Baryon asymmetry
Encyclopedia
The baryon asymmetry problem in physics
refers to the apparent fact that there is an imbalance in baryonic matter and antibaryonic matter in the universe
. Neither the standard model
of particle physics
, nor the theory of general relativity
provide an obvious explanation for why this should be so; and it is a natural assumption that the universe be neutral with all conserved charge
s. The Big Bang
should have produced equal amounts of matter
and antimatter
, as such, there should have been total cancellation of both. In other words, proton
s should have cancelled with antiproton
s, electron
s with antielectrons (positron
s), neutron
s with antineutron
s, and so on for all elementary particle
s. This would have resulted in a sea of photon
s in the universe with no matter. Since this is evidently not the case, after the Big Bang
, some physical laws must have acted differently for matter
and antimatter
.
There are competing hypotheses to explain the matter-antimatter imbalance that resulted in baryogenesis
, but there is as yet no one consensus theory to explain the phenomenon.
of particle physics
, to allow for some reactions (specifically involving the weak nuclear force) to proceed more easily than their opposite. This is called "violating CP symmetry
" in weak interaction
s. Such a violation could allow matter to be produced more commonly than antimatter in conditions immediately after the Big Bang
. However, as of yet, no theoretical consensus has been reached regarding this, and there is no experimental evidence of an imbalance in the creation rates of matter and antimatter.
in which matter is dominant, and other regions of the universe in which antimatter is dominant, and these are widely separated. The problem therefore becomes a matter/antimatter separation problem, rather than a creation imbalance problem. Antimatter atoms would appear from a distance indistinguishable from matter atoms, as both matter and antimatter atoms would produce light (photons) in the same way. Only in the border between a matter dominated region and an antimatter dominated region would the antimatter's presence be detectable, as only there would matter/antimatter annihilation (and the subsequent production of gamma radiation) occur. How easy such a boundary would be to detect would depend on its distance and what the density of matter and antimatter is along it. Presumably such a boundary would lie (almost by necessity) in deep intergalactic space, and the density of matter in intergalactic space is reasonably well established at about one atom per cubic metre. Assuming this is the typical density of both matter and antimatter near a boundary, the gamma ray luminosity of the boundary interaction zone is easily calculated. Approximately 30 years of scientific research have placed boundaries on how far away, at a minimum, any such boundary interaction zone would have to be, as no such zones have been detected. Hence, it is now considered unlikely that any region within the observable universe is antimatter dominated.
At least one more major scientific study, called the Alpha Magnetic Spectrometer
, is planned that would, among other things, advance our capability of detecting very distant antimatter dominated regions.
Another possibility is that antimatter dominated regions exist within the universe, but outside our observable universe
. Inflationary cosmology models suggest that there may be more to the universe than can be seen from the Earth, if only for the simple reason that the universe isn't old enough for light from the most distant parts of the universe to have reached us yet. If so, radiation from the boundary of matter and antimatter dominated regions may simply still "be on its way" to Earth, and so cannot be observed.
Yet another possibility is that antimatter repels ordinary matter rather than attracting it gravitationally, this would prevent observable interactions, see Motivations for antigravity. However it appears more likely that matter and antimatter attract each other gravitationally, see Antimatter gravity debate.
moment in electron
s which would result in an unequal distribution of the electron's negative charge. Such a property would allow matter and antimatter to decay at different rates. A new ceramic material, called europium barium titanate
, was created to test this theory, with experiments being underway as of 2010.
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
refers to the apparent fact that there is an imbalance in baryonic matter and antibaryonic matter in the universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...
. Neither the standard model
Standard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...
of particle physics
Particle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...
, nor the theory of general relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...
provide an obvious explanation for why this should be so; and it is a natural assumption that the universe be neutral with all conserved charge
Charge (physics)
In physics, a charge may refer to one of many different quantities, such as the electric charge in electromagnetism or the color charge in quantum chromodynamics. Charges are associated with conserved quantum numbers.-Formal definition:...
s. The Big Bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...
should have produced equal amounts of matter
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...
and antimatter
Antimatter
In particle physics, antimatter is the extension of the concept of the antiparticle to matter, where antimatter is composed of antiparticles in the same way that normal matter is composed of particles...
, as such, there should have been total cancellation of both. In other words, proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
s should have cancelled with antiproton
Antiproton
The antiproton is the antiparticle of the proton. Antiprotons are stable, but they are typically short-lived since any collision with a proton will cause both particles to be annihilated in a burst of energy....
s, electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s with antielectrons (positron
Positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1e, a spin of ½, and has the same mass as an electron...
s), neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s with antineutron
Antineutron
The antineutron is the antiparticle of the neutron with symbol . It differs from the neutron only in that some of its properties have equal magnitude but opposite sign. It has the same mass as the neutron, and no net electric charge, but has opposite baryon number...
s, and so on for all elementary particle
Elementary particle
In particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not known to be made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic building blocks of the universe from which...
s. This would have resulted in a sea of photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s in the universe with no matter. Since this is evidently not the case, after the Big Bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...
