Absence of "Ghost Images" Excludes Large Values of the Cosmological   Constant
  We used the 1.4 GHz NRAO NVSS survey to search for ghost images of radio sources, expected in cosmologies with a positive cosmological constant and positive space curvature. No statistically significant evidence for ghost images was found, placing constraints on the values of L, the space curvature or the duration of the radio-luminous phase of extragalactic radio sources. 
J. M. Eppley and R. B. Partridge