Dark energy: investigation and modeling (1004.1493v1)

Abstract: Constantly accumulating observational data continue to confirm that about 70% of the energy density today consists of dark energy responsible for the accelerated expansion of the Universe. We present recent observational bounds on dark energy constrained by the type Ia supernovae, cosmic microwave background, and baryon acoustic oscillations. We review a number of theoretical approaches that have been adopted so far to explain the origin of dark energy. This includes the cosmological constant, modified matter models (such as quintessence, k-essence, coupled dark energy, unified models of dark energy and dark matter), modified gravity models (such as f(R) gravity, scalar-tensor theories, braneworlds), and inhomogeneous models. We also discuss observational and experimental constraints on those models and clarify which models are favored or ruled out in current observations.

• The paper identifies observational evidence that dark energy makes up about 70% of the Universe’s energy density while evaluating multiple models to explain cosmic acceleration.
• The paper employs data from type Ia supernovae, CMB, and BAO to constrain the equation of state and test the viability of models including the cosmological constant and scalar fields.
• The paper discusses the implications of quintessence, coupled dark energy, and f(R) gravity in addressing fine-tuning challenges and meeting local gravity constraints.

Dark Energy: Investigation and Modeling

In the context of cosmology, the concept of dark energy has become a central focus of scientific inquiry due to its major role in driving the accelerated expansion of the Universe. This essay provides a comprehensive overview of various theoretical models, observational constraints, and future prospects within the paper of dark energy, as outlined in the paper "Dark energy: investigation and modeling".

Observational Constraints and Models

The paper starts by discussing the foundation laid by observational data that confirms the presence of dark energy, which constitutes approximately 70% of the energy density of the Universe today. These observations are derived from type Ia supernovae, cosmic microwave background (CMB), and baryon acoustic oscillations (BAO). The observational data indicates that the equation of state parameter $w_{\rm DE}$ of dark energy is close to $-1$, suggesting that the cosmological constant ($\Lambda$) is a plausible candidate for dark energy. However, the source and precise nature of dark energy remain unresolved.

Cosmological Constant and Alternatives

The paper reviews the cosmological constant model alongside various alternatives in order to address the enigmatic nature of dark energy. The simplest model, $\Lambda$, is faced with fine-tuning issues as its energy scale from particle physics expectations is vastly different from observations. The paper explores other models, including scalar fields (quintessence and k-essence), coupled dark energy, Chaplygin gas, and potential modifications to General Relativity.

Quintessence and K-Essence

Quintessence models deploy scalar fields with potential energies that evolve dynamically, potentially resolving issues related to the cosmological constant. Different potentials such as inverse power-law and exponential models are discussed, providing predictions on the variation of the $w_{\rm DE}$ parameter. The paper implies that k-essence, utilizing non-canonical kinetic terms, offers a mechanism to produce cosmic acceleration without invoking negative pressure or a cosmological constant.

Coupled Dark Energy and Unified Models

The coupling of dark energy to dark matter is considered as a way to mediate interactions between these components, potentially alleviating the coincidence problem. Models such as the generalized Chaplygin gas seek to unify dark energy and dark matter, yet face stringent constraints particularly from large-scale structure data.

Modified Gravity: $f(R)$ and Scalar-Tensor Theories

Modifications to General Relativity, such as $f(R)$ gravity, are also explored as a means of explaining dark energy phenomena. The paper highlights the constraints and conditions necessary for $f(R)$ models to be viable both cosmologically and locally. These models have the potential to mimic cosmic acceleration effects without necessitating a cosmological constant.

Observational Signatures and Local Gravity Constraints

Metric $f(R)$ theories can potentially distinguish themselves from $\Lambda$CDM by predicting certain signatures in the growth rate of cosmic perturbations and weak lensing surveys. The chameleon mechanism, which adjusts the field's effective mass based on local density, can provide compliance with solar system experiments.

Theoretical Implications and Future Directions

The paper emphasizes the importance of devising theoretical models that effectively resolve the cosmological constant problem while fitting observational data. Both scalar-field and modified gravity approaches remain promising, though significant challenges persist, including consistency with local gravity tests and observational evidence.

Concluding Thoughts

As the nature of dark energy continues to elude definitive explanation, this paper offers a systematic review of both established and emerging models. The quest for understanding dark energy involves rigorous standard model testing, innovative model building, careful observational scrutiny, and the persistence required to eventually reveal the true dynamics behind cosmic acceleration. Future developments in cosmological observations and theoretical frameworks promise to deepen our understanding of dark energy and its role in the Universe.