Classification of dark matter: cold.2C warm or hot Dark matter

1 classification of dark matter: cold, warm or hot

1.1 alternative definitions
1.2 cold dark matter
1.3 warm dark matter
1.4 hot dark matter

classification of dark matter: cold, warm or hot

dark matter can divided cold, warm , hot categories. these categories refer velocity rather actual temperature, indicating how far corresponding objects moved due random motions in universe, before slowed due cosmic expansion – important distance called free streaming length (fsl). primordial density fluctuations smaller length washed out particles spread overdense underdense regions, while larger fluctuations unaffected; therefore length sets minimum scale later structure formation. categories set respect size of protogalaxy (an object later evolves dwarf galaxy): dark matter particles classified cold, warm, or hot according fsl; smaller (cold), similar (warm), or larger (hot) protogalaxy.

mixtures of above possible: theory of mixed dark matter popular in mid-1990s, rejected following discovery of dark energy.

cold dark matter leads bottom-up formation of structure while hot dark matter result in top-down formation scenario; latter excluded high-redshift galaxy observations.

alternative definitions

these categories correspond fluctuation spectrum effects , interval following big bang @ each type became non-relativistic. davis et al. wrote in 1985:

another approximate dividing line warm dark matter became non-relativistic when universe approximately 1 year old , 1 millionth of present size , in radiation-dominated era (photons , neutrinos), photon temperature 2.7 million k. standard physical cosmology gives particle horizon size 2ct (speed of light multiplied time) in radiation-dominated era, 2 light-years. region of size expand 2 million light years today (absent structure formation). actual fsl 5 times above length, since continues grow particle velocities decrease inversely scale factor after become non-relativistic. in example fsl correspond 10 million light-years or 3 mpc today, around size containing average large galaxy.

the 2.7 million k photon temperature gives typical photon energy of 250 electron-volts, thereby setting typical mass scale warm dark matter: particles more massive this, such gev – tev mass wimps, become non-relativistic earlier 1 year after big bang , have fsls smaller protogalaxy, making them cold . conversely, lighter particles, such neutrinos masses of few ev, have fsls larger protogalaxy, qualifying them hot .

cold dark matter

cold dark matter offers simplest explanation cosmological observations. dark matter composed of constituents fsl smaller protogalaxy. focus dark matter research, hot dark matter not seem capable of supporting galaxy or galaxy cluster formation, , particle candidates slowed early.

the constituents of cold dark matter unknown. possibilities range large objects machos (such black holes) or rambos (such clusters of brown dwarfs), new particles such wimps , axions.

studies of big bang nucleosynthesis , gravitational lensing convinced cosmologists machos cannot make more small fraction of dark matter. according a. peter: ... plausible dark-matter candidates new particles.

the 1997 dama/nai experiment , successor dama/libra in 2013, claimed directly detect dark matter particles passing through earth, many researchers remain skeptical, negative results similar experiments seem incompatible dama results.

many supersymmetric models offer dark matter candidates in form of wimpy lightest supersymmetric particle (lsp). separately, heavy sterile neutrinos exist in non-supersymmetric extensions standard model explain small neutrino mass through seesaw mechanism.

warm dark matter

warm dark matter refers particles fsl comparable size of protogalaxy. predictions based on warm dark matter similar cold dark matter on large scales, less small-scale density perturbations. reduces predicted abundance of dwarf galaxies , may lead lower density of dark matter in central parts of large galaxies; researchers consider better fit observations. challenge model lack of particle candidates required mass ~ 300 ev 3000 ev.

no known particles can categorized warm dark matter. postulated candidate sterile neutrino: heavier, slower form of neutrino not interact through weak force, unlike other neutrinos. modified gravity theories, such scalar-tensor-vector gravity, require warm dark matter make equations work.

hot dark matter

hot dark matter consists of particles fsl larger size of protogalaxy. neutrino qualifies such particle. discovered independently, long before hunt dark matter: postulated in 1930, , detected in 1956. neutrinos mass less 10 of electron. neutrinos interact normal matter via gravity , weak force, making them difficult detect (the weak force works on small distance, neutrino triggers weak force event if hits nucleus head-on). makes them weakly interacting light particles (wilps), opposed wimps.

the 3 known flavours of neutrinos electron, muon, , tau. masses different. neutrinos oscillate among flavours move. hard determine exact upper bound on collective average mass of 3 neutrinos (or of 3 individually). example, if average neutrino mass on 50 ev/c (less 10 of mass of electron), universe collapse. cmb data , other methods indicate average mass not exceed 0.3 ev/c. thus, observed neutrinos cannot explain dark matter.

because galaxy-size density fluctuations washed out free-streaming, hot dark matter implies first objects can form huge supercluster-size pancakes, fragment galaxies. deep-field observations show instead galaxies formed first, followed clusters , superclusters galaxies clump together.