Resolving the cosmological missing energy problem - art. no. 063005

Some form of missing energy may account for the difference between the obse rved cosmic matter density and the critical density. Two leading candidates are a cosmological constant and quintessence (a time-varying, inhomogeneou s component with negative pressure). We show that an ideal, full-sky cosmic background an isotropy experiment may not be able to distinguish the two, even when non-linear effects due to gravitational lensing are included. Bec ause of this ambiguity, microwave background experiments alone may not dete rmine the matter density or Hubble constant very precisely. We further show that degeneracy may remain even after considering classical cosmological t ests and measurements of large scale structure. [S0556-2821(99)00706-7].