Baryon asymmetry

Short description: Imbalance of matter and antimatter in the observable universe

Not to be confused with Missing baryon problem.

Antimatter

Annihilation

Devices

* Particle accelerator
* Penning trap
* Wilson chamber

Antiparticles

* Positron
* Antiproton
* Antineutron

Uses

* Positron emission tomography
* Fuel

Bodies

* ALPHA Collaboration
* ATHENA
* ATRAP
* CERN
* RHIC

People

* Paul Dirac
* Carl David Anderson
* Andrei Sakharov

* v
* t
* e

| Unsolved problem in physics:

What is the source of imbalance of matter and antimatter?

(more unsolved problems in physics)

|

In physical cosmology, the baryon asymmetry problem, also known as the matter asymmetry problem or the matter–antimatter asymmetry problem,[1][2] is the observed imbalance in baryonic matter and antibaryonic matter in the observable universe. As the two form and behave in nearly identical ways, it is expected that they would have been created in near equal portions by the Big Bang, where in reality matter makes up the vast majority of the universe (including the Earth and humanity). Neither the Standard Model of particle physics nor the theory of general relativity provides a known explanation for why this should be so, and it is a natural assumption that the universe is neutral with all conserved charges.[3] Since this does not seem to have been the case, it is likely some physical laws must have acted differently or did not exist for matter and/or antimatter. Several competing hypotheses exist to explain the imbalance of matter and antimatter that resulted in baryogenesis. However, there is as of yet no consensus theory to explain the phenomenon, which has been described as "one of the great mysteries in physics".[4]

## Contents

* 1 Sakharov conditions
* 1.1 Baryon number violation
* 1.2 CP-symmetry violation
* 1.3 Interactions out of thermal equilibrium
* 2 Other explanations
* 2.1 Regions of the universe where antimatter dominates
* 2.2 Mirror anti-universe
* 2.3 Cyclic cosmology
* 3 Baryon asymmetry parameter
* 4 See also
* 5 References

## Sakharov conditions

Main article: Physics:Baryogenesis

In 1967, Andrei Sakharov proposed[5] a set of three necessary conditions that a baryon-generating interaction must satisfy to produce matter and antimatter at different rates. These conditions were inspired by the recent discoveries of the Cosmic microwave background[6] and CP violation in the neutral kaon system.[7] The three necessary "Sakharov conditions" are:

* Baryon number B violation.
* C-symmetry violation and CP-symmetry violation.
* Interactions out of thermal equilibrium.

Baryon number violation is a necessary condition to produce an excess of baryons over anti-baryons. But C-symmetry violation is also needed so that the interactions which produce more baryons than anti-baryons will not be counterbalanced by interactions which produce more anti-baryons than baryons. CP-symmetry violation is similarly required because otherwise equal numbers of left-handed baryons and right-handed anti-baryons would be produced, as well as equal numbers of left-handed anti-baryons and right-handed baryons. Finally, the interactions must be out of thermal equilibrium, since otherwise CPT symmetry would assure compensation between processes increasing and decreasing the baryon number.[8]

### Baryon number violation

Currently, there is no experimental evidence of particle interactions where the conservation of baryon number is broken perturbatively: this would appear to suggest that all observed particle reactions have equal baryon number before and after. Mathematically, the commutator of the baryon number quantum operator with the (perturbative) Standard Model hamiltonian is zero: [B,H]=BH−HB=0. However, the Standard Model is known to violate the conservation of baryon number: a U(1)B Adler–Bell–Jackiw anomaly captured by a triangle Feynman diagram U(1)B-SU(2)w-SU(2)w with SU(2)w weak interaction gauge group. To account for baryon violation in baryogenesis, such events (including proton decay) can occur in Grand Unification Theories (GUTs) and supersymmetric (SUSY) models via hypothetical massive bosons such as the X boson.

