The Effective Field Theory of Dark Energy (1210.0201v2)

Abstract: We propose a universal description of dark energy and modified gravity that includes all single-field models. By extending a formalism previously applied to inflation, we consider the metric universally coupled to matter fields and we write in terms of it the most general unitary gauge action consistent with the residual unbroken symmetries of spatial diffeomorphisms. Our action is particularly suited for cosmological perturbation theory: the background evolution depends on only three operators. All other operators start at least at quadratic order in the perturbations and their effects can be studied independently and systematically. In particular, we focus on the properties of a few operators which appear in non-minimally coupled scalar-tensor gravity and galileon theories. In this context, we study the mixing between gravity and the scalar degree of freedom. We assess the quantum and classical stability, derive the speed of sound of fluctuations and the renormalization of the Newton constant. The scalar can always be de-mixed from gravity at quadratic order in the perturbations, but not necessarily through a conformal rescaling of the metric. We show how to express covariant field-operators in our formalism and give several explicit examples of dark energy and modified gravity models in our language. Finally, we discuss the relation with the covariant EFT methods recently appeared in the literature.

• The paper presents a universal framework that unifies all single-field models of dark energy and modified gravity using effective field theory.
• It employs the unitary gauge with time-dependent operators to analyze cosmological perturbations, ensuring stability and accurate gravitational dynamics.
• The framework is validated against models like quintessence, k-essence, and F(R) gravity, offering a consistent approach for future observational tests.

Summary of "The Effective Field Theory of Dark Energy"

The paper "The Effective Field Theory of Dark Energy" by Giulia Gubitosi, Federico Piazza, and Filippo Vernizzi presents a structured and comprehensive framework for modeling dark energy (DE) and modified gravity through the lens of effective field theory (EFT). By extending methodologies initially applied to inflationary cosmology, the authors propose a formalism tailored to encompass all single-field models of DE and modified gravity, thus providing a universal description of these phenomena.

Theoretical Framework

Central to the paper is the employment of the unitary gauge to construct the most general action consistent with unbroken spatial diffeomorphism symmetries, which effectively captures cosmological perturbations up to linear order. The structural core of this setup relies on three time-dependent operators that dictate the background evolution, while higher-order operators describe perturbations without affecting the homogenous background dynamics. This choice facilitates a systematic exploration of the implications of various operators within a cosmological context, offering a direct path to paper their effects on both quantum and classical stability, sound speed of fluctuations, and the renormalization of the Newton constant.

Mixing and Perturbations Analysis

Within this theoretical construct, the interaction and mixing between gravity and scalar fields are scrutinized. Two particularly salient aspects govern this interplay: the operator proportional to the time derivative of the function $f(t)$ and the operator $m_1$ typical in galileon and kinetic braiding models. By examining these interactions, the authors illustrate scenarios with and without the Einstein-frame decoupling where scalar fields can kinetically mingle with gravity, considerably enriching the theoretical understanding of scalar dynamics in DE models. The analysis proceeds to elaborate on the conditions under which the scalar fields can be demixed from gravity via local transformations, albeit at the cost of introducing interactions with matter fields.

Application to Existing DE Models

The formalism is tested against a wide array of existing DE models, including quintessence, k-essence, DGP models, and F(R) gravity, demonstrating its flexibility and generality. For instance, the authors show how various conventional models translate into the EFT language, offering insight into their cosmological perturbations and stability without complex recalibration of background equations.

Implications and Advancements

The construction of this framework not only broadens the scope for model comparison using a uniform language but also enhances the capability to predict and analyze cosmological phenomena across differing DE theories within a consistent schema. Furthermore, the paper outlines how their framework can inform future observational tests, particularly from large-scale structure surveys such as EUCLID and BigBOSS, by correlating theoretical parameters with potential astrophysical and cosmological signatures.

Future Directions

Looking forward, the research proposes intriguing pathways for theoretical developments and experimental engagements. It highlights open questions related to the dynamics of perturbations beyond the high-energy decoupling limit and the exploration of potential screening mechanisms in the IR regime. The framework promises to remain a cornerstone for devising and testing novel DE models against observed cosmic acceleration and gravitational phenomena.

In conclusion, this paper provides a rigorous EFT-based approach that holds significant potential in advancing both theoretical and observational cosmology, offering a robust lens through which to examine the rich phenomenology of dark energy and its interaction with gravity.