Johannes Noller and Scott Melville talk about their recent paper exploring the impacts of certain positivity bounds on cosmological parameters.

Positivity bounds are restrictions on low energy effective parameters that arise from requiring the full high energy fundamental theory to satisfy certain criteria. It is possible to show that if, e.g. all the interactions of a full theory satisfies unitarity (conservation of information/probability), causality and locality, then a specific class of low energy theories must have the speed of light less than the speed of gravity.

The specific interactions Johannes, Scott (and collaborator Claudia) used to show this are interactions between dark energy and standard model matter.

This condition actually ends up lying right in the region that observations prefer for this model, effectively cutting the allowed parameter space in half.

Johannes: http://www.icg.port.ac.uk/author/nollerj/
Scott: http://www.scottamelville.com/

Paper: https://arxiv.org/abs/2103.06855
Supplementary video: https://www.youtube.com/watch?v=Z3Lx7VXB78E