Computing the Universe: Simulating Cosmology and Galaxy Formation

1
Computing the Universe Simulating Cosmology and
Galaxy Formation

• Prof. Romeel Davé
• University of Arizona

2
Overview

• Meet Our Universe.
• Quantum Genesis.
• Cosmos on a Computer.
• The Dark Side.
• Baryons Are Cool.

3
Overview

• Meet Our Universe.
• Quantum Genesis.
• Cosmos on a Computer.
• The Dark Side.
• Baryons Are Cool.

4
Overview

• Meet Our Universe.
• Quantum Genesis.
• Cosmos on a Computer.
• The Dark Side.
• Baryons Are Cool.

5
Overview

• Meet Our Universe.
• Quantum Genesis.
• Cosmos on a Computer.
• The Dark Side.
• Baryons Are Cool.

6
Overview

• Meet Our Universe.
• Quantum Genesis.
• Cosmos on a Computer.
• The Dark Side.
• Baryons Are Cool.

7
Meet Our Universe

• We live in an accelerating, dark matter
  dominated, inflationary, Big Bang universe.

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Big Bang

• Universe began 13.8 billion years ago, in a Big
  Bang of super-hot super-dense plasma.
• Our laws of physics cannot extrapolate prior to
  10-43s after the Bang we do not have a Grand
  Unified Theory.
• Space itself expands, so that objects farther
  away are receding at a faster rate (Hubbles
  Law).
• In small regions, expansion is halted (and
  reversed) where bound by gravity (e.g on Earth,
  or in our Galaxy).

9
Big Bang

• Universe began 13.8 billion years ago, in a Big
  Bang of super-hot super-dense plasma.
• Our laws of physics cannot extrapolate prior to
  10-43s after the Bang we do not have a Grand
  Unified Theory.
• Space itself expands, so that objects farther
  away are receding at a faster rate (Hubbles
  Law).
• In small regions, expansion is halted (and
  reversed) where bound by gravity (e.g on Earth,
  or in our Galaxy).

10
Big Bang

• Universe began 13.8 billion years ago, in a Big
  Bang of super-hot super-dense plasma.
• Our laws of physics cannot extrapolate prior to
  10-43s after the Bang we do not have a Grand
  Unified Theory.
• Space itself expands, so that objects farther
  away are receding at a faster rate (Hubbles
  Law).
• In small regions, expansion is halted (and
  reversed) where bound by gravity (e.g on Earth,
  or in our Galaxy).

11
Big Bang

• Universe began 13.8 billion years ago, in a Big
  Bang of super-hot super-dense plasma.
• Our laws of physics cannot extrapolate prior to
  10-43s after the Bang we do not have a Grand
  Unified Theory.
• Space itself expands, so that objects farther
  away are receding at a faster rate (Hubbles
  Law).
• In small regions, expansion is halted (and
  reversed) where bound by gravity (e.g on Earth,
  or in our Galaxy).

If the Universe is expanding, why cant I ever
find a parking space?
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Inflation

• A fraction of a second after the Big Bang, the
  Universe suddenly inflated by x1060 in a fraction
  of a second. It then resumed normal expansion.
• Quantum vacuum fluctuations were frozen in.
• We see these fluctuations today as Cosmic
  Microwave Background (CMB) Radiation.

?
Vacuum is not empty!
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Inflation

• Less than a second after the Big Bang, the
  Universe suddenly inflated by x1060 in a fraction
  of a second. It then resumed normal expansion.
• Quantum vacuum fluctuations were frozen in.
• We see these fluctuations today as Cosmic
  Microwave Background (CMB) Radiation.

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Inflation

• Less than a second after the Big Bang, the
  Universe suddenly inflated by x1060 in a fraction
  of a second. It then resumed normal expansion.
• Quantum vacuum fluctuations were frozen in.
• We see these fluctuations today as Cosmic
  Microwave Background (CMB) Radiation.

Full-sky WMAP satellite data. Typical
fluctuation 1 part in 106
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Dark Matter

• We are made of ordinary matter, or what
  Astronomers call baryonic matter. Baryonic
  matter interacts with electromagnetic radiation
  (light), via emission, reflection, refraction, or
  absorption.
• But we are in the minority 90 of the Universes
  mass is non-baryonic dark matter that only
  interacts via gravity.

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Accelerating Universe

• Recently, astronomers have observed that our
  Universe is not just expanding, but accelerating!
• The Universe contains a vacuum energy, causing
  space to have pressure that drives acceleration.

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Quantum Genesis

• How did we get from this (smoooooth)

The Universe at z1189
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Quantum Genesis

• to this (chunky)?

The Universe today (z0) HST GOODS Survey data
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Cosmos on a Computer

• Structure formation is highly nonlinear. Our
  solar systems overdensity is 108!
• Easiest way to model is to numerically follow
  growth of perturbations into nonlinearity.
• Model random sub-volume of Universe using many
  particles, each representing a bit of mass
  (typically millions/billions of Suns!).
• Simulate Compute forces on particles from
  gravity, pressure advance particles velocities
  and positions repeat until end!

MCR Cluster at Livermore
Fi mi S(agravahydro)
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Cosmos on a Computer

• Structure formation is highly nonlinear. Our
  solar systems overdensity is 108!
• Easiest way to model is to numerically follow
  growth of perturbations into nonlinearity.
• Model random sub-volume of Universe using many
  particles, each representing a bit of mass
  (typically millions/billions of Suns!).
• Simulate Compute forces on particles from
  gravity, pressure advance particles velocities
  and positions repeat until end!

