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What is meant by the term "flat universe"? How is this flatness supported by measurements of the cosmic microwave background?

SKIP HULL, SAN DIEGO
Space is flat if the three angles of any triangle add up to 180°. For example, Earth's surface is a positively curved two-dimensional space. When you go from the North Pole to Ecuador to Congo and back to the North Pole, you make a triangle whose angles add up to about 270°. All other triangles on this spherical surface also give more than 180°. A saddle-shape surface is negatively curved. Draw a triangle on it, and the angles will sum to less than 180°.

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