Moritz Haslbauer and Indranil Banik talk about the Keenan, Barger and Cowie (KBC) void and the νHDM model of cosmology.

The KBC void is a locally observed ~300 Mpc scale under-density that appears to be impossible within ΛCDM (under-densities shouldn’t have emptied out this much by now).

νHDM is a model that has sterile neutrinos as a hot dark matter component and enhanced gravity in environments with a weak gravitational field. This dark matter adequately explains the CMB and expansion history of the universe, but doesn’t cluster on the smallest scales. The modified gravity (essentially Milgromian dynamics, or MOND) then kicks in on these scales to produce phenomena like the correct rotation curves in galaxies.

Moritz and Indranil give an intro to both KBC and νHDM, and then explain how this model is consistent with the main tent-poles of modern cosmology (e.g. the CMB anisotropies, nucleosynthesis, the displacement of the gas and weak lensing in the bullet cluster, galaxy rotation curves, the clustering of galaxies) and can also alleviate some of the tensions in the standard ΛCDM model.

They focus on two specific tensions. The size and depth of the KBC void, and the Hubble tension. νHDM predicts stronger gravity in under-dense regions, so allows the KBC void to exist as-measured. This has implications for the locally measured Hubble parameter because a) the void itself would increase the local expansion rate but b) in νHDM this void would also be expanding faster than it would if it were placed in a ΛCDM universe.

At any specific point in space the exact strength of the enhancement of gravity will depend on the local environment due to the “external field effect” (an integral part of MOND since its foundation in the 1980s). In principle this is predictable by measuring the local environment, but this would require better measurements than we currently have. It is also in principle predictable statistically using a large cosmological simulation in the νHDM paradigm. So far such simulations only go up to a 750 Mpc box size (https://iopscience.iop.org/article/10.1088/0004-637X/772/1/10​), not sufficient to address the KBC void (which the current study considers semi-analytically). Smaller hydrodynamical cosmological simulations in vHDM are currently underway in Bonn to address galaxies.

Therefore, in current empirical fits, the size of this effect, at each point in space, is essentially a free parameter. Still, it is only one free parameter and while it remains free the important question is, ‘does this parameter have enough explanatory power to justify its existence?’ – Moritz and Indranil argue ‘absolutely yes!’

The paper: https://arxiv.org/abs/2009.11292
Moritz: https://moritzhaslbauer.jimdofree.com/
Indranil: https://www.youtube.com/channel/UCwO0bEeE6oNahkt8dWQFcXw

A blog post by Indranil, Moritz (and co-author Pavel) on the same topic: https://tritonstation.com/2020/10/23/big-trouble-in-a-deep-void/