Fig. 1
Illustration of the proper distance dP and transverse comoving distance dM in a closed universe (ΩK< 0). The source is AB, and observer is at O. It is obvious that the transverse comoving distance dM is shorter than the proper distance dP. In a flat universe, they are the same, however. The cosmic curvature can therefore be tested by comparing dP and dM.
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