### CP-symmetry violation

Main article: Physics:CP violation

See also: Biography:Chien-Shiung Wu and Wu Experiment

The second condition for generating baryon asymmetry—violation of charge-parity symmetry—is that a process is able to happen at a different rate to its antimatter counterpart. In the Standard Model, CP violation appears as a complex phase in the quark mixing matrix of the weak interaction. There may also be a non-zero CP-violating phase in the neutrino mixing matrix, but this is currently unmeasured. The first in a series of basic physics principles to be violated was parity through Chien-Shiung Wu's experiment. This led to CP violation being verified in the 1964 Fitch–Cronin experiment with neutral kaons, which resulted in the 1980 Nobel Prize in Physics (direct CP violation, that is violation of CP symmetry in a decay process, was discovered later, in 1999). Due to CPT symmetry, violation of CP symmetry demands violation of time inversion symmetry, or T-symmetry. Despite the allowance for CP violation in the Standard Model, it is insufficient to account for the observed baryon asymmetry of the universe (BAU) given the limits on baryon number violation, meaning that beyond-Standard Model sources are needed.

A possible new source of CP violation was found at the Large Hadron Collider (LHC) by the LHCb collaboration during the first three years of LHC operations (beginning March 2010). The experiment analyzed the decays of two particles, the bottom Lambda (Λb0) and its antiparticle, and compared the distributions of decay products. The data showed an asymmetry of up to 20% of CP-violation sensitive quantities, implying a breaking of CP-symmetry. This analysis will need to be confirmed by more data from subsequent runs of the LHC.[9]

One method to search for additional CP-violation is the search for electric dipole moments of fundamental or composed particles. The existence of electric dipole moments in equilibrium states requires violation of T-symmetry. That way finding a non zero electric dipole moment would imply the existence of T-violating interactions in the vacuum corrections to the measured particle. So far all measurements are consistent with zero putting strong bounds on the properties of the yet unknown new CP-violating interactions.

### Interactions out of thermal equilibrium

In the out-of-equilibrium decay scenario,[10] the last condition states that the rate of a reaction which generates baryon-asymmetry must be less than the rate of expansion of the universe. In this situation the particles and their corresponding antiparticles do not achieve thermal equilibrium due to rapid expansion decreasing the occurrence of pair-annihilation.

## Other explanations

### Regions of the universe where antimatter dominates

Another possible explanation of the apparent baryon asymmetry is that matter and antimatter are essentially separated into different, widely distant regions of the universe. The formation of antimatter galaxies was originally thought to explain the baryon asymmetry, as from a distance, antimatter atoms are indistinguishable from matter atoms; both produce light (photons) in the same way.[11] Along the boundary between matter and antimatter regions, however, annihilation (and the subsequent production of gamma radiation) would be detectable, depending on its distance and the density of matter and antimatter. Such boundaries, if they exist, would likely lie in deep intergalactic space. The density of matter in intergalactic space is reasonably well established at about one atom per cubic meter.[12][13] Assuming this is a typical density near a boundary, the gamma ray luminosity of the boundary interaction zone can be calculated. No such zones have been detected, but 30 years of research have placed bounds on how far they might be. On the basis of such analyses, it is now deemed unlikely that any region within the observable universe is dominated by antimatter.[4]

### Mirror anti-universe

In the mirror anti-universe theory, the Big Bang generated a universe–antiuniverse pair. This universe flows forward in time, while the mirror counterpart flows backward.

The state of the universe, as it is, does not violate the CPT symmetry, because the Big Bang could be considered as a double sided event, both classically and quantum mechanically, consisting of a universe-antiuniverse pair. This means that this universe is the charge (C), parity (P) and time (T) image of the anti-universe. This pair emerged from the Big Bang epochs not directly into a hot, radiation-dominated era. The antiuniverse would flow back in time from the Big Bang, becoming bigger as it does so, and would be also dominated by antimatter. Its spatial properties are inverted if compared to those in our universe, a situation analogous to creating electron–positron pairs in a vacuum. This model, devised by physicists from the Perimeter Institute for Theoretical Physics in Canada, proposes that temperature fluctuations in the cosmic microwave background (CMB) are due to the quantum-mechanical nature of space-time near the Big Bang singularity.[14] This means that a point in the future of our universe and a point in the distant past of the anti-universe would provide fixed classical points, while all possible quantum-based permutations would exist in between. Quantum uncertainty causes the universe and antiuniverse to not be exact mirror images of each other.[15]

