Quasars as tracers
of the dark energy

About the project

Dark energy is the greatest challenge of the present day cosmology. According to our current knowledge, the well known standard, radiating, barionic matter contributes only a few per cent to the Universe content, the next 25 per cent is the misterious dark matter, vigorously searched for in the laboratories, and the remaining 70 per cent is the dark energy, an invisible medium with strange properties, not understood by physicists. This medium, however, can be traced observationally looking at the effect it has on the motion of distant objects made out of a usual radiating matter.

In our project we will use quasars as the probes of the dark energy. Quasars discovered (or, more precisely, identified) in 1963 are truly cosmological objects at distances of order of a few bilions of light years, so observing quasars we reach the epochs when the Universe was much younger than it is today. The distances to quasars are usually given through the specification of their cosmological redshifts caused by the expansion of the Universe, and the most distant currently known quasar has the redshift.

Quasars are objects belonging to a general class of active galaxies. Practically all regular galaxies contain supermassive black holes in their centers, with masses from a milion to a few bilions of solar masses. The surrounding matter flows into those black holes. In most galaxies this inflow is weak (those are non-active galaxies, like the Milky Way) but in some galaxies it is vigorous (those are active galaxies), and it is particularly strong in quasars. Before this gas flows into a black hole, it strongly radiates, and this radiation can exceed thousans of times the emission of stars in the quasar host galaxy. This is why we see quasars from such huge distances.

We plan to use quasars to measure the darm energy, and the method is to some extent analogous to the use of the Supernovea Ia. The key point is the determination of the absolute luminosity of a given quasar. As soon as we have it, the next steps are simple: we have to measure the observed luminosity, and the redshift. In this way we construct a Hubble diagram for quasars: we measure independently the distance (from the comparison of the absolute and measured luminosities) and the expansion rate of the Universe (from redshift) between the moment of the light emission from quasar and the current epoch. Other measurement methods than our show that the expansion of the Universe accelerates (this is the signature of the dark energy dominance) but we want to measre this effect independently, and more accurately.

The determination of the absolute luminosity is difficult. In our project we will use the method based on the theory of the formation of the Broad Line Region, which we formulated in the paper by Czerny & Hryniewicz (2011). We showed that the formation of the low ionization broad emission lines is caused by dust formation in the atmosphere of an accretion disk surrounding the black hole. Generally known dust properties imply that the dust cannot form or survive in temperatures significantly above 1000 K. Disk temperature, in turn, depends on the distance from the black hole, as well as on the absolute luminosity of a quasar. Therefore, measuring the distance from a black hole to the Broad Line Region we are able to calculate the requested quasar absolute luminosity.