I was thinking about the shape of the universe and how normally when people think of a closed universe they think of the universe as being a four dimensional hypersphere. In 4d it's possible for the universe to close back on it self but not without distorting space time and so it would in principle be possible to figure out if the universe is a hypersphere as a triangle drawn on a hypersphere would have angles that add up to more than 180 degrees. It is possible though for the universe to be closed without being a hypersphere.
Instead of being the 4d equivalent of a sphere another way the universe could be closed is if it is the 6d equivalent of a duocylinder. While hypersphers have distorted surfaces "hyperduocylinders" do not have distorted surfaces and the only difference between their surface and a regular flat surface is that they close on themselves and so they can be cut into flat surfaces without any stretching or squeezing. If you draw a triangle on the 6d equivalent of a duocylinder it's angles will add up to 180 degrees meaning that if the universe is the six dimensional equivalent of the duocylinder it will pass the triangle test for flatness. A closed universe that is the six dimensional equivalent of a duocylinder could be much smaller than a universe that is the 4d equivalent of a sphere without us figuring out that the universe is a 6d equivalent of a duocylinder as the only way we could figure out that the universe is the 6d equivalent of a duocylinder is if we can either detect extra dimensions, or by having the 6d equivalent of a duocylinder have a circumference smaller than the diameter of the visible universe
It's also possible that the universe could close on itself but in such a way that if you go around the universe once everything is inverted to it's mirror image relative to you.