'Shaving' here has a similar meaning to slicing. Imagine slicing food. You can cut big chunks or make the slices very thin. This is now being used to think about discussing a subject (cosmology) and making ever 'finer' and more refined distinctions between things. The fashionable word today is 'nuanced' I hope this helps, Richard

More specifically, this appears to be about the first second after the big bang. Cosmologist studying what happened in the first second after the big bang in ever smaller fractions of a second.