Probing the Unknown Nature of Gravitation and Dark Matter in the Universe

Dear Colleagues,

The Big Bang cosmological model is an excellent achievement of contemporaneous fundamental physics. Such a model can explain the formation of the structure observed in the present-day universe from planets and stars up to galaxies and galaxy clusters. Nevertheless, this success is mitigated by two tough predicaments in the model, i.e., the postulation of the existence of two gravitational sources of unknown origin: dark matter and dark energy.

The impact of dark matter in structure formation manifests itself mostly at small cosmological scales in the present-day universe. The bottom–up approach in structure formation (small structures are formed first, and bigger structures are formed later on) implies that hot dark matter is ruled out, although a very small percentage can be tolerated. All current data support the idea that most of the nonrelativistic matter in the universe is of nonbaryonic nature. Consequently, standard model neutrinos cannot be the dominant contribution of dark matter. Furthermore, some puzzles at galactic scales indicate that dark matter may be self-interacting. Although, presently, many different candidates and models do exist in the literature (bosons or fermions, collisionless or self-interacting, thermal relics from the big-bang or non-thermally produced, etc.), the nature and origin of dark matter still remains a mystery.

Dark energy, sometimes referred to as an antigravity force, becomes dominant in the last 4 billion years in which the Universe has experimented with a cosmic acceleration in its expansion. Furthermore, although Einstein’s general relativity has passed numerous tests and is considered to be both beautiful and successful, it is widely accepted that it may be the low energy limit of a more complicated theory of gravity, which allows for additional terms of modified general relativity, either in the early universe to describe cosmological inflation, or at later times to explain the current cosmic acceleration.

Since those two unknown sources of gravity will impact the formation and evolution of stars in unique ways, stars can be used as first probes of such a new cosmological model. Relativistic compact objects are characterized by ultradense matter as well as strong gravitational fields and must be studied assuming both a certain theory of gravity and an equation-of-state derived from particle and nuclear physics. Therefore, the properties of compact stars may be modified either due to the presence of dark matter particles inside the object or due to unconventional structure equations coming from a modified theory of gravity.

Therefore relativistic stars may be viewed as ideal cosmic laboratories to explore the unknown nature of gravitation and dark matter in the universe.

Prof. Dr. Ilídio Lopes
Dr. Grigorios Panotopoulos
Guest Editors

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