Perfect Cosmological Principle? We not only do not occupy a preferred place in the Universe, we also do not occupy a preferred time. The Universe appears homogeneous and isotropic and the same to all observers at all times in the Universe.

The PCP states that there are preferred locations in either space or time in the Universe. This is a very restrictive assumption in that it does not allow room for evolution of things in the Universe. However, philosophically speaking, this is very attractive as it removes the necessity for having a birth and/or death of the Universe -- the Universe is and has always been. This notion gave rise to the steady-state models of the Universe in the 1940's.

The idea that our Universe obeys the PCP is hard to swallow given that we see evolution in our Universe in terms of the evolution of Quasars, normal galaxies, the CMBR, and mass-energy conservation.

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Comment 1

Given that our Universe is expanding, the idea of a steady-state universe requires that mass be continously created in order that the Universe appear the same at all times (expansion ===> increase in volume ===> if the density is to remain constant that mass must be created to fill the larger volume).

Interestingly, the amount of mass which is required to fulfill this condtion is very small -- 0.3 atoms per cubic kilometer per year -- and is essentially undetectable.

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Comment 2

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 from collapsing. He parameterized this force introducing the Lambda parameter, to represent an unknown repulsive force which permeated the Universe. Einstein made this assertion before Slipher and 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, Lambda was unnecessary. Einstein immediately took back his suggestion, considering it to be one of his greatest blunders. Interestingly, today, it is suggested that there is some unknown force, the Dark Energy which may, in fact, be the repulsive force which is parameterized by the Lambda Einstein added to his theory. We do not yet know if this is correct, however.

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