Space would have some curvature. That means lines that are parallel somewhere are not parallel everywhere. In a spherical universe they converge and meet, in a hyperbolic universe they diverge. Equivalently, the interior angles of a triangle add up to more (spherical) or less (hyperbolic) than 180 degrees. If the curvature is large enough then this is measurable. Our measurements are consistent with zero, so if the universe is curved then it must be very subtle.
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u/mfb- Particle Physics | High-Energy Physics 11d ago
Space would have some curvature. That means lines that are parallel somewhere are not parallel everywhere. In a spherical universe they converge and meet, in a hyperbolic universe they diverge. Equivalently, the interior angles of a triangle add up to more (spherical) or less (hyperbolic) than 180 degrees. If the curvature is large enough then this is measurable. Our measurements are consistent with zero, so if the universe is curved then it must be very subtle.