1Hot & Cold Dark Matter• Dark Matter = matter not coupled to electromagnetic field– unable to condense by dissipation.– growth of structure depends on properties of Dark Matter.• Hot Dark Matter (HDM) – relativistic for T ~105K– can “free-stream” out of galaxy-sized matter concentrations. – erases small-scale structures early in life of universe. top-down structure formation, starting from structures >1013Msun.• Cold Dark Matter (CDM)– slow moving (non-relativistic)– does not erase small concentrations.– preponderance of low-mass structures predicted by inflation survive. lower mass concentrations form first (bottom up structure formation).Early U. contained more small condensations than large ones. N-body simulations  CDM• Start with perturbation spectrum at time of decoupling • Follow perturbations into highly non-linear regime.• HDM models become too highly clustered over observed lifetime of galaxiesCDM HDMStandard CDM = SDCM,replaced by CDM model2Cold Dark Matter (CDM)• slow moving• mass power spectrum only slightly modified by free-streaming• less massive concentrations form first (bottom up).CDM candidates• Axions– zero momentum– very light ==> huge number density needed to make up WM– should be detectable within a few years if present.•WIMPs– Weakly Interacting Massive Particles– 50x proton mass• set by the weak interaction cross-section• Leftovers from GUT era– Expansion, cooling of U frozen out of equilibrium reactions– Lots of theories  lots of candidatesFig. 30.2Grand Unified TheoryCDMMenu of the DayAxionsAxinosGravatinosNeutralinosWimpzillas...Cold Dark Matter in the LabCDM candidates• Axions– zero momentum– very light ==> huge number density needed to make up M•WIMPs– Weakly Interacting Massive Particles– 50x proton mass• set by the weak interaction cross-section–  neutralino is best candidate• Can be detected through elastic scattering off various target nuclei– measure recoil energy imparted to target– look for seasonal variation due to Earth’s orbital motion– these WIMPS are the MW halo– Massive neutrinos (m ~ 100-1000 mp) already ruled out.• Hope is to identify CDM, then manufacture it in Large Hadron ColliderCDMMenu of the DayAxionsAxinosGravatinosNeutralinosWimpzillas...3Log MJeansLog time [CO Fig. 30.7]CollapsesCollapsesOscillates4100001.00001.10000.1WMAPDensity fluctuations at t = 379,000 yr1Current large-scale structuret = t0= 13.7 GyrEarly U. contained condensations of many different sizes. The Growth of StructureRead [CO 30.2]The Simplest Picture of Galaxy Formation and Why It Fails(chapter title from Longair, “Galaxy Formation”)Will a condensation collapse?The Jeans criterion:(see [CO Sect. 12.2 and pg. 1250] Jeans mass MJ2533 GMmMkTH2/12/3343534ooHmGkTMUnstable to collapse if2K < -ULog MJeansLog time [CO Fig. 30.7]CollapsesCollapsesOscillatesMass = MTemp = TDensity = o2Pressure < Gravity4Mass = MTemp = TDensity = o2How fast will it collapse?In a static medium (e.g. star formation):Perturbation analysis showsM < MJ/ exp(-ir/-it )  OscillationsM > MJ/ exp(-ir/+ Kt )  Exponential growthRadiation era:40)( tR2/1)( ttR 2/3Matter era:30)( tR3/2)( ttR In an expanding medium (e.g. the universe):PerturbationExpanding U.density = See [CO pg. 1249]Outside the perturbation (flat universe):Inside the perturbation (closed mini-universe):(Friedman Eqn)Rad.Mattertt2/3log tlog SlowGrowth• Cosmic Microwave Background is smooth to a few parts in 105/~ 10-4• Yet high contrast structures (QSOs, galaxies) by z ~ 6./>> 1• Adiabatic perturbations grow as / t 2/3  R(t)  1/(1+z)• Expect onlyBlue = 0oKRed = 4oKBlue = 2.724oKRed = 2.732oKDipole Anistropy~ 1 part in 300After removing dipoleRed – blue = 0.0002oK~ 1 part in 105The Simplest Picture of Galaxy Formation and Why It Fails01.010711001)1(4CMBQSOCMBQSOzzSo where did galaxies and clusters come from?5Log MJeans,bLog time [CO Fig. 30.7]CollapsesCollapsesOscillatesIn an expanding universe, will a cloud collapse?The Jeans criterion Version 2:3,4433)()(3TMTtRTtRcvbJTbs2/3,0035TMTTmkTvbJbTbHsRadiation era After decouplingTsGv4522TTTsTsTGGGMvGMvM23222543/4535353212Collapse if 2K < -U= [CO eq. 30.27]2/33,3/2.43TsbbJbbGvconstMMDecouplingRadiation pressure keeps these clouds fluffed up.Radiation pressure has disappeared. Clouds now collapse.2K < -U Pressure support < gravityLog MJeans,bLog time [CO Fig. 30.7]CollapsesCollapsesOscillatesIn an expanding universe, will a cloud collapse?DecouplingRadiation pressure keeps these clouds fluffed up.Radiation pressure has disappeared. Clouds now collapse.2K < -U Pressure support < gravityWMAP image • Snapshot of oscillating condensations on all size scales.• Taken at the moment of decoupling.• Brightness 