ m) is far smaller than the size of a proton.

Ever since Maxwell showed that electricity and magnetism were two aspects of the same underlying electromagnetic force, theoretical physicists have asked whether all the forces in nature could be understood in the context of a single comprehensive theory. After he formulated the general theory of relativity, Einstein spent much of the rest of his life trying to produce such a unified field theory, with little success. One of the more obvious problems is how to account for the very different strengths of the four forces. In the 1970s, however, Sheldon Glashow, Abdus Salam, and Steven Weinberg succeeded in unifying electromagnetism and the weak nuclear force into an "electroweak" interaction. They were awarded the 1979 Nobel Prize in physics for this remarkable accomplishment. One key prediction of the theory was the existence of the W^± and Z⁰ force-carrying particles, subsequently discovered in 1983. These play the same role for the weak force that the photon plays for the electromagnetic force. In the electroweak theory, the two forces appear to be very different at everyday low energies but merge into a single interaction at a unification energy equal to about 100 GeV, the rest-mass energy of the Z⁰. The equations describing this are complicated, but the basic idea is that the uncertainty principle allows for the production of virtual Z⁰ particles by "borrowing" energy from the universe. They are called "virtual" since they exist only for the brief amount of time allowed by the uncertainty principle. This "borrowing" becomes easier at higher energies since less energy must be borrowed, and at the unification energy Z⁰s become common since no extra energy is required to produce them. Under these conditions, quantum mechanics predicts that Z⁰s and photons "mix" together to form a combined particle that transmits the electroweak interaction. Also, electrons and neutrinos mix together to form combined particles that are neither electron nor neutrino. These concepts have been verified in great detail by experiments at particle accelerators, such as the (former) Tevatron at Fermilab, which could reach sufficiently high energies to directly produce Z⁰s. The relevance for cosmology is that at a very early stage of the Big Bang, when the temperature of the universe corresponds to an energy above the unification energy of 100 GeV (~10¹⁵ K), the electromagnetic and weak forces were combined into one single, unified interaction. At about 10⁻¹² s after the Big Bang, the temperature drops below this critical value and the separate forces begin to emerge.

Electroweak theory provides a framework for the discussion of progress toward unification of the electroweak and strong nuclear forces, which will be deferred until the next section. The unification with gravity, however, presents its own problems. Einstein's general relativity theory describes gravity as resulting from the local deformation of the "spacetime continuum" by a mass. In order to describe it in the same way as the electroweak interaction, a quantum theory of gravity is necessary, but no such theory currently exists. One of the main reasons (apart from the extreme weakness of gravity) is that a quantum theory involves the exchange of a force-carrying particle (the graviton). At high energies this is not compatible with a smooth spacetime continuum, which is instead roiled by fluctuations that become increasingly violent as the energy increases. The result has been dubbed the "quantum foam " (Figure 10.2).

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Attempts to deal with this situation have not to date been successful, although some progress has been made as will be discussed further in later chapters. However, on very general grounds it can be argued that unification with gravity will occur at about the "Planck time," equal to about 10⁻⁴³ s, which is derived from a particular combination of Newton's gravitational constant and Planck's constant (which is characteristic of quantum theories). The temperature of the universe at the Planck time after the Big Bang was approximately 10³² K. At this temperature it's presumed that all four forces were unified, but as the universe cooled gravity split off from the other forces.

