http://en.wikipedia.org/wiki/Cosmological_constant
Observations announced in 1998 of distanceâredshift relation for Type Ia supernovae[6][7] indicated that the expansion of the universe is accelerating. When combined with measurements of the cosmic microwave background radiation these implied a value of \Omega_{\Lambda} \simeq 0.7,[8] a result which has been supported and refined by more recent measurements. There are other possible causes of an accelerating universe, such as quintessence, but the cosmological constant is in most respects the simplest solution. Thus, the current standard model of cosmology, the Lambda-CDM model, includes the cosmological constant, which is measured to be on the order of 10â52 mâ2, in metric units. Multiplied by other constants that appear in the equations, it is often expressed as 10â35 sâ2, 10â47 GeV4, 10â29 g/cm3.[9] In terms of Planck units, and as a natural dimensionless value, the cosmological constant, λ, is on the order of 10â122.[10]
As was only recently seen, by works of 't Hooft, Susskind[11] and others, a positive cosmological constant has surprising consequences, such as a finite maximum entropy of the observable universe (see the holographic principle).