Abraham "Avi" Loeb a theoretical physicist who works on astrophysics and cosmology. Loeb is the Frank B. Baird Jr. Professor of Science at Harvard University. He had been the longest-serving chair of Harvard's Department of Astronomy (2011–2020), founding director of Harvard's Black Hole Initiative (since 2016) and director of the Institute for Theory and Computation (since 2007) within the Harvard-Smithsonian Center for Astrophysics.

Loeb is a fellow of the American Academy of Arts and Sciences, the American Physical Society, and the International Academy of Astronautics. In July 2018, he was appointed as chair of the Board on Physics and Astronomy (BPA) of the National Academies, which is the Academies' forum for issues connected with the fields of physics and astronomy, including oversight of their decadal surveys.

In June 2020, Loeb was sworn in as a member of the President's Council of Advisors on Science and Technology (PCAST) at the White House. In December 2012, Time magazine selected Loeb as one of the 25 most influential people in space. In 2015, Loeb was appointed as the science theory director for the Breakthrough Initiatives of the Breakthrough Prize Foundation. In 2018, he attracted media attention for suggesting that alien spacecraft may be in our solar system, using the anomalous behaviour of ʻOumuamua as an example. In 2019, together with his Harvard undergraduate student, Amir Siraj, Loeb reported discovering a meteor that potentially originated outside the Solar System.