Self- interacting Dark Matter 2.0:

1. Self-interacting Dark Matter 2.0: Back and Better Than Ever! Annika Peter McCue Fellow uCirvine M. Rocha, AP+ 1204.XXXX AP+ 1204.XXXX

2. The Universe as a cupcake Baryons: ~4% Dark matter: ~23% Image credits: NASA/JPL; NASA, Jeff Hester, and Paul Scowen (Arizona State University); NASA, ESA, S. Beckwith (STScI), and the HUDF team Dark energy: ~72% ???

3. The cold dark matter orthodoxy • “Born cold”. • Late-time behavior: collisionless and boring. overdensity r Millennium simulation Image credits: M. Blanton and the SDSS

4. Small-scale issues (circa 2000) Missing satellites problem (Moore et al., Klypin et al. 1999) Dwarf core problem (Kuzio de Naray et al. 2008)

5. Self-interacting dark matter (SIDM) Elastic scattering---need cross section ~1 cm2/g (>1012 times stronger than weak force) to be interesting. Original formulation (Spergel & Steinhardt 2000): hard-sphere elastic scattering. In vogue now: on particle side (hidden-sector models, Sommerfeld-enhanced dark matter)---generally velocity-dependent.

6. Phenomenology Looks exactly like CDM on large scales: 10 Mpc/h slice, CDM 10 Mpc/h slice, σ/m = 1 cm2 /g

7. Phenomenology • Generic predictions when : • Rounder halo in inner parts. • Cored (less dense) halo density profiles. • Fewer satellites close to the center. SIDM CDM

8. Version 1.0 nail in the coffin Miralda-Escude (2002) Requires a non-circularly-symmetric surface density at r > 70 kpc. Assume ε=0 if . σ/m < 0.02 cm2/g. MS 2137-23 Sand et al. 2008 Tightest constraint by far (by > 10x)!

9. The problem with shapes • We see surface density (or gravitational potentials) in projection. • If inner parts have flattened density, outer parts have even greater weight. σ/m=1 cm2/g CDM

10. SIDM 2.0: It’s back! σ/m=1 cm2/g allowed!

11. Density profile

12. Cores Milky Way dwarfs Milky Way 1015 M clusters Dwarf galaxies ~hundred pc ~kpc

13. Observations Galaxy cluster densities ρ ~ r-β Richard Ellis and co. (Newman et al. 2011) “Too big to fail” Milky Way dwarfs (Boylan-Kolchin et al. 2011) Dwarf core problem (Kuzio de Naray et al. 2008) Need cores in ~100 kpc in 1015 M halos Need less DM in ~100 pc in 109-1010 M halos Need cores in ~1 kpc in 1011 M halos

14. Cores! Milky Way dwarfs Milky Way 1015 M clusters Dwarf galaxies ~hundred pc ~kpc

15. Subhalos

16. Takeaway points • “Vanilla” SIDM is far from dead! • Moreover, clinging to one particular unproven model (cough, cough, CDM) may be dangerous. Try to constrain general phenomenology! With at least a modicum of rigor! • A reanalysis of the old constraints shows σ/m=1 cm2/g OK! (AKA, do not believe everything you read) • Suggestive core sizes! • Cross sections that give interesting-sized cores do NOT substantially reduce subhalo mass function. • Clusters remain an interesting environment for constraints.