ΛCDM Gets Galactic Lensing Wrong

The ΛCDM Standard Model of Cosmology overestimates gravitational lensing measurements around massive galaxies by 30%. This is particularly significant because gravitational lensing data are often used as a key empirical observation supporting the ΛCDM paradigm.

The paper below argues that this is because the models of the massive galaxies used to make the predictions is flawed, rather than because ΛCDM is itself flawed. 

If this were the only thing that ΛCDM got wrong, this would be a credible argument. But, since ΛCDM also gets so many other things wrong, and since the models of galaxy structure blamed are so complex and susceptible to manipulation to fit a desired outcome, the possibility that the "lensing is low" problem reflects a fundamental problem with the paradigm shouldn't be unduly discounted.

It is now well-established that ΛCDM predictions overestimate gravitational lensing measurements around massive galaxies by about 30%, the so-called lensing is low problem. Using a state-of-the-art hydrodynamical simulation, we show that this discrepancy reflects shortcomings in standard structure formation models rather than tensions within the ΛCDM paradigm itself. Specifically, this problem results from ignoring a variety of galaxy formation effects in simple models, including assembly bias, segregation of satellite galaxies relative to dark matter, and baryonic effects on the matter distribution. Each of these contributes towards overestimating gravitational lensing and, when combined, these explain the amplitude and scale dependence of the lensing is low problem. 

We conclude that simplistic structure formation models are inadequate to interpret lensing and clustering together, and that it is crucial to employ more sophisticated models for the upcoming generation of large-scale surveys.

Jonas Chaves-Montero, Raul E. Angulo, Sergio Contreras "The galaxy formation origin of the lensing is low problem" arXiv:2211.01744 (November 3, 2022).