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I have a question:

i am trying to understand how we find out that the curvature of the universe is zero using the angular size of the hot spots of the d microwave background radiation.

http://scienceblogs.com/startswithabang/2012/07/18/how-big-is-the-entire-universe/

there is a picture in this blog showing the light rays from the opposite sides of the hot spots.

My question is: doesn't the proof that the curvature is zero require that the rays are parallel to each other? If they are parallel in a positively curved universe they converge. But in a flat one they should remain parallel and in a negatively curved they should diverge. Then, in the last two cases, how do they meet in the eye?

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# Are rays from opposite sides of a hot spot of CMB parallel or not?

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