Cosmic radiation: "Amaterasu" particle with extreme energy poses a mystery
No known physical process can explain the enormous energy of a particle detected in 2021. Experts are discussing exotic causes, such as unknown forms of matter or even properties of space-time itself.
A cosmic particle with the second-highest energy ever recorded has reignited the debate about physical phenomena beyond the known. On 27 May 2021, 23 detectors at the Telescope Array in Utah registered cosmic rays with an energy of 2.4 × 10^20 electron volts, which is roughly equivalent to a kick in the butt. Only a particle detected in 1991 with 3.2 × 10^20 electron volts had more energy, which was christened the "Oh-My-God particle". Similarly, the team named the new particle "Amaterasu particle" after the Japanese sun goddess. To date, no physical process is known that can accelerate individual particles to such high energies.
"Processes that are generally understood to be high-energy, such as supernovae, are not nearly energetic enough for this," says John Matthews from the University of Utah and one of the spokespersons for the telescope collaboration that detected the particle. "You need extremely strong magnetic fields to confine the particle while it is being accelerated." As the team now reports in the journal "Science", the source of the particle is also a complete mystery. It hit the detectors from roughly the direction of the local void, an enormous, largely empty region of space near the Local Group of galaxies, to which our Milky Way system belongs.
In addition, the particle must have come from the immediate neighbourhood; as the working group calculates, it could not have travelled more than a few tens of millions of light years, otherwise it would have lost its energy through interactions with magnetic fields and the cosmic microwave background. Such a high-energy source in the relative neighbourhood should actually be visible. "These particles have such high energies that they should not be deflected by galactic or extragalactic magnetic fields," explains Matthews. "You should be able to show where in the sky they come from." However, the 30 or so extremely energetic particles above 10^20 electron volts registered by the telescope network so far appear to have come from all areas of the sky without any recognisable accumulation.
Experts are therefore at a complete loss as to what processes could be behind the enormously high energies. In addition to processes relating to relatively conventional objects such as rotating extremely massive black holes, gamma-ray bursts or neutron stars, research groups have also put forward exotic explanations for the high energies that go beyond currently known physics. For example, stars from previously unknown states of matter based on strange quarks or other subatomic particles or even more unusual processes in free space. "It could be defects in the structure of space-time or colliding cosmic strings," says co-author John Belz from the University of Utah. "I'm just throwing around crazy ideas that other people have come up with because there's no conventional explanation."
Spectrum of science
We are partners of Spektrum der Wissenschaft and want to make well-founded information more accessible to you. Follow Spektrum der Wissenschaft if you like the articles.
[[small:]]
Cover photo: Osaka Metropolitan University / L-Insight, Kyoto University / Ryuunosuke Takeshige
Experts from science and research report on the latest findings in their fields – competent, authentic and comprehensible.