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Matter

Our knowledge of the universe remains very limited, but we are continuing to make remarkable observations.

Composition of the Universe

To the best of present knowledge, the universe is mostly dark energy and dark matter.

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Source: 1. The nature of dark energy and dark matter is not well-established, and even their very existence is not universally accepted.

By element, the baryonic matter is estimated as follows

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Source: 2.

By macroscopic structure, baryonic matter is estimated as follows

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Source: 3. Main sequence stars include red dwarves, such as Proxima Centauri, and giant stars such as Betelgeuse.

Additionally, the full mass-energy of the universe comprises small quantities of neutrinos, radiation, nuclear binding energy, gravitational binding energy, cosmic rays, magnetic fields, and kinetic energy 3.

Dark Energy

Dark energy is a hypothetical substance that, although at a low density, permeates the universe and comprises most of its mass-energy balance. Dark energy imparts an expansive force and was posited in the late 1990s to explain the observed acceleration of the universe's expansion 4, 5. However, dark energy has not yet been observed in a laboratory and its nature remains mysterious. Following are some leading hypotheses.

HypothesisExplanation
Cosmological Constant 6An intrinsic positive energy of space that exercises a repulsive force countering gravity.
Quintessence 7A fifth fundamental force of nature. Differences from the cosmological constant in that this form of dark energy is dynamic rather than constant.
Modified Gravity 8The observation of dark energy may stem from inaccuracies in our understanding of general relativity, but this seems unlikely 9.
Observational Error 10This is possible but viewed as unlikely 11.

The above represents a partial list of explanations offered for dark energy and is not intended to be complete.

Dark Matter

Dark matter is a hypothesized form of matter that interacts with ordinary matter through gravity but not through electromagnetism, hence we cannot see it 12. The existence of dark matter is proposed as an explanation for the way galaxies rotate 13, gravitational lensing observations 14, and other phenomena, which cannot be explained with ordinary matter that is observed. Several candidate explanations have been offered.

ExplanationNotes
Weakly Interacting Massive Particles (WIMPs) 15A leading explanation, but no evidence for WIMPs has yet been found.
Supersymmetric Particles 16The theoretical framework of supersymmetry predict dark matter particles. Particle accelerator research is ongoing.
Primordial Black Holes 17Hawking radiation expected from primordial black holes not observed 18.
Modified Gravity 8No adequate theory of modified gravity has been found that explains dark matter observations 9.

A partial list of possible explanations of dark matter. Physicists lean toward a non-baryonic explanation, such as WIMPs, supersymmetric particles, or others not listed above. Additional explanations are considered by Bertone and Tait 19.

Matter and Antimatter

Every particle is associated with an antiparticle, which has the same mass but opposite electric charge and other physical charges. Antimatter is similar to ordinary matter but composed of antiparticles instead of ordinary particles. Matter and antimatter are extremely reactive with each other, and only a few billionths of a gram have been produced in particle accelerators 20. Antimatter, specifically positrons (the antiparticles of electrons) are used today in positron emission tomography (PET) scanning for medical imaging 21.

The baryonic portion of the universe appears to be almost entirely regular matter and not antimatter, and why this is the case is a major open question in cosmology. There is one surviving matter particle for every billion matter-antimatter reactions in the early universe 22.

Research has found CP (charge conjugation parity) violations, indicating that the laws of physics are different between matter and antimatter 23, 24, 25. However, these asymmetries are insufficient to explain the predominance of matter in the universe. It is hoped that further particle accelerator research will yield more information 26.

References

  1. National Aeronautics and Space Administration. "WMAP - Content of the Universe". January 2014. ↩

  2. Gray, T., Mann, N., Whitby, M. "Abundance in the Universe of the elements". Accessed August 22, 2020. ↩

  3. Fukugita, M., Peebles, P. J. E. "The Cosmic Energy Inventory". The Astrophysical Journal 616(2). December 2004. ↩ ↩2

  4. Perlmutter, S. et al. "Measurements of Ω and Λ from 42 High-Redshift Supernovae". The Astrophysical Journal 517(2). 1999. ↩

