We examine whether the Hubble tension, the mismatch between early and late measurements of $H_0$, can be alleviated by ultralight scalar fields in the early universe, and we assess its plausibility within UV physics. Since their energy density needs to rapidly redshift away, we explore decays to massless fields around the era of matter-radiation equality. We highlight a concrete implementation of ultralight pseudo-scalars, axions, that decay to an abelian dark sector. This scenario circumvents major problems of other popular realizations of early universe scalar models in that it uses a regular scalar potential that is quadratic around the minimum, instead of the extreme fine-tuning of many existing models. The idea is that the scalar is initially frozen in its potential until $H\sim m$, then efficient energy transfer from the scalar to the massless field can occur shortly after the beginning of oscillations due to resonance. We introduce an effective fluid model which captures the transition from the frozen scalar phase to the radiation dark sector phase. We perform a fit to a combined Planck 2018, BAO, SH$_0$ES and Pantheon supernovae dataset and find that the model gives $H_0=69.9_{-0.86}^{+0.84}$ km/s/Mpc with $\Delta\chi^2 \approx -9$ compared to $\Lambda$CDM; while inclusions of other data sets may worsen the fit. Importantly, we find that large values of the coupling between fields is required for sufficiently rapid decay: For axion-gauge field models $\phi F\tilde{F}/\Lambda$ it requires $\Lambda\lesssim f/80$, where $2\pi f$ is the field range. We find related conclusions for scalar-scalar models $\sim\phi\,\chi^2$ and for models that utilize perturbative decays. We conclude that these sorts of ultralight scalar models that purport to alleviate the Hubble tension, while being reasonable effective field theories, require features that are difficult to embed within UV physics.

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超轻标量衰减和哈勃张力

我们研究了哈勃张力（$H_0$ 早期和晚期测量值之间的不匹配）是否可以通过早期宇宙中的超轻标量场来缓解，并评估其在紫外线物理学中的合理性。由于它们的能量密度需要迅速红移，我们探索了物质辐射平等时代的无质量场衰变。我们强调了超轻伪标量、轴子的具体实现，它衰减到阿贝尔暗区。这种情况避免了早期宇宙标量模型的其他流行实现的主要问题，因为它使用了围绕最小值二次方的常规标量势，而不是对许多现有模型进行极端微调。这个想法是标量最初被冻结在其潜力中，直到 $H\sim m$，那么在共振开始振荡后不久，就会发生从标量到无质量场的有效能量转移。我们引入了一个有效的流体模型，它可以捕捉从冻结标量相到辐射暗区相的转变。我们对合并的 Planck 2018、BAO、SH$_0$ES 和 Pantheon 超新星数据集进行拟合，发现该模型给出了 $H_0=69.9_{-0.86}^{+0.84}$ km/s/Mpc 和 $\ Delta\chi^2 \approx -9$ 与 $\Lambda$CDM 相比；而包含其他数据集可能会使拟合恶化。重要的是，我们发现足够快的衰减需要场之间的大耦合值：对于轴子-规范场模型 $\phi F\tilde{F}/\Lambda$ 它需要 $\Lambda\lesssim f/80$，其中 $2\pi f$ 是字段范围。我们找到标量-标量模型 $\sim\phi\ 的相关结论，\chi^2$ 和使用微扰衰减的模型。我们得出的结论是，这些旨在减轻哈勃张力的超轻标量模型虽然是合理的有效场理论，但需要难以嵌入紫外线物理学的特征。