Anisotropy

Anisotropy is the property whereby a physical quantity varies depending on the direction in which it is measured. It is the opposite of isotropy, which refers to a uniform distribution in all directions. In astronomy and cosmology, the study of anisotropies reveals crucial information about the structure of the universe.

Anisotropies of the cosmic microwave background

The most famous anisotropies are those of the cosmic microwave background— tiny temperature variations, on the order of one hundred-thousandth of a degree, observed in the fossil radiation from the Big Bang. These slight differences, mapped by the COBE, WMAP, and Planck satellites, correspond to the seeds from which the first galaxies and all the large-scale structures of the present-day universe formed.

Sources of Anisotropy

Several phenomena produce anisotropies. Quantum fluctuations during cosmic inflation created the primordial anisotropies. The Sachs-Wolfe effect describes how photons from the cosmic microwave background interact with gravitational potential wells. The Sunyaev-Zel’dovich effect modifies the spectrum of the cosmic microwave background as it passes through hot galaxy clusters.

Significance for cosmology

A precise analysis of the spectrum of anisotropies allows us to determine the fundamental parameters of the cosmological model: the age of theuniverse, the density of ordinary matter, the density of dark matter, the density of dark energy, and the overall geometry. In 2013, the Planck satellite provided the most precise measurements ever made of these parameters.

Did you know?

The anisotropies in the cosmic microwave background are so small— about one-hundred-thousandth of the average signal—that they were not detected untilin 1992 by the COBE satellite, nearly 30 years after the discovery of the cosmic microwave background itself. This detection earned John Mather and George Smoot the 2006 Nobel Prize in Physics.

Frequently Asked Questions

What is the difference between anisotropy and isotropy?

Isotropy means that something is the same in all directions. Anisotropy, on the other hand, refers to variations depending on direction. The universe is generally isotropic on large scales, but exhibits small anisotropies that tell its story.

How are the anisotropies of the cosmic microwave background measured?

Using specialized satellites such as Planck, which measured the temperature of the cosmic microwave background in millions of different directions. The difference between these directions yields a map of the anisotropies, which is then analyzed as an angular power spectrum.

Why are anisotropies important?

They contain all the information about the initial state of the universe and its subsequent evolution. They allow us to test cosmological models with phenomenal precision and to detect possible new physics.