Dark matters on the scale of galaxies (2007.15539v1)

Abstract: The cold dark matter model successfully explains both the emergence and evolution of cosmic structures on large scales and, when we include a cosmological constant, the properties of the homogeneous and isotropic Universe. However, the cold dark matter model faces persistent challenges on the scales of galaxies. {Indeed,} N-body simulations predict some galaxy properties that are at odds with the observations. These discrepancies are primarily related to the dark matter distribution in the innermost regions of the halos of galaxies and to the dynamical properties of dwarf galaxies. They may have three different origins: (1) the baryonic physics affecting galaxy formation is still poorly understood and it is thus not properly included in the model; (2) the actual properties of dark matter differs from those of the conventional cold dark matter; (3) the theory of gravity departs from General Relativity. Solving these discrepancies is a rapidly evolving research field. We illustrate some of the solutions proposed} within the cold dark matter model, and solutions when including warm dark matter, self-interacting dark matter, axion-like particles, or fuzzy dark matter. { We also illustrate some modifications of the theory of gravity: Modified Newtonian Dynamics (MOND), MOdified Gravity (MOG), and $f(R)$ gravity.

Dark Matters on the Scale of Galaxies

The cold dark matter (CDM) model offers a framework to describe cosmic structures and the Universe's expansion through a cosmological constant. However, the model faces challenges, particularly on galactic scales. This paper by Ivan de Martino and collaborators examines these discrepancies and proposes solutions within the dark matter paradigm and alternative theories of gravity.

Challenges with the Cold Dark Matter Model

The CDM model successfully accounts for large-scale cosmic structures but struggles with galaxy-scale phenomena. Notable discrepancies include the expected distribution and behavior of dark matter in galaxy halos, particularly the innermost regions and dwarf galaxies. These discrepancies can originate from incomplete understanding of baryonic physics, alternate properties of dark matter, or deviations from General Relativity.

Galaxy Rotation Curves and Baryonic Scaling Relations

CDM predictions are at odds with observed galaxy rotation curves and baryonic scaling relations like the Baryonic Tully-Fisher Relation (BTFR), the Mass Discrepancy Acceleration Relation (MDAR), and the Radial Acceleration Relation (RAR). The small scatter found in these relations contrasts with expectations from CDM, suggesting a unexplained connection between dark and baryonic matter distributions.

Cusp/Core and Missing Satellites Problems

N-body simulations predict cuspy density profiles for dark matter, whereas observations favor cores, particularly in dwarf galaxies. This central density discrepancy is a well-known issue termed the "cusp/core problem." Moreover, CDM simulations forecast a larger number of satellite galaxies than observed, known as the "missing satellites problem." Similar mismatches in halo mass and galaxy formation efficiency suggest modifications might be necessary to the CDM model.

Proposed Solutions

The paper explores solutions within and outside of the CDM paradigm:

• Warm Dark Matter (WDM): Proposes non-baryonic particles with properties between CDM and neutrinos, mitigating small-scale structure formation challenges. However, constraints from Lyman-alpha forest data place stringent bounds on WDM viability, questioning its ability to address the cusp/core issue effectively.
• Self-Interacting Dark Matter (SIDM): Offers a framework where dark matter self-interactions lead to lower central density, helping alleviate cusp/core issues and explaining galaxy halo shapes. Velocity-dependent interactions are particularly promising but require further understanding.
• Axions and Fuzzy Dark Matter: Ultra-light axions are suggested as dark matter candidates that form Bose-Einstein condensates, which could potentially explain dark matter distribution in galaxies. Fuzzy dark matter models predict large-scale coherence due to quantum effects, offering a novel explanation for the small-scale issues in CDM.
• Modified Gravity Theories: This paper explores Modified Newtonian Dynamics (MOND), a phenomenological modification of gravity via an acceleration parameter, providing solutions to galactic rotation discrepancies. MOND's predictive power at galactic scales has been substantial, although its cosmological applicability remains limited. The paper also reviews MOdified Gravity (MOG) and $f(R)$ gravity as theoretical frameworks capable of addressing dark matter effects without necessitating the dark matter hypothesis.

Implications and Future Prospects

The discrepancies presented compel reconsideration of the CDM model or exploration of alternative theories. Solutions such as WDM and SIDM offer potential routes forward but must overcome stringent observational constraints. Axions and fuzzy dark matter open new avenues in both theoretical physics and astrophysics, bridging high-energy physics and cosmology.

Further robust, high-resolution observational and simulation efforts are necessary to refine these models, particularly in addressing the small-scale challenges of CDM. Novel experiment approaches and deeper astrophysical surveys will likely play a crucial role in solving these intricate cosmic puzzles. Understanding these aspects could redefine the standard model of cosmology, potentially integrating particle physics, astrophysics, and modified gravitational models.