This model has not shown if it can reproduce certain observations regarding the inflation scenario, such as explaining the uniformity of the cosmos on large scales. However, it provides a natural and straightforward explanation for dark matter. Such a universe-antiuniverse pair would produce large numbers of superheavy neutrinos, also known as sterile neutrinos. These neutrinos might also be the source of recently observed bursts of high-energy cosmic rays.[16]

### Cyclic cosmology

In cyclic cosmology (for example the Big Bounce) the initial baryon asymmetry is order of magnitudes smaller, because it (cyclic cosmology) is entropic and not a perfect spatiotemporal defaulting,[17] thus previous conditions generate a boost bias increasing the rate of matter over antimatter.

## Baryon asymmetry parameter

The challenges to the physics theories are then to explain how to produce the predominance of matter over antimatter, and also the magnitude of this asymmetry. An important quantifier is the asymmetry parameter,

η=nB−nB¯nγ.

This quantity relates the overall number density difference between baryons and antibaryons (nB and nB, respectively) and the number density of cosmic background radiation photons nγ.

According to the Big Bang model, matter decoupled from the cosmic background radiation (CBR) at a temperature of roughly 3000 kelvin, corresponding to an average kinetic energy of 3000 K / (10.08×103 K/eV) = 0.3 eV. After the decoupling, the total number of CBR photons remains constant. Therefore, due to space-time expansion, the photon density decreases. The photon density at equilibrium temperature T per cubic centimeter, is given by

nγ=1π2(kBTℏc)3×∫0∞x2ex−1dx=1π2(kBTℏc)3×2ζ(3)≈20.3(T1K)3cm−3,

with kB as the Boltzmann constant, ħ as the Planck constant divided by 2π and c as the speed of light in vacuum, and ζ(3) as Apéry's constant. At the current CBR photon temperature of 2.725 K, this corresponds to a photon density nγ of around 411 CBR photons per cubic centimeter.

Therefore, the asymmetry parameter η, as defined above, is not the "good" parameter. Instead, the preferred asymmetry parameter uses the entropy density s,

ηs=nB−nB¯s

because the entropy density of the universe remained reasonably constant throughout most of its evolution. The entropy density is

s =def entropyvolume=p+ρT=2π245g*(T)T3

with p and ρ as the pressure and density from the energy density tensor Tμν, and g* as the effective number of degrees of freedom for "massless" particles (inasmuch as mc2 ≪ kBT holds) at temperature T,

g*(T)=∑i=bosonsgi(TiT)3+78∑j=fermionsgj(TjT)3,

for bosons and fermions with gi and gj degrees of freedom at temperatures Ti and Tj respectively. Presently, s = 7.04nγ.

