Albert Einstein (1879 - 1955), a great thinker and creator at a level with Newton and Maxwell, created a general, classical field theory of gravity in 1915 that has been a mainstay of physics and astronomy ever since. Einstein's nonlinear tensor field theory of gravity for the first time made it possible to construct realistic models of entire universes... including our own.

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!