Georges Lemaitre (1894 - 1966) was one of the very first astronomers or physicists to realize that the universe was expanding, and the first to create a model of a realistically expanding universe using Einstein's theory of gravity. He is properly considered the father of Big Bang cosmology.
Willem de Sitter (1872 - 1934)
famously found a solution to Einstein's field equations that
describes a universe that is empty except for what we call Dark
Energy these days, and therefore expands explosively. The de
Sitter universe, unlike a later expanding-universe
model that was created jointly by de Sitter and Einstein, was a
footnote to physics until the 1980s when people realized that
our actual universe is very close to the de Sitter universe...
our own universe is dominated by Dark Energy, and evolves very
much like an empty universe, because its total energy is
zero. An anti-de Sitter universe (collapsing explosively)
is the major component of a new development in fundamental
theory, called AdS-CFT, which depends on a perfect (?)
correspondence between scale-invariant quantum field
theories and quantum gravity (described using string theory) in
an anti-de Sitter universe. Too bad that no realistic
fundamental quantum field theory (for example QCD) is
scale-invariant... in fact, just the opposite, with the
realistic field theories giving totally different results at
various scales... remember asymptotic freedom!