The CosmoVerse White Paper: Addressing observational tensions in cosmology with systematics and fundamental physics (2504.01669v2)

Abstract: The standard model of cosmology has provided a good phenomenological description of a wide range of observations both at astrophysical and cosmological scales for several decades. This concordance model is constructed by a universal cosmological constant and supported by a matter sector described by the standard model of particle physics and a cold dark matter contribution, as well as very early-time inflationary physics, and underpinned by gravitation through general relativity. There have always been open questions about the soundness of the foundations of the standard model. However, recent years have shown that there may also be questions from the observational sector with the emergence of differences between certain cosmological probes. In this White Paper, we identify the key objectives that need to be addressed over the coming decade together with the core science projects that aim to meet these challenges. These discordances primarily rest on the divergence in the measurement of core cosmological parameters with varying levels of statistical confidence. These possible statistical tensions may be partially accounted for by systematics in various measurements or cosmological probes but there is also a growing indication of potential new physics beyond the standard model. After reviewing the principal probes used in the measurement of cosmological parameters, as well as potential systematics, we discuss the most promising array of potential new physics that may be observable in upcoming surveys. We also discuss the growing set of novel data analysis approaches that go beyond traditional methods to test physical models. [Abridged]

• The paper reviews major observational tensions in cosmology like H0 and S8, assesses the role of systematics, and explores how new physics could resolve these discrepancies.
• It details tensions in parameters like the Hubble constant (H0) and S8, comparing measurements from early-time probes like CMB with late-time data from supernovae and weak lensing.
• The document explores potential theoretical resolutions such as Early Dark Energy or modified gravity and recommends future research integrating advanced data analysis methods and upcoming survey data.

Addressing Observational Tensions in Cosmology with Systematics and Fundamental Physics: Overview of the CosmoVerse White Paper

The "CosmoVerse White Paper" by the CosmoVerse Network is a comprehensive document that reviews current challenges in observational cosmology, particularly focusing on systematic tensions and the prospect of new physics beyond the standard cosmological model, $\Lambda$CDM. This paper is a result of collective efforts from numerous researchers under the CosmoVerse COST Action initiative, stemming from previous discussions like those during SNOWMASS 2021. The White Paper discusses the increasing discrepancies in cosmological measurements and lays out objectives for future investigations to resolve these tensions.

The Standard Model of Cosmology and Emerging Tensions

The $\Lambda$CDM model, despite being remarkably successful in explaining various phenomena across cosmic scales, faces significant challenges due to discrepancies in measurements of key parameters such as the Hubble constant, $H_0$, and the parameter $S_8$ related to the amplitude of matter fluctuations. These are captured through various observational probes: the cosmic microwave background (CMB), baryon acoustic oscillations (BAO), galaxy cluster counts, and Type Ia Supernovae, among others. The paper underscores that while systematics in these measurements might account for some discrepancies, there is growing evidence that points towards physics beyond the standard model.

Reviewing Observational Techniques and Systematics

The White Paper provides a detailed review of the principal observational probes and their associated systematics affecting cosmological measurements. For example, the Planck satellite provides unparalleled precision for the CMB, but there exist tensions when comparing its $H_0$ measurements with those obtained from the local Universe, such as from supernovae and Cepheids which are systematically higher. Similarly, the $S_8$ parameter diverges between early-time (CMB) and late-time (weak gravitational lensing) measurements. The document discusses not just these discrepancies but also examines the potential biases due to astrophysical processes and instrumentation, indicating a comprehensive approach to understanding and reducing observational systematics.

Exploring New Theoretical Models

Another core focus of the paper is on theoretical developments that could address these tensions. It highlights potential solutions such as Early Dark Energy (EDE), modifications of gravity, and interactions between dark matter and dark energy (IDE). These models propose alterations to cosmological dynamics either in the early or late Universe that could reconcile the observed tensions. The White Paper also stresses the importance of novel data analysis methodologies, such as machine learning techniques, which can complement traditional approaches in forecasting and analyzing survey data, thereby enhancing model testing.

Recommendations for Future Research

The document concludes with a series of recommendations for the cosmology community, emphasizing the integration of diverse observational and theoretical efforts to address these challenges. It stresses the need for more precise and expansive datasets from upcoming surveys such as Euclid and the Rubin Observatory's LSST to provide a better empirical foundation to test these frameworks. Moreover, it advocates for exploratory work in the development of advanced statistical methods and collaborative initiatives that may bridge various independent lines of evidence.

Implications and Future Developments

Overall, the work set forth by the CosmoVerse Network highlights the multi-faceted approach required to resolve the observational tensions in cosmology. If accurate, the tension may signal new physics, like modifications to gravity or an adjustment in the understanding of dark matter and energy interactions. Thus, addressing these tensions carries profound implications for our understanding of the Universe and its fundamental components. This also promises broader impacts on related fields, including particle physics and astronomy, potentially leading to a redefinition of standard assumptions about the Universe's composition and evolution.

Conclusion

The CosmoVerse White Paper serves as a crucial document in cosmological research, emphasizing both the intricacies of current observations and the opportunities for theoretical innovations. By providing a detailed roadmap and recognizing the collaborative efforts required, it sets the stage for future explorations that may ultimately lead to solving some of the most persistent puzzles in contemporary cosmology.