William de Sitter

William de Sitter was a Dutch mathematician and physicist renowned for his work in cosmology. He developed the first mathematical model describing an expanding universe, which laid the groundwork for understanding how galaxies move away from each other over time. His equations, known as the de Sitter universe, helped scientists grasp the idea that the universe is not static but dynamic, expanding due to a form of energy known as dark energy. De Sitter's contributions are fundamental to modern cosmology and our understanding of the universe's large-scale structure.