Does the Hubble constant tension call for new physics? (1801.07260v1)

Abstract: The $\Lambda$ Cold Dark Matter model ($\Lambda$CDM) represents the current standard model in cosmology. Within this, there is a tension between the value of the Hubble constant, $H_0$, inferred from local distance indicators and the angular scale of fluctuations in the Cosmic Microwave Background (CMB). We investigate whether the tension is significant enough to warrant new physics in the form of modifying or adding energy components to the standard cosmological model. We find that late time dark energy explanations are slightly disfavoured whereas a pre-CMB decoupling extra dark energy component has a marginally positive Bayesian evidence. A constant equation of state of the additional early energy density is constrained to 0.086$^{+0.04}_{-0.03}$. Although this value deviates significantly from 1/3, valid for dark radiation, the latter is not disfavoured based on the Bayesian evidence. If the tension persists, future estimates of $H_0$ at the 1$\%$ level will be able to decisively determine which of the proposed explanations is favoured.

• The paper evaluates both late- and early-time modifications using a Bayesian framework, finding early dark energy models slightly favored by observational data.
• It demonstrates that late-time dark energy modifications conflict with BAO and supernova constraints, limiting their potential to resolve the H₀ tension.
• Forecasts indicate that future 1% precision measurements of H₀ could decisively test model viability and enhance our understanding of cosmic expansion.

Investigating the Hubble Constant Tension and Its Implications for New Physics

The paper "Does the Hubble constant tension call for new physics?" addresses the discrepancy between local measurements and Cosmic Microwave Background (CMB) inferences of the Hubble constant ($H_0$). This tension has prompted questions about whether it signifies the need for new physics beyond the standard Lambda Cold Dark Matter ($\Lambda$CDM) cosmological model. The paper provides a comprehensive analysis using various cosmological observations and Bayesian evidence to explore both late-time and early-time potential modifications to the $\Lambda$CDM model.

Introduction to the Problem

The Hubble constant ($H_0$) represents the current expansion rate of the Universe. Historically, determining its value has been pivotal in cosmology. The $\Lambda$CDM model, which includes a cosmological constant ($\Lambda$) to account for dark energy, has been the standard framework. However, measurements of $H_0$ from local distance indicators (e.g., Cepheid variable stars, Type Ia supernovae) typically yield higher values than those inferred from the CMB, leading to an apparent tension. This paper investigates possible extensions or modifications to the $\Lambda$CDM model that could resolve this discrepancy.

Methodology

The authors utilize a Bayesian framework to evaluate various models' consistency with empirical data. Observational inputs include local measurements, gravitational lensing, CMB anisotropies, Type Ia supernovae, Baryon Acoustic Oscillations (BAO), and Big Bang Nucleosynthesis (BBN). The paper tests models of late-time and early-time modifications to dark energy and other cosmological parameters that might alleviate the tension between different measurements of $H_0$.

Key Findings

1. Late-Time Modifications:
   • Several late-time dark energy models are considered, including phantom dark energy (with $w < -1$), bimetric gravity, and phenomenological modifications. However, models that postulate deviations at redshifts below the CMB surface of last scattering (e.g., negative dark energy densities or (dis)appearing dark energy) are generally disfavored because they contradict constraints from BAO and SN Ia data.
2. Early-Time Modifications:
   • Early dark energy (EDE) models, with influences before CMB decoupling, show more promise. These include additions of dark radiation or altering the equation of state for early dark energy ($w_{\text{EDE}}$). The analysis indicates potential relativity of models incorporating early dark energy components, thus increasing the Bayesian evidence compared to the standard $\Lambda$CDM, though not decisively.
3. Bayesian Evidence:
   • By calculating Bayesian evidence, the paper quantitatively compares the fit of different models to the observational data. Some early dark energy models exhibit a slight Bayesian preference over $\Lambda$CDM, suggesting that they might account for the $H_0$ tension without substantial conflict with established cosmological observations.
4. Sensitivity to Future Precision:
   • The paper forecasts that more precise future measurements of the Hubble constant, potentially down to 1% precision, will significantly affect the favorability of these cosmological models. Improved measurements could decisively verify or nullify the early dark energy hypothesis, offering stronger insights into the possible necessity of new physical paradigms.

Implications and Future Perspectives

The findings underscore the challenges in reconciling local and CMB-derived $H_0$ measurements within the $\Lambda$CDM paradigm without introducing additional physics. The paper's implications are significant for theoretical cosmology, as resolving the $H_0$ tension could illuminate unknown facets of dark energy or indicate new physics. Future data from potential sources like gravitational waves, improved standard candles, and other cosmological experiments will likely refine these theoretical frameworks and potentially support or contradict the presence of an extra dark energy component.

In conclusion, while the current data does not decisively support abandoning $\Lambda$CDM for scenarios involving minor energy adjustments, early dark energy remains an intriguing possibility. Persistent tension in $H_0$ measurements could ultimately call for augmented models that incorporate early Universe dynamics, potentially advancing our understanding of fundamental physics in cosmology.