Correlation function astronomy: Encyclopedia - Correlation function astronomy

Correlation function (astronomy)

Astronomers describe the distribution of galaxies in the universe by means of a correlation function. By default, correlation function refers to the two-point autocorrelation function. For a given distance, the two-point autocorrelation function is a function of one variable (distance) which describes the probability that two galaxies are separated by this particular distance. It can be thought of as a lumpiness factor - the higher the value for some distance scale, the more lumpy the universe is at that distance scale.

The following definition (from Peebles 1980) is often cited:

However, it can only be correct in the statistical sense that it is averaged over a large number of galaxies chosen as the first, random galaxy. If just one random galaxy is chosen, then the definition is no longer correct, firstly because it is meaningless to talk of just one "random" galaxy, and secondly because the function will vary wildly depending on which galaxy is chosen, in contradiction with its definition as a function.

The n-point autocorrelation functions for n greater than 2 or cross-correlation functions for particular object types are defined similarly to the two-point autocorrelation function.

The correlation function is important for theoretical models of cosmology because it provides a means of testing models which assume different things about the contents of the universe. Computer models which calculate the formation of galaxies seem to favor cold dark matter as the model with the most support.

Correlation function astronomy - External references

• Peebles, P.J.E. 1980, The large scale structure of the universe

Category: Astronomy

Adapted from the Wikipedia article "Correlation function astronomy", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki