This method compares
measurements of large numbers of galaxies (thousands
to hundreds of millions) with predictions of the
density of galaxies per unit of volume for various cosmological
models. When the universe was denser and hotter,
sound waves were transmitted across the cosmos, with
higher- and lower-density portions of the universe reflecting
the wavelength of the sound. This is entirely analogous
to how familiar sound is transmitted in air. However, at
about 380,000 years after the Big Bang, the universe cooled
enough to decouple the matter and energy existing at the
time, stopping the transmission of sound waves and freezing
the pattern of rarefication and concentration. Gravitational
attraction amplified these variations in density in
the early universe, resulting in variations in the location
of galaxy formation. In essence, galaxies preferentially
formed in regions of high density and were less likely to be
formed in regions of low density, with separations that are
easy to calculate. The expansion of the universe has caused
these patterns of concentrations of galaxies to similarly expand.
Studying the actual size of these patterns as a function
of distance from Earth (and, consequently, time) tells
us a lot of the history of the universe and how its size has
varied