BASEL, Switzerland, May 12 -- Universe, a peer-reviewed open access journal that says it focuses on fundamental principles in physics, published research articles, including the following topics, in its May 2021 edition:

Reviews:

* Feedback from Active Galactic Nuclei in Galaxy Groups

* Gamma Rays as Probes of Cosmic-Ray Propagation and Interactions in Galaxies

* Recent Observations of Gravitational Waves by LIGO and Virgo Detectors