The Universe in a box:
how do galaxies form?

Dr. Corentin Cadiou ­— Lund University

Credits: NASA & ESA

Sombrero galaxy

M83

“Penguin” galaxy

Magellanic clouds

M87 galaxy

What is a galaxy?

Credits: A. Russell/ESO

What is a galaxy?

Credits: A. Russell/ESO

  • Hundreds of billions of stars*
  • Dust
  • Gas
  • Dark matter
  • Supermassive black holes
  • Planets with humans on them!

* only true for Milky Way, may vary by orders of magnitude

Dust in the horsehead nebula, credits: HST

Star-forming region (Westerlund 2), credits: HST

SMBH in galaxy M87, credit: EHT

DM (in blue) in a simulation

Credits: Adams Evans

Observations in astronomy

Credits: Adams Evans

Observations in astronomy

Three key issues

  1. Distances are hard to measure

Credits: Adams Evans

Observations in astronomy

Three key issues

  1. Distances are hard to measure
  2. Times — seemingly no evolution

Credits: Adams Evans

Observations in astronomy

Three key issues

  1. Distances are hard to measure
  2. Times — seemingly no evolution

Today

Credits: Adams Evans

Observations in astronomy

thousand of kilometers

billions of billions  kilometers

Three key issues

  1. Distances are hard to measure
  2. Times — seemingly no evolution
  3. Maaaany scales involved!

a few meters

NASA; ESA; and F. Summers

We use simulations like experiments

example: what will happen between the Vintergatan and the Andromeda galaxies?

Andromeda

NASA; ESA; and F. Summers

We use simulations like experiments

Milky Way (Vintergatan)

NASA; ESA; and F. Summers

... but how do we know?

© Ventusky

“If we have clouds and there is wind, the clouds move in the direction of the wind”

“If there is water and it is warm enough, clouds form”, …

Each bright dot is one galaxy

?

HST

  1. Create a box for space and time
  2. Include known physics
  3. Set some initial conditions
  4. Move time forward
  5. ☕*
  6. SCIENCE!
    • Compare to observations
    • Make predictions
    • …

How to simulate the Universe?

... and galaxies therein

* of the order of \( 10^6-10^7\ \mathrm{hr} \approx 100-1000\ \mathrm{yr} \)

  1. Create a box for space and time
  2. Include known physics
  3. Set some initial conditions
  4. Move time forward
  5. ☕*
  6. SCIENCE!
    • Compare to observations
    • Make predictions
    • …

How to simulate the Universe?

... and galaxies therein

* of the order of \( 10^6-10^7\ \mathrm{hr} \approx 100-1000\ \mathrm{yr} \)

  1. Create a box for space and time
  2. Include known physics
  3. Set some initial conditions
  4. Move time forward
  5. ☕
  6. SCIENCE !
    • Compare to observations
    • Make predictions
    • …

How to simulate the Universe?

... and galaxies therein

What recipes do we need?

The universe is expanding!

The universe is expanding!

Dark matter

\(80\%\) dark matter

\(20\%\) “gas”

<\(1\%\) light and other stuff

✅

Dark matter

Gas

\(80\%\) dark matter

\(20\%\) “gas”

<\(1\%\) light and other stuff

✅

✅

Gas

Stars

Dark matter

\(80\%\) dark matter

\(20\%\) “gas”

<\(1\%\) light and other stuff

✅

✅

✅

Gas

Stars

Dark matter

\(80\%\) dark matter

\(20\%\) “gas”

<\(1\%\) light, neutrinos, black holes, …

✅

✅

✅

We have a scenario for galaxy formation

We have a scenario for galaxy formation

  1. Formation of dark matter "halos"...
  2. ... and of the cosmic web
  3. Gas falls in, galaxies merge
  4. Gas condenses to form stars & black holes
  5. Stars & black holes expel gas

3C 348 seen by HST & VLA

Crab nebula seen by HST

Forming an elliptical galaxy...

Forming a spiral galaxy...

We can simulate galaxies on (super)computers

→ we can watch them in 3D!

→ we don't have to wait millions of years!

We can simulate galaxies on (super)computers

→ we can watch them in 3D!

→ we don't have to wait millions of years!

climate change

tens

We can simulate galaxies on (super)computers

→ we can watch them in 3D!

→ we don't have to wait millions of years!

climate change

tens

Resources (På engelska, förlåt!)

Lund's astronomy podcast: TheMeridian

Youtube channels:
@DrBecky, @CoolWorldsLab, @kurzgesagt

Tack för att du lyssna!

Same initial conditions
Same “physical model”

They will diverge eventually (simulations running at different paces + numerical errors)

3 parameters:

  • coherence
  • separation
  • alignment

Same initial conditions
Different parameters

Same “Universe”

Different physical models

We have a scenario for galaxy formation

Can we test it?

Effect of physical parameters

Formation of stars

Formation of black holes

Feedback effects

[...]

Effect of physical parameters

Dubois+16

Black holes prevent the formation

of large spiral galaxies

Effect of physical parameters

Dubois+16

Different initial conditions
Same physical parameters

Different “Universe”

Same physical models

The initial conditions of the Universe

Our whole universe was in a hot, dense state*
Then nearly fourteen billion years ago expansion started, wait […]

*(and homogenous)

The initial conditions of the Universe

Planck satellite. Credits: ESA/NASA/JPL-Caltech

The initial conditions of the Universe

The initial conditions of the Universe

Initial conditions

Evolved Universe (+ galaxies)

The initial conditions of the Universe

Evolved Universe (+ galaxies)

The Universe is determined

by its initial conditions

Initial conditions = DNA of galaxies
Genetically modified galaxies?
 

\(14\ \mathrm{Gyr}\)

Adaptive Mesh Refinment

Domain decomposition

The initial conditions of the Universe

Initial conditions:

  • Tiny density fluctuations \(\sim  0.001\% \)
  • Very well described mathematically*
    → easy to generate

*By a Gaussian random field with known spectrum

The initial conditions of the Universe

Initial conditions:

  • Tiny density fluctuations \(\sim  0.001\% \)
  • Very well described mathematically*
    → easy to generate & constrain

"Splicing" method, Cadiou+ in prep.

The initial conditions of the Universe

"Splicing" method, Cadiou+ in prep.

\(14\ \mathrm{Gyr}\)

The initial conditions of the Universe

Galaxies are influenced by a region at least \( 100\times\) larger

"Splicing" method, Cadiou+ in prep.

Galaxies are influenced by a region at least \( 100\times\) larger

"Splicing" method, Cadiou+ in prep.

\(100\ \mathrm{kly}\)

\(10 \ \mathrm{Mly}\)

Forming galaxies...

New Horizon simulation — IAP, CNRS

50 million light year

50 million light year

Galaxies

  1. form in a "cosmic web"
  2. grow by accreting
  3. merge together

1 million light year

100,000 light year