Romain Petrov (Lagrange) - Interferometric Cosmography

Interferometric Cosmography

Interferometric Cosmography, based on the combination of angular measurements from spectro-astrometry (SA) with the VLTI and linear measurements from Reverberation Mapping (RM) of the size of broad-line regions (BLRs), offers a new class of direct geometric measurements of angular diameter distances to active galactic nuclei up to  and beyond. In addition to its contribution to the Hubble tension, this approach opens the possibility of constraining cosmological models and their parameters, including the Etherington relation, the sound horizon at recombination, and the dark energy equation of state, in combination with large-scale structure probes such as DESI and Euclid.

This seminar will present the principles and recent advances of SA with the Very Large Telescope Interferometer and its GRAVITY+ instrument, including the first measurements at high redshift, and will discuss how SA and RM can be combined into a “BLR parallax,” thereby providing a direct geometric measurement of angular diameter distances. Possible systematic uncertainties of the method will be discussed, with particular attention given to the limitations of RM as well as uncertainties related to BLR geometry and modelling.

The seminar will also present the instrumental developments required to extend these measurements to the  range. I will discuss the new instruments required to improve the sensitivity and angular resolution of the VLTI current infrastructure, as well as the potential scientific impact of future extensions of this interferometer toward longer baselines.

Such developments would not only enable the cosmological applications of optical interferometry but also yield major advances in the study of active galactic nuclei, including precise measurements of supermassive black hole masses and the search for binary super massive black holes.