Despite the success of the standard $Λ$CDM model of cosmology, recent data improvements have made tensions emerge between low- and high-redshift observables, most importantly in determinations of the Hubble constant $H_0$ and the (rescaled) clustering amplitude $S_8$. The high-redshift data, from the cosmic microwave background (CMB), crucially relies on recombination physics for its interpretation. Here we study how small-scale baryon inhomogeneities (i.e., clumping) can affect recombination and consider whether they can relieve both the $H_0$ and $S_8$ tensions. Such small-scale clumping, which may be caused by primordial magnetic fields or baryon isocurvature below kpc scales, enhances the recombination rate even when averaged over larger scales, shifting recombination to earlier times. We introduce a flexible clumping model, parametrized via three spatial zones with free densities and volume fractions, and use it to study the impact of clumping on CMB observables. We find that increasing $H_0$ decreases both $Ω_m$ and $S_8$, which alleviates the $S_8$ tension. On the other hand, the shift in $Ω_m$ is disfavored by the low-$z$ baryon-acoustic-oscillations measurements. We find that the clumping parameters that can change the CMB sound horizon enough to explain the $H_0$ tension also alter the damping tail, so they are disfavored by current Planck 2018 data. We test how the CMB damping-tail information rules out changes to recombination by first removing $\ell>1000$ multipoles in Planck data, where we find that clumping could resolve the $H_0$ tension. Furthermore, we make predictions for future CMB experiments, as their improved damping-tail precision can better constrain departures from standard recombination. Both the Simons Observatory and CMB-S4 will provide decisive evidence for or against clumping as a resolution to the $H_0$ tension.
The Lumina Project: Intergalactic Clumping and Recombination Sinks
The University of Texas at Dallas（德克萨斯大学达拉斯分校）；Institute for Basic Science（韩国基础科学研究所）；Massachusetts Institute of Technology（麻省理工学院）；York University（约克大学）；Max Planck Institute for Astrophysics（马克斯·普朗克天体物理学研究所）
Combining pre- and post-recombination new physics to address cosmological tensions: case study with varying electron mass and sign-switching cosmological constant
Hot New Early Dark Energy bridging cosmic gaps: Supercooled phase transition reconciles (stepped) dark radiation solutions to the Hubble tension with BBN
The Serendipitous Axiodilaton: A Self-Consistent Recombination-Era Solution to the Hubble Tension
University of Sheffield（谢菲尔德大学）；Kavli Institute for the Physics and Mathematics of the Universe (IPMU)（宇宙物理与数学卡弗里研究所）；Perimeter Institute for Theoretical Physics（理论物理珀利默研究所）；McMaster University（麦克马斯特大学）；Dublin Institute for Advanced Studies（都柏林高等研究院）
Double the axions, half the tension: multi-field early dark energy eases the Hubble tension
University of Trento（特伦托大学）；Université de Montpellier（蒙彼利埃大学）；Trento Institute for Fundamental Physics and Applications (TIFPA)-INFN（特伦托基础物理与应用研究所（TIFPA）-意大利国家核物理研究所）
Exploring the interplay of late-time dynamical dark energy and new physics before recombination
Departament de Física Quàntica i Astrofísica (FQA) and Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (UB)（巴塞罗那大学量子物理与天体物理学系和宇宙科学研究所）
The $H_0$ world cup. II. A comprehensive competition between proposed Hubble tension solutions
Ludwig-Maximilians-Universität（路德维希-马克西米利安大学）；Excellence Cluster ORIGINS（ORIGINS卓越集群）；Laboratoire Univers et Particules de Montpellier (LUPM), Université de Montpellier & CNRS（蒙彼利埃大学与法国国家科学研究中心宇宙与粒子物理实验室（LUPM））；INFN–Bologna（意大利国家核物理研究所博洛尼亚分部）；INAF–Osservatorio di Astrofisica e Scienza dello Spazio di Bologna（意大利国家天体物理研究所博洛尼亚天文与空间科学观测站）；Institute for Theoretical Particle Physics and Cosmology (TTK), RWTH Aachen University（亚琛工业大学理论粒子物理学与宇宙学研究所（TTK））；Dipartimento di Fisica e Astronomia “Augusto Righi”, Università di Bologna（博洛尼塔大学“奥古斯托·里基”物理与天文学系）；Institute for Theoretical Particle Physics (TTP), Karlsruhe Institute of Technology (KIT)（卡尔斯鲁厄理工学院理论粒子物理学研究所（TTP））
BAO miscalibration cannot rescue late-time solutions to the Hubble tension
University of Trento（特伦托大学）；Trento Institute for Fundamental Physics and Applications (TIFPA)-INFN（特伦托基础物理与应用研究所（TIFPA）-意大利国家核物理研究所）；Istanbul Technical University（伊斯坦布尔技术大学）；University of Sheffield（谢菲尔德大学）；Universidade Federal do Espírito Santo（圣埃斯皮里图联邦大学）；INAF – Osservatorio Astronomico di Trieste（意大利国家天体物理研究所的里雅斯特天文台）；IFPU – Institute for Fundamental Physics of the Universe（宇宙基础物理研究所）；University of Ioannina（约阿尼纳大学）
Running into tension: primordial black holes from ultra-slow-roll inflation, spectral running, and the Hubble tension
University of Trento（特伦托大学）；Trento Institute for Fundamental Physics and Applications (TIFPA)-INFN（特伦托基础物理与应用研究所（TIFPA）-意大利国家核物理研究所）；New York University Abu Dhabi（纽约大学阿布扎比分校）
The $H_0$ World Cup. I. Summary of the baseline group stage results
Ludwig-Maximilians-Universität（路德维希-马克西米利安大学）；Excellence Cluster ORIGINS（ORIGINS卓越集群）；Université de Montpellier & CNRS（蒙彼利埃大学与法国国家科学研究中心）；INFN–Bologna（意大利国家核物理研究所博洛尼亚分部）；INAF–Osservatorio di Astrofisica e Scienza dello Spazio di Bologna（意大利国家天体物理研究所博洛尼亚天文与空间科学观测站）；RWTH Aachen University（亚琛工业大学）；Università di Bologna（博洛尼亚大学）；Karlsruhe Institute of Technology (KIT)（卡尔斯鲁厄理工学院）
