The origin of the Ultra High Energy Cosmic Rays [UHECR] (E > 5*1019 eV) is still unknown. Theoriticians have developed numerous models in order to give an explanation for the arrival on the athmosphere of such energetic particules.
We can split these models in two groups:
- The so called "bottom up" models:
Low energy particules localized in the plasma of some astrophysical objets are accelerated via well known mechanisms. The most favored scenario was proposed by Enrico Fermi as soon as 1949.
The main idea is the following: cosmic rays are diffused on magnetized gaz clouds moving in the interstellar environment. If a cloud is initialy approaching the particule, this one is accelerated. It is decelerated otherwise. The average value on all the incident angles shows that, statistically, the particules increased their energy at each bounce by a fraction proportionnal to E (so the spectrum generated by such a mechanism is a power law).
The original theory was modified at the end oh the seventies. The magnetised clouds with a random movement are replaced by a unique moving magnetised shock (relativistic or not). This theory leads also to a power law with a slowly variable spectral index, which is an interesting property.
Possible sources where such mechanisms can occur are (non exhaustive list):
These astrophysical models are suffering some weakness: none of them allow to easily over take the 1020 eV threshold and most of them would fail if the GZK cut off (Greisen-Zatsepin-Kuz'min) is not observed.
- supernovae remnants,
- neutrons stars,
- gamma ray bursts,
- large scale shocks in the univers.
- The "top down" models:
In that case, the cosmic rays will be issued from some more "exotic" mechanisms, requiring physics different of the standard model one. Such models have been developed for different reasons: it is justified to refer to new physics when describing particules whose energy is between TeV and Plank mass; the previous models, even considering the most violent process in the univers, have trouble to reach such energies; last, but not least, some experiments like AGASA seem to see no GZK cut off.
A large number of alternative models, relying mainly on unknown physics, have been proposed. Among them, we can quote super heavy dark matter ans topological defects.
They are, of course, extremely speculative. However, these models predict some very precise phenomena:
In conclusion, even if some of these exotic models are not excluded for the moment, they are less attractive than some years ago. That's why the astrophysical UEHCR origin is actually favored, even if some important problems have to be solved in order to fully understand the involved sources.
- UHECRs nature : in spite of incertainties on desintegration modes, quarks combine more easily to give mesons than baryons. So, generated particules are mainly photons ans neutrinos issued from pion desintegration.
- UHECRs spectrum : typical predicted spectra are harder than observed one (E-1.9 typically). So these scenarios can explain the spectrum only for energies greater than GZK cut off. A very fine tune of the parameters is then necessary to connect sub-GZK and sup-GZK spectra, which will be a surprizing coincidence.
- UHECRs anisotropy : in super heavy dark matter scenarios, the UHECRs are coming from our galaxy halo (so no need of GZK effect). So we expect a very strong large scale anisotropy. The actual statistics is almost suffisant to invalidate this prediction.
- Gamma photons diffuse background : The important gamma flux at tipically 1020 eV generated by all these models should produce a bacground of secondary photons with energies ranging fronmGeV to Tev. On quasi cosmologic distances, UHE photons generate electromagnetic showers. The predicted flux is og the same ordre of magnitude, even perhaps larger, than the extra galatic bacground measured by EGRET (after corrections). Some of these models are, for that reason, already excluded.