A cosmologist was a scientist who studied in the field of theoretical cosmology.
According to Kathryn Janeway, Federation cosmologists had a theory, following the discovery of the Omega particle in the 23rd century, that revealed that "the molecule once existed in nature for an infinitesimal period of time at the exact moment of the Big Bang." She added, "Some claimed Omega was the primal source of energy for the explosion that began our universe." Seven of Nine, however, dismissed it as "a creation myth like any other." (VOY: "The Omega Directive")