Perfect Cosmological Principle (PCP) We occupy neither a preferred place in the Universe nor exist at a preferred time. Not only does the Universe appear homogeneous and isotropic to all observers at all times in the Universe, the properties of the Universe are the same at all times. The PCP is dismissed by observing that galaxies evolve (see figure to right) and by the expansion of the Universe if one doesn't invoke rather hard-to-take processes. Philosophically speaking, the PCP is very attractive as it removes the necessity for a birth and/or death of the Universe -- the Universe is and has always been. The PCP gave rise to the steady-state model for the Universe in the 1940s.

Comment and Einstein

Einstein was seduced by the notion of an unchanging in space and time Universe (a Universe in a stationary state). He recognized that because gravity which he thought dominated the Universe on large scales, was always attractive, that the Universe must contain some unknown repulsive force in order to keep itself from collapsing. He parameterized this force. Einstein introduced the Lambda parameter (the Greek Letter Lambda = Λ) to represent this mysterious and unknown repulsive force which he assumed to permeate the Universe. Einstein made this assertion before Hubble made the discovery that the Universe was expanding. After Hubble made his announcement, Einstein immediately recognized that because the Universe was not stationary (it was expanding) that there was no need to postulate a repulsive force, Λ was unnecessary. Einstein immediately took back his suggestion considering it to be one of his greatest blunders. Interestingly, it is now suggested that the unknown force given by Dark Energy may, in fact, be the repulsive force that Einstein added to his theory. We do not yet know if this is correct, however.

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