It's interesting how much brain power and observations are going into trying to figure out if the geometry of the universe if "flat". What this really means is - do the angles of a triangle add to 180 degrees, or more, or less?
Every single test has shown that the universe is Euclidian. Flat. It's 180 degree triangles all the way down. A²+B²=C² is the law of the universe.
The funny part is that they seem to not have noticed that relativity, which has been thoroughly tested, only works in an Euclidian universe. That's right. Relativity rests upon the assumption that sin² + cos² = 1.
When you examine the space-time field, you will see that it does not stretch. It does not bend. It does not expand. It simply exists, with various energy densities. Space-time is the quantum field from which all other fields draw their energy. So, the more energy an area possesses, the less dense the space-time field. We experience the difference in density as gravitational attraction.
If you imagine the energy density on a plane, it looks like a graph of a parabola. Normally, the parabola is remarkably shallow. In the vicinity of black holes and neutron stars, the parabola is quite steep.
Now take a look at the slope of the curve at any given point. The sine of that slope is the gravitational attraction, based on c=1. Particles gain energy by moving from a higher to a lower energy level. Energy is conserved, after all.
To cosine of that slope is the perceived time of a particle at that point, where 1 is the theoretical maximum possible experience of time's passage. The universal tick, as it were.
Please note that sin²+cos²=1, where c=1. Relativity only works in an Euclidian universe where A²+B²=C². It really is just this simple.