  How much can we know about our Universe? All of our observations are
restricted to a finite volume, and therefore our estimates of presumably global
cosmological parameters are necessarily based on incomplete information. Even
assuming that the Standard Model of cosmology is correct, this means that some
cosmological questions may be unanswerable. For example, is the curvature
parameter Omega_K positive, negative, or identically zero? If its magnitude is
sufficiently small, then due to cosmic variance no causal observation can ever
answer that question. In this article, we first describe the gauge problems
associated with defining the cosmic variance of cosmological parameters, then
describe a solution involving the use of parameters defined on the surface of
last scattering, and finally calculate the statistical variance of ideal
measurements of the matter, radiation, and curvature density parameters. We
find that Omega_K cannot be measured to better than about 1.5x10^(-5) (1
sigma), and that this limit has already begun to decrease due to the flattening
effect of dark energy. Proposed 21 cm hydrogen experiments, for example, make
this limit more than just a theoretical curiosity.
