Cosmology boasts a successful standard cosmological model. While its success rests on its ability to explain diverse observations, its practical use derives from its framing of open questions. One central open question is: How does the cosmic ecosystem of dark matter and baryons co-evolve? The proposed thesis research will examine this question by modeling the cross-correlation of the Euclid mission’s galaxy and lensing survey with cosmic microwave background (CMB) surveys - the Atacama Cosmology Telescope (ACT), the South Pole Telescope (SPT), and the Simons Observatory (SO). The importance of this question was emphasized by the U.S. decadal survey Astro2020, which identified it as one of the three fundamental questions for the coming decade in astrophysical research. 

The mystery at the heart of this question is why only 10% of baryons (ordinary atoms) end up in stars, i.e., the stellar mass in galaxies, by today. Left alone, the majority of baryons would cool to form stars, dramatically violating observational constraints - energy feedback is required to prevent this ‘cosmic cooling catastrophe’. In other words, baryons flow as cooling gas into galaxies to fuel star formation and are also expelled by feedback into the gaseous phase in and around galaxy halos. Specific unanswered questions about the evolution of this cosmic ecosystem are: 1) What are the feedback mechanisms and how do they vary with galaxy properties, and 2) How does the diffuse gas influence the visible galaxies? 

The context of this issue can be more fully appreciated by noting that all theories of galaxy formation and evolution to date have relied solely on observations of the 10% of baryons making stars! The primary reason for this is that the other 90% is very difficult to observe, being very diffuse and extremely faint. 

This situation is changing. In the millimeter, CMB surveys probe the diffuse gas through the thermal and kinetic Sunyaev-Zeldovich (tSZ and kSZ) effects, and they also probe the total matter distribution via gravitational lensing of primordial CMB anisotropies. Cross correlations of galaxy positions with tSZ, kSZ, and gravitational lensing (both CMB and galaxy-galaxy lensing) are beginning to tell us about the joint distribution of dark matter, gas, and galaxies. This is the key to revealing the co-evolution of these components.

The thesis project consists of:

Measuring the cross-correlation function of galaxy positions and lensing with maps of tSZ, kSZ, and CMB lensing;

Modeling these cross-correlation functions with the Halo Model and semi-analytical models.

The research will build on the supervisor’s previous work in this area [Planck Collaboration 2013, A&A 557, 52; Kou and Bartlett 2023, A&A 675 149; Kou, Murray, & Bartlett 2024, A&A 686, 193] by using higher quality data from the Euclid mission and SO, as well as new data from ACT and SPT. The modeling effort will involve collaborators with sophisticated simulation infrastructure at the University of California, Berkeley, and Stanford University.

The host laboratory, the Centre Pierre Binétruy (IRL 2007), is ideally suited for this project. The Center has been a leading contributor CMB research, including collaboration with the University of California, Berkeley (UCB, which hosts the Center), and the Lawrence Berkeley National Laboratory (LBNL). Their is a strong effort simulating cosmological observations at LBNL, and also at Stanford University in close proximity. The thesis director has close collaboration with colleagues at these institutions who are working in this new field. 

Cross-correlation studies are driving a novel and rapidly growing research area in extragalactic astrophysics. The new ability to simultaneously probe all matter components and their co-evolution enables us to finally answer a central, unanswered question about cosmic evolution and the mystery surrounding the fate of the baryons. A central element of the project, the high-quality imaging and infrared spectra from space furnished by the Euclid mission will enable precision measurements not attainable by other means in the foreseeable future. The thesis project is timely given the upcoming Euclid data releases.

The project schedule is well matched to incoming data. It is as follows:

- Year 1: Collect and manipulate Euclid DR1 galaxy and lensing data (publication date 11/2026), and new maps from ACT, SPT, and first observations from SO.

- Year 2: Extract cross-correlation functions; begin model development with the Halo Model and more sophisticated semi-analytical models with collaborators.

- Year 3: Interpret the measured correlation functions with the models. Expected results: relation between observed galaxy properties and surrounding gas and dark matter distributions. This will provide answers to the questions above, constrain feedback mechanisms, and provide valuable data for galaxy theories.

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For more Information about the topics and the co-financial partner (found by the lab!); contact Directeur de thèse - bartlett@apc.in2p3.fr

Then, prepare a resume, a recent transcript and a reference letter from your M2 supervisor/ engineering school director and you will be ready to apply online before March 13th, 2026 Midnight Paris time!