Review of Observational Evidence for Dark Matter in the Universe and in upcoming searches for Dark Stars (0812.4005v1)

Abstract: Over the past decade, a consensus picture has emerged in which roughly a quarter of the universe consists of dark matter. The observational evidence for the existence of dark matter is reviewed: rotation curves of galaxies, weak lensing measurements, hot gas in clusters, primordial nucleosynthesis and microwave background experiments. In addition, a new line of research on Dark Stars is presented, which suggests that the first stars to exist in the universe were powered by dark matter heating rather than by fusion: the observational possibilities of discovering dark matter in this way are discussed.

Overview of Observational Evidence for Dark Matter and Dark Stars

Katherine Freese's paper provides a comprehensive evaluation of observational evidence supporting the existence of dark matter in the universe, examining multiple studies and methodologies that have shaped the consensus understanding of dark matter. Furthermore, it explores the innovative concept of Dark Stars, proposing a scenario where dark matter plays a fundamental role in stellar formation and evolution.

Observational Evidence for Dark Matter

The research begins by delineating the Concordance Model of cosmology, in which dark matter constitutes approximately 23% of the universe's total composition. Dark matter's existence is primarily inferred from its gravitational influence on visible matter and radiation. Several key lines of evidence are examined:

1. Galactic Rotation Curves: Unlike expectations from luminous mass alone, galaxy rotation curves remain flat at large radii, implying a substantial unseen mass. This mass necessitates the presence of extensive dark matter halos enveloping galaxies, making up more than 95% of their total mass.
2. Gravitational Lensing: Observations of lensing effects in galaxies and clusters align with predictions under General Relativity, further indicating the presence of dark matter. Lensing studies, such as those conducted in the Sloan Digital Sky Survey, illustrate that galaxies are more massive and extend farther than previously understood.
3. Hot Gas in Clusters: X-ray emissions from clusters such as the Coma Cluster highlight the presence of hot gas that requires dark matter's gravitational well to remain bound, providing another line of evidence for dark matter's significant role.
4. Cosmic Microwave Background (CMB) and Primordial Nucleosynthesis: CMB measurements, notably by the WMAP collaboration, offer cosmological-scale evidence for dark matter, while elemental abundances from primordial nucleosynthesis restrict the baryonic content of the universe, corroborating dark matter's dominant presence.
5. Bullet Cluster Observations: The dynamics observed in the Bullet Cluster—where dark matter and baryonic matter separate during cluster collisions—defy modified gravity theories devoid of dark matter, underscoring dark matter's fundamental role distinct from baryonic matter.

Dark Matter Candidates

The paper explores potential candidates for dark matter, including non-baryonic candidates like WIMPs and axions, and questions the adequacy of baryonic MACHOs to account for dark matter. WIMPs remain a focus due to their theoretical backing from supersymmetry and their consistency with dark matter's required relic density observed today—a phenomenon described as the "WIMP miracle." Current and future detection experiments aim to substantiate these theoretical candidates through direct and indirect methods.

The Concept of Dark Stars

Freese introduces the Dark Star hypothesis, positing that the first stars may have been powered not by nuclear fusion but by the energy from dark matter annihilation. This concept suggests that during the early universe, the concentration of WIMPs within primordial stars could provide sufficient heating to sustain stellar equilibrium. Dark Stars would have distinct features compared to traditional Population III stars, with implications for their size, brightness, and impact on subsequent cosmic events.

The hypothesis centers around three criteria: high dark matter density within the stars, efficient trapping of annihilation products, and dominance of dark matter heating over other processes. The potential for dark matter capture even after the initial supply is exhausted suggests an enduring phase of growth and evolution for these stellar objects.

Implications and Future Directions

The paper reinforces the significance of dark matter in astrophysical phenomena and its integral role in cosmological models. The conception of Dark Stars invites further exploration into the intersection of dark matter particle physics and cosmology. Future developments may include observational quests for detecting such objects with advanced telescopic technologies, offering a novel approach to indirectly validate dark matter theories.

In conclusion, Freese's paper underlines the extensive and varied evidence for dark matter's existence while introducing a fascinating line of inquiry into its cosmological consequences. The continued convergence of theory, observation, and experimentation promises to refine our understanding of dark matter and its profound influence on the universe.