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.
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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.
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.
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, howerer.
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