Ultra-High-Energy Cosmic Rays
Ultra-high-energy cosmic rays (UHECRs) are extremely energetic subatomic particles arriving from space. Scientists debate their origins, propagation limits, and composition.
Scientist 1
UHECRs are protons originating from extragalactic active galactic nuclei (AGNs). Because protons have a low charge (), they experience minimal deflection by intergalactic magnetic fields, allowing their arrival directions to correlate with the positions of nearby AGNs. However, these protons must travel through extragalactic space, meaning their energy is limited by interactions with the Cosmic Microwave Background (CMB), a threshold known as the GZK limit (approx. ), which prevents UHECRs from traveling distances greater than 50 megaparsecs (Mpc) without losing significant energy.
Scientist 2
UHECRs are heavy nuclei (specifically iron, ) originating from starburst galaxies (SBGs). Due to their high charge, iron nuclei are highly deflected by magnetic fields, which explains why UHECR arrival directions do not point directly back to their source galaxies. Like Scientist 1, Scientist 2 maintains that UHECRs are extragalactic and thus their propagation over vast distances is strictly constrained by photodisintegration interactions with the CMB, limiting their sources to nearby SBGs within 50 Mpc.
Scientist 3
UHECRs are produced by the decay of supermassive dark matter particles residing in our own Milky Way's galactic halo. Because these particles originate locally within our galaxy rather than across extragalactic space, UHECRs do not travel through the intergalactic medium. Consequently, their flux is not subject to the GZK limit or photodisintegration by the CMB. Their arrival directions are expected to be isotropic, showing a slight dipole anisotropy toward the galactic center.
Based on the passage, which of the following statements best describes a core claim of Scientist 3 that directly distinguishes their hypothesis from those of Scientist 1 and Scientist 2?
- UHECRs originate within the Milky Way's galactic halo, meaning they do not undergo energy-reducing interactions with the Cosmic Microwave Background during propagation.Answer
- BUHECRs are composed of heavy nuclei that undergo heavy magnetic deflection, allowing them to bypass the GZK limit entirely.
- CUHECRs are protons that are accelerated by active galactic nuclei and are shielded from the GZK limit by galactic magnetic fields.
- DUHECRs originate from distant extragalactic sources beyond 50 megaparsecs, which explains the isotropic distribution of their arrival directions.