The strong discrepancy between local and early-time (inverse distance ladder) estimates of the Hubble constant H0 could be pointing towards new physics beyond the concordance ΛCDM model. Several attempts to address this tension through new physics rely on extended cosmological models, featuring extra free parameters beyond the six ΛCDM parameters. However, marginalizing over additional parameters has the effect of broadening the uncertainties on the inferred parameters (including H0), and it is often the case that within these models the H0 tension is addressed due to larger uncertainties rather than a genuine shift in the central value of H0. In this paper I consider an alternative viewpoint: what happens if a physical theory is able to fix the extra parameters to a specific set of nonstandard values? In this case, the degrees of freedom of the model are reduced with respect to the standard case where the extra parameters are free to vary. Focusing on the dark energy equation of state w and the effective number of relativistic species Neff, I find that physical theories able to fix w≈-1.3 or Neff≈3.95 would lead to an estimate of H0 from cosmic microwave background, baryon acoustic oscillation, and type Ia supernovae data in perfect agreement with the local distance ladder estimate, without broadening the uncertainty on the former. These two nonstandard models are, from a model-selection perspective, strongly disfavored with respect to the baseline ΛCDM model. However, models that predict Neff≈3.45 would be able to bring the tension down to 1.5σ while only being weakly disfavored with respect to ΛCDM, whereas models that predict w≈-1.1 would be able to bring the tension down to 2σ (at the cost of the preference for ΛCDM being definite). Finally, I estimate dimensionless multipliers relating variations in H0 to variations in w and Neff, which can be used to swiftly repeat the analysis of this paper in light of future more precise local distance ladder estimates of H0, should the tension persist. As a caveat, these results were obtained from the 2015 Planck data release, but these findings would be qualitatively largely unaffected were I to use more recent data.

中文翻译：

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基于 H0 张力的新物理学：另一种观点

哈勃常数 H0 的局部和早期（反距离阶梯）估计之间的强烈差异可能指向 ΛCDM 模型之外的新物理学。通过新物理学解决这种紧张局势的几种尝试依赖于扩展的宇宙学模型，这些模型具有超出六个 ΛCDM 参数的额外自由参数。然而，对附加参数的边缘化会扩大推断参数（包括 H0）的不确定性，通常情况下，在这些模型中，H0 张力是由于更大的不确定性而不是中心值的真正变化而得到解决的H0。在本文中，我考虑了另一种观点：如果物理理论能够将额外参数固定为一组特定的非标准值，会发生什么？在这种情况下，相对于额外参数可以自由变化的标准情况，模型的自由度有所降低。关注状态 w 的暗能量方程和相对论物种 Neff 的有效数量，我发现能够确定 w≈-1.3 或 Neff≈3.95 的物理理论将导致从宇宙微波背景、重子声学振荡、和 Ia 型超新星数据与局部距离阶梯估计完全一致，没有扩大前者的不确定性。从模型选择的角度来看，这两个非标准模型相对于基线 ΛCDM 模型非常不受欢迎。然而，预测 Neff≈3.45 的模型将能够将张力降低到 1.5σ，而在 ΛCDM 方面仅略微不利，而预测 w≈-1 的模型。1 将能够将张力降低到 2σ（代价是对 ΛCDM 的偏好是确定的）。最后，我估计了将 H0 的变化与 w 和 Neff 的变化相关联的无量纲乘数，如果张力持续存在，可以根据未来更精确的 H0 局部距离阶梯估计，快速重复本文的分析。需要注意的是，这些结果是从 2015 年普朗克数据发布中获得的，但如果我使用更新的数据，这些发现在定性上基本上不受影响。如果紧张局势持续下去。需要注意的是，这些结果是从 2015 年普朗克数据发布中获得的，但如果我使用更新的数据，这些发现在定性上基本上不受影响。如果紧张局势持续下去。需要注意的是，这些结果是从 2015 年普朗克数据发布中获得的，但如果我使用更新的数据，这些发现在定性上基本上不受影响。