Abstract: Dark matter is one of the greatest unsolved mysteries in cosmology at the present time. About 80% of the universe's gravitating matter is non-luminous, and its nature and distribution are for the most part unknown. In this paper, we will outline the history, astrophysical evidence, candidates, and detection methods of dark matter, with the goal to give the reader an accessible but rigorous introduction to the puzzle of dark matter. This review targets advanced students and researchers new to the field of dark matter, and includes an extensive list of references for further study.
The paper synthesizes historical astronomical observations and theoretical particle models to detail evidence for dark matter.
It explores detection methodologies, including both direct nuclear recoil experiments and indirect gamma ray and neutrino searches.
The work emphasizes the interdisciplinary nature of dark matter research, highlighting the need for continued experimental and theoretical advancements.
Dark Matter: A Comprehensive Introduction
The paper "Dark Matter: A Primer" by Katherine Garrett and Gintaras Duˉda offers a detailed examination of one of cosmology's most persistent enigmas, dark matter. The authors endeavor to synthesize the historical background, astrophysical evidence, potential candidates, and detection methodologies into a cohesive review aimed at providing foundational knowledge to researchers new to the field.
Historical Context and Observational Evidence
Initial hypotheses surrounding dark matter stemmed from astronomical observations in the early 20th century. J. H. Oort's work on stellar motion in the Milky Way and Fritz Zwicky's studies of the Coma cluster both suggested a significant discrepancy between the observable luminous mass and the gravitational mass required to explain observed dynamics. Notably, Zwicky employed the virial theorem in his analyses, which presaged the discovery of vast quantities of mass that were non-luminous.
Vera Rubin's pivotal research on spiral galaxy rotation curves further substantiated the necessity for dark matter. Her observations revealed that galactic rotational velocities remained constant, contravening expectations from Newtonian dynamics, which would predict a decrease in velocity proportional to 1/r​ at large radii. Rubin's work indicated that most galactic mass is not radiating light, binding stars beyond their observable mass.
Efforts to detect massive compact halo objects (MACHOs) via microlensing, such as by the MACHO and EROS-2 collaborations, have largely discounted them as major components of dark matter, thus directing focus to non-baryonic candidates.
Gravitational lensing observations, particularly of the Bullet cluster, have served as robust evidence for dark matter, demonstrating mass concentrations offset from luminous matter. Such direct empirical proofs suggest collisionless dark matter properties, inconsistent with modified Newtonian dynamics (MOND) models, which attempt to explain missing mass without invoking dark matter.
Particle Candidates and Theoretical Frameworks
The authors thoroughly discuss particle physics extensions beyond the Standard Model that propose viable dark matter candidates, specifically WIMPs (Weakly Interacting Massive Particles). Supersymmetry (SUSY) emerges as a resonant solution, introducing the lightest supersymmetric partner (LSP), notably the neutralino, as a dark matter candidate. The neutralino, not yet detected in laboratory settings, is stable due to R-parity conservation and fits the cold dark matter profile consistent with known astrophysical and cosmological constraints.
Additionally, the paper explores alternative theories, including axions, produced through spontaneous Peccei-Quinn symmetry breaking to resolve the strong CP problem, and Kaluza-Klein particles in scenarios with extra spatial dimensions. Each candidate brings forth different experimental implications and challenges.
Detection Methods and Experimental Pursuits
On the experimental front, the review explores both direct and indirect detection methodologies. Direct detection efforts, aiming to observe nuclear recoils from dark matter particles, include notable experiments like CDMS II and XENON10, which set stringent limits on WIMP-nucleon cross sections. Despite these advancements, direct detection has yet to unequivocally confirm the existence of WIMPs.
Indirect detection programs analyze potential annihilation products of dark matter, such as gamma rays and neutrinos. While the PAMELA, Fermi, and other experiments provide suggestive evidence of WIMP interactions, no clear confirmation has yet been obtained. Atmospheric neutrino detectors like IceCube continue to refine constraints on high-energy neutrino fluxes originating from WIMP annihilation.
Conclusion
This paper underscores the multifaceted and interdisciplinary nature of dark matter research, merging cosmology, particle physics, and astrophysics. Although significant progress has been made in understanding dark matter's role in cosmic evolution, its precise nature remains an open question. Continued theoretical development and increasingly sensitive experimental approaches are imperative to unlocking the mysteries of dark matter, a component critical to the universe's mass-energy budget and structure formation. As this field of research advances, new insights are anticipated, potentially heralding transformative understandings of more fundamental principles in both cosmology and particle physics.