BASEL, Switzerland, April 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 April 2021 edition:

Review:

* A Gravitational-Wave Perspective on Neutron-Star Seismology

* 20 Years of Indian Gamma Ray Astronomy Using Imaging Cherenkov Telescopes and Road Ahead

* Heavy Tetraquarks in the Relativistic Quark Model