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).
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