Cosmic background fluctuations 99Figure 2.21.The geometrical degeneracy in the CMB means that models with fixedmh^2
andbh^2 can be made to look identical by varying the curvature against vacuum energy,
while also varying the Hubble parameter. This degeneracy is illustrated here for the case
ωm≡mh^2 = 0 .2. Models along a given line are equivalent from a CMB point of view;
corresponding lines in the upper and lower panels have the same line style. The sensitivity
to curvature is strong: if the universe appears to be flat, then it really must be so, unless
it is very heavily vacuum dominated. Note that supplying external information abouth
breaks the degeneracy. This figure assumes scalar fluctuations only; allowing tensor modes
introduces additional degeneracies—mainly between the tensor fraction and tilt.