Dottorato - PhD thesis
Area Tematica: RSN 1 - Galassie e cosmologia
Referente: Stefano Cristiani (
Titolo: The Sandage test of the cosmological redshift drift - from ESPRESSO to ANDES and SKA
Decorrenza: 15/05/2024
The Sandage test (Sandage 1962) is based on the principle that the expansion of the Universe will cause the redshift of a given astrophysical object to drift over time. This drift provides a direct, real-time and model-independent mapping of the expansion rate of the universe. It is fundamentally different from other cosmological probes: instead of mapping our (present-day) past light-cone, it directly compares different past light-cones. Being independent of any assumptions on gravity, geometry or clustering, it directly tests the pillars of the Lambda-CDM paradigm. The Sandage Test is seen as the next step in high-precision cosmic geometric observations and a direct and irrefutable test for cosmic acceleration. It has shown remarkable constraints on the matter density parameter today on every tested model and a great complementarity with Type Ia Supernovae (SNe) and Baryon Acoustic Oscillations (BAO) data regarding dark energy parameters (Martins et al. 2016; Alves et al. 2019; Esteves et al. 2021). A measurement of the redshift drift is a flagship objective of the Extremely Large Telescope (ELT; Liske et al. 2008), namely of its high-resolution spectrograph, ANDES. The expected signal is extremely small (ca. 5 cm/s per decade) so the experiment time is long, and vulnerable to time-dependent systematics. On the other hand, the experiment has a high legacy value because the signal grows linearly with time, and provided that adequate calibration procedures are followed - e.g. based on Laser Frequency Combs - first epoch measurements can be already obtained with the ESPRESSO instrument at the ESO VLT, while waiting for the construction of ANDES. There are three published attempts to measure the redshift drift (Darling 2012; Cooke et al. 2020; Lu et al. 2021). All are based on archival data and have a systematics-dominated uncertainty 1000 times larger than the expected signal. This is a photon-starved science case, the limiting factor being the availability of bright high-redshift quasars. The QUBRICS survey (Boutsia et al. 2020), has produced new bright quasars in the Southern hemisphere that have allowed to produce a new Golden Sample and reduce to 1500 hours the ELT telescope time necessary to detect the signal (Cristiani et al. 2023). Besides, the combination of the measurement of the redshift drift with the QSOs Lyman Forest, at z > ~2.5, will be plausibly complemented at z <~2.5 with radio observations by the SKAO, CHIME and FAST (Moresco et al. 2022), providing an extended redshift leverage of the drift and significantly shortening the observing time required. Given these developments, the Ph.D. Student would be involved in the first dedicated redshift drift experiment with ESPRESSO. This is presently being executed over four ESO periods (P110-P113) and relies on two QUBRICS quasars, with a one-year experiment timescale. While this will not detect the drift signal, it will improve current bounds by a factor of ten and obtain the first statistics-limited constraint. It will also provide a full end-to-end proof of concept for the ANDES experiment at the ELT, optimizing methods and identifying any bottlenecks. In addition to achieving the primary goal, the data has unique legacy value as a first epoch for the ELT experiment, effectively increasing the sensitivity of the ELT experiment (or, conversely, reducing the amount of ELT time necessary to reach a chosen sensitivity). Finally, the data enables ancillary science, pertaining to the IGM metallicity and temperature, the primordial Deuterium abundance and tests of the stability of fundamental constants. The Ph.D. student will carry out a complex research involving theoretical Cosmology, observations, data reduction and interpretation, and in this process will be exposed to the development of cutting-edge instrumentation (e.g. new laser frequency combs) and techniques (e.g. advanced metrology, data processing with spectral perfectionism - Bolton & Schlegel 2010) and will address the hot topic of spectral fidelity. Ideally, this Ph.D. Fellowship will help the student develop the skills needed to play a leading role in the exploitation of the ANDES GTO.
Progetto: ESPRESSO - ANDES
Allegato: download pdf
