Direct answer: Cosmologists classify the universe as open, flat, or closed based on spatial curvature. Current cosmic microwave background (CMB) measurements indicate the universe is very close to flat, but small curvature remains observationally allowed. Whether the universe is ultimately finite or infinite cannot be settled by theory alone, since all three curvature types are...

The Einstein Field Equations (EFE) are ten coupled, nonlinear differential equations that describe how matter and energy determine the curvature of spacetime. First published by Albert Einstein in 1915, they form the mathematical core of general relativity, linking spacetime geometry to its energy and matter content through the equation [itex]G_{\mu\nu} = \frac{8\pi G}{c^4} T_{\mu\nu}[/itex]. Key...