hkyriazi said:
I may have to display some ignorance here, as I'm unclear on the functional difference between "cold" matter and "dark" matter. Clearly we can postulate a new type of matter that doesn't interact with photons regardless of its vibrational, rotational, or linear kinetic energy (temperature). (I assume this is why we call it "dark.") Must it be cold so that it can form postulated charge neutral "dark matter molecules" that can then coalesce gravitationally (rather than colliding and recoiling from each other)? Just guessing here...
Whether dark matter is "hot", "warm", or "cold" is a function of the mean velocity of dark matter particles of that type. And, because dark matter has little or no interactions with other matter, by hypothesis, once dark matter forms and reaches an equilibrium state, presumably not long after the Big Bang, this mean velocity should stay more or less constant. (This isn't perfectly true, even for truly "sterile" dark matter that has interactions only via gravity, because gravitational interaction can alter the mean velocity of dark matter in dark matter halos, if the dark matter is distributed in a particular way. Cosmologists and astrophysicists usually assume, however, as a first order approximation that this effect is small, even over times on the order of billions of years.)
In the case of "
thermal freeze-out" dark matter, mean velocity is related by a virial theorem to the mass of the dark matter particle:
In the universe, today, [assume] there exists some non-zero amount of dark matter. How did it get here? Has this same amount always been here? Did it start out as more or less earlier in the universe? The so-called “freeze out” scenario is one explanation for how the amount of dark matter we see today came to be.
The freeze out scenario essentially says that there is some large amount of dark matter in the early universe that decreases to the amount we observe today. This early universe dark matter
is in thermal equilibrum with the particle bath
, meaning that whatever particle processes create and destroy dark matter, they happen at equal rates,
, so that the net amount of dark matter is unchanged. We will take this as our “initial condition” and evolve it by letting the universe expand. For pedagogical reasons, we will name processes that
create dark matter
“production” processes, and processes that
destroy dark matter
“annihilation” processes.
Now that we’ve established our initial condition, a large amount of dark matter in thermal equilibrium with the particle bath, let us evolve it by letting the universe expand. As the universe expands, two things happen:
- The energy scale of the particle bath decreases. The expansion of the universe also cools down the particle bath. At energy scales (temperatures) less than the dark matter mass, the production reaction becomes kinematically forbidden. This is because the initial bath particles simply don’t have enough energy to produce dark matter. The annihilation process though is unaffected, it only requires that dark matter find itself to annihilate. The net effect is that as the universe cools, dark matter production slows down and eventually stops.
- Dark matter annihilations cease. Due to the expansion of the universe, dark matter particles become increasingly separated in space which makes it harder for them to find each other and annihilate. The result is that as the universe expands, dark matter annihilations eventually cease.
"Hot" dark matter is dark matter whose mean velocity is in the relativistic range or close (perhaps 0.1 c or more, or even as little as 0.001 c). The classic example of "hot" dark matter would be sterile neutrinos with masses comparable to the masses of the neutrinos or perhaps to the electron mass. Basically in the ballpark of 10 eV or less for thermal freeze out dark matter.
"Warm" dark matter in the case of thermal freeze-out dark matter is dark matter with a particle mass on the order of 1-100 keV/c
2.
"Cold" dark matter in the case of thermal freeze out dark matter is dark matter with a particle mass on the order of 1 GeV or more, with the favored region being the electroweak mass scale of perhaps 10 GeV to 500 GeV.
This range of dark matter particle masses was initially favored because when the cold dark matter hypothesis was formulated, electroweak scale supersymmetry, which would have created new particles in this mass range that interacted only via the weak force, was expected by many theorists to be right around the corner. At that mass the relic abundance of dark matter, the mass and properties of the predicted lightest supersymmetric particle in a thermal freeze out scenario seemed like an excellent fit to the properties that dark matter needed to have to fit astronomy observations.
Many decades later, we know that this hypothesis has not panned out. Supersymmetric particles with the properties expected from the electroweak scale supersymmetry theories that were favored at the time have been ruled out by collider experiments and direct dark matter detection experiments. And, collisionless cold dark matter (or dark matter that interacts only via gravity and the weak force), generically, leads to dark matter halo shapes that are materially different from what is inferred from the dynamics of ordinary matter.
The
Wikipedia explanation of the concept is as follows:
Dark matter can be divided into
cold,
warm, and
hot categories.These categories refer to velocity rather than an actual temperature, indicating how far corresponding objects moved due to random motions in the early universe, before they slowed due to cosmic expansion – this is an important distance called the
free streaming length (FSL). Primordial density fluctuations smaller than this length get washed out as particles spread from overdense to underdense regions, while larger fluctuations are unaffected; therefore this length sets a minimum scale for later structure formation.
The categories are set with respect to the size of a
protogalaxy (an object that later evolves into a
dwarf galaxy): Dark matter particles are classified as cold, warm, or hot according to their FSL; much smaller (cold), similar to (warm), or much larger (hot) than a protogalaxy. Mixtures of the above are also possible: a theory of
mixed dark matter was popular in the mid-1990s, but was rejected following the discovery of
dark energy.
Cold dark matter leads to a bottom-up formation of structure with galaxies forming first and galaxy clusters at a latter stage, while hot dark matter would result in a top-down formation scenario with large matter aggregations forming early, later fragmenting into separate galaxies; the latter is excluded by high-redshift galaxy observations.
Low mass dark matter candidates were initially disfavored because of the concern that it would be hot dark matter in a thermal freeze out scenario which was the widespread assumption in cosmology circles. But, eventually, it was recognized that dark matter could be created by processes that didn't involve thermal freeze out, so long as the amount of dark matter created and destroyed balanced out neatly, thus breaking the tight link between dark matter particle mass and the mean velocity of particles of that type, which in turn, determines if it is cold, warm, or hot.
Warm dark matter particles with very low mass are attractive because the wave-like behavior of particles starts to become pronounced relative to particle-like behavior of particles at low masses, and this wave-like behavior can help to address some of the problems generic to the dark matter halo shapes we would generically expect in the case of heavier and cold dark matter.
If dark matter particles are indeed low in mass (sometimes called, somewhat ironically "light dark matter") relative to say, thermal freeze-out warm dark matter, then dark matter particles presumably came to be by a means other than thermal freeze-out that may continue to be occurring in equilibrium today.
, meaning that whatever particle processes create and destroy dark matter, they happen at equal rates,
, so that the net amount of dark matter is unchanged. We will take this as our “initial condition” and evolve it by letting the universe expand. For pedagogical reasons, we will name processes that create dark matter
“production” processes, and processes that destroy dark matter
“annihilation” processes.
