Dark matter and antimatter in cosmology
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Dark Matter in Cosmology: Evidence, Candidates, and Theoretical Models
Dark matter is a key component of the universe, making up about 27% of its total content, while ordinary matter accounts for only about 5% and dark energy dominates with roughly 68% 48. The existence of dark matter is supported by a wide range of astrophysical and cosmological observations, including galaxy rotation curves, gravitational lensing, the cosmic microwave background (CMB), and large-scale structure formation 110. These observations indicate that the Standard Model of particle physics is incomplete, as it cannot account for dark matter or the observed matter-antimatter asymmetry .
Several theoretical candidates for dark matter have been proposed. Well-motivated possibilities include Weakly Interacting Massive Particles (WIMPs), axions, and sterile neutrinos, as well as more speculative options like fuzzy dark matter . Axions, in particular, are of interest because they can be produced through different mechanisms, with their abundance depending on parameters like the axion mass and initial conditions . Some models also propose that dark matter could be composed of macroscopic objects, such as "macros" made of antimatter, which would interact with ordinary matter primarily through annihilation, though much of the parameter space for such objects remains unconstrained .
Matter-Antimatter Asymmetry: Baryogenesis and Cosmological Implications
The universe exhibits a clear asymmetry between matter and antimatter, with matter vastly dominating in the observable universe 17. This asymmetry is believed to have arisen in the early universe through a process known as baryogenesis, which requires physics beyond the Standard Model 17. Theoretical frameworks for baryogenesis often involve mechanisms that can also explain neutrino oscillations and, in some cases, provide a link to dark matter .
Observational evidence for the matter-antimatter asymmetry comes from the lack of significant amounts of antimatter in the present-day universe and the measured baryon density . Some models suggest that the dark sector could carry baryon number, effectively hiding the antimatter counterpart of the observed asymmetry and maintaining overall baryon number conservation . In these scenarios, dark matter particles could interact with the Standard Model through specific portals, such as the Neutron Portal, and transfer asymmetry between sectors .
Connections Between Dark Matter and Antimatter in Cosmology
There is growing interest in models that connect the origins of dark matter and the matter-antimatter asymmetry. Some frameworks propose that both dark and visible matter have a common origin in quantum chromodynamics (QCD), forming at the same epoch and being proportional to the same fundamental scale . In such models, dark matter may consist of both axions and quark nuggets (which can be made of matter or antimatter), naturally explaining the observed similarity in the abundances of dark and visible matter .
Other theories explore the possibility that dark matter is composed of mirror matter, a hypothetical form of matter that could oscillate into ordinary antimatter under certain conditions. These oscillations could have significant astrophysical consequences and might even be detectable experimentally . Additionally, alternative cosmological models, such as the Dirac-Milne universe, propose a symmetric universe with equal amounts of matter and antimatter, where antimatter has negative gravitational mass. This model offers solutions to several cosmological problems and remains consistent with key observations .
Distinguishing Dark Matter from Antimatter
It is important to distinguish between dark matter and antimatter, as they are fundamentally different concepts. Antimatter consists of particles with opposite charges to their matter counterparts and annihilates upon contact with matter, releasing energy 48. In contrast, dark matter is defined by its lack of electromagnetic interaction, making it invisible to light and detectable only through its gravitational effects . While some dark matter candidates may involve antimatter or mirror matter, most models treat them as separate entities.
Conclusion
Dark matter and antimatter are central to our understanding of cosmology, with both playing crucial roles in the evolution and composition of the universe. While dark matter remains undetected directly, its gravitational effects are well established, and a variety of theoretical candidates are under investigation. The matter-antimatter asymmetry is a profound puzzle, with several models linking its origin to the properties and abundance of dark matter. Ongoing and future experiments, along with cosmological observations, will continue to test these ideas and may eventually reveal the true nature of both dark matter and antimatter in the cosmos.
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Cosmology and Dark Matter
The Standard Model of particle physics is incomplete, with dark matter and matter-antimatter asymmetry remaining unsolved, and future cosmological observations may help resolve these issues.
The universe could be symmetric: dark matter stability and matter-antimatter asymmetry from baryon number conservation
The universe could be symmetric, with dark matter carrying baryon number and hiding the counterpart of the measured asymmetry, consistent with cosmological observations and collider constraints.
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