Hot & Cold Dark Matter

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1 Hot & Cold Dark atter Dark atter = matter not coupled to electromagnetic field unale to condense y dissipation growth of structure depends on properties of Dark atter Early U contained more small condensations than large ones Hot Dark atter (HD) relativistic for ~10 5 K 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 >10 1 sun Cold Dark atter (CD) slow moving (non-relativistic) does not erase small concentrations preponderance of low-mass structures predicted y inflation survive lower mass concentrations form first (ottom up structure formation) N-ody simulations CD Start with perturation spectrum at time of decoupling Follow perturations into highly non-linear regime Standard CD = SDC, replaced y CD model HD CD models ecome too highly HDclustered over oserved lifetime of galaxies 1

2 slow moving mass power spectrum only slightly modified y free-streaming less massive concentrations form first (ottom up) Cold Dark atter (CD) CD enu of the Day Axions Axinos Gravatinos Neutralinos Wimpzillas CD candidates Axions zero momentum very light ==> huge numer density needed to make up W should e detectale within a few years if present WIPs Weakly Interacting assive Particles 50x proton mass set y the weak interaction cross-section Grand Unified heory Leftovers from GU era Expansion, cooling of U frozen out of equilirium reactions Lots of theories lots of candidates Fig 0 Cold Dark atter in the La CD CD candidates enu of the Day Axions Axions zero momentum Axinos Gravatinos very light ==> huge numer Neutralinos density needed to make up Wimpzillas WIPs Weakly Interacting assive Particles 50x proton mass set y the weak interaction cross-section neutralino is est candidate Can e detected through elastic scattering off various target nuclei measure recoil energy imparted to target look for seasonal variation due to Earth s orital motion these WIPS are the W halo assive neutrinos (m ~ m p ) already ruled out Hope is to identify CD, then manufacture it in Large Hadron Collider

3 WAP Density fluctuations at t = 79,000 yr he Growth of Structure Read [CO 0] Early U contained condensations of many different sizes Current large-scale structure t = t 0 = 17 Gyr 1 Log Jeans [CO Fig 07] Log time he Simplest Picture of Galaxy Formation and Why It Fails (chapter title from Longair, Galaxy Formation ) Will a condensation collapse? he Jeans criterion: ass = emp = Density = o (see [CO Sect 1 and pg 150] Unstale to collapse if K < -U Pressure < Gravity k G m H 5 4 5k o G m H / 4 Jeans mass J o 1/ Log Jeans [CO Fig 07] Log time

4 How fast will it collapse? In a static medium (eg star formation): Perturation analysis shows Expanding U density = See [CO pg 149] < J / exp(-ir/ -it ) Oscillations > J / exp(-ir/ + Kt ) Exponential growth In an expanding medium (eg the universe): Outside the perturation (flat universe): (Friedman Eqn) ass Perturation = emp = Density = o Rad atter t / Inside the perturation (closed mini-universe): log t Slow Growth log t Radiation era: 4 1/ 0 R( R( t atter era: 0 R( / R( t / he Simplest Picture of Galaxy Formation and Why It Fails Cosmic icrowave Background is smooth to a few parts in 10 5 / ~ 10-4 Yet high contrast structures (QSOs, galaxies) y z ~ 6 / >> 1 Blue = 0 o K Red = 4 o K Adiaatic perturations grow as / t / R( 1/(1+z) Expect only QSO ( 1 z) CB z 7 QSO CB So where did galaxies and clusters come from? Blue = 74 o K Red = 7 o K Dipole Anistropy ~ 1 part in 00 After removing dipole Red lue = 0000 o K ~ 1 part in

5 In an expanding universe, will a cloud collapse? he Jeans criterion Version : Collapse if K < -U Radiation era c vs R( R( J, 1 G vs G G 4 / 4 vs G J, / const 5v s 4G v s G / After decoupling v s J, 5k m 0 = [CO eq 07] H / 0 Log Jeans, [CO Fig 07] K < -U Pressure support < gravity keeps these clouds fluffed up Decoupling Log time has disappeared Clouds now collapse In an expanding universe, will a cloud collapse? K < -U Pressure support < gravity WAP image Snapshot of oscillating condensations on all size scales aken at the moment of decoupling Brightness (/) Log Jeans, keeps these clouds fluffed up has disappeared Clouds now collapse Decoupling [CO Fig 07] Log time 5