From the time of the discovery of the atomic nucleus by Ernest Rutherford in 1911 until the development of powerful particle accelerators in the 1950s, it appeared that all the matter in the universe could be completely described using only a small number of "elementary particles," beginning with the proton and the electron that are the fundamental constituents of all atoms. However, measurements of the masses of atoms in the 1920s resulted in the discovery that atoms of a given element may have different masses (as mentioned above, these are called the "isotopes" of the element). The measured masses were always approximately an integer multiple of the proton mass. In 1932, Chadwick proposed that the solution to this conundrum involved the existence of yet another elementary particle, the neutron, which is very similar to a proton except that it has zero electrical charge. He subsequently performed a series of measurements demonstrating the validity of this hypothesis. In the same year, Carl Anderson discovered the positron, the antiparticle of the electron, which had been predicted in 1928 by Paul Dirac. The Dirac theory also predicted that every particle had its own corresponding antiparticle (though the antiproton and antineutron weren't actually discovered until much later). In 1930, Wolfgang Pauli proposed the existence of yet another particle, the neutrino, in order to explain features of the radioactive decay of certain elements. The neutrino was eventually discovered by Reines and Cowan and their collaborators in 1956. (Reines was awarded the Nobel Prize in physics almost forty years later, in 1995, for this work.) The number of particles was growing, but not so much that the idea of "elementary particles" was questioned. Then, with the development of high-energy accelerators in the 1950s, the situation changed dramatically. Experiments using these accelerators eventually resulted in the discovery of hundreds of different types of particles, each with their own peculiar characteristics. The field of elementary particle physics came to resemble taxonomy in biology: the discovery and classification of new species. It was difficult to see how anything in this zoo of particles could be considered to be elementary, and in fact the name of the field was changed to "high energy physics" to reflect this. Something was clearly missing: an underlying principle to explain all these objects. Then, in 1962, Murray Gell-Mann introduced the "Eightfold Way," a classification scheme that ordered all the particles known at the time. This led him to predict the existence of a yet-undiscovered particle, now called the Ω⁻, which was subsequently found and shown to have the predicted properties. Gell-Mann was awarded the 1969 Nobel Prize in physics for this work, which became the basis of an even more comprehensive theory known as the "quark model," first proposed by Gell-Mann and George Zweig in 1964. In this model, the known heavy particles or hadrons (a classification which excludes light particles such as electrons, neutrinos, and photons) are composed of truly fundamental particles which Gell-Mann called quarks after a phrase in Finnegan's Wake by James Joyce. These objects had some very unusual properties, such as the fact that they were supposed to have electrical charges that are integer multiples of 1/3 the charge of the electron. Fractionally-charged particles had never been observed, so at first the quark model created an industry of high-energy physicists looking for such bizarre objects. Nothing of the sort was ever found and they have never been observed even up to the present day despite the fact that the quark model is the accepted theory of hadrons. (The reason for this will be discussed further below.) Another feature of the theory was that it required the existence of three different types of quarks. Two of these were called the "up" (u) and "down" (d) quarks, having charges of +2/3 and −1/3 respectively. The proton and neutron are made of specific combinations of three of these quarks: uud and udd, respectively. In addition antiquarks, having the opposite charge, exist. For example, the "quark structure" of the antiproton is $\bar{u}\bar{u}\bar{d}$, where the bar above a particle symbol indicates that it's an antiparticle. The third type of quark, the strange quark (s) having electrical charge −1/3, was necessary to explain the decay properties of certain elementary particles which took much longer to decay to lighter particles than expected. These were called strange particles and their quark structure contains strange quarks. For example, the quark structure of the Ω⁻ is sss. The various types of quarks are called the quark flavors. The hadrons have another property that allows them to be classified into two distinct groups, and that is their spin. Spin is a property related to rotation, as might be guessed from the name, but it's much more abstract. It turns out that particles may have either integer or half-integer multiples of the fundamental unit of spin h/2π, the same quantity that appears in Heisenberg's uncertainty principle. These two classes are called bosons (integer spin) and fermions (half-integer spin), named after Satyendra Bose and Enrico Fermi who discussed their properties. Quarks are fermions, so particles like the proton, neutron, Ω⁻, etc, which are formed from three quarks, must have half-integer spin according to the rules of quantum mechanics. The proton and neutron, for example, have spin 1/2 while the Ω⁻ has spin 3/2. Note that the light particles (leptons) also have spin, as do force-carrying particles such as the photon. With regard to the hadrons, however, there exists another class of particle known as "mesons," for historical reasons having to do with the fact that the first-known members had masses midway between those of the electron and the proton. These particles are all bosons and their quark structure contains a combination of a quark and an antiquark. Quarks are baryons (see above) and mesons therefore have baryon-number zero since they are combinations of a baryon with an antibaryon. The other hadrons, made up of three quarks, are all baryons (the baryon number of a quark is 1/3, so three of them yield a baryon number of one). Note that very recent work at the Large Hadron Collider (LHC) has resulted in the discovery of pentaquark objects consisting of four quarks and one antiquark (Figure 10.3). Since quarks have a baryon number of +1/3, and antiquarks have −1/3, the pentaquark has a total baryon number of 1 and is therefore a baryon. Note that it, like all other subatomic particles in the current universe, is "colorless" (Section 10.6). (Yellow is the anti-color of blue, and they combine to produce white. The red–green–blue combination also produces white.)

