Dark matter, dark energy, and alternate models: A review

Abstract: The nature of dark matter (DM) and dark energy (DE) which is supposed to constitute about 95% of the energy density of the universe is still a mystery. There is no shortage of ideas regarding the nature of both. While some candidates for DM are clearly ruled out, there is still a plethora of viable particles that fit the bill. In the context of DE, while current observations favour a cosmological constant picture, there are other competing models that are equally likely. This paper reviews the different possible candidates for DM including exotic candidates and their possible detection. This review also covers the different models for DE and the possibility of unified models for DM and DE. Keeping in mind the negative results in some of the ongoing DM detection experiments, here we also review the possible alternatives to both DM and DE (such as MOND and modifications of general relativity) and possible means of observationally distinguishing between the alternatives.

• The paper provides a comprehensive review of dark matter and dark energy, which constitute 95% of the universe's energy density, and evaluates various theoretical models and observational evidence.
• Key dark matter topics discussed include astrophysical evidence, candidate particles like WIMPs and axions, and the challenges faced by ongoing direct and indirect detection experiments.
• The review explores dark energy through models like the cosmological constant and quintessence, considers potential unified dark matter-dark energy theories, and critically examines alternative modified gravity theories.

Dark Matter, Dark Energy, and Alternate Models: A Review

The study titled "Dark Matter, Dark Energy, and Alternate Models: A Review" presents a comprehensive evaluation of dark matter (DM) and dark energy (DE), which collectively constitute approximately 95% of the universe's energy density. Despite extensive research, their precise nature remains uncertain. This paper scrutinizes potential candidates for DM and alternative DE models, with a focus on observational evidence, theoretical frameworks, and the viability of alternative theories to replace DM and DE.

Dark Matter Analysis

Evidence and Classification

Dark matter's presence is inferred from a variety of astrophysical observations, including cosmic microwave background anisotropies, galaxy cluster dynamics, gravitational lensing, and X-ray emissions from galaxy clusters. Key categorizations of DM include cold, warm, and hot varieties, each with distinct characteristics influencing cosmic structure formation. Cold dark matter (CDM) is favored for its ability to explain galaxy formation from a bottom-up perspective, aligning with empirical data.

Potential Candidates

The paper evaluates various DM candidates, extending from baryonic MACHOs, whose potential contribution is largely negated by nucleosynthesis constraints, to non-baryonic particles such as WIMPs, axions, primordial black holes, and exotic entities like "Fermi balls" and "mirror dark matter". These candidates are assessed in light of their mass range, interaction characteristics, and detectability. Particle physics extensions, like supersymmetry, offer plausible candidates like neutralinos for CDM.

Detection Methods

Despite ongoing attempts through both direct and indirect detection techniques—such as cryogenic detectors, xenon-based experiments (e.g., LUX, XENON1T), and cosmic ray observatories—conclusive evidence for DM particles has been elusive. These experiments strive to observe the scattering events or annihilation products anticipated from DM interactions.

Dark Energy Exploration

Theoretical Models

DE is largely characterized by its role in the universe's accelerated expansion, evident from supernova observations and CMB measurements. Proposed models include the cosmological constant (Λ), quintessence, phantom energy, and hybrid models like quintom dark energy. Quintessence, for instance, posits a dynamically evolving scalar field, while phantom energy entails potential future cosmological phenomena such as a "Big Rip".

Unified Theories

The paper investigates the hypothesis that DM and DE might share a common origin, exemplified by models like the Chaplygin and Dieterici gases, which suggest a potential unification under varying equations of state. Such approaches aim to transcend fine-tuning issues inherent in conventional models.

Alternative Theories

The review acknowledges the limitations and challenges of conventional DM and DE paradigms, thus contemplating modified theories of gravity, such as MOND, TeVeS, and modifications to the Einstein-Hilbert action. These theories propose alterations in gravitational dynamics on galactic or cosmological scales to address phenomena typically ascribed to DM or DE. Although promising in some respects, experimental evidence, such as observations of the Bullet Cluster, often challenges these theories, affirming the necessity of DM presence.

Implications and Future Directions

The detailed examination of this paper underscores the profound complexity and evolving understanding of DM and DE. These phenomena are central to cosmology and theoretical physics, with implications for understanding the universe's fundamental structure and destiny. Future research should prioritize innovative experimental techniques and theoretical developments to elucidate these dark components. Enhanced sensitivity in detection efforts and novel astronomical observations may gradually reveal the universe's elusive constituents, marking substantial progress in cosmological and particle physics research. As the paper indicates, the upcoming years hold promise for significant discoveries or paradigm shifts in comprehending the cosmos.