Weltman became known when she co-authored a 2004 paper titled "Chameleon Cosmology" with Justin Khoury, which proposed a theory to explain dark energy. She was a 24-year-old graduate student at Columbia University at the time. Dark energy is proposed as an explanation for the accelerating expansion of the universe. Khoury and Weltman proposed the existence of a new force that drove this expansion, which changed depending on the environment it was in. It would be weak when particles were densely packed together, and strong when they were far apart. Thus, the theory suggests that in regions where matter is relatively dense, the chameleon force is difficult to detect; but in empty regions of space, it acts to push bodies apart and expand the universe. The theory was such that it could not be tested for ten years after it was proposed; however, in 2014 experimenters at the University of Nottingham developed a method that had the potential to test the theory. The theory evolved by Khoury and Weltman has been described as leading to "entire sub-fields in cosmology and experimental physics." Her work has been described as a continuation of the work of Albert Einstein.