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At first the quark model was regarded as a cute mathematical construct but the reality of the underlying quarks was doubted, especially since nobody had ever seen a free quark. That all changed dramatically in 1974 when a particle that didn't fit into the Eightfold Way was discovered, nearly simultaneously, by groups at Brookhaven National Laboratory (BNL) and the Stanford Linear Accelerator Center (SLAC). Known as the J/ψ particle, it was soon realized that it signaled the existence of a fourth quark flavor, now called charm. Actually, this quark had been predicted in 1970 by Sheldon Glashow on the basis of some anomalies in the behavior of the known particles, but his suggestion was ignored until the J/ψ appeared. This discovery was honored with the 1976 Nobel Prize in physics. The electrical charge of the charm quark is +2/3, and together with the strange quark it forms a second generation of quarks, parallel to the first generation (u,d). The reason that the charm quark had not been discovered earlier is the fact that it's significantly more massive than the other quarks and so it takes higher energy accelerators to produce particles containing charm. At this point, the search was on for other "charmed" particles. An extension of the quark model predicted the properties of all these particles, and they were all eventually found. In addition, a third, even more massive, generation of quarks, the bottom quark (b, charge = −1/3) and the top quark (t, charge = +2/3) were discovered, and essentially all the particles containing these quarks have been seen. For a time, it seemed that the number of "elementary" quarks might be growing without bounds, similar to what happened to the number of particles in the 1960s. However, there are now good theoretical and experimental grounds to believe that there are only three generations of quarks.

In parallel to what was happening with the heavy particles, the family of leptons was also growing. The first addition was the "muon" (symbol: μ⁻) discovered by Carl Anderson and Seth Neddermeyer in 1936. It appeared to be identical to the electron, having the same electrical charge and spin, but it was 210 times heavier. Anderson won the 1936 Nobel Prize in physics for this discovery. Another new lepton, the "tau" particle (symbol: τ⁻), was discovered in 1974. Found to be 3500 times (!) as massive as the electron, it also has the same charge and spin as the electron. The τ is almost twice as heavy as a proton, so why is it classified as a lepton? It turns out that the real distinguishing feature of leptons is that they, unlike hadrons, don't "feel" the strong nuclear force and only interact via the electroweak interaction. The number of neutrinos was also proliferating, and it was discovered that each electron-like particle has its own unique neutrino, established by looking at reactions involving them. As a result, neutrinos are given a subscript indicating which particle they are associated with (ν_e, ν_μ, ν_τ), and the leptons are grouped into three generations just like the quarks. Until recently it was assumed that all the neutrinos had zero mass, but this is now known to be wrong. They have a very small mass, but their actual masses have not yet been determined. Finally, as with the quarks, all the leptons have their corresponding anti-leptons.

As previously discussed, gluons are the force-transmitting particles of the strong nuclear force, which holds protons and neutrons in the nucleus and also binds the quarks together to produce hadrons. Theoretical investigations of this force ultimately revealed the reason why free quarks (and gluons) have never been seen. It turns out that, unlike the other forces we've discussed, the strength of the strong force actually increases with distance. The gluon interaction between two quarks is more like the force exerted by a rubber band...the more it stretches the greater the force it generates. Of course, a rubber band will ultimately break if it's stretched too far. That also happens with gluons, but instead of the quarks coming free the energy of the interaction is large enough to create a particle–antiparticle pair. This unique property of the strong nuclear force is modeled by associating a "color charge" (either blue, green, or red) with quarks and gluons. The analogy here is to the result of mixing primary colors to make white, but of course the color in this case is just an abstract property of the particles. As an example of the principle, the three quarks in a baryon each have one of the three primary colors associated with them, so the baryon is white (or colorless). Gluon interactions change the colors of the individual quarks, but always in such a way as to preserve this property since the main requirement of the theory is that observable particles in the present-day universe must all be colorless. In the case of a meson, which contains a quark and an antiquark, the associated color and anti-color are such as to produce a colorless particle. Gluons are also colored, which means that they feel the strong nuclear force. This is different from the case of the photon, which is electrically neutral and therefore doesn't feel the electromagnetic force it generates. This property of the gluons accounts for the very unusual nature of the gluonic force. Again, however, free colored objects can't exist in the present universe and free gluons are never observed. All of this sounds very abstract, and it is, but it has been incorporated into a theory that's similar to Feynman's quantum electrodynamics. Since the theory involves color charges, it's been called "quantum chromodynamics" or QCD. Though many of the predictions of QCD have been verified, the theory itself is quite complicated and it can be formulated in several different ways. As a result, it's not yet as well established as QED and more work is necessary before it will be possible to completely understand all its implications.

