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.

8. 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 (Hubble’s Law). • In small regions, expansion is halted (and reversed) where boundby gravity (e.g on Earth, or in our Galaxy).

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

10. Big Bang • Universe began 13.8 billion years ago, in a “Big Bang” ofsuper-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 (Hubble’s Law). • In small regions, expansion is halted (and reversed) where boundby gravity (e.g on Earth, or in our Galaxy).

11. Big Bang • Universe began 13.8 billion years ago, in a “Big Bang” ofsuper-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 (Hubble’s Law). • In small regions, expansion is halted (and reversed) whereboundby gravity(e.g on Earth, or in our Galaxy). “If the Universe is expanding, why can’t I ever find a parking space?

12. Inflation • A fraction of a second after the Big Bang, the Universe suddenly inflated byx1060in 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!

13. Inflation • Less than a second after the Big Bang, the Universe suddenly inflated byx1060in 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.

14. 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 asCosmic Microwave Background (CMB) Radiation. Full-sky WMAP satellite data. Typical fluctuation: 1 part in 106

15. 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 Universe’s mass is non-baryonic dark matter that only interacts via gravity.

16. 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.

17. Quantum Genesis • How did we get from this (smoooooth)… The Universe at z=1189

18. Quantum Genesis • …to this (chunky)? The Universe today (z~0): HST GOODS Survey data

19. Cosmos on a Computer • Structure formation is highly nonlinear. Our solar system’s 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 = miS(agrav+ahydro)

20. Cosmos on a Computer • Structure formation is highly nonlinear. Our solar system’s 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 system’s 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

22. The Dark Side:The 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.

23. The Dark Side:Halting 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.

24. The Dark Side:Hierarchical 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.

25. Galaxy Redshift Surveys • The hardest problem in astronomy: What is the distance to an observed object? • Can obtain a good estimate using Hubble’s Law (vr=H0d): 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).

26. Observed Large-Scale Structure • Galaxy redshift surveys observe the Cosmic Web! • 2dF survey detects most galaxies out to ~1000 Mpc, z~0.3, ~3 Gyr in lookback time, to bJ<19.45. Total of 250,000 galaxies in 2000 deg2. 2dF Redshift Survey: 250,000 galaxies

27. Baryons Are Cool • That’s nice, but it still doesn’t look much like this:

28. 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 Forming Galaxies Simulating Dark Matter Gravity: Gm1m2 / r2

29. 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 Forming Galaxies 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

30. 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 spiraldisk.

31. A Galaxy’s Life • Most galaxies don’t 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.

32. The Antennae Galaxy (HST) Morphological Transformation • 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.

33. 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.

34. vrot Observed rotation curve Dark matter rotation curve inferred by subtraction Rotation curve from visible baryons (stars and gas) Our position in the Milky Way Radius Galaxy Rotation Curves: A Dark Matter Indicator

35. 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: Wm=0.27 • Baryonic matter density: Wb=0.04 • Vacuum energy density: WL=0.73 • Hubble constant (today): H0=72 km/s/Mpc

36. 24 h-1Mpc 6 h-1Mpc 1.5 h-1Mpc 375 h-1kpc

37. Radius Accelerating (L>0) Open (W<1) Critical (W=1) Closed (W>1) NOW time 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.