Cosmological Solutions in Bimetric Gravity and their Observational Tests (1111.1655v2)

Abstract: We obtain the general cosmological evolution equations for a classically consistent theory of bimetric gravity. Their analytic solutions are demonstrated to generically allow for a cosmic evolution starting out from a matter dominated FLRW universe while relaxing towards a de Sitter (anti-de Sitter) phase at late cosmic time. In particular, we examine a subclass of models which contain solutions that are able to reproduce the expansion history of the cosmic concordance model inspite of the nonlinear couplings of the two metrics. This is demonstrated explicitly by fitting these models to observational data from Type Ia supernovae, Cosmic Microwave Background and Baryon Acoustic Oscillations.

• The paper derives cosmological evolution equations within bimetric gravity and demonstrates consistency with observational data.
• It uncovers solutions that transition from a matter-dominated FLRW universe to de Sitter or anti-de Sitter phases, mimicking cosmic acceleration.
• The analysis highlights a 'minimal' bimetric model that replicates the standard expansion dynamics, offering an alternative explanation to dark energy.

Cosmological Solutions in Bimetric Gravity: A Theoretical Examination

The research paper titled "Cosmological Solutions in Bimetric Gravity and their Observational Tests" explores a sophisticated extension of general relativity known as bimetric gravity. The authors rigorously derive cosmological evolution equations within this framework and demonstrate compatibility with observational data. The paper provides a comprehensive analytical approach to determine whether bimetric gravity can serve as a viable candidate to explain the accelerated expansion of the universe, an observation typically attributed to the concept of dark energy.

The research proceeds by formulating the evolution equations for bimetric theories, characterized by two distinct metrics $g_{\mu\nu}$ and $f_{\mu\nu}$. These metrics, though nontrivially interacting, adhere to specific constraints that avoid ghost instabilities, a known problem in theories that introduce mass to the gravitational field. By solving these equations, the authors uncover a class of solutions that transition from a matter-dominated Friedmann-Lemaître-Robertson-Walker (FLRW) universe towards a de Sitter or anti-de Sitter phase, analogous to the late-time cosmic acceleration observed in our universe.

A significant portion of the analysis is devoted to understanding the cosmological implications of a subclass of models that reduce to a quadratic form, making them analytically tractable. Notably, one identified model, referred to as the "minimal" bimetric model, replicates the expansion dynamics of the standard concordance model of cosmology when tested against datasets from Type Ia supernovae, the Cosmic Microwave Background (CMB), and Baryon Acoustic Oscillations (BAO). This demonstrates the potential of bimetric gravity to embody the cosmological constant within its theoretical structure while mimicking general relativity's predictions on large scales.

The authors explore technical details outlining how the bimetric action is structured to ensure the absence of the Boulware-Deser ghost. This is achieved by incorporating intricate geometric constraints and employing an action which includes terms that account for nonlinear and non-derivative interactions between the two metrics. The research extends into theoretical predictions concerning the behavior of cosmological perturbations within this framework, an area that suggests practical ways to distinguish between general relativity and bimetric theories through future cosmological surveys.

The implications of this work are both practical and theoretical. In practice, bimetric gravity offers an alluring alternative to the cosmological constant problem by potentially obviating the need for fine-tuning the vacuum energy. Theoretically, it contributes to a broader understanding of modified gravity theories that remain consistent at both classical and quantum levels. Speculations for future developments in artificial intelligence motivated analyses can involve leveraging machine learning algorithms to further classify the parameter space of solutions, thus enhancing our understanding of gravitational interactions across different cosmological scales.

This paper adds valuable insight into the landscape of modified gravity theories, showcasing bimetric gravity not only as a feasible alternative to general relativity at cosmological scales but also as a consistent framework that fundamentally revises our understanding of the gravitational force. Far from merely extending theoretical boundaries, this research paves the path for observational confrontations that could unveil new physics underlying dark energy and cosmic acceleration phenomena. Future endeavors could focus on adapting this theory to encompass local tests of gravity or scrutinizing the stability of solutions under cosmological perturbations, thereby providing a more complete picture of its viability.