The universe in a computer | 9am PT, Tues March 10, 2026

[Grigory Bronevetsky]

Modeling Talks

The universe in a computer Joop Schaye, Leiden University Tues, March 10, 2026 | 9am PT Youtube Stream Hi all, The presentation will be via Meet and all questions will be addressed there. If you cannot attend live, the event will be recorded and can be found afterward at https://sites.google.com/modelingtalks.org/entry/the-universe-in-a-computer More information on previous and future talks: https://sites.google.com/modelingtalks.org/entry/home Abstract: About 14 billion years ago there were no galaxies. Since that time, the universe has expanded enormously, and a cosmic web of galaxies has developed. How did this happen? Why do galaxies exist in so many sizes and shapes? How do they regulate their growth? Questions like these cannot be answered using laboratory experiments, and the formation of galaxies proceeds too slowly to observe in real time. Computer simulations therefore play an important role in the interpretation of observations. I will discuss how simulations contribute to our understanding of the evolution of galaxies and the large-scale distribution of matter in the universe.   Bio: After obtaining his PhD from Cambridge University, Joop Schaye spent 4 years at the Institute for Advanced Study in Princeton as a long-term member before taking up a faculty position at Leiden University in 2005. Schaye works on simulations and observations of galaxies, the intergalactic medium, and large-scale structure. He has led influential, international simulation projects such as OWLS, EAGLE, FLAMINGO, and COLIBRE. Schaye was awarded the 2010 Pastoor Schmeits prize and the 2022 Royal Astronomical Society Group Award  (to the EAGLE team led by him). He is a member of the Royal Netherlands Academy of Arts and Sciences (KNAW).

[Grigory Bronevetsky]

Video Recording: https://youtube.com/live/_V95_JC2mpI

Slides: https://drive.google.com/file/d/1XvH7sVfC859OUYzYuiY86vIJnD7b96WS/view?usp=sharing

Summary:

• History of the universe

• Big Bang: rapid acceleration / stretching of space

• Slowing expansion era

• Today: accelerated expansion (last 2 Billion years)

• We can see the cosmic microwave background: 300k years after Big Bang

• Today’s cosmic distribution of galaxies originated 200m years after Big Bang

• Time period between cosmic microwave background and first stars is “Dark Ages”

• Cosmic microwave background radiation field fluctuates a little (average temperature is ~3K and fluctuation is <.3mK

• We think these small fluctuations led local differences in density

• These caused a run-away gravitational effect of mass accumulating in a few places

• Caused current distribution of galaxies and cosmic filaments

• Our cosmological models captures this distribution very accurately across many scales

• Universe’s energy distribution:

• Dark energy: 70%

• Dark matter: 25%

• Ordinary matter (us): 5%

• 90% intergalactic gas

• 10% stars

• OR

• 75% Hydrogen / 25% Helium (mostly created in the Big Bang) / <<1% all other elements

• Galaxy formation:

• Dark matter / intergalactic gas / primordial fluctuations

• Galaxy hallows

• Nebulae/start dynamics

• Major questions about structure of the universe

• Statistical properties of the primordial density fluctuations

• How does large-scale structure emerge?

• When and how do galaxies form?

• Galaxy shapes?

• How to regulate growth?

• When/how supermassive black holes form?

• Simulations are a major tool for this since we cannot create test universes in lab

• Test theories, validate against astronomical observations

• Run quickly

• Help design new experiments / observations

• Create informative visualizations

• Cosmological simulations evolve representative parts of the universe

• Create a cubic volume with periodic boundary conditions

• Equations to evolve motion of particles

• Particles represent chunks of matter

• Initial conditions constrained by observations

• Equations: (magneto)hydrodynamics, radiative transfer, chemical networks, gravity

• Subgrid models for finer-scale processes

• Wide range of length scales: 

• 10-8m: inter-particle distances in starts

• …

• 1011: stellar radii

• …

• 1028m: observable universe

• Galaxy formation process:

• Distribution of galaxy masses vs count of galaxies with this mass

• Uniform distribution for dark matter halos (straight line of mass vs count)

• Galaxy distribution is heavily bent with much fewer very large and small galaxies than dark matter halos would suggest

• Small: hard to form because individual supernovas blow out gas too far, killing off star formation

• Large: very large galaxies have supermassive black holes, which consume a lot of gas; this gas gets very hot and energetic and blows out of the galaxy, also killing off start formation

• Simulations

• FLAMINGO: https://flamingo.strw.leidenuniv.nl/simulations.html

• Very large (whole universe) but also coarse

• Galaxies are smaller than pixel size

• COLIBRE: https://colibre.strw.leidenuniv.nl

• Smaller volumes at a higher resolution <400cMPC

• Model bits of the cosmic web

• Capable of forming realistic galaxies

• 1 particle: 105 solar masses

• Challenges:

• Model finer-grained processes directly, not via sub-grid and model larger systems 

• Include radiation transport, magnetic fields

• Explore assumption space of dark matter and energy