All of the phenomena discussed above, the three generations of quarks and leptons as well as the force-carrying particles and their properties, together make up the standard model of high-energy physics, illustrated in Figure 10.4. Actually, there was until fairly recently one remaining unobserved particle predicted by the theory: the "Higgs boson" which was necessary to account for the fact that particles have mass (as will be discussed further below). This last missing object was discovered in 2012.

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With this background in the standard model, it's possible to extend the description of the history and evolution of the universe given before to much earlier times after the Big Bang:

• The Planck era: (t = 10⁻⁴³ s). Until this time all of the forces of the universe, including gravity, were presumably unified into one superforce and spacetime itself was a quantum foam. A viable quantum theory of gravity is necessary to describe the condition of the universe at this and earlier times, but none yet exists and so the descriptions are speculative and to some extent metaphysical. However, as the universe cooled, gravity separated from the remaining forces and the quantum foam evolved into a spacetime continuum.
• Grand unification: (t = 10⁻³⁴ s; T = 10²⁷ K). The unification of the strong and electroweak forces is called grand unification, which is somewhat misleading since gravity remains separate in these theories. At present, there are several grand unified theories (GUTs) that make somewhat different predictions. However, in all of these the quarks, electrons, and neutrinos are mixed together, as are the photons, Z⁰, W^± and gluons. As the temperature of the universe cooled below 10²⁷ K, however, quarks and gluons emerged as the strong force separated from the electroweak force. In addition, a small asymmetry between the numbers of quarks and antiquarks arose at this time, as will be discussed further below. Eventually, the quarks and antiquarks annihilated leaving only a few unpaired survivors that today make up all the matter in the universe (10⁻⁹ particles per CMB photon). Another important property of the quarks in this era is that, unlike the case in the present universe, they don't interact very strongly with each other. As mentioned above, the "color force" between quarks is similar to the force exerted by a rubber band: it grows stronger as the separation between the quarks becomes larger, and it becomes very weak at short distances. This property, known as asymptotic freedom, implies that quarks and gluons in this early, high-density phase of the universe form a quark–gluon plasma which is colorful. In other words, the restriction that objects must be colorless doesn't yet apply and is imposed only at a much later time when the density of the universe drops to a value closer to that of a hadron and the quarks and gluons begin to interact very strongly. (Of course, it's well to remember that color charge is an abstract quantity unrelated to what we normally think of as color.)
• The end of the electroweak era: (t = 10⁻¹⁰ s; T = 10¹⁵ K). At about this time electromagnetism and the weak force, previously united into the electroweak force, part ways. During this transition electrons and neutrinos emerge from their previously-mixed phase, as do the photon and Z⁰. Light as we know it only exists after this time! In contrast with the earlier eras discussed above, this transition could be studied in the laboratory because the corresponding temperature could be reached (in a small volume of space) with existing high energy particle accelerators such as the (former) Tevatron at Fermilab. As a result, it's been very-well studied and there is little doubt remaining about the state of the universe at this time.
• Hadronization of the quark–gluon plasma: (t = 10⁻⁶ s; T = 10¹³ K). By this time, the density of the universe had become low enough that the quarks and gluons interact very strongly and (colorless) hadrons begin to be formed out of the quark–gluon plasma. This process was essentially complete by t = 10⁻⁴ s, after which the universe is entirely colorless as at present. Also, annihilation of quarks and antiquarks occurs since the temperature is now insufficient to keep their numbers in balance via pair production. As a result, only the remaining few unpaired quarks (about one in a billion) are left to form hadrons, and the present universe is made up almost entirely of matter rather than antimatter. The quark–gluon plasma has been studied using the RHIC accelerator at Brookhaven National Laboratory, and more recently at the ALICE facility at CERN, which can produce the required temperatures and densities over a (relatively) large region of space by colliding big nuclei such as those of gold or lead that have been accelerated to a sufficiently high energy. One interesting feature that emerged from these studies is that the plasma seems to behave like a perfect fluid rather than a gas as was originally expected.

