Inflationary cosmology 43Figure 2.5.The Hubble diagram produced by the High-zSupernova search team (Riesset
al1998). The lower panel shows the data divided by a default model (m= 0 .2,v=0).
The results lie clearly above this model, favouring a non-zero. The lowest line is the
Einstein–de Sitter model, which is in gross disagreement with observation.
2.5.6 Cosmic coincidence
This is an astonishing result—an observational detection of the physical reality of
vacuum energy. The error bars continue to shrink, and no convincing systematic
error has been suggested that could yield this result spuriously; this is one of the
most important achievements of 20th century physics.
And yet, accepting the reality of vacuum energy raises a difficult question. If
the universe contains a constant vacuum density and normal matter withρ∝a−^3 ,
there is a unique epoch at which these two contributions cross over, and we seem
to be living near to that time. This coincidence calls for some explanation. One
might think of appealing to anthropic ideas, and these can limitto some extent:
if the universe became vacuum-dominated atz>1000, gravitational instability as
discussed in the next section would have been impossible—so that galaxies, stars
and observers would not have been possible. However, Weinberg (1989) argues