Is the Cosmological Constant Zero?
The discovery of the expansion removed the immediate need for the cosmological constant. It is still possible that there is a cosmological constant but it must be very small in the present Universe to be consistent with the data. An important unresolved issue in astronomy is whether the cosmological constant is identically zero, or just very small in the present Universe. Let us also note that an effective cosmological constant plays a role in the inflationary theories that we shall discuss later. The adjacent image shows one of the most distant type 1a supernovae yet observed, SN1997cj, in the constellation Ursa Major at a distance of about 5 billion light years (Ref). It was discovered in 1997 by the Canada-France-Hawaii Telescope on Mauna Kea, and the Hubble Space Telescope was then used to resolve it from its host galaxy and study the decay of its light curve. By using the standard candle properties of the type 1a lightcurve, the distance to the supernova could then be determined.
