Multi-messenger high-energy astrophysics is the extension of the multi-wavelength exploration of the cosmos with multiple messengers with a common origin, including neutrinos, gravitational waves, and cosmic rays. This branch of astrophysics has currently achieved the potential to unravel
the origin of cosmic rays and how sources accelerate them, their relation to the diffuse radiation in the extra-galactic space, and their role to forge their galaxies of origin while they wander in their magnetic fields for millions of years.
Neutrino astronomy produced its major scientific milestone with the discovery by IceCube of a diffuse flux at energies above 60 TeV with intensity comparable to a predicted upper limit to the
flux from extra-galactic sources of ultra-high energy cosmic rays. More recent results provide the first strong evidence of a standalone neutrino source and a highly probable coincidence of a neutrino alert with gamma rays. These results of IceCube indicate that neutrino astronomy can complement photon astronomy also providing insights into opaque sources of high-energy radiation. Starburst galaxies and jetted black holes in active galaxies are favored candidates to explain the diffuse cosmic neutrino background at > 60 TeV energies and its relation to the extragalactic background light. Additionally, gamma-ray bursts remain an intriguing mystery now
enriched by joint observations of gamma-rays and gravitational waves. Ground-based detection of gamma-ray burst emissions with energies up to more than 10 TeV challenges the standard fireball model as well as the non-observation of neutrinos.
The galactic diffuse flux, produced by cosmic ray interactions on the interstellar matter of our galaxy and peaking at lower energies, is within the reach of neutrino detectors. Together with the measured galactic gamma-ray flux up to PeV energies, they will shed light on the knee region of
cosmic rays and the possible existence of dark matter in the Galactic plane.
In the future, more work will be done in IceCube and deep sea and lake neutrino telescopes to use further low-energy cascades for cosmic source searches thanks to improved descriptions of
detection media and deep learning methods.
These aspects were discussed at the conference and are summarised in this write-up, and when necessary more recent results will be referred to.