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.
Now, here, you see, it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run twice as fast as that.
The Red Queen in Lewis Carroll's Through the Looking Glass