  5. Riess, A. G., Filippenko, A. V., Challis, P., Clocchiatti, A., Diercks, A., Garnavich, P. M., Gilliland, R. L., Hogan, C. J., Jha, S., Kirshner, R. P., Leibundgut, B., Phillips, M. M., Reiss, D., Schmidt, B. P., Schommer, R. A., Smith, R. C., Spyromilio, J., Stubbs, C., Suntzeff, N. B., Tonry, J. "Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant". The Astronomical Journal 116(3). 1998. ↩

  6. O’Raifeartaigh, C., O’Keeffe, M., Nahm, W., Mitton, S. "One hundred years of the cosmological constant: from "superfluous stunt" to dark energy". The European Physical Journal H 43, pp. 73-117. March 2018. ↩

  7. Caldwell, R. R., Dave, R., Steinhardt, P. J. "Cosmological Imprint of an Energy Component with General Equation of State". Physical Review Letters 80(1582). February 1998. ↩

  8. Milgrom, M. "A modification of the Newtonian dynamics as a possible alternative to the hidden mass hypothesis". Astrophysical Journal 270, pp. 365-370. July 1983. ↩ ↩2

  9. Cho, A. "To explain away dark matter, gravity would have to be really weird, cosmologists say". Science Magazine. November 2020. ↩ ↩2

  10. Kang, Y., Lee, Y., Kim, Y., Chung, C., Ree, C. H. "Early-type Host Galaxies of Type Ia Supernovae. II. Evidence for Luminosity Evolution in Supernova Cosmology". The Astrophysical Journal 889(1). January 2020. ↩

  11. Gohd, C. "Has Dark Energy Been Debunked? Probably Not.". space.com. January 2020. ↩

  12. Copi, C. J., Schramm, D. N., Turner, M. S. "Big-Bang Nucleosynthesis and the Baryon Density of the Universe". Science 267: 5195, pp. 192-199. January 1995. ↩

  13. Rubin, V. C., Ford, W. K. Jr., Thonnard, N. "Rotational properties of 21 SC galaxies with a large range of luminosities and radii, from NGC 4605 (R=4kpc) to UGC 2885 (R=122kpc)". Astrophysical Journal 238, pp. 471-487. June 1980. ↩

  14. Hong, S. E., Jeong, D., Hwang, H. S., Kim, J. "Revealing the Local Cosmic Web from Galaxies by Deep Learning". The Astrophysical Journal 913(1). May 2021. ↩

  15. Gibney, E. "Last chance for WIMPs: physicists launch all-out hunt for dark-matter candidate". Nature 586, pp. 344-345. October 2020. ↩

  16. ATLAS Collaboration. "ATLAS sets strong constraints on supersymmetric dark matter". CERN. April 2019. ↩

  17. Overduin, J. M., Wesson, P. S. "Dark matter and background light". Physics Repots 402(5-6): pp. 267-406. November 2004. ↩

  18. Cho, A. "Aging Voyager 1 spacecraft undermines idea that dark matter is tiny black holes". Science Magazine. January 2019. ↩

  19. Bertone, G., Tait, T. M. P. "A new era in the search for dark matter". Nature 562, pp. 51-56. October 2018. ↩

  20. Kwon, D. "Ten things you might not know about antimatter". Symmetry: dimensions of particle physics: A joint Fermilab/SLAC publication. April 2015. ↩

  21. Mayo Clinic. "Positron emission tomography scan". Accessed September 13, 2021. ↩

  22. Sather, E. "The Mystery of the Matter Asymmetry". Spring/Summer 1996. ↩

  23. Aaij R. et al. "Observation of CP Violation in Charm Decays". Physical Review Letters 122: 211803. May 2019. ↩

  24. Christenson, J. H., Cronin, J. W., Fitch, L., Turlay, R. "Evidence for the 2π Decay of the K20 Meson". Physical Review Letters 13(138). July 1964. ↩

  25. Fanti, V. et al. "A new measurement of direct CP violation in two pion decays of the neutral kaon". Physics Letters B 465(1-4), pp. 335-348. October 1999. ↩

  26. CERN. "The matter-antimatter asymmetry problem". Accessed September 13, 2021. ↩