The cosmological constant and cold dark matter

THE cold dark matter (CDM) model^1-4 for the formation and distribution of galaxies in a universe with exactly the critical density is theoretically appealing and has proved to be durable, but recent work^5-8 suggests that there is more cosmological structure on very large scales (l> 10 h ^-1 Mpc, where h is the Hubble constant H ₀ in units of 100 km s^-1 Mpc^-1) than simple versions of the CDM theory predict. We argue here that the successes of the CDM theory can be retained and the new observations accommodated in a spatially flat cosmology in which as much as 80% of the critical density is provided by a positive cosmological constant, which is dynamically equivalent to endowing the vacuum with a non-zero energy density. In such a universe, expansion was dominated by CDM until a recent epoch, but is now governed by the cosmological constant. As well as explaining large-scale structure, a cosmological constant can account for the lack of fluctuations in the microwave background and the large number of certain kinds of object found at high redshift.