Cosmology Intertwined II: The Hubble Constant Tension (2008.11284v4)

Abstract: The current cosmological probes have provided a fantastic confirmation of the standard $\Lambda$ Cold Dark Matter cosmological model, that has been constrained with unprecedented accuracy. However, with the increase of the experimental sensitivity a few statistically significant tensions between different independent cosmological datasets emerged. While these tensions can be in portion the result of systematic errors, the persistence after several years of accurate analysis strongly hints at cracks in the standard cosmological scenario and the need for new physics. In this Letter of Interest we will focus on the $4.4\sigma$ tension between the Planck estimate of the Hubble constant $H_0$ and the SH0ES collaboration measurements. After showing the $H_0$ evaluations made from different teams using different methods and geometric calibrations, we will list a few interesting new physics models that could solve this tension and discuss how the next decade experiments will be crucial.

• The paper identifies a significant 4.4σ tension between CMB-based H0 estimates (67.27±0.60 km/s/Mpc) and local measurements (74.03±1.42 km/s/Mpc).
• The study systematically examines multiple datasets and methodologies, ruling out simple systematic errors and challenging the standard ΛCDM model.
• The research explores innovative theoretical frameworks, including early dark energy and dark matter interactions, and emphasizes future observational tests with platforms like the Simons Observatory and gravitational wave standard sirens.

Cosmology Intertwined: Addressing the Hubble Constant Tension

The paper, "Cosmology Intertwined II: The Hubble Constant Tension," encapsulates a vital discourse in modern cosmology, specifically addressing the statistically significant tension between the Cosmic Microwave Background (CMB) measurements by the Planck satellite and direct local distance ladder measurements of the Hubble constant $H_0$. The discrepancy between the Planck estimate of $H_0 = 67.27 \pm 0.60$ km/s/Mpc and the SH0ES collaboration's value of $H_0 = 74.03 \pm 1.42$ km/s/Mpc signifies a tension of approximately $4.4\sigma$, suggesting potential inadequacies in our current understanding of cosmology and hinting at the need for new physics.

Problematic Discrepancies

This paper identifies multiple datasets and methodologies contributing to the $H_0$ tension:

• CMB and Early Universe Probes: Measurements from the Planck satellite, ACT+WMAP, and Baryon Acoustic Oscillation (BAO) data, among others, consistently note smaller $H_0$ values.
• Local Distance Ladder Measurements: Techniques utilizing various astronomical phenomena, such as Cepheids, Mira variables, and strong gravitational lensing, typically infer higher values for $H_0$.

Several attempts have been made to identify systematic errors that could account for this tension. However, the uniformity of results over time and across varied methodologies suggests a more profound deviation from the standard ΛCDM model.

Theoretical Explanations

In response to this tension, the paper explores several theoretical frameworks, notably:

• Dark Energy Models: Proposals involving a dynamic dark energy component, especially early dark energy (EDE) models, attempt to reconcile measurements with theoretical expectations.
• Interacting Dark Energy and Dark Matter Models: These hypothesize non-gravitational interactions between dark matter and dark energy, potentially offering a resolution to the observed $H_0$ values.
• Increased Relativistic Degrees of Freedom: This involves augmenting the number of effective light neutrino species, $N_{\rm eff}$, facilitating an $H_0$ adjustment.

Future Prospects and Observational Needs

The resolution of the $H_0$ tension will likely require comprehensive data from next-generation telescopes alongside advanced theoretical models. The paper emphasizes upcoming observational platforms such as the Simons Observatory and CMB-S4, which promise more precise $H_0$ constraints and the potential validation or refutation of various cosmological models.

Moreover, gravitational wave standard sirens (GWSS) hold promise as independent $H_0$ estimators free from traditional cosmic distance ladder uncertainties. The paper highlights the significance of future gravitational wave observations in refining $H_0$ measurements and resolving longstanding tensions.

Conclusion

The "Cosmology Intertwined II" paper underscores a pivotal challenge in cosmology—the Hubble constant tension. It presents a robust overview of observational inconsistencies, explores theoretical paradigms for resolution, and points to future observational endeavors. This ongoing investigation into the $H_0$ discrepancy could profoundly impact our understanding of the universe, potentially necessitating paradigm shifts in cosmology or comprehensive updates to the ΛCDM model. Such developments would pave the way for advancements in theoretical physics and cosmological models in the upcoming years.