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

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?

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

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

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 v max /2 Points are estimates from real galaxies Dashed curves are from standard  CDM

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

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

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

Serious problem #4 (cont’d) Formula is MOND from Milgrom Postulates a departure from Newtonian gravity in very weak fields g  (|g|/a 0 ) = g n Stronger forces when |g|  a 0 (  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

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

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

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

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

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)

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

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

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

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?

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

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