Beyond the Cosmological Standard Model (1407.0059v2)

Abstract: After a decade and a half of research motivated by the accelerating universe, theory and experiment have a reached a certain level of maturity. The development of theoretical models beyond \Lambda, or smooth dark energy, often called modified gravity, has led to broader insights into a path forward, and a host of observational and experimental tests have been developed. In this review we present the current state of the field and describe a framework for anticipating developments in the next decade. We identify the guiding principles for rigorous and consistent modifications of the standard model, and discuss the prospects for empirical tests. We begin by reviewing attempts to consistently modify Einstein gravity in the infrared, focusing on the notion that additional degrees of freedom introduced by the modification must screen themselves from local tests of gravity. We categorize screening mechanisms into three broad classes: mechanisms which become active in regions of high Newtonian potential, those in which first derivatives become important, and those for which second derivatives are important. Examples of the first class, such as f(R) gravity, employ the familiar chameleon or symmetron mechanisms, whereas examples of the last class are galileon and massive gravity theories, employing the Vainshtein mechanism. In each case, we describe the theories as effective theories. We describe experimental tests, summarizing laboratory and solar system tests and describing in some detail astrophysical and cosmological tests. We discuss future tests which will be sensitive to different signatures of new physics in the gravitational sector. Parts that are more relevant to theorists vs. observers/experimentalists are clearly indicated, in the hope that this will serve as a useful reference for both audiences, as well as helping those interested in bridging the gap between them.

• The paper introduces modified gravity and screening mechanisms as key alternatives to explain the universe's accelerated expansion beyond the standard cosmological model.
• The authors systematically classify screening approaches—chameleon, kinetic, and Vainshtein—to bridge theoretical models with both laboratory and astrophysical tests.
• Empirical tests, including cosmological probes and local experiments, are emphasized to rigorously assess deviations from the ΛCDM framework.

An Expert Overview of "Beyond the Cosmological Standard Model"

The paper "Beyond the Cosmological Standard Model," authored by Austin Joyce, Bhuvnesh Jain, Justin Khoury, and Mark Trodden, provides a comprehensive review of the current state and future prospects of theoretical cosmology beyond the standard model. It focuses on addressing the challenges posed by cosmic acceleration, highlighting modified gravity and new physics in the gravitational sector as potential solutions. The paper serves as a thorough reference for researchers interested in bridging theoretical advances with empirical tests.

Theoretical Developments

The authors begin by discussing the limitations of the cosmological constant problem within the standard model, emphasizing the need for new physics. Modified gravity, often motivated as an alternative to dark energy, has garnered attention as a means to explain the universe's accelerated expansion without invoking an additional component like the cosmological constant.

The paper categorizes screening mechanisms into three classes based on how they operate in high-density regions:

1. Chameleon Mechanisms: These involve a scalar field whose mass depends on the local density, becoming large in high-density environments and effectively "screening" its effects. The chameleon mechanism can be realized in theories like $f(R)$ gravity and has implications for laboratory and astrophysical tests.
2. Kinetic Screening: This includes theories where the kinetic terms in the Lagrangian lead to non-linear dynamics that hide the effects of extra degrees of freedom. For instance, Dirac-Born-Infeld (DBI) theories exhibit such behavior. The paper notes the quantum stability and technical challenges of these models.
3. Vainshtein Mechanism and Galileons: These arise primarily in the context of massive gravity theories and rely on non-linear kinetic interactions to screen modifications to gravity. The galileon's unique properties, such as symmetry requirements and radiative stability, are highlighted, along with their implications for local and cosmological tests.

Empirical Tests and Observational Implications

The paper emphasizes the critical role of empirical tests in validating these theoretical models. It outlines several key observational avenues:

• Cosmological Probes: These include the cosmic microwave background (CMB), baryon acoustic oscillations (BAO), and type Ia supernovae as means to constrain the expansion history and growth of structure in the universe. The synergy of these observations provides a powerful consistency check against the $\Lambda$CDM model.
• Astrophysical Tests: The paper highlights the potential of using galaxy dynamics, stellar evolution, and gravitational lensing to probe modified gravity theories. The discrepancy between dynamical and lensing masses in galactic halos is a valuable diagnostic for modifications to GR.
• Laboratory Experiments: These focus on testing the inverse square law and the weak equivalence principle. Eöt-Wash experiments, Casimir force measurements, and neutron interferometry are mentioned as promising methods to constrain chameleon and similar theories.

Future Directions and Challenges

The paper identifies several challenges and directions for future research:

• Parameterizing Modifications: The authors discuss the need for a systematic framework to parameterize deviations from $\Lambda$CDM, akin to the parameterized post-Friedmann (PPF) framework.
• Connecting Scales: Bridging the gap between solar system tests and cosmological observations remains a significant challenge. The authors note the importance of understanding screening mechanisms across different scales and environments.
• Theoretical Consistency and Radiative Stability: Ensuring that modified gravity theories remain free of pathologies, such as ghost or gradient instabilities, is crucial for their viability. The paper emphasizes the need for careful consideration of quantum corrections in these models.

In conclusion, "Beyond the Cosmological Standard Model" provides an insightful and detailed examination of the landscape of theoretical cosmology beyond standard $\Lambda$CDM. It highlights the interplay between theoretical innovation and observational constraints, offering a roadmap for future research in understanding cosmic acceleration. The paper's comprehensive approach makes it a valuable resource for researchers seeking to explore the frontiers of cosmology and modified gravity.