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Both the lack of observation of ultra-high energy (UHE) photons and the limitations of the state-of-the-art methodology being applied for their identification motivate studies on alternative approaches to the relevant simulations and the related observational strategies. One such new approach is proposed in this report and it concerns new observables allowing indirect identification of UHE photons through cosmic ray phenomena composed of many spatially correlated extensive air showers or primary cosmic rays observed at one time. The study is based on simulations of interactions of UHE photons with the magnetic field of the Sun using the PRESHOWER program with some essential modifications. One of the expected results of such interactions is a generation of cosmic ray ensembles (CREs) in the form of very thin and very elongated cascades of secondary photons of energies spanning the whole cosmic ray energy spectrum. Upon entering the Earth's atmosphere, these cascades or their parts may generate uniquely characteristic walls of spatially correlated extensive air showers, and the effect is expected also in cases when primary UHE photons are not directed towards the Earth. Particle distributions in these multi-primary UHE photon footprints are expected to have thicknesses of the order of meters and elongations reaching even hundreds of millions of kilometers, making them potentially observable with a global, multi-experiment approach, including re-exploring of the historical data, with the expected event rate exceeding the capabilities of even very large cosmic ray observatories. In this report, we introduce for the first time the methods allowing for simulating the isotropic flux of UHE photons in the Sun's vicinity. Presented methods were verified and optimised in such a way that they would successfully model the cumulative spatial distribution of secondary photons at the top of the atmosphere. The preliminary results of simulations for the UHE photon flux of energy 100 EeV demonstrate the possibility of simulating potentially observable quantities related to CRE induced by UHE photons: densities, energy spectra and geographical orientations of secondary particles at the top of the Earth's atmosphere. A measurement of at least one of these quantities would be equivalent to a confirmation of the existence of UHE photons, which would give an insight into fundamental physics processes at unprecedentedly high energies, far beyond the reach of man-made accelerators. On the other hand, a lack of such an observation would allow for further constraining of these fundamental processes with the physically new upper limits on UHE photon fluxes after careful analysis of the technical observation ability. The novel advantage of such an approach would lay in the purely electrodynamical character of the underlying simulations which are fully independent on extrapolations of hadronic interaction models by many orders of magnitude. Such extrapolations are necessary in the UHE photon identification methods based on the analyses of properties of individual extensive air showers presently used to determine the UHE photon upper limits. [ABSTRACT FROM AUTHOR]

Warm dark matter is consistent with the observations of the large-scale structure, and it can also explain the cored density profiles on smaller scales. However, it has been argued that warm dark matter could delay the star formation. This does not happen if warm dark matter is made up of keV sterile neutrinos, which can decay into X-ray photons and active neutrinos. The X-ray photons have a catalytic effect on the formation of molecular hydrogen, the essential cooling ingredient in the primordial gas. In all the cases we have examined, the overall effect of sterile dark matter is to facilitate the cooling of the gas and to reduce the minimal mass of the halo prone to collapse. We find that the X-rays from the decay of keV sterile neutrinos facilitate the collapse of the gas clouds and the subsequent star formation at high redshift. [ABSTRACT FROM AUTHOR]