Astrophysical probes

The term astrophysical probes can refer to particles emitted in astrophysical processes, usually referred to as astrophysical messengers, or to astrophysical processes whose dynamics could be affected by the quantum nature of gravity. Messengers are particles or waves produced in astrophysical sources. They tend to reach energies far beyond those attainable in terrestrial accelerators. These messengers are cosmic rays, gravitational waves, neutrinos, and photons (or gamma rays if we focus on high energy regime). They carry information about the physical processes in sources through which they were created; about the spacetime effects and interaction with radiation and matter they experience on their way through Cosmos; and about the processes through which they are detected. Any of these processes can be used to probe quantum gravity, and can be classified in three stages: production, propagation, and detection [DePi18, AlCoVi18].

The production stage is strongly influenced by the nature of the source. In sources dominated by hadronic processes, interactions involving accelerated protons or nuclei lead to the simultaneous production of cosmic rays, gamma rays, and neutrinos, whereas in leptonic environments the emission is largely driven by accelerated electrons and their radiative losses, resulting in characteristic photon spectra with no accompanying neutrino flux. These differences directly affect the interpretation of multimessenger observations and the type of quantum gravity effects that can be tested.

The location of the source also plays a key role during propagation, through both the distance traveled and the environments encountered by the messengers. Nearby Galactic sources typically allow access to very high energies with limited attenuation, while extragalactic sources combine extreme energies with long propagation distances and interactions with diffuse photon backgrounds. Quantum-gravity effects may manifest themselves through modifications of standard interactions, such as changes in reaction thresholds, cross sections, or effective opacities, altering, for instance, the attenuation of high-energy gamma rays during propagation. Modified kinematics can also allow or forbid reactions with respect to special relativity, leading to qualitative changes in particle stability or decay channels at high energies A review can be found in [Ad22].

Finally, at the detection stage, messengers are observed through different experimental techniques depending on their nature and energy, such as air-shower measurements, Cherenkov radiation, neutrino telescopes, or interferometric detectors. Since different messengers are linked by the particle processes that govern their production and interactions, their origin can be investigated in a correlated way. Multimessenger astrophysics is developed through studies involving more than one messenger, and is a very hot topic in current investigations of the nature of astrophysical objects, as well as searches for signatures of quantum gravity.

See also:

• Astrophysical sources

• Production stage (acceleration mechanisms, elementary particle interactions, time delays)

• Propagation stage (elementary particle interactions, time delays, magnetic fields)

• Detection stage (detection techniques)

• Multimessenger astrophysics (connections of different messengers and energy ranges)

• Quantum gravity effects (list of relevant effects)

List of Authors

References

[DePi18] De Angelis A and Pimenta M 2018 Introduction to Particle and Astroparticle Physics: Multimessenger Astronomy and its Particle Physics Foundations [doi:10.1007/978-3-319-78181-5]

[AlCoVi18] Aloisio R, Coccia E, and Vissani F eds. 2018 Multiple Messengers and Challenges in Astroparticle Physics [doi:10.1007/978-3-319-65425-6]

[Ad22] Addazi A et al. 2022 Quantum gravity phenomenology at the dawn of the multi-messenger eraA review Prog. Part. Nucl. Phys. 125 103948 [doi:10.1016/j.ppnp.2022.103948]