Think of the large-scale universe as a web: Filamentary structures—threads of galaxies, gas, and dark matter—crisscross space. Where they intersect, gravitationally collapsing material forms galaxy clusters that can then merge through some still-unknown process. Some of the energy released by cluster mergers helps power relativistic particles that spiral around magnetic field lines.

One way that astronomers observe what happens during a merger is by detecting emissions that come from superheated plasma known as the intracluster medium (ICM). The density of the ICM is so low that the only way for matter to interact is through collisionless dealings between the plasma’s electric and magnetic fields. X rays are emitted from the interaction of the plasma’s electric field with free electrons; synchrotron radiation is emitted from cosmic-ray particles traveling through magnetic fields. (See the article by Lawrence Rudnick, Physics Today, January 2019, page 46.)

Astronomers have long wondered whether the...

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