Cosmology Intertwined: A Review of the Particle Physics, Astrophysics, and Cosmology Associated with the Cosmological Tensions and Anomalies

Abstract: In this paper we will list a few important goals that need to be addressed in the next decade, also taking into account the current discordances between the different cosmological probes, such as the disagreement in the value of the Hubble constant $H_0$, the $\sigma_8$--$S_8$ tension, and other less statistically significant anomalies. While these discordances can still be in part the result of systematic errors, their persistence after several years of accurate analysis strongly hints at cracks in the standard cosmological scenario and the necessity for new physics or generalisations beyond the standard model. In this paper, we focus on the $5.0\,\sigma$ tension between the {\it Planck} CMB 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 list a few interesting new physics models that could alleviate this tension and discuss how the next decade's experiments will be crucial. Moreover, we focus on the tension of the {\it Planck} CMB data with weak lensing measurements and redshift surveys, about the value of the matter energy density $\Omega_m$, and the amplitude or rate of the growth of structure ($\sigma_8,f\sigma_8$). We list a few interesting models proposed for alleviating this tension, and we discuss the importance of trying to fit a full array of data with a single model and not just one parameter at a time. Additionally, we present a wide range of other less discussed anomalies at a statistical significance level lower than the $H_0$--$S_8$ tensions which may also constitute hints towards new physics, and we discuss possible generic theoretical approaches that can collectively explain the non-standard nature of these signals.[Abridged]

• The paper presents a comprehensive review of the H0 and S8 tensions that challenge the standard ΛCDM cosmological model.
• It evaluates early-time solutions like early dark energy and late-time modifications such as interacting dark energy to reconcile these discrepancies.
• The study emphasizes the role of upcoming experiments in refining cosmological models and exploring dark sector dynamics.

Cosmology Intertwined: A Synthesis of Observational Tensions

The paper "Cosmology Intertwined: A Review of the Particle Physics, Astrophysics, and Cosmology Associated with the Cosmological Tensions and Anomalies" constitutes a foundational examination of contemporary issues in particle physics, astrophysics, and cosmology, particularly emphasizing the cosmological tensions and anomalies. This comprehensive review, developed as part of the Snowmass 2021 community planning exercise, presents a critical assessment of the challenges facing the standard cosmological model, commonly known as $\Lambda$CDM.

Cosmological Tensions

The Hubble constant ($H_0$) debate stands at the forefront of current cosmological tensions, manifesting as a stark disagreement between the value inferred from early Universe observations (for example, CMB data by the {\it Planck} satellite) and the value obtained through direct late Universe measurements (such as distance ladder techniques). This discrepancy has persisted across numerous independent datasets, challenging the robustness of the $\Lambda$CDM model.

Similarly, the $S_8$ tension arises from the differences in the measurements of the present-day amplitude of matter fluctuations on large scales, derived from large-scale structure surveys, and those predicted by the CMB observations. Together, these tensions question the completeness of our standard cosmological model and suggest avenues for theoretical innovation and experimental refinement.

Addressing Cosmological Tensions

The review recognizes two broad categories of solutions to address these tensions:

1. Early-Time Solutions: These propose adjustments to the pre-recombination cosmological conditions. For example, the Early Dark Energy (EDE) model posits an additional, transient contribution to the energy density of the Universe that modifies the sound horizon prior to recombination, potentially reconciling the $H_0$ measurements. This model, however, requires careful examination of its implications for large-scale structure and other cosmological parameters.
2. Late-Time Solutions: These investigate modifications to the late-time dynamics or constituents of the Universe. Models like interacting dark energy propose interactions between dark matter and dark energy to alleviate the $S_8$ tension. Additionally, bulk viscous models introduce a pressure component associated with bulk viscosity in cosmic fluids to address deviations from $\Lambda$CDM predictions.

Implications and Future Directions

The implications of resolving these tensions extend beyond mere parameter adjustments within the standard model. They touch upon fundamental questions in cosmology, such as the nature of dark energy, potential interactions within the dark sector, and modifications to our understanding of basic gravitational dynamics.

Furthermore, the paper discusses broader experimental and observational initiatives critical to advancing our understanding and refining theoretical models. It highlights upcoming observations from the next generation of cosmic microwave background experiments, large-scale structure surveys, and gravitational wave detectors as pivotal in constraining and testing these theoretical proposals.

Conclusion

The reviewed collective research effort is indicative of the expansive nature of this inquiry, involving collaboration across multiple domains, from theoretical physics to observational cosmology. While acknowledging the challenges, the review is optimistic about the capability of current and upcoming studies to reconcile these tensions, leading to a more profound and comprehensive understanding of the Universe.

In conclusion, the paper by the Snowmass community lays a foundational framework for building cosmological models that can potentially resolve existing tensions, offering a roadmap for future theoretical and observational exploration in cosmology.