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Because Science isn't very explicit, I also went through one of the numerous reports put out by LBNL (arXiv: astro-ph/03030428 v1 18 Mar 2003), titled, Measuring Cosmology with Supernovae, Saul Perlmutter, Brian P. Schmidt. From the Abstract:
``...Two teams have used type Ia supernovae to trace the expansion of the Universe to a look-back time more than 60% of the age of the Universe. These observations show an accelerating Universe which is currently best explained by a cosmological constant or other form of dark energy with an equation of state near w = ... -1. While there are many possible remaining systemtic effects, none appears large enough to challenge these current results....''
In the article they give a 99% confidence to the statement that the Universe has a non-zero cosmological constant:
``The easiest solution to explain the observed acceleration is to include an additional component of matter with an equation of state parameter more negative than w < -1/3; the most familiar being the cosmological constant (w = -1).... If we assume the Universe is composed only of normal matter and a cosmological constant, then with greater than 99.9% confidence the Universe has a non-zero cosmological constant or some other form of dark energy. Of course we do not know the form of dark energy which is leading to the acceleration, and it is worthwhile investigating what other forms of energy are possible additional components...'' (19p)
The original cosmological constant was explicitly an artificial term used to balance the global geometry equations so that in Einstein's first GR model the universe would be static and not expand or contract. When it was originally put in, it had a non-zero value to keep the universe static. Leaving it out originally lead to an expanding universe but of a wrong time length---too short.
Setting the constant with various other non-zero values results in an oscillating, or expanding universe with various expansion curves. The interesting part is that it is not just a numerical factor. It is expressed in cm^-2 units, so it has to be postulated as a `force' of some sort. In effect, by fine tunning this term, you can give the universe various dynamic outcomes.
Chuck Grimes