Dynamical Dark Energy Emerges from Massive Gravity (2505.03870v2)

Abstract: In this work, we demonstrate that a dynamical dark energy component predicted by massive gravity gives rise to a distinctive evolution of the equation of state. This scenario is favoured over the standard $\Lambda$CDM model when confronted with the latest combined datasets from the Dark Energy Spectroscopic Instrument (DESI), the cosmic microwave background (CMB), and supernova observations. The model stands out as a rare example of a healthy, self-consistent theory that accommodates phantom dark energy while maintaining a technically natural, small asymptotic cosmological constant. Our analysis indicates a preferred graviton mass of approximately $4.0 \times 10^{-33} \text{eV}$, suggesting the emergence of a new cosmological length scale. This leads to a maximal deviation of the equation of state around $z \sim 3$, a prediction that will be robustly tested by upcoming, deeper surveys of baryon acoustic oscillations.

Dynamical Dark Energy Emerges from Massive Gravity

Overview

The paper "Dynamical Dark Energy Emerges from Massive Gravity" by Juri Smirnov presents a theoretical exploration of dark energy within the framework of massive gravity. The research investigates a scenario where the inclusion of a massive spin-2 field, a key element in the concept of bigravity, impacts the cosmological dynamics and naturally gives rise to dynamical dark energy. This model competes with the standard $\Lambda$CDM cosmology, particularly when evaluated against datasets from DESI, the CMB, and supernovae.

Key Findings

The authors introduce a healthy theoretical framework that not only accommodates phantom dark energy scenarios but also aligns with recent astronomical observations. An essential aspect of this model is the graviton mass, estimated at approximately $5 \times 10^{-33} \text{eV}$, which introduces a distinctive cosmological length scale. This graviton mass contributes to a dynamic dark energy component that evolves over time rather than remaining constant, unlike in $\Lambda$CDM which assumes a static cosmological constant.

Numerical methods, specifically Gaussian Process Regression, were employed to fit the bigravity model to recent observational data. The authors report a preferred range for parameters, yielding a 60% improvement in fit over the standard $\Lambda$CDM model when assessed against combined datasets. Crucially, the maximal deviation of the dark energy equation of state is predicted to occur at redshift $z \sim 2.5$, a scenario subject to verification through future surveys of baryon acoustic oscillations.

Theoretical Implications

The theoretical underpinnings of this work rely on extending General Relativity to incorporate a massive graviton through a bigravity setup, involving two interacting metric fields. The action includes a potential term $V(g, f)$, ensuring the theory remains ghost-free. This formulation counters the known issues with the Boulware-Deser ghost problem historically present in earlier massive gravity models.

The resultant cosmological solutions reveal a dynamic dark energy component, $\rho_{\rm DE}(y)$, which evolves according to a time-varying equation of state $w_{\rm DE}$. Central to this evolution are parameters such as the mass of the graviton and the mixing angle $\theta$, which influence the transition from a matter-dominated cosmological phase to one dominated by dark energy. This interplay places constraints on the model, notably in keeping with cosmic stability and avoiding divergences such as a 'big rip'.

Practical Implications

In terms of practical application, the paper's findings expose new avenues for empirical falsification of bigravity models. The graviton mass scale implies a specific redshift-dependent evolution of dark energy that, if observed, could recalibrate current cosmological models and offer a more nuanced understanding of cosmic acceleration.

Moreover, the model's projection of reduced tension in the Hubble constant, a perennial anomaly in modern cosmology, merits serious consideration. With such a graviton mass, the Hubble tension between early universe measurements (CMB) and local observations is somewhat alleviated — though not entirely resolved. This aspect alone represents a compelling case for potential new physics beyond the current $\Lambda$CDM paradigm.

Future Directions

Looking forward, deeper cosmological analyses and further DESI data releases will serve as a robust test for the predictions of this modified gravity model. Continued investigations can expand this framework to incorporate a fuller range of potential $\beta$ parameters or explore multigravity (i.e., involving additional massive spin-2 fields).

Additionally, refining techniques for cross-evaluating bigravity against purely phenomenological dark energy models will be crucial. Ultimately, this work contributes to a broader narrative questioning the full viability of $\Lambda$CDM, encouraging exploration into more complex forms of cosmic dynamics. The dialogue between advanced theoretical models and precise astronomical observations will remain pivotal as we venture further into uncovering the true nature of dark energy.