3.7σ evidence for suppression of growth of structure! (and S₈ resolved?) - Minh Nguyen

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Minh Nguyen tells us about his recent work looking for evidence of modified gravity in cosmological observations. Rather than look at a specific model, Minh and his collaborators look for evidence of a deviation in the "growth index" γ.
In general relativity and ΛCDM γ≃0.55 but in a more general theory of gravity it could deviate. Minh looks at measurements of peculiar velocity, clustering and Planck CMB data (temperature, polarisation and lensing) and finds 3.7σ evidence for γ greater than 0.55.
This is not perhaps surprising, as it ties in with the S₈ tension. γ quantifies how fast structure grows in the universe and where we already know there is a deficit of structure in the late universe. Moreover γ greater than 0.55 would cause a deficit.
However, it is still impressive that this one parameter is able to provide a good fit to the various data sets simultaneously. This doesn't look like a "resolution" that takes CMB and late universe probes into bad fitting regions of parameter space. In fact, Planck CMB alone even has weak evidence for γ greater than 0.55 because it appears to also help with the Aₗₑₙₛ tension.
A fascinating development to keep an eye on...
Paper: journals.aps.org/prl/abstract... (arxiv.org/abs/2302.01331)
Minh: minhmpa.github.io/
Code: github.com/MinhMPA/CAMB_Gamma...

[@talks_curator728]

*Index to Key Parts of the Talk*
[00:00] Introductory comments by Shaun; Figure 3 from paper
[01:17] Minh's opening comments on the paper
[03:50] Two things you would like people to remember from this talk
[05:02] What motivated this work?
[07:14] Why wasn't it done earlier? Figure 2
[11:30] Matter density fluctuations grow over time
[12:27] Growth rate parameterization
[15:05] Planck 2018 CMB data
[19:14] fσ8 from Peculiar Velocity
[23:55] fσ8(z) from RSD - BAO data
[25:51] DES(Y1) data
[26:43] Constraints on the growth index; Figure 1
[28:07] Implications for S8 tension; Figure 3
[33:30] Implications for Omega_k, A_lens; Fig. 5 with Fig. 4 overlay
[39:54] Conclusion (not) driven by fσ8? (conclusion appears to be robust)
[44:15] Where to next? (Future LSS x CMB data will settle this)
[49:51] Q&A: What are compelling reasons to be skeptical about this?
[53:30] What current work in cosmology is particularly underappreciated by the community?

[@NhatMinhNguyen111]

This tension reaches 4.5sigma today, as indendently reported by a different team in arXiv:2406.13388, led by Shi-Fan Chen.

[@CosmologyTalks]

Very exciting!