Cosmology and Dark Matter IV: Problems with our current picture

1. Cosmology and Dark Matter IV: Problems with our current picture Jerry Sellwood

2. The story so far • Once the universe becomes neutral, dark matter halos start to form • Simulations show a clustering hierarchy of DM halos that resembles the distribution of galaxies • Galaxies form inside DM halos as gas cools, settles to a disk, and makes stars • Do the properties of the predicted galaxies match up with observation?

3. Serious problem #1 • Predicted galaxy rotation curves have the wrong shape • Too much mass in the “bulge” • Gas has too little angular momentum • Also never form bulgeless galaxies, which are common in nature

4. Serious problem #2 • Dark matter halos have too much substructure • Why is there not a small galaxy inside every clump? • May be able to explain them away by re-ionization

5. Serious problem #3 • Dark matter halos are not as dense as predicted (Alam et al) • v/2 is the mean density inside the radius at which rotn curve reaches vmax/2 • Points are estimates from real galaxies • Dashed curves are from standard CDM

6. Serious problem #3 (cont’d) • Better data are in worse agreement • Weiner’s work

7. Serious problem #3 (cont’d) • Weiner’s work gets around uncertainty in M/L • Better data are in worse agreement • Halos are under-dense by more than one order of magnitude • Plenty of work for SALT

8. Serious problem #4 • There is a formula that predicts rotation curves from the baryons only with no dark matter

9. Serious problem #4 (cont’d) • Formula is MOND from Milgrom • Postulates a departure from Newtonian gravity in very weak fields g(|g|/a0) = gn • Stronger forces when |g|  a0 (10-8 cm s-2) – a new constant of nature • Ad hoc, but not been shot down in >20years! • If DM exists, it is very hard to understand why the formula works so well

10. Serious problem #5 • Tully-Fisher relation does not depend on surface brightness • Data from Zwaan et al • Incredibly severe fine-tuning problem

11. Evidence for dark matter • Could come soon from any one of 3 on-going experiments • WMAP • Dark matter would be indicated if 3rd peak in final data is higher than 2nd

12. Evidence for dark matter • Could come soon from any one of 3 on-going experiments • WMAP • Direct detection in laboratory experiments • CDMS team in underground mine • Only upper limits so far

13. Evidence for dark matter • Could come soon from any one of 3 on-going experiments • WMAP • Direct detection in laboratory experiments • -rays form dark matter annihilations • EGRET data – very weak • GLAST will be better

14. What is Dark Energy? • The cosmological constant is the energy density of vacuum particle + antiparticle ↔ radiation • Heisenberg uncertainty principle energy uncertainty × duration > h (Planck’s const)

15. What is Dark Energy? • The cosmological constant is an energy density of vacuum particle + antiparticle ↔ radiation • Heisenberg uncertainty principle energy uncertainty × duration > h (Planck’s const) • Quantum fluctuations in vacuum • Energy of them detected experimentally • Casimir effect

16. Expected energy of vacuum • Know protons, electrons, neutrinos, quarks, gluons, etc. all have anti-particles • Count up all contributions to vacuum energy density • Result is huge – 120 orders of magnitude larger than observed! • Physicists have no idea why • First major headache

17. Second headache • Why is dark energy about 70% of the critical density? • Almost 0% or almost 100% expected at most times • We live at a special time in the history of the universe • anti-Copernican

18. Our Preposterous Universe • Our model for the universe is now very ugly • 70% dark energy • 25% dark matter • 4% normal atoms • < 2% neutrinos (may be much less) • No natural explanation why they should all contribute so significantly • Our only evidence so far for the two dark components is gravitational • could another modification to gravity, for ultra-weak fields, make them both go away?

19. Conclusions • Cosmology has come a long way in the past 30 years • But we still have plenty of unsolved problems!

20. Generalized Dark Energy • Einstein’s cosmological constant has a single, fixed value • Can consider dark energy that varies in time and space – coined Quintessence • Still an energy density that has repulsive gravity and negative pressure • Equation of state: pressure = w × energy density • w = 0 for cold matter • w = +⅓ for radiation • w = –1 for cosmological constant • –1 < w < 0 for quintessence