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 NO 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.

Einstein had developed an early version of the Steady-State Universe, before the expansion of the Universe was discovered, wherein the Universe was static. However to be static meant that some unknown repulsive force had to exist so that the natural effect of gravity to make the Universe contract could be counteracted. Einstein introduced the Cosmological Constant to represent this unknown repulsive force. Once it was discovered that the Universe was expanding (and not static), the need for this unknown force disappeared and Einstein quickly tried to remove the Cosmological Constant from the lore. Interetingly, this idea (or a similar effect) was resurrected in the late 1990s to explain some new observations. Today, we consider that some form of Dark Energy which produces a repulsive force like the Cosmological Constant exists in the Universe. In fact, current models suggest that 70 % of the Universe is made up of Dark Energy!

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.

Comment

Also, 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.