MCR Cluster at Livermore
vnew vold aDt
21
Cosmos on a Computer

• Structure formation is highly nonlinear. Our
  solar systems overdensity is 108!
• Easiest way to model is to numerically follow
  growth of perturbations into nonlinearity.
• Model random sub-volume of Universe using many
  particles, each representing a bit of mass
  (typically millions/billions of Suns!).
• Simulate Compute forces on particles from
  gravity, pressure advance particles velocities
  and positions repeat until end!

MCR Cluster at Livermore
rnew rold vDt
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The Dark SideThe Growth of Structure

• On largest scales, the Universe evolution is
  governed by dark matter and gravitational forces.
• Fluctuations grow via gravitational instability
  Dense regions attract more matter,
  becoming more dense, and so on (the rich get
  richer).
• Simulation shown contains 4 million particles,
  each one about 1010M?.

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The Dark SideHalting the Collapse

• So what stops the runaway collapse?
• Dark matter must conserve energy and angular
  momentum (it cannot dissipate!)
• Torques are generated by tidal forces.
• Gravitational instability is halted by dynamical
  pressure.

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The Dark SideHierarchical Structure Formation

• Matter collapses into pancakes, then onto
  filaments, creating a Cosmic Web.
• At the intersection of filaments, matter breaks
  away from Hubble expansion and collapses back on
  itself, forming a galaxy.

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Galaxy Redshift Surveys

• The hardest problem in astronomy What is the
  distance to an observed object?
• Can obtain a good estimate using Hubbles Law
  (vrH0d) If we measure recession velocity, we
  know distance!
• Recession results in characteristic atomic
  emissions (e.g. HI Lya) being redshifted (i.e.
  Doppler shifted) to longer wavelengths.
• Redshift surveys are thus used to map the galaxy
  distribution.
• Examples CfA1 (original stick man, 1986), 2dF
  (2002), Sloan Digital Sky Survey (2003).

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Observed Large-Scale Structure

• Galaxy redshift surveys observe the Cosmic Web!
• 2dF survey detects most galaxies out to 1000
  Mpc, z0.3, 3 Gyr in lookback time, to bJlt19.45.
  Total of 250,000 galaxies in 2000 deg2.

2dF Redshift Survey 250,000 galaxies
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Baryons Are Cool

• Thats nice, but it still doesnt look much like
  this

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Simulating Forming Galaxies

• Baryons can radiate , i.e. convert their
  potential energy into light, and thus achieve
  super-high densities necessary to form galaxies
  (106), stars (108), planets (109), etc.

Simulating Dark Matter Gravity Gm1m2 / r2
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Simulating Forming Galaxies

• Baryons can radiate , i.e. convert their
  potential energy into light, and thus achieve
  super-high densities necessary to form galaxies
  (108), stars (1010), planets (1011), etc.

Simulating Dark Matter Gravity Gm1m2 / r2
Simulating Baryons Gravity Gm1m2 / r2 Pressure
-?P/r Shocks Viscosity Cooling
L(r,T) Photoionization Jn(r,T,r) Heuristic star
formation Supernova feedback/winds Heavy element
production Magnetic fields Kitchen sink
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Forming Spiral Galaxies

• Dark matter is forced to stay in a halo because
  it cannot dissipate its gravitational potential
  energy by emitting light (marble in a basin).
• Baryons are dissipative, so they collapse down to
  center of halo. But they still have angular
  momentum, so the centrifugal force results in a
  spiral disk.

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A Galaxys Life

• Most galaxies dont have such a fortunate,
  peaceful existence instead undergoing mergers
  and harrassment.
• Generically, interactions tend to drive gas
  stars to the center, forming a more concentrated
  galaxy.

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Morphological Transformation
The Antennae Galaxy (HST)

• Mergers of galaxies can change the morphology of
  galaxies, turning spirals into ellipticals.
• Minor mergers make disks smaller and thicker.
• The Antennae galaxy is a merger caught in the
  act.
• Interactions can move galaxies along the Hubble
  Sequence.

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Summary

• We live in a accelerating, dark matter-dominated,
  inflationary Big Bang Universe. The quantum
  fluctuations from this early epoch are the seeds
  of galaxies, stars, and planets.
• The growth of structure can be modeled using
  high-performance supercomputers, allowing
  theories to be tested and new phenomena to be
  elucidated.
• Growth of large-scale structure is dominated by
  dark matter driven by gravitational instability.
• Galaxies form from dissipative baryons within
  halos of dark matter.
• Once galaxies are born, many processes can change
  their morphology and color.
• Understanding all these processes within a
  cosmological framework remains a great unsolved
  challenge for astronomers today.

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Galaxy Rotation Curves A Dark Matter Indicator
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The Constituents of Our Universe

• Densities are measured in terms of the critical
  density, i.e. the amount of mass-energy it would
  take to just halt the expansion due to
  self-gravity.
• Matter density Wm0.27
• Baryonic matter density Wb0.04
• Vacuum energy density WL0.73
• Hubble constant (today) H072 km/s/Mpc

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Meet Our Accelerating Universe

• Recently, astronomers have observed that our
  Universe is not just expanding, but accelerating!
• The Universe contains a vacuum energy, causing
  space to have pressure that drives acceleration.