Observing Quasars as Probes of Dark Energy

On October 21, 2015, Prof. Bozena Czerny, from the Center for Theoretical Physics and Copernicus Astronomical Center in Warsaw, Poland, visited NAOC and gave a lecture about her efforts to use quasars to study the expansion rate of the Universe at large distances. She started her talk by giving an overview about historical studies of dark energy and what is currently known about this topic. She mentioned that astronomers know that dark energy smoothly fills the Universe and has a negative pressure, which acts as a cosmological constant in the Einstein equation. She also described efforts to measure effects of dark energy, including mapping large scale structure, the cosmic microwave background, type Ia supernovae and gravitational lensing. However, she also listed some shortcomings associated with those methods, especially type Ia supernovae arising from possible different types of progenitors which lead to differing luminosities and difficulties determining redshift. 

As an alternative, Prof. Czerny said that, since they are so energetic, quasars can potentially be observed over even greater distances than type Ia supernovae, leading to measurements of expansion rates for the Universe at larger redshifts. She went on to describe how comparisons between emission lines and the continuum of a quasar can be used to measure the luminosity of the quasar. She and her collaborators are currently pursuing this line of research that combines theoretical modeling of emission properties of a quasar with observational campaigns, including using the South African Large Telescope (SALT). Prof. Czerny also described the possibility of using the LSST in the future for continued monitoring of quasars. This avenue of research may lead to a better understanding of dark energy in coming years.