## See also

* Baryogenesis
* CP violation
* List of unsolved problems in physics

## References

1. ↑ "The matter-antimatter asymmetry problem". https://home.cern/topics/antimatter/matter-antimatter-asymmetry-problem.
2. ↑ Sather, Eric. "The Mystery of the Matter Asymmetry". https://www.vanderbilt.edu/AnS/physics/panvini/babar/sakharov.pdf.
3. ↑ Sarkar, Utpal (2007). Particle and astroparticle physics. CRC Press. p. 429. ISBN 978-1-58488-931-1.
4. ↑ 4.0 4.1 Canetti, L.; Drewes, M.; Shaposhnikov, M. (2012). "Matter and Antimatter in the Universe". New J. Phys. 14 (9). doi:10.1088/1367-2630/14/9/095012. Bibcode: 2012NJPh...14i5012C.
5. ↑ A. D. Sakharov (1967). "Violation of CP invariance, C asymmetry, and baryon asymmetry of the universe". Journal of Experimental and Theoretical Physics Letters 5: 24–27. http://www.jetpletters.ac.ru/ps/1643/article_25089.shtml. and in Russian, A. D. Sakharov (1967). "Violation of CP invariance, C asymmetry, and baryon asymmetry of the universe". ZhETF Pis'ma 5: 32–35. http://www.jetpletters.ac.ru/ps/808/article_12459.shtml. Retrieved 2017-12-06. republished as A. D. Sakharov (1991). "Violation of CP invariance, C asymmetry, and baryon asymmetry of the universe" (in ru, en). Soviet Physics Uspekhi 34 (5): 392–393. doi:10.1070/PU1991v034n05ABEH002497. Bibcode: 1991SvPhU..34..392S. https://ufn.ru/en/articles/1991/5/h/.
6. ↑ A. A. Penzias; R. W. Wilson (1965). "A Measurement of Excess Antenna Temperature at 4080 Mc/s". Astrophysical Journal 142: 419–421. doi:10.1086/148307. Bibcode: 1965ApJ...142..419P.
7. ↑ J. W. Cronin; V. L. Fitch (1964). "Evidence for the 2π decay of the K02 meson". Physical Review Letters 13 (4): 138–140. doi:10.1103/PhysRevLett.13.138. Bibcode: 1964PhRvL..13..138C.K02+meson&rft.jtitle=[[Physics:Physical+Review+Letters|Physical+Review+Letters]]&rft.aulast=J.+W.+Cronin&rft.au=J.+W.+Cronin&rft.au=V.+L.+Fitch&rft.date=1964&rft.volume=13&rft.issue=4&rft.pages=138–140&rft_id=info:doi/10.1103/PhysRevLett.13.138&rft_id=info:bibcode/1964PhRvL..13..138C&rfr_id=info:sid/en.wikibooks.org:Astronomy:Baryon_asymmetry">
8. ↑ M. E. Shaposhnikov; G. R. Farrar (1993). "Baryon Asymmetry of the Universe in the Minimal Standard Model". Physical Review Letters 70 (19): 2833–2836. doi:10.1103/PhysRevLett.70.2833. PMID 10053665. Bibcode: 1993PhRvL..70.2833F.
9. ↑ "New source of asymmetry between matter and antimatter | CERN" (in en). https://home.cern/about/updates/2017/01/new-source-asymmetry-between-matter-and-antimatter.
10. ↑ A. Riotto; M. Trodden (1999). "Recent progress in baryogenesis". Annual Review of Nuclear and Particle Science 49: 46. doi:10.1146/annurev.nucl.49.1.35. Bibcode: 1999ARNPS..49...35R.
11. ↑ Kragh, Helge (January 9, 2025). "Antimatter in astronomy and cosmology: the early history" (in en). Annals of Science: 1–25. doi:10.1080/00033790.2025.2449861. ISSN 0003-3790. https://www.tandfonline.com/doi/full/10.1080/00033790.2025.2449861.
12. ↑ Davidson, Keay; Smoot, George (2008). Wrinkles in Time. New York: Avon. pp. 158–163. ISBN 978-0-06-134444-2.
13. ↑ Silk, Joseph (1977). Big Bang. New York: Freeman. p. 299. ISBN 978-0-8050-7256-3. https://books.google.com/books?id=XLwe1lUmz5kC.
14. ↑ "Our universe has antimatter partner on the other side of the Big Bang, say physicists" (in en-GB). 2019-01-03. https://physicsworld.com/a/our-universe-has-antimatter-partner-on-the-other-side-of-the-big-bang-say-physicists/.
15. ↑ Boyle, Latham; Finn, Kieran; Turok, Neil (2018-12-20). "C P T -Symmetric Universe" (in en). Physical Review Letters 121 (25). doi:10.1103/PhysRevLett.121.251301. ISSN 0031-9007. PMID 30608856. Bibcode: 2018PhRvL.121y1301B.
16. ↑ Boyle, L.; Finn, K.; Turok, N. (2018-12-20). "Synopsis: Universe Preceded by an Antiuniverse?" (in en). Physics 121 (25). doi:10.1103/PhysRevLett.121.251301. PMID 30608856. Bibcode: 2018PhRvL.121y1301B.
17. ↑ P. C. M. Delgado, M. B. Jesus, N. Pinto-Neto, T. Mourão & G. S. Vicente, “Baryogenesis in cosmological models with symmetric and asymmetric quantum bounces”, Physical Review D (2020), arXiv:2010.04807.

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