Dark matter
Another form of cold dark matter which shows promise are axions. Axions are hypothetical particles first theorized in 1977 as a solution to the strong CP 9 -problem. In QCD, 10 the study of the strong force, there should be a violation of CP- symmetry. Flipping the charge and parity of the quarks should not obtain the same results. However, this is not the case. The constant θ had to be a value of zero or near zero for CP - symmetry to be observed. But there was no logical reason for this. Peccei and Quinn suggested that θ should be promoted to a field which would naturally fall to and can oscillate around zero. These oscillations are axions. Scientists have
Figure 8: Events / (tonne x year x keV) versus Energy recorded at the XENON1T detector
calculated specific ranges on the properties of axions with them being neutrally charged, with incredibly small mass range, 1 µeV / c 2 to 1 eV / c 2 , a very low interaction cross-sections for weak and strong forces resulting in minimal interactivity with normal matter just like dark matter. Ultralight axions, with masses on the lower end of scale, around 10 µeV / c 2 , would have been produced in high enough quantities, with low enough interaction rates to be a perfect explanation for dark matter. 11 Unlike WIMPs, the search for axions only began recently and detection efforts are relatively new. 12 Early results potentially support the axion theory (see figure 4 for current range). The XENON1T detector revealed discrepancies between the expected number of events (the grey line on the graph) and the actual number recorded at the low energy scale of around 1 keV to 7 keV (see figure 4). 13 This could be a random fluctuation but that is statistically unlikely. More interestingly for this essay, it could be the first glimmer of the existence of axions. Other explanations for the results exist, such as the contamination of the water within the detector by tritium (which would result in the red line on the graph) or that neutrinos have a magnetic moment, although two independent sources of the XENON detector have placed constraints on themagnetic moment of a neutrino at less than required to produce the results recorded. 14 Though WIMPs and axions are the most likely source of dark matter, other explanations do exist. One is MACHOs, which are astronomical bodies disassociated from planetary systems. They emit negligible, if any, radiation making their detection incredibly hard. MACHOs are unlikely to explain most dark matter in the universe as they are unlikely to be able to clump in large enough numbers to form the halos of dark matter as seen in figure 2. 15 Furthermore, the current understanding of the big bang, as well as observations on the large-scale structure of galaxies, places limits on the ratio of baryonic matter to non-baryonic matter, suggesting that MACHOs can only be at most a small part of the story of dark matter. 9 Charge-parity. 10 Quantum Chromodynamics. 11 Falk (2020). 12 Wood (2019). 13 Seigel (2020); Rayner (2020). 14 The magnetic moment of the neutrino would have to have been as high as billion times greater than predicted by the standard model. 15 Ashfaque (2015).
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