As the universe expands and is per definition gravitationally decoupled on long distances and the overall metric therefore is "flat" and apparently no gravitational background exists, the question in some discussion arose: Can GWs propagate in in a gravitational empty space at all? If not, and as an outcome of the recent detection of GWs, I then suppose that the universe cannot be flat. There should be a kind of an overall and isotropic gravitational bias (of course only positive, as negative gravitation does not exist) in which e.g. the grav. oscillating "glitch" of heavy bodies orbiting each other can propagate (by the speed of light).