Dark Energy vs. Modified Gravity (1601.06133v4)

Abstract: Understanding the reason for the observed accelerated expansion of the Universe represents one of the fundamental open questions in physics. In cosmology, a classification has emerged among physical models for the acceleration, distinguishing between Dark Energy and Modified Gravity. In this review, we give a brief overview of models in both categories as well as their phenomenology and characteristic observable signatures in cosmology. We also introduce a rigorous distinction between Dark Energy and Modified Gravity based on the strong and weak equivalence principles.

• The paper distinguishes between Dark Energy and Modified Gravity by clarifying their theoretical foundations and key observational differences.
• It employs detailed analysis of quintessence and scalar-tensor theories, highlighting screening mechanisms and tracking behaviors in cosmic evolution.
• Empirical tests using CMB, BAO, LSS, and gravitational lensing are evaluated to differentiate the models and guide future cosmological surveys.

An Analytical Evaluation of Dark Energy and Modified Gravity

The paper "Dark Energy vs. Modified Gravity" by authors Austin Joyce, Lucas Lombriser, and Fabian Schmidt offers a comprehensive examination of the two major theoretical frameworks designed to explain the accelerated expansion of the universe: Dark Energy (DE) and Modified Gravity (MG). This research focuses explicitly on differentiating these two paradigms based on their theoretical constructs and observable signatures, providing both a classification and an evaluative lens through which these models can be studied.

The acceleration of the universe's expansion, initially discovered in 1998, continues to prompt significant scientific inquiry due to its implications for cosmology and fundamental physics. This paper delineates between DE and MG as principal categories used to model this acceleration. DE models attribute acceleration to modifications in the stress-energy content of the universe, introducing components with an equation of state approximating $w \simeq -1$. In contrast, MG models explore the possibility of modifying gravity itself, altering the structures of General Relativity (GR).

Theoretical Frameworks and Distinctions

The paper establishes a theoretical distinction based on the strong and weak equivalence principles. DE adheres to the Strong Equivalence Principle (SEP), indicating gravitational interaction modeled as per Einstein's gravity with a universal application of free fall, independent of an object's internal structure or composition. MG, in contrast, lacks conformity with SEP, allowing for a different gravitational force experience as a function of additional fields (such as a scalar in scalar-tensor theories), often mediated by a fifth force.

Analysis of Models

The exploration into quintessence, a framework under DE, presents it as a canonical scalar field influencing cosmic expansion through potential-driven dynamics. The distinction between models with a time-dependent state of evolution, such as "thawing" or "freezing," expands our understanding of cosmic acceleration behaviors. Furthermore, the paper discusses the tracking behavior of quintessence fields, explaining how they might provide a resolution to the "coincidence problem" where DE density tracks dark matter density.

For MG, the paper discusses scalar-tensor theories such as the Brans-Dicke theory, and generalizes to frameworks like Horndeski and f(R) gravity. These theories introduce modifications in the gravitational interaction, incorporating screening mechanisms to reconcile large-scale cosmological behaviors with local gravitational tests. Notably, mechanisms such as the chameleon effect, symmetron/dilaton field dynamics, and Vainshtein screening are elaborated upon for their roles in metric alteration in dense environments like the solar system or on cosmological scales.

Observational Implications and Empirical Testing

The paper affirms the empirical significance of cosmological observables in distinguishing DE and MG models. Observational data leveraged from cosmic microwave background (CMB) studies, baryon acoustic oscillations (BAO), and large-scale structure (LSS) analyses are pivotal. These data sets are integral in setting constraints or exploring extensions to canonical models. Tests like E-G probe and its utilization of gravitational lensing provide critical methodologies to detect MG's fifth force effects by comparing gravitational potentials used in lensing (sensitive to $\Phi+\Psi$) against dynamical mass measurements (sensitive to $\Psi$).

Furthermore, the authors detail cosmic degeneracies where phenomena such as massive neutrinos or baryonic feedback can mimic the predicted effects of MG models. This insight is critical in formulating targeted tests that specifically differentiate MG from its counterparts, offering a robust strategy to disambiguate subtle cosmological signatures amidst complex nonlinear astrophysical processes.

Conclusion and Prospective Research Directions

In summary, this paper provides not only a rigorous theoretical distinction between DE and MG but also a structured overview of their respective phenomenologies and observational constraints. While current and forthcoming large-scale surveys are expected to place DE and MG models under tighter scrutiny, uncovering deviations from $\Lambda$CDM will necessitate an advancement in theoretical modeling and precision cosmology. The integration of gravitational wave detections offers an additional dimension for constraint, potentially affording unprecedented insights into the nature of gravity and cosmic acceleration, paving the way for breakthroughs in understanding cosmic evolution.