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 A_lens tension.

A fascinating development to keep an eye on…

Paper: https://arxiv.org/abs/2302.01331
Minh: https://minhmpa.github.io/
Code: https://github.com/MinhMPA/CAMB_GammaPrime_Growth