Thesis Topic: Measuring the redshift drift with quasars
Thesis Supervisor: Luca Pasquini
Abstract
We now know that the universe has been expanding ever since the Big Bang. As a consequence, we expect to observe the redshift of all objects in the universe to change, or drift with time. The rate of the redshift drift is set by the fundamental properties of our universe and its measurement is therefore an excellent way to constrain cosmological models (Sandage 1962, ApJ, 136, 319). The measurement principle is simple: measure the change in the redshift of distant objects between different epochs and compare it to the theoretical predictions. This measurement has two unique advantages with respect to any other method:
1) It is direct: i.e. it does not require assumptions, modelling of astrophysical systems or special astrophysical calibrations;
2) The signal increases with time: the longer one waits, the stronger the signal.
The shift predicted by the present cosmological model is extremely tiny: δz ~ 10-9 over 10 years, or, in velocity, a few cm s-1/yr, comparable with the radial velocity variation induced by the Earth on the Sun. In order to measure it, a super- stable high-resolution spectrograph, a full understanding of the data chain, and high-quality spectra are necessary. This will require the use of HIRES at the ELT (Maiolino et al. 2013, 2013arXiv1310.3163M; Marconi et al. 2021, Messenger,182, 27), for which the redshift drift measurement is one of the main science cases. Isolated systems with narrow absorption lines, such as the intergalactic medium (IGM) clouds seen in quasar spectra, are expected to provide the strongest constraints; the theory and simulations can be found in Liske et al. (2008, MNRAS, 396, 1192).