CosmicVariance

"Cosmic variance" is a term used in cosmology and astrophysics to refer to the statistical uncertainty that arises when measurements of a physical quantity, such as the density of galaxies or the cosmic microwave background radiation, are made from a limited sample of the universe.

The concept stems from the fact that when we observe the universe, we are essentially sampling from a single realization of all possible universes. The universe is both isotropic (the same in every direction) and homogeneous (the same at every location) on sufficiently large scales, but on smaller scales, there are fluctuations and inhomogeneities. Thus, if we take a different sample, or observe a different region of the universe, we might get a different measurement due to these fluctuations.

Cosmic variance is particularly significant when observing rare objects or large-scale structures, since there are fewer such objects or structures within our observable universe.

In essence, cosmic variance is the measure of the uncertainty in our observations due to the fact that we only have one universe to observe. This uncertainty is intrinsic and cannot be reduced by simply taking more measurements or improving our measurement precision. It's an inherent limitation of observational cosmology.