From this time on, the state of the universe continues to evolve as described in the previous chapter. This entire history is diagrammed in Figure 10.5, which summarizes almost everything that modern cosmology has revealed about the evolution of the universe since the time of the Big Bang. It's truly remarkable that we now have a nearly complete understanding of the state of the universe at 10⁻¹⁰ s, and reasonably good information about periods as early as only 10⁻³⁴ s, after the Big Bang.

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As mentioned above, the stuff of the present universe is predominantly what we call matter. Until recently it was assumed that antimatter was produced only in radioactive decay processes, or in violent astrophysical events resulting in its detection in cosmic rays. However, the PAMELA spacecraft found that a belt of antielectrons (positrons) and antiprotons exists in orbit around the Earth, possibly resulting from the collisions of cosmic rays with the Earth's atmosphere. Nevertheless, antimatter is extremely rare and it's interesting to discuss why this is the case. In the standard model of high-energy physics, heavy particles (baryons) are assigned a baryon number B equal to +1/3 for a quark and −1/3 for an antiquark. Thus, for example, the proton and neutron, which each contain three constituent quarks, have B = 1. Originally, it appeared that baryon number was a conserved quantity. Consider, for example, the process of pair production from photons as previously discussed. In the initial state of this process there are no baryons so B = 0. Conservation of baryon number means that B must also equal zero in the final state, which can only happen if a particle ( B = 1) is produced together with its antiparticle ( B = −1) and that is what actually happens. However, in 1964 a phenomenon called "CP violation" was observed by Val Fitch and James Cronin, who won the 1980 Nobel Prize in physics for this discovery. A naive interpretation of the process is that antiquarks can very rarely decay into quarks, but not vice versa, so B is not exactly conserved. Certain theories predict that this process will occur with a probability ranging from as much as 10⁻⁴ (one in ten thousand) to as little as 10⁻¹² (one in a trillion) depending on the mechanism and the particles involved, and experiments (such as the BABAR experiment at the former Stanford Linear Accelerator Center) were carried out to test them. Soon after the discovery of CP violation, Andrei Sakarov proposed that this was the explanation for the predominance of matter in the universe. Sakarov, the father of the Soviet atomic bomb, was a prominent Russian nuclear physicist, Soviet dissident, and human rights activist who was awarded the Nobel Peace Prize in 1975. In 1967, he proposed that the asymmetry between matter and antimatter occurred at a very early stage in the history of the Big Bang, during the transition from grand unification at about 10⁻³⁴ s. In order to account for the observed amount of matter, a CP violation probability of about 3 × 10⁻⁹ is required and this is within the range of both theory and experiment. Sakarov's point was that pair production in this early phase of the Big Bang would produce equal amounts of matter and antimatter, but CP violation would ultimately result in a small excess of matter. Then the vast majority of the produced quarks annihilate with their corresponding antiquarks, producing photons that ultimately end up in the CMB. The remaining excess quarks find no partner and remain as the matter we see today. This accounts for the 10⁻⁹ ratio of baryons to photons, as well as the asymmetry between matter and antimatter observed in the present universe. Very recently (2020), the T2K experiment at the Super-Kamiokande Neutrino Observatory in Japan has discovered that CP violation occurs for neutrinos. This is a remarkable technical achievement since neutrinos are very difficult to detect in the first place and only a small fraction of them undergo CP violation. This discovery adds further weight to the hypothesis that CP violation is the ultimate explanation for why there is something rather than nothing in our universe.