Early dark energy, the Hubble-parameter tension, and the string axiverse (1608.01309v2)

Abstract: Precise measurements of the cosmic microwave background (CMB) power spectrum are in excellent agreement with the predictions of the standard $\Lambda$CDM cosmological model. However, there is some tension between the value of the Hubble parameter $H_0$ inferred from the CMB and that inferred from observations of the Universe at lower redshifts, and the unusually small value of the dark-energy density is a puzzling ingredient of the model. In this paper, we explore a scenario with a new exotic energy density that behaves like a cosmological constant at early times and then decays quickly at some critical redshift $z_c$. An exotic energy density like this is motivated by some string-axiverse-inspired scenarios for dark energy. By increasing the expansion rate at early times, the very precisely determined angular scale of the sound horizon at decoupling can be preserved with a larger Hubble constant. We find, however, that the Planck temperature power spectrum tightly constrains the magnitude of the early dark-energy density and thus any shift in the Hubble constant obtained from the CMB. If the reionization optical depth is required to be smaller than the Planck 2016 $2\sigma$ upper bound $\tau\lesssim 0.0774$, then early dark energy allows a Hubble-parameter shift of at most 1.6 km~s$^{-1}$~Mpc$^{-1}$ (at $z_c\simeq 1585$), too small to fully alleviate the Hubble-parameter tension. Only if $\tau$ is increased by more than $5\sigma$ can the CMB Hubble parameter be brought into agreement with that from local measurements. In the process, we derive strong constraints to the contribution of early dark energy at the time of recombination---it can never exceed $\sim2\%$ of the radiation/matter density for $10 \lesssim z_c \lesssim 10^5$.

• The paper investigates whether early dark energy, acting like a cosmological constant before recombination, can reconcile the Hubble parameter tension.
• It uses Fisher-matrix analysis of Planck’s 2016 CMB data to show that EDE’s contribution is constrained to about 2% of the pre-recombination energy density.
• The study finds that even with a 5σ shift in reionization optical depth, the resulting H0 change of up to 1.6 km/s/Mpc leaves the discrepancy largely unresolved.

Analysis of Early Dark Energy and the Hubble Parameter Tension

The paper investigates the role of an exotic form of energy density, termed "early dark energy" (EDE), in addressing the well-documented tension between values of the Hubble parameter $H_0$ inferred from the cosmic microwave background (CMB) and those deduced from local measurements at lower redshifts. The authors explore whether EDE, which would act as a cosmological constant at early times before rapidly decaying, can reconcile these differing values of $H_0$.

Key Insights and Methodology

The paper assesses a phenomenological model inspired by string theory's axiverse scenarios. In this framework, EDE behaves like a cosmological constant at high redshifts before decaying rapidly at a critical redshift $z_c$. This exotic component ostensibly increases the early Universe's expansion rate, modifying the sound horizon at recombination and potentially allowing an increased present-day Hubble constant that aligns better with local measurements.

The paper employs Fisher-matrix analysis using Planck's 2016 CMB temperature power spectrum data to mathematically constrain this model's parameters. The authors focus on evaluating whether introducing EDE can shift the Hubble parameter $H_0$ sufficiently to alleviate the tension.

Findings and Implications

The analysis determines that the Planck power spectrum data tightly constrains the magnitude of this early dark energy density. Even when allowing for a 5$\sigma$ deviation from the Planck best-fit value for the reionization optical depth ($\tau$), early dark energy contributes at most approximately 2% of the total pre-recombination energy density. This contribution results in a shift in the Hubble parameter of at most 1.6 km/s/Mpc, insufficient to fully resolve the discrepancies between the CMB-inferred and local measurement-derived $H_0$.

The research suggests that even if the reionization optical depth were increased beyond its current 5$\sigma$ level, the introduction of EDE alone might only bridge the disparity partially. This indicates that while EDE serves as a viable component in attempting to understand and resolve the Hubble tension, it cannot solely account for the complete discrepancy within the limits set by current CMB observations.

Theoretical and Practical Implications

The implications of this work are twofold. Theoretically, it underscores the necessity for more comprehensive models that may include different types of distinctions or interactions among early cosmic fields and components. Practically, the paper directs attention to better measurements of $\tau$ and further investigations into systematic uncertainties that could impact both CMB and local $H_0$ measurements.

Future developments may include the integration of polarization data and Planck data at multiple multipoles to refine this analysis. Moreover, it could be beneficial to explore other forms of exotic energy models or incorporate broader cosmological parameters, like the variation of the equation of state for dark energy or the effects of additional relativistic species.

In conclusion, while this paper highlights the constraints imposed by current data on exotic early dark energy, it opens pathways for future work in refining cosmological models to elucidate the fundamental nature of the Hubble parameter discrepancy. The nuanced exploration of EDE serves as a basis for further theoretical and observational inquiries in resolving the intricacies of our Universe’s expansion history.