How can we tell whether dark energy is composed by multiple fields?

Dark energy is often assumed to be composed by a single scalar field. The background cosmic expansion is not sufficient to determine whether this is true or not. We study multi-field scalar-tensor models with a general dark matter source and write the observable modified gravity parameters (effective gravitational constant and anisotropic stress) in the form of a ratio of polynomials in the Fourier wavenumber k of order 2N, where N is the number of scalar fields. By comparing these observables to real data it is in principle possible to determine the number of dark energy scalar fields coupled to gravity. We also show that there are no realistic non-trivial cases in which the order of the polynomials is reduced.

On the Possibility of Medium-Energy Compact X-ray Free-Electron Laser

The problem of X-ray Free-Electron Laser operating on self-amplified spontaneous emission in irregular microundulator is considered. The case when the spectrum width of spontaneous radiation is conditioned by the spatial distribution of sources creating the undulating field is considered. In this case gain function of the stimulated radiation is dozens of times higher than that of the conventional undulators. We propose a model of irregular microundulator, which can be used to construct a drastically cheap and compact X-ray free-electron laser operating on medium energy electron bunch.

Radiation of the electron bunch moving in non-regular fields

The problem of spontaneous radiation of the electron bunch grazing into a charged metallic surface with randomly distributed needle shaped asperities is considered. Distances between two neighboring asperities have been described by gamma distribution. Being repealed by highly charged asperities the electrons of the bunch move along non-regular periodical trajectories in the planes parallel to the metallic surface. The spatial periods of the trajectories are random quantities which are described by the same gamma distribution. The radiation characteristics of the bunch have been obtained. It is shown that the angular distributions of the number of photons radiated from the bunch and from a single electron are the same but the frequency distribution of the bunch is being drastically changed at the hard frequency region. It is proposed to develop a new non-destructive method for investigation of the metal surface roughness. The frequency distribution of the number of photons radiated under the zero angle has been obtained. That allows to find the gain expression of the stimulated radiation.