I wonder how one would test this. I suppose you could look for stars that seem to be older than the universe, in places that are significantly less densely populated with matter?
Gravity waves from a kilonova are affected by the distance through space they have to travel but since they have no mass in themselves, they do not have their own "clock" and would not be affected by time differences?
It's a "speed of causality" question that breaks my brain.
Maybe that next-gen space interferometer (LISA) can help answer that, well if it's not cancelled, sigh, oh wait it's Europe-only project now so should happen
https://en.m.wikipedia.org/wiki/Laser_Interferometer_Space_A...
ie. "standard candles" are no longer consistent?
adding link to actual paper: