We've measured the flatness of the universe on the scale of the cosmic microwave background, which is essentially as big a ruler as could possibly exist. https

We've measured the flatness of the universe on the scale of the cosmic microwave background, which is essentially as big a ruler as could possibly exist. https://map.gsfc.nasa.gov/mission/sgoals_parameters_geom.html https://map.gsfc.nasa.gov/mission/sgoals_parameters_geom.htm... https://wmap.gsfc.nasa.gov/universe/uni_shape.html https://wmap.gsfc.nasa.gov/universe/uni_shape.html