ΛCDM Tensions: Localising Missing Physics through Consistency Checks

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors examined the ΛCDM model by investigating tensions related to two key parameters: H₀ (the Hubble constant) and S₈. These tensions arise from comparisons across various cosmological datasets. On one hand, the H₀ tension emerges due to discrepancies between the value derived from Planck data and local measurements provided by the SHOES collaboration. On the other hand, the S₈ tension, at approximately 2.6 sigma significance, exists between the Planck data within the ΛCDM framework and datasets like KiDS-450.

Weak galaxy lensing surveys consistently report lower S₈ values compared to the predictions of the Planck lambda cold dark matter (ΛCDM) cosmology. The authors focus on resolving these tensions by examining consistency tests within a restricted redshift window (a localized view) or by introducing new physics in the cosmological era preceding the mentioned redshift window.

In the following, I highlight the main critiques of this manuscript:

1. While the paper is well-written, including a diagram summarizing the main approaches discussed would benefit the reader.

2. Given that most issues comes from comparing Planck data with other datasets, it would be prudent to acknowledge open questions regarding the Planck data in more detail. Some scientists within the Planck collaboration have recently raised concerns about potential foreground contamination. Specifically, they propose that nearby late spiral galaxies could explain both the cold and hot spots observed. Importantly, this explanation cannot be attributed solely to the integrated Sachs-Wolfe effect hypothesis. I recommend the authors include the following article in their references: “The CMB Cold Spot as Predicted by Foregrounds around Nearby Galaxies” by D. Lambas et al., A&A 681, A2 (2024).

I look forward to reviewing the revised version once these modifications are incorporated

 

Author Response

We thank the referee for the recommendations. Changes to the manuscript are in red text. 

Point 1: We have opted not to include images. This leads to a wordy document, but the idea being put forward is easily grasped: in any good dynamical model in physics, the fitting parameters cannot evolve with time, or here redshift. We review works in the literature where the Lambda-CDM cosmological parameters have become redshift dependent. 

Point 2: We added references where we discuss systematics in Planck data. Foregrounds could certainly be impacting the CMB and it is worth raising this concern. 

Reviewer 2 Report

Comments and Suggestions for Authors

In this paper, the authors are making an important question about how to define and solve the tensions existing in LambdaCDM focusing on the assumptions implicitly done in LambdaCDM. So this discussion is an important question in Cosmology where the repetition of the experiments to compare to a predictive model is a  base to determine the validity or not of a model is not possible.  The paper is well written and brings elements to the debate over how to treat the tensions existing in LambdaCDM. 

In conclusion I recommend to accept this paper in its present form to be published in Universe.

Author Response

We thank the referee for the nice comments. 

Reviewer 3 Report

Comments and Suggestions for Authors

The tensions of the Lambda Cold Dark Matter (ΛCDM) cosmological model with a focus on the H0 and S8 tensions are reviewed in the manuscript. Consistency checks using redshift and scale to search for any unknown physics to fix these problems are suggested. The introduction of the paper is written in an appropriate manner. This study reviews the ΛCDM model's assumptions so that recent observational data support the detailed verification of them. The reconsideration of these accepted assumptions can help to clarify the mentioned tensions. Consistency checks at various redshifts and scales can be a possible approach for future explorations. These checks help the improvement in understanding of unknown cosmological physics and show the limits of the standard model of cosmology. However, to improve the understanding of the reader, authors can include some figures and explain how upcoming observational missions can help the resolution of the disputes between H0 and S8 measurements. I have a few comments and questions:

 

• I would suggest the authors add a few plots for the current bounds on the parameters they are discussing (\Omega_m, S_8, \sigma_8, etc.).

 

• Can you explain how baryonic feedback and other scale-dependent physics might clarify the S8 tension?

• Which (and why) redshift ranges are the most important for recognising the unknown physics in the ΛCDM model ?

 

• Assuming that certain datasets show a descending trend of H0 with redshift, what particular modifications to the ΛCDM model may be able to explain this behaviour?

 

• What particular consistency checks or redshift-dependent analyses are required  to identify the redshift ranges where the ΛCDM model may be problematic or require new physics, given the complexity of H0 tension and its implications for the  ΛCDM model?

I would like to ask the authors to consider the above points if it is possible. By addressing these points, the paper can be in a form to be published in the journal.

Author Response

We thank the referee for the comments and recommendations. Changes to the manuscript are in red text. 

Point 1: The manuscript is designed to reinforce a simple idea, namely that a good dynamical model in physics cannot have time-dependent fitting parameters. We review works in the literature where time-dependent cosmological parameters (H0, \Omega_m, S8) are evident and in those papers one finds plots. We are reluctant to reproduce plots in this work. 

Point 2: It cannot. There is no shortage of papers in the literature stating that this is the case. We have added two recent papers (DESI+ACT) that show S8 is robust to scale and that baryonic feedback cannot explain the discrepancy because only linear scales are probed, yet the tension with Planck persists. As a bonus, both papers show that S8/\sigma8 increases with bin redshift, as claimed in this paper. Actually, our paper predates these papers, which have just appeared. We stress that there is considerable confusion in the S8 literature, some of which is driven by personalities in the field. 

Point 3: This point is implicit in the manuscript. Ultimately, we advocate studying subsectors of the LCDM model. The upshot is that one can fit the model with a smaller subset of parameters and this makes evolution (if it is present) more evident. At the background level, one can study LCDM physics using geometric probes and only consider the parameters H0 and \Omega_m. Given the irrelevance of radiation in the Hubble parameter, this can be done up to redshifts far beyond the scope of current probes. 

Point 4: As we see it, H0 can only decrease if \Omega_m or some other parameter compensates. What we are observing is that \Omega_m increases, and that the changes become more pronounced at higher redshifts beyond the acceleration-deceleration transition redshift. What this implies is that there is a problem with the pressureless matter assumption of the LCDM model. We have added a comment to that effect. As the referee will appreciate, the possibilities for modifying the matter sector, where the majority of matter is dark (!), are endless. 

Point 5: This is explained in the text. The proposal is to bin data first by redshift, then by scale (on the basis that redshift is more fundamental than scale), confront the data to the LCDM model and check if the cosmological parameters are robust constants. If they are not, and one sees the signal across multiple probes (to be checked), then physics is missing in the redshifts/scales probed. The idea is simple. Executing it effectively is best done by focussing on subsectors of the model as advocated in the paper. 

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

After minor modifications, the paper is now suitable for its publication in its current form.

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have addressed my comments and questions. I suggest the manuscript for publication in Universe journal.