Annalisa Pillepich receives ERC Consolidator Grant for cosmological simulations

January 31, 2023

Annalisa Pillepich, who is a group leader in the department "Galaxies and Cosmology" at the Max Planck Institute for Astronomy (MPIA), has been awarded one of the prestigious ERC Consolidator Grants. With a total amount of 2 million euros, the grant will be used to finance 4 post-doctoral positions and 2 PhD students at MPIA over the coming five years. In a project Pillepich has called COSMIC-KEY, she and her colleagues will be developing the next generation of simulations of the universe as a whole – a key to understanding the observational data that is set to become available over the next years.

Our understanding of the history of our universe is based on models of an expanding cosmos. At their core, these cosmological models are based on Einstein's general theory of relativity, and they describe the framework for the history of our universe in terms of comparatively few parameters. Such parameters include the mean matter density in our cosmos and the mean density of the so-called dark energy that makes cosmic expansion accelerate. Another parameter measures how quickly the universe, which was initially almost perfectly homogeneous, evolved its first irregularities.

Determining the values of these parameters is one of the key tasks of modern cosmology. But this poses quite a challenge – the parameter values can only be deduced indirectly, from systematic observations of different kinds of cosmic objects. At present, the main limitation is "statistical error" – the fact that we have not yet been able to observe suitably many objects to obtain a representative picture. But with missions like the X-ray observatory eROSITA, or the upcoming space telescope Euclid, that is bound to change: In the near future, the main source of uncertainty will be that we do not know enough about the relationship between the fundamental parameters and the observational data.

This is where cosmological simulations play a key role. Such simulations amount to running "model universes”, and in programming such simulations, we encode our understanding of what is going on as the universe evolves: the physical laws by which dark matter clumps and forms voids, filaments and numerous halos under the influence of its own gravity, the laws by which gas flows, heats up or cools, and — on less elementary levels — the conditions under which stars are formed, emit light, and explode as supernovae, as well as models for the way that the central black holes in galaxies form and inject radiation, energy or momentum into the surrounding gas.

Once the billion-years-history of such a model has been simulated, we can find out what an observer in such a universe, with the same capabilities as those of today's observational astronomer, would see – and importantly, we can also find out how these virtual observations depend on the values of the various cosmological parameters. In turn, by comparing virtual observations for different cosmological parameter values with real data, we can deduce the values these cosmological parameters have in our own universe.

In the chain linking observational data, simulations and our physical understanding of the inner workings of the Universe as a whole, up to now, the available observational data was the "weakest link". In the near future, that role is set to shift to the theoretical models, namely to numerical simulations. Annalisa Pillepich says: "We measure cosmological parameters by comparing observations with theoretical predictions. That is why the precision and especially the accuracy of the cosmological inferences are only as good as the model predictions adopted to fit the data!" Strengthening that link, and taking cosmological simulations to the next level, is the aim of the COSMIC-KEY project for which Pillepich has now obtained her ERC grant.

Pillepich is in a unique position to tackle that challenging task. After studying physics at the Università di Pisa, Italy, she completed her PhD in cosmology at the ETH Zurich. After post-doctoral positions at the University of California in Santa Cruz and at Harvard University, she joined MPIA as an independent research group leader in 2016, and recently made the transition to a permanent research group leader and staff member in MPIA's Galaxies and Cosmology Department. Over the course of her career as a numerical astrophysicist and cosmologist, she has worked on the development and analysis of supercomputer simulations of galaxies in large representative portions of virtual universes. She is one of the lead developers of the IllustrisTNG simulations (, the most comprehensive and detailed representative "model universe" available at this time.

The goal of COSMIC-KEY is to create the most realistic simulations yet of galaxy formation and its consequences for cosmic evolution and cosmic structures. This includes the influence of the central black holes not only on the rate of star formation in galaxies but, crucially, on the physical properties of the gas that surrounds galaxies and permeates dark-matter halos. These models will be based on fundamental physics, notably the equations of hydrodynamics, and will require the development of novel and more efficient ways of performing the underlying calculations, and of translating the physics of galaxy formation into numerical code. The comparison of model versions with different cosmological parameter values with the available observational data will focus on the information from measuring the X-ray and Sunyaev-Zel’dovich signatures of massive groups and clusters of galaxies – information of the kind that eROSITA and other observatories can supply.

The European Research Council (ERC) was set up by the European Union in 2007. It is the premier European funding organisation for excellent frontier research, and funds creative researchers of any nationality and age to run projects based across Europe. Pillepich's consolidator grant consists of total funding of 2 million Euros. It will be used to fund four post-doctoral positions and two positions for PhD students, all of them based at MPIA, over the coming five years.

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