Sebastian Bocquet

Survey Science Cosmology

I am actively involved in cosmological surveys that explore the sky and our universe at different wavelengths: the South Pole Telescope (SPT, millimeter-waves or microwaves), the Dark Energy Survey (DES, optical), and eROSITA (extended ROentgen Survey with an Imaging Telescope Array, X-ray). I am also involved in the preparation for upcoming surveys such as Euclid and the Rubin Observatory Legacy Survey of Space and Time (LSST) (both are optical surveys) and CMB-S4, a next-generation ground-based mm-wave survey.
Some of the key questions I am trying to answer with these survey data are related to the origin of the observed accelerated late-time expansion of the universe, the mass of neutrinos, and our ability to provide a coherent description of all cosmological observations.

Abundance of dark matter halos

The abundance (or number) of dark matter halos (and the galaxy clusters they host) above a given mass and at a given redshift has long been recognized to be extremely sensitive to some cosmological parameters such as the matter density Ω_m and the variance in the matter density field σ₈. Performing this measurement over a range of redshifts thus also allows to constrain the dark energy equation of state parameter w. Finally, the combination of abundance measurements with measurements of the primary anisotropies in the cosmic microwave background (CMB) radiation allows to constrain the sum of neutrino masses. Plenty of discoveries to go after! However, in practice, all these measurements are limited by our ability to accurately measure the masses of dark matter halos.

Multi-probe cosmology using clusters of galaxies

The key for a successful cosmological analysis using halo abundances is to adopt a multi-probe multi-observable mass calibration strategy. In the following I describe such an analysis.

Galaxy cluster selection using the South Pole Telescope survey

The South Pole Telescope detects galaxy clusters through the Sunyaev-Zel'dovich effect: low-energy CMB photons (remember, the CMB is 2.7 K) scatter off the hot gas that is trapped by the halo's gravitational potential. This leads to a small but measurable spectral distortion in the CMB along the line of sight of a cluster. The Sunyaev-Zel'dovich effect is independent of redshift. On the one hand, this means that we need other data to obtain cluster redshifts, but it also means that the Sunyaev-Zel'dovich effect allows us to detect clusters at high redshifts where other means of detection run out of sensitivity.

The mass and redshift distributions of the SPT, Planck, and ACT cluster samples (Bleem, Bocquet et al. 2020).

In 2024, the collaboration released the full catalog of galaxy clusters detected in the first-generation SPT-SZ data and in the second-generation SPTpol dataset. The analyses represent a milestone and prerequisite for the ongoing analysis of SPT-3G data.

The mass and redshift distributions of the clusters detected by SPT-SZ and SPTpol, Planck, and ACT (Bleem et al. 2024).

Weak-lensing observations of galaxy clusters

The most accurate approach to measure the mass scale of galaxy clusters is to exploit the effect of gravitational lensing, which is sensitive to the full mass of a halo (and not just its visible galaxies or hot gas). The advent of large optical weak-lensing surveys such as DES make this approach particularly attractive (Bocquet et al. 2024 I). At higher redshifts, targeted observations using the largest ground-based telescopes and the Hubble Space Telescope allow us to cover redshifts up to about 1.1 (e.g., Schrabback, Bocquet et al. 2020).

The outline of the main fields of the SPT cluster surveys. We also show the a mass map based on DES Y3 weak-lensing data, and SPT clusters with z<0.5 for which we can measure DES lensing shear in the overlap region (Bocquet et al. 2024 I).

Cosmological Constraints

Cosmological constraints from different probes (Bocquet et al. 2019).

Evolution of structure growth with time (Bocquet et al. 2024 II).

Theoretical modeling using numerical simulations

On large scales, cosmic structure formation can accurately be described by perturbation theory. However, scales smaller than a few megaparsecs evolve non-linearly as matter collapses under its own gravitational pull and forms filaments and dark matter halos. A lot of cosmological information is contained in these non-linear scales. To access these scales from a theoretical modeling perspective, we use numerical cosmological simulations. In these, the matter density field is evolved and allow us to track its evolution well into the non-linear regime.

Cosmic emulation

As discussed, we have to rely on numerical simulations to describe the non-linear matter density field. Originally, this approach was used to calibrate a correction of the linear theory. However, this correction may itself depend on the cosmological parameters we are trying to measure. The solution is emulation: a suite of cosmological simulations is run for a range of different cosmologies, and an interpolation technique then allows to make predictions for the quantity of interest at any new cosmology.
Using the Mira-Titan Universe suite of cosmological simulations, I constructed an emulator for the halo mass function. The 111 input cosmologies cover 8 cosmological parameters, including the effects of dynamical dark energy and massive neutrinos. I did not "teach" the emulator anything about the halo mass function being approximately universal; the emulator is purely data-driven. The accuracy is at the few-percent level in the cluster mass range at low redshifts (Bocquet et al. 2020).

Cosmologies of the Mira-Titan Universe (111 black markers) which are used to construct an emulator for the halo mass function (Bocquet et al. 2020).

Halo mass maps

To interpret the weak-lensing measurements discussed above, we need reliable models of the mass distribution within massive halos. We have used numerical simulations to establish such models. Our latest cosmological analysis is indeed based on our halo mass model ( Grandis, Bocquet et al. 2021).

Projected mass map of a massive halo in the Magneticum simulation (figure from Grandis, Bocquet et al. 2021).