Thesis Topic: Cepheid stars, Ho and Dark Energy: precision stellar astrophysics for precision cosmology

Thesis Supervisor: Martino Romaniello




Thesis Supervisor: Martino Romaniello


Understanding the physics that drives the acceleration of the Universe is a formidable challenge, with the simplest theoretical expectations failing to match the observations by 120 orders of magnitude. In combination with other data, a high precision local measurement of the local expansion rate of the Universe, the Hubble constant Ho, provides direct insight into the physics of the accelerating Universe. In this context, The Hubble Space Telescope is currently used to measure Ho at percent-level precision by building from scratch a clean, simple three-rung stellar distance ladder: Cepheid stars are used to calibrate the peak luminosity of Supernovae type Ia that are, then, used to measure Ho in the Hubble flow (SH0ES project, Riess et al 2011).

The LACES collaboration (PI Romaniello), which includes the SH0ES PI Adam Riess, is now coming to fruition to determine the effects of chemical composition on the properties of Cepheids on which the SH0ES distance ladder is built. At an unprecedented accuracy of 1% we will settle the long standing debate on this subject and probe whether the current indications of an intriguing tension between the SH0ES direct value of Ho and Planck indirect estimates based on the concordance LambdaCDM cosmological model indicates the need for new physics, or is due to random errors or systematics not yet accounted for.

The LACES initial dataset includes data from two multi-object spectrographs at ESO’s VLT: FLAMES-GIRAFFE in the optical and KMOS in the infrared. The proposed PhD project aims at analyzing and interpreting these datasets, thus exposing the candidate to a variety of cutting-edge data and analysis techniques, working in an international collaboration within the context of one of the hottest and most relevant topics in contemporary science: the quest for the very nature of Dark Energy and Cosmic Acceleration.