Photons as tracers of space-time curvature and mass distribution in the Universe

Context

In a few months, the telescope at the Vera Rubin Observatory in Chile will start taking its first images of the sky, after two decades of design and construction. Over a period of 10 years, it will scan the southern sky with the goal of conducting a survey, the LSST (Legacy Survey of Space and Time). This ambitious project will compile an unprecedented catalog, including 20 billion galaxies and a thousand times more transient luminous objects. The colossal amount of data collected will help reduce statistical errors in cosmological measurements. We are therefore entering a new era, where systematic uncertainties will be central in analyses, potentially leading to tensions between measurements if they are incorrectly assessed. To avoid this pitfall, we propose to put general relativity at the core of cosmology.

General relativity predicts a distortion of spacetime associated with the presence of matter. Currently, cosmological parameters describing the evolution and composition of the universe are obtained, among other methods, through the observation of the most massive structures, such as galaxy clusters, by making assump- tions about their shapes and masses. Rather than relying on such assumptions, which introduce significant systematic uncertainties, we propose here an alternative approach: directly measuring the distortion of space- time by studying the geodesics followed by photons near these massive structures.

Master 2 internship

Starting from the equations of general relativity, the goal will be to develop a simulation tool in C++ to trace the path of light given a mass distribution, and deducing the observational effects. A proposed application will be to study the impact of the shape of galaxy clusters on the gravitational lensing phenomenon observed in distant objects. This phenomenon will be simulated for increasingly complex cluster configura- tions, ranging from perfect spheres to more realistic structures. The resulting lensing will be compared to that produced by the official simulation of the DESC LSST collaboration, which assumes spherical symmetry of the clusters. This comparison will allow us to estimate the uncertainties related to the measurement of cluster mass, a crucial element for the precise determination of cosmological parameters.

This work is suspected to last 3-6 months, corresponding to the duration of an internship, that will occur before the PhD, in the period March-July 2025, as it is traditional in France.

Follow-up PhD research

The internship can be followed by a PhD thesis (if internship candidate is fitting the requirements), that would continue to investigate the use of photon propagation to study spacetime curvature. This method is indeed a powerful tool that allows testing the observational effects of general relativity in various regimes and with multiple observables, while bypassing many of the assumptions typically required.

The goal of the PhD will be to address fundamental questions related to dark energy and dark matter, and it will provide an opportunity to explore several complementary areas, including general relativity theory, simulations, statistical data analysis, and the treatment of systematic uncertainties. This will offer an ideal opportunity for the selected student to gain a comprehensive overview of the field and to better understand the entire process, from theory to the interpretation of observations. The student will have the freedom to choose the areas they wish to delve deeper into, based on their preferences.

One interesting application of the tool will be to predict the effects of strong gravitational lensing caused by a variable light source (such as a supernova or a quasar) passing through a massive object, such as a galaxy cluster. This strong lensing can lead to the formation of multiple images of the same source, with each light ray following distinct paths and consequently exhibiting different redshifts due to the expansion of the universe. This method, combining LSST transient object catalogs with spectroscopic data from DESI or Euclid, will allow for the estimation of the temporal variation in redshift (redshift drift), providing an independent measure of the universe’s expansion rate and offering a deeper understanding of dark energy.

Furthermore, this tool can be used to trace the distribution of matter purely through gravitational effects. In this context, artificial intelligence techniques could be developed and applied to facilitate the generation of such a map, replacing current classical methods that are often computationally expensive. The uncertainties related to assumptions about the shapes of objects involved in gravitational lensing will also be considered, by testing various configurations and assessing their impact on the observed distortions. This approach will allow for the statistical extraction of the distribution and abundance of dark matter by comparing visible and gravitational masses for a large number of objects, all without making assumptions about the nature of dark matter.

Scientific environment

Vincent Reverdy and Narei Lorenzo Martinez will co-supervise this PhD, drawing on their respective expertise in theoretical computations, simulations, and statistical data analysis. They are both members of the Dark Energy Science Collaboration (DESC) within LSST and part of the LSST research team at LAPP in Annecy. Several physicists from the team are experienced members of DESC and experts in both observational and theoretical cosmology. This environment will provide a supportive framework for the PhD student, allowing them to navigate the complexities of the collaboration and tackle intricate cosmological questions.

PhD funding

A PhD funding proposal has been submitted, with results expected in February 2025. Other funding opportunities are possible with the local doctoral school.

Location

The PhD will be based at Laboratoire d'Annecy de Physique des Particules (LAPP), in Annecy, France.

Contact, questions, and application

Please send your questions and/or your application (1-2 reference letters, cover letter) by email to vincent.reverdy@lapp.in2p3.fr and lorenzo@lapp.in2p3.fr with the subject "STAGE COSMO 2025 : FirstName LastName".