A mathematical device used by Einstein to keep the Universe from falling in on itself. He later called this his "greatest blunder", because it kept him from predicting the expansion of the Universe. Astrophysicists now believe there may be a use for the cosmological constant.
Come, you lost Atoms, to your Centre draw,
And be the Eternal Mirror that you saw:
Rays that have wander'd into Darkness wide,
Return and back into your Sun subside.
From Farid al-Din Attar's
The Conference of the Birds