Late time cosmic acceleration: ABCD of dark energy and modified theories of gravity (1309.4188v2)

Abstract: We briefly review the problems and prospects of the standard lore of dark energy and notice that the cosmological constant problems are often over emphasized. In order to keep the discussion pedagogical aimed at a wider audience, we have avoided technical complications in several places and resorted to heuristic arguments based on physical perceptions. We presented underlying ideas of modified theories based upon chameleon mechanism and Vainshtein screening. We have given a lucid illustration of recently investigated ghost free non linear massive gravity. Again we have sacrificed rigor and confined to the basic ideas that led to the formulation of $dRGT$. The review ends with a brief discussion on the difficulties of the theory applied to cosmology with an optimism that a consistent solution of the problem is round the corner.

• The paper critically evaluates dark energy models and modified gravity theories to address the challenges of cosmic acceleration.
• It compares the chameleon mechanism with Vainshtein screening, emphasizing how local density and nonlinear interactions affect gravitational modifications.
• The authors advocate for innovative approaches to overcome issues like fine-tuning, quantum corrections, and inconsistencies in massive gravity theories.

Overview of Late Time Cosmic Acceleration and Modified Gravity Theories

The paper by M. Sami and R. Myrzakulov critically examines the established perspective on dark energy and explores the implications of modified gravity theories in explaining late-time cosmic acceleration. The authors argue that scalar fields fail to solve the cosmological constant problem and instead focus on potentials for modifying gravity at large scales, including mechanisms such as chameleon screening and Vainshtein mechanisms.

Problems and Prospects of Dark Energy

The cosmological constant is seen as a pivotal component of Einstein's theory of general relativity; however, its application to cosmology presents substantial theoretical challenges, such as fine-tuning and the coincidence problem. The authors emphasize that scalar field models, including quintessence and others, are hindered by naturalness problems which undermine their ability to alleviate these issues.

Modified Gravity Theories

Sami and Myrzakulov explore two frameworks for modifying gravity:

1. Chameleon Mechanism: This involves scalar fields adapting their mass in response to local density, potentially addressing observational discrepancies locally. While this mechanism can effectively screen gravitational modifications in dense environments such as the solar system, it is insufficient to induce late-time cosmic acceleration due to the stringent local gravity constraints.
2. Vainshtein Screening and Massive Gravity: The Vainshtein mechanism employs the galileon field to nonlinear derivative interactions that serve to hide modifications to gravity locally by dynamically suppressing the additional degrees of freedom. This screening is central to theories such as dRGT massive gravity, which attempt to link the cosmological constant to graviton mass.

Challenges with Modified Gravity Theories

Despite theoretical sophistication, modified gravity theories face substantial challenges:

• Quantum Corrections: Both chameleon and galileon frameworks face difficulties related to quantum corrections.
• Massive Gravity: Nonlinear formulations like dRGT struggle with cosmological consistency, such as the absence of Friedmann-Robertson-Walker (FRW) cosmology despite successful local gravity tests.
• Superluminality and Stability: Concerns such as superluminality further complicate these theories.

Implications and Future Directions

While dRGT presents an elegant formulation potentially unifying cosmological constants with graviton mass, its practical relevance remains uncertain due to foundational cosmological inconsistencies. There is a pressing need for innovative approaches such as extending massive gravity to bi-gravity or incorporating novel fields. The current stance is that cosmological constant, despite challenges, fits observations without requiring extensive conceptual alterations.

Conclusion

Sami and Myrzakulov underscore the complexities and uncertainties in the quest to understand cosmic acceleration. While theoretical explorations of modified gravity provide intriguing perspectives, they have yet to offer compelling alternatives to the cosmological constant that align with observed phenomena and overcome entrenched theoretical problems effectively. Enhanced precision in future observational surveys and novel theoretical developments are imperative for resolving these cosmological enigmas.