Brief Review of Recent Advances in Understanding Dark Matter and Dark Energy (2111.00363v1)

Abstract: Dark sector, constituting about 95 percent of the Universe, remains the subject of numerous studies. There are lots of models dealing with the cause of the effects assigned to dark matter and dark energy. This brief review is devoted to the very recent theoretical advances in these areas: only to the advances achieved in the last few years. For example, in section devoted to particle dark matter we overview recent publications on sterile neutrinos, self-interacting dark matter, dibarions, dark matter from primordial bubbles, primordial black holes as dark matter, axions escaping from neutron stars, and dark and usual matter interacting via the fifth dimension. We also overview the second flavor of hydrogen atoms: their existence was proven by analyzing atomic experiments and is also evidenced by the latest astrophysical observations of the 21 cm spectral line from the early Universe. While discussing non-particle models of the cause of dark matter effects, we refer to modified Newtonian dynamics and modifications of the strong equivalence principles. We also consider exotic compact objects, primordial black holes, and retardation effects. Finally, we review recent studies on the cause of dark energy effects. Specifically, we cover two disputes that arose in 2019 and 2020 on whether the observations of supernovas, previously interpreted as the proof of the existence of dark energy, could have alternative explanations. Besides, we note a study of 2021, where dark energy is substituted by a new hypothetical type of dark matter having a magnetic-type interaction. We also refer to the recent model of a system of nonrelativistic neutral gravitating particles providing an alternative explanation of the entire dynamics of the Universe expansion. without introducing dark energy or new gravitational degrees of freedom.

• The paper categorizes dark matter hypotheses into particle and non-particle models, detailing their theoretical advances and experimental challenges.
• It presents emerging evidence, such as the second flavor of hydrogen atoms, which may resolve discrepancies in cosmic observations.
• The review explores alternative dark energy models including dynamic scalar fields and modified gravity, fueling ongoing debates in cosmology.

Advances in Understanding Dark Matter and Dark Energy

The paper by Eugene Oks presents a comprehensive review of recent theoretical advances in our understanding of dark matter and dark energy, two predominant components of the universe's dark sector. This review is timely given the ongoing debates and developments surrounding these enigmatic cosmic elements, which together constitute approximately 95% of the universe.

Dark Matter Models

The paper categorizes dark matter hypotheses into two primary groups: particle dark matter and non-particle models. Particle dark matter models involve new particles as potential dark matter candidates, including sterile neutrinos, self-interacting dark matter, dibaryons, dark matter from primordial bubbles, primordial black holes, axions, and interactions through a fifth dimension.

Each of these hypotheses offers unique insights and confronts specific challenges. For instance, sterile neutrinos have not been detected experimentally but are proposed based on symmetry arguments and potential mirror-universe models. Self-interacting dark matter, contrasting with the collisionless cold dark matter model, attempts to account for observed discrepancies in galaxy dynamics but requires unknown dark forces. Novel suggestions such as dark matter from primordial bubbles incorporate early universe phase transitions, while hypotheses involving primordial black holes suggest alternative formation mechanisms which remain observationally unverified. The second flavor of hydrogen atoms presents a particularly intriguing candidate, supported by experimental evidence and potentially resolving observational anomalies without the need for exotic new physics.

Alternative Theories and Exotic Proposals

Several non-particle models, including modified gravity and exotic compact objects, seek to explain dark matter effects without invoking new particles. Modified gravity theories, such as Modified Newtonian Dynamics (MoND), attempt to reconcile observed galactic rotation curves without unseen mass, although they struggle with galaxy cluster scales. Exotic compact objects like quark stars and fuzzballs offer alternative baryonic configurations carrying the potential for dark matter roles. These hypotheses, however, often extend beyond standard physics and face significant empirical challenges.

Dark Energy Hypotheses

Understanding dark energy focuses on elucidating the universe's accelerated expansion. The cosmological constant remains the simplest solution proposed, though fraught with theoretical discrepancies regarding its magnitude. Other propositions like quintessence, phantom energy, and k-essence involve dynamic scalar fields extending the standard model's framework, though these too entail significant theoretical challenges.

Counterpoints include hypotheses negating the need for dark energy altogether, favoring modified gravity or alternative cosmological models. Such perspectives question traditional supernova evidence for dark energy via statistical critiques or propose alternative cosmic dynamics potentially driven by hitherto overlooked mechanisms like universal rotation or spin direction disparity in galaxies.

Conclusions and Implications

Oks' review underscores the rich diversity and contentious nature of current theoretical advances in dark sector research. It highlights both well-trodden and novel ideas, each exhibiting distinct advantages and hurdles. Particle dark matter models generally require extensions to the standard model, whereas recent experimental data supporting the second flavor of hydrogen atoms promise insights consistent with established physics yet capable of explaining key cosmic observations.

In contrast, dark energy remains more enigmatic, with models necessitating either new physics or complex cosmological reconstructions. Notably, the paper stresses ongoing debates around observational interpretations and theoretical proposals questioning the very existence of dark energy.

Future Directions

The continued development of observational techniques, including advanced telescopes and gravitational wave observatories, seeks to test these diverse hypotheses. Future research will likely further delineate viable models from those requiring revision or rejection. The paper also invites reconsideration of foundational cosmological assumptions in light of emerging evidence and theoretical models, promoting a dynamic reevaluation of our cosmic understanding.