5.2. Open Universe, or Vacuum Energy?
The two currently-favoured options seem to be:
(A) an open model, or else
(B) a flat model where vacuum energy (or some negative-pressure component that didn't participate in clustering) makes up the balance.
If the universe is a more complicated place than some people hoped, which of these options is the more palatable? Opinions here may differ: How ``contrived'' are the open-inflation models? Is it even more contrived that the vacuum-energy should have the specific small value that leads it to start dominating just at the present epoch?
Either of these models involves a specific large number. In case (A) this is the ratio of the Robertson Walker curvature scale to the Planck scale; in (B) it is the ratio of vacuum energy to some other (much higher) energy density. At present, (A) seems to accord less well than (B) with the data. In particular, the angular scale of the ``doppler peaks'' in the CMB angular fluctuations seems to favour a flat universe; and the supernova Hubble diagram indicates an actual acceleration, rather than merely a slight deceleration (as would be expected in the open model).
We will certainly hear a great deal about the mounting evidence for (or one of its time-dependent generalisations): the claimed best fit to all current data suggests a non-zero energy in the vacuum. However we should be mindful of the current large scatter in all CMB measurements relevant to the doppler peak, and the various uncertainties (especially those that depend on composition, etc.) in the supernovae from which a cosmic acceleration has been inferred. I think the jury is still out. However, CMB experiments are developing fast, and the high-z supernova sample is expanding fast too; so within two years we should know whether there is a vacuum energy, or whether systematic intrinsic differences between high-z and low-z supernovae are large enough to render the claims spurious. (On the same timescale we should learn whether the Universe actually is flat).