, some physical laws must have acted differently for matter
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...
and antimatter
Antimatter
In particle physics, antimatter is the extension of the concept of the antiparticle to matter, where antimatter is composed of antiparticles in the same way that normal matter is composed of particles...
.
There are competing hypotheses to explain the matter-antimatter imbalance that resulted in baryogenesis
Baryogenesis
In physical cosmology, baryogenesis is the generic term for hypothetical physical processes that produced an asymmetry between baryons and antibaryons in the very early universe, resulting in the substantial amounts of residual matter that make up the universe today.Baryogenesis theories employ...
, but there is as yet no one consensus theory to explain the phenomenon.
CP violations
Most explanations involve modifying the standard modelStandard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...
of particle physics
Particle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...
, to allow for some reactions (specifically involving the weak nuclear force) to proceed more easily than their opposite. This is called "violating CP symmetry
CP violation
In particle physics, CP violation is a violation of the postulated CP-symmetry: the combination of C-symmetry and P-symmetry . CP-symmetry states that the laws of physics should be the same if a particle were interchanged with its antiparticle , and left and right were swapped...
" in weak interaction
Weak interaction
Weak interaction , is one of the four fundamental forces of nature, alongside the strong nuclear force, electromagnetism, and gravity. It is responsible for the radioactive decay of subatomic particles and initiates the process known as hydrogen fusion in stars...
s. Such a violation could allow matter to be produced more commonly than antimatter in conditions immediately after the Big Bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...
. However, as of yet, no theoretical consensus has been reached regarding this, and there is no experimental evidence of an imbalance in the creation rates of matter and antimatter.
Regions of the universe where antimatter predominates
Another possible explanation of the apparent baryon asymmetry is that there are regions of the universeUniverse
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...
in which matter is dominant, and other regions of the universe in which antimatter is dominant, and these are widely separated. The problem therefore becomes a matter/antimatter separation problem, rather than a creation imbalance problem. Antimatter atoms would appear from a distance indistinguishable from matter atoms, as both matter and antimatter atoms would produce light (photons) in the same way. Only in the border between a matter dominated region and an antimatter dominated region would the antimatter's presence be detectable, as only there would matter/antimatter annihilation (and the subsequent production of gamma radiation) occur. How easy such a boundary would be to detect would depend on its distance and what the density of matter and antimatter is along it. Presumably such a boundary would lie (almost by necessity) in deep intergalactic space, and the density of matter in intergalactic space is reasonably well established at about one atom per cubic metre. Assuming this is the typical density of both matter and antimatter near a boundary, the gamma ray luminosity of the boundary interaction zone is easily calculated. Approximately 30 years of scientific research have placed boundaries on how far away, at a minimum, any such boundary interaction zone would have to be, as no such zones have been detected. Hence, it is now considered unlikely that any region within the observable universe is antimatter dominated.
At least one more major scientific study, called the Alpha Magnetic Spectrometer
Alpha Magnetic Spectrometer
The Alpha Magnetic Spectrometer, also designated AMS-02, is a particle physics experiment module that is mounted on the International Space Station. It is designed to search for various types of unusual matter by measuring cosmic rays. Its experiments will help researchers study the formation of...
, is planned that would, among other things, advance our capability of detecting very distant antimatter dominated regions.
Another possibility is that antimatter dominated regions exist within the universe, but outside our observable universe
Observable universe
In Big Bang cosmology, the observable universe consists of the galaxies and other matter that we can in principle observe from Earth in the present day, because light from those objects has had time to reach us since the beginning of the cosmological expansion...
. Inflationary cosmology models suggest that there may be more to the universe than can be seen from the Earth, if only for the simple reason that the universe isn't old enough for light from the most distant parts of the universe to have reached us yet. If so, radiation from the boundary of matter and antimatter dominated regions may simply still "be on its way" to Earth, and so cannot be observed.
Yet another possibility is that antimatter repels ordinary matter rather than attracting it gravitationally, this would prevent observable interactions, see Motivations for antigravity. However it appears more likely that matter and antimatter attract each other gravitationally, see Antimatter gravity debate.
Electron dipole moment
There is a theory which suggests the existence of a dipoleDipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...
moment in electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s which would result in an unequal distribution of the electron's negative charge. Such a property would allow matter and antimatter to decay at different rates. A new ceramic material, called europium barium titanate
Europium barium titanate
Europium barium titanate is a chemical compound composed of barium, europium, titanium, and oxygen. It is magnetic and ferroelectric.It is a ceramic material which was used in 2010 experiments on a new theory on baryon asymmetry....
, was created to test this theory, with experiments being underway as of 2010.