As the highest energy cosmic rays travel through the cosmos, they encounter the photons of the 2.7 K microwave radiation, a remnant of the ‘Big Bang’. These photons look like high-energy γ-rays in the rest frame of the proton and a reaction takes place in which new particles are formed and the proton losses energy:

This reaction limits the distance from which such energetic particles can have come to about 150 million light years at 10²⁰ eV.

Because the highest-energy cosmic rays are deflected very little by the magnetic fields in our galaxy -- and even less by the much weaker fields in intergalactic space -- we ought to be able to look back in the direction of the cosmic rays to find their origin. So far, however, none of the cosmic ray events with energies above 10²⁰ eV point back to a possible source in the cosmos! Where have they come from?

Wherever they come from, the highest-energy particles hold secrets to the beginning of the universe, because of the mystery of their enormous energies, so many millions of times greater than any earthbound particle accelerator can create. We know of no source in the cosmos that could produce such energies, not even the power released by the most violent exploding stars although there is much speculation.
Fermi's acceleration mechanism provides an explanation for cosmic ray energies perhaps as high as 10¹⁵eV. However, the size of the galaxy and the magnetic field strengths available within it are not great enough to accelerate particles to energies higher than that. More powerful natural accelerators, therefore, must be responsible for the cosmic rays observed at still higher energies; and these powerful accelerators must lie outside our galaxy. They may be produced in active galactic nuclei or colliding galaxies, examples of which are below. In fact, cosmic rays of energies beyond 10²⁰ eV cannot be easily contained by the weak magnetic fields within the galaxy, a fact that also suggests their extragalactic origin.

Cosmologists -- scientists who study the structure and dynamics of the universe -- offer another possible explanation for the mysterious source of the highest-energy cosmic rays. Cosmologists postulate a universe filled with relics left over from the Big Bang -- hypothetical objects, called topological defects, with names like "cosmic strings," "domain walls," and "monopoles" or super-heavy relic particles with exotic names such as cryptons, vortons or wimpzillas. Although these strange objects figure prominently in theories of the evolution of the universe, we have no experimental evidence to show that they really exist. However, if they do, and if they sometimes collapse or decay, they could release enough energy to create very high-energy cosmic rays. If we could make the connection between high-energy cosmic rays and the collapse of topological defects, for example, it would provide experimental evidence for these topological defects and a great step forward in understanding the early universe. Super-heavy relic particles would cluster in the haloes of galaxies and could be identified by spotting an excess from this direction.