Evidence for Emergent Dark Energy (2001.05103v2)

Abstract: We introduce a generalised form of an emergent dark energy model with one degree of freedom for the dark energy sector that has the flexibility to include both $\Lambda$CDM model as well as the Phenomenologically Emergent Dark Energy (PEDE) model proposed by Li & Shafieloo (2019) as two of its special limits. The free parameter for the dark energy sector, namely $\Delta$, has the value of $0$ for the case of the $\Lambda$ and $1$ for the case of PEDE. Fitting the introduced parametric form to Planck CMB data and most recent $H_0$ results from local observations of Cepheids and Supernovae, we show that the $\Delta=0$ associated with the $\Lambda$CDM model would fall out of 4$\sigma$ confidence limits of the derived posterior of the $\Delta$ parameter. Moreover, $H_0$ tensions will be alleviated with emergent dark energy model and this model can satisfy the combination of Planck CMB data and local $H_0$ observations with $\Delta{\rm{DIC}}\,=\,-2.88$ compared with $\LCDM$ model.

• The paper introduces the GEDE model by parameterizing dark energy with a single free parameter Δ to interpolate between ΛCDM and PEDE.
• It employs combined CMB and local H0 measurements, statistically rejecting the ΛCDM model at over 4σ and reducing the DIC by -2.88.
• The study implies that an emergent dark energy framework offers a promising alternative to reconcile observational discrepancies in cosmology.

Analyzing the Implications of the Generalised Emergent Dark Energy (GEDE) Model

The paper "Evidence for Emergent Dark Energy" presents an intriguing development in the field of cosmology, introducing the Generalised Emergent Dark Energy (GEDE) model. This model seeks to address the long-standing tensions in cosmological observations by incorporating a more flexible representation of dark energy. With the introduction of this model, the authors provide a framework that includes the $\Lambda$CDM model and the Phenomenologically Emergent Dark Energy (PEDE) model as specific limiting cases, thereby enhancing the exploration capabilities in understanding dark energy dynamics.

Technical Overview

The GEDE model is parameterized by a single free parameter, $\Delta$. This parameter effectively modulates the behavior of dark energy by allowing the model to interpolate between the $\Lambda$CDM model, where $\Delta = 0$, and the PEDE model, where $\Delta = 1$. The approach involves fitting this parameter to observational data, including the CMB data from the Planck mission and recent $H_0$ measurements derived from local observations using Cepheids and Supernovae.

Crucially, the paper illustrates that by introducing this generalized form of dark energy, the $\Lambda$CDM model falls outside the 4$\sigma$ confidence limits based on combined datasets. This suggests that traditional cosmological constant assumptions may not fully capture the complexity of dark energy phenomena. The implications for alleviating the $H_0$ tension—discrepancies in the Hubble constant estimations between local measurements and cosmic microwave background predictions—are significant, with the GEDE model offering a statistically preferable fit.

Key Numerical Findings

Several key results underscore the potential impact of the GEDE model. Importantly, when using the combined CMB and local $H_0$ data, the model yields $\Delta = 1.13 \pm 0.28$, statistically rejecting the $\Lambda$CDM model with $\Delta=0$. This lends credence to an emergent behavior in dark energy that veers away from a simple cosmological constant.

Further, the GEDE model demonstrates superior performance over the $\Lambda$CDM model in terms of deviance information criterion (DIC), with a $\Delta\rm{DIC} = -2.88$ when combining CMB and $H_0$ datasets. The reduction in DIC indicates a better balance between model complexity and fit to the data.

Implications and Future Directions

The GEDE model holds substantial theoretical and practical implications for the field of cosmology. The model's ability to accommodate an emergent dark energy behavior challenges the conventional paradigms and supports the notion that modifications to $\Lambda$CDM might be necessary for reconciling observational discrepancies.

On a theoretical level, this framework broadens the scope for exploring alternate dark energy models without restricting them to a static cosmological constant. Such flexibility is vital for testing a broader range of cosmological scenarios and avoiding potential biases inherent in less comprehensive models.

Practically, the model underscores the necessity of high-precision measurements and the integration of diverse observational data to enhance our understanding of cosmic acceleration. As future observational efforts, like CMB Stage 4 projects and improved local measurements via Gaia and other methods, provide more refined data, the GEDE model could serve as a pivotal tool in deriving more precise cosmological constraints.

In conclusion, the GEDE model's introduction signifies an important step in modeling cosmic acceleration and addressing the tensions between disparate observational datasets. While embracing this model is not without its challenges, it presents a promising avenue for enhancing our comprehension of the complex nature of dark energy and its role in the evolution of the universe. Further research and observational efforts will be essential to substantiate its validity and explore its full potential within the broader landscape of cosmological research.