Early Dark Energy Can Resolve The Hubble Tension (1811.04083v2)

Abstract: Early dark energy (EDE) that behaves like a cosmological constant at early times (redshifts $z\gtrsim3000$) and then dilutes away like radiation or faster at later times can solve the Hubble tension. In these models, the sound horizon at decoupling is reduced resulting in a larger value of the Hubble parameter $H_0$ inferred from the cosmic microwave background (CMB). We consider two physical models for this EDE, one involving an oscillating scalar field and another a slowly-rolling field. We perform a detailed calculation of the evolution of perturbations in these models. A Markov Chain Monte Carlo search of the parameter space for the EDE parameters, in conjunction with the standard cosmological parameters, identifies regions in which $H_0$ inferred from {\it Planck} CMB data agrees with the SH0ES local measurement. In these cosmologies, current baryon acoustic oscillation and supernova data are described as successfully as in \LCDM, while the fit to {\it Planck} data is slightly improved. Future CMB and large-scale-structure surveys will further probe this scenario.

• The paper introduces two EDE models (oscillating and slowly-rolling scalar fields) that reduce the sound horizon and increase the inferred H0 from CMB data.
• It employs comprehensive MCMC analyses using BAO, supernova, and Planck CMB data to pinpoint parameter regions that reconcile the Hubble constant measurements.
• Results indicate that a ~5% EDE contribution at redshift around 5000 alleviates the Hubble tension while maintaining consistency with other cosmological observables.

Analysis of Early Dark Energy as a Solution to the Hubble Tension

The paper "Early Dark Energy Can Resolve The Hubble Tension" by Poulin, Smith, Karwal, and Kamionkowski explores a promising approach to address the longstanding discrepancy between local and cosmic microwave background (CMB) measurements of the Hubble constant $H_0$. The authors propose that introducing a form of early dark energy (EDE) that mimics a cosmological constant in the early universe and then diminishes at a rate faster than radiation could reconcile the two measurements.

Core Contribution

The primary contribution of this paper is the introduction of two distinct models of EDE: one featuring an oscillating scalar field and another involving a slowly-rolling scalar field. These models aim to reduce the sound horizon at the time of decoupling, thereby increasing the inferred value of $H_0$ from CMB data to align it more closely with the local measurement provided by the Supernova H0 for the Equation of State (SH0ES) team. This adjustment potentially resolves the tension without significant disruption to other cosmological observables.

Methodology

The research employs a thorough analysis incorporating Markov Chain Monte Carlo (MCMC) methods to explore the parameter space of these EDE models alongside standard cosmological parameters. This approach allows the authors to pinpoint regions in the parameter space that bring predictions from Planck CMB data in harmony with local $H_0$ measurements. The evaluation includes a rich set of observational data, such as baryon acoustic oscillation (BAO) measurements, supernova datasets, and Planck CMB data.

Results

The results reveal that by allowing a $\sim5\%$ contribution from early dark energy at redshifts around 5000, then permitting it to dissipate faster than radiation, the models not only align $H_0$ values from CMB and local measurements but also maintain consistency with BAO and supernova data. Furthermore, the paper shows that the fit to Planck data improves slightly with the introduction of EDE, contrasting with the constraints faced by other resolutions involving late-time modifications like phantom dark energy or additional relativistic species.

Implications and Future Developments

This research presents potential implications for the theoretical landscape of cosmology. By demonstrating that EDE could account for the observed tension, it invites further exploration into the nature of dark energy and its role throughout cosmological history. The authors suggest that future CMB and large-scale structure surveys could further refine these models, potentially providing more clues about the underlying physics of dark energy.

The paper discerns that the success of the EDE resolution does not heavily depend on the detailed mechanisms underlying the field; both oscillating and slowly-rolling scalar field models exhibit satisfactory outcomes. This flexibility suggests that the early universe might contain dynamics capable of influencing the cosmic expansion history in currently-underexplored ways.

The EDE resolution stands positioned for future empirical tests. The characteristic oscillations and changes in cosmological parameters such as $n_s$, $A_s$, and the ratio of sound horizons warrant focused observational campaigns, with expectations set for new insights from upcoming missions like Simons Observatory, CMB-S4, and other large-scale surveys. This endeavor could clarify the occurrence of episodic periods of dark energy dominance during cosmic evolution.

Overall, this paper sheds light on a plausible resolution to the Hubble tension by invoking new physics related to early dark energy, proposing an intriguing avenue for ongoing and future research in the field of cosmology.