The Universe in a box

Understanding the emergence of galaxy properties through numerical simulations

Dr. Corentin Cadiou

University College London

@cphyc

@corentin-cadiou

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
  • Cosmic rays, planets, …

* 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 — no parallax

Credits: Adams Evans

Observations in astronomy

Three key issues

  1. Distances — no parallax
  2. Times — seemingly no evolution

Credits: Adams Evans

Observations in astronomy

Three key issues

  1. Distances — no parallax
  2. Times — seemingly no evolution

Andromeda in 1899 by  Isaac Roberts

Credits: Adams Evans

Observations in astronomy

Three key issues

  1. Distances — no parallax
  2. Times — seemingly no evolution
  3. Scale coupling

mm

km

10,000 km

Credits: Adams Evans

Observations in astronomy

Three key issues

  1. Distances — no parallax
  2. Times — seemingly no evolution
  3. Scale coupling

km

10,000 km

\(10^{-3} \ \mathrm{ly} \)

Credits: Adams Evans

Observations in astronomy

Three key issues

  1. Distances — no parallax
  2. Times — seemingly no evolution
  3. Scale coupling

\(10\ \mathrm{ly}\)

10,000 km

\(10^{-3} \ \mathrm{ly} \)

Credits: Adams Evans

Observations in astronomy

Three key issues

  1. Distances — no parallax
  2. Times — seemingly no evolution
  3. Scale coupling

\(10\ \mathrm{ly}\)

\(100,000 \ \mathrm{ly}\)

\(10^{-3} \ \mathrm{ly} \)

NASA; ESA; and F. Summers

Simulations are proxy for experiments

Merging of the Milky Way (our own galaxy)
with Andromeda [in a few billion years]

NASA; ESA; and F. Summers

Simulations are proxy for experiments

NASA; ESA; and F. Summers

How to simulate the Universe?

... and galaxies therein

Energy content of the Universe

We need to simulate an expanding Universe

Energy content of the Universe

Energy content of the Universe

We need to simulate an expanding Universe

Energy content of the Universe

We need to simulate an expanding Universe

Matter content of the Universe

✅ We need to simulate an expanding Universe

\(80\%\) dark matter

\(20\%\) “ordinary matter”

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

Dark matter

\(80\%\) dark matter

\(20\%\) “ordinary matter”

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

✅

Dark matter

Gas

\(80\%\) dark matter

\(20\%\) “ordinary matter”

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

✅

✅

Gas

Stars

Dark matter

\(80\%\) dark matter

\(20\%\) “ordinary matter”

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

✅

✅

✅

Gas

Stars

Dark matter

\(80\%\) dark matter

\(20\%\) “ordinary matter”

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

✅

✅

✅

Gas

Stars

Dark matter

\(80\%\) dark matter

\(20\%\) “ordinary matter”

<\(1\%\) 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

We have a scenario for galaxy formation

Can we test it?

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

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?
 

\(14\ \mathrm{Gyr}\)

We can simulate galaxies on (super)computers

→ insight on actual galaxies

→ better understanding of physics

→ testbench of astrophysics

Questions?

Forming an elliptical galaxy...

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.

\(100\ \mathrm{kly}\)

\(10 \ \mathrm{Mly}\)

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

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