Is "heating" the right way to think about the power from dark matter? Isn't "heating" a function of regular energy and matter?
https://en.wikipedia.org/wiki/Cold_dark_matter
https://www.symmetrymagazine.org/article/is-dark-matter-cold...
https://en.wikipedia.org/wiki/Thermal_velocity
The article from Symmetry Magazine says the same thing as the Phys.org article:
>Light, fast particles are known as hot dark matter; heavy, slow ones are cold dark matter; and warm dark matter falls in between.
The dark matter in the early universe would have been at the same temperature whether of the "warm" or "cold" type, it is just that the speed of the "warm"/non-heavy types would have been too fast to have caused the clumping we see. Or at least that's how I understood it. But that Symmetry article is a good one as well.
Now, it turns out there's an opposing effect where, depending on how the DM couples to normal matter, the heavier particle decouples earlier (and thus at a hotter temperature) because creation/annihilation have to stop once the temperature drops below the energy scale associated with the rest mass, and this effect can often dominate the previous one.
The thing that actually matters, functionally, (and thus the thing that is used to classify DM types) is the thermal velocity relative to the speed of the Hubble flow during structure formation. It is of course true that, as you say, at fixed temperature a species of low mass will have higher velocity than a species of high mass, but CDM and HDM are not at the same temperature due to the opposing expansion and decoupling effects mentioned above (among other things).
My original point, in response to andsoitis, was that heat is an abstract thermodynamic concept that does not depend on the particle species. The names "hot" and "cold" were not metaphorical, even if the boundaries between CDM and HDM are not literally drawn using a threshold temperature. For instance, per the symmetry article:
> “Even though the universe was very hot at the time, axions would have been very cold at birth and would stay cold forever, which means that they are absolutely cold dark matter.” Even though axions are very light, Graham says, “because they exist at close to absolute zero, the temperature where all motion stops, they are essentially not moving. They’re kind of this ghostly fluid, and everything else moves through it.”
This is why it's wrong for phys.org to say that "cold", "warm", and "hot" do not refer not to the temperatures of the matter itself. Likewise, even if "hard" and "soft" cheeses are formally defined by whether they are dried and aged (such that there could be a few "hard" cheeses that are softer than the hardest "soft" cheeses), it would be wrong to say "'hard' and 'soft' do not refer to the firmness of the cheese itself".
But if the DM runs out, then you'd expect the same to happen for non-DM matter, which it doesn't.
Just like a neutral atom can have a neutral antiatom.
As for why dark antimatter/matter pairs would persist longer rather than run out (I guess that's the opposite of what you're saying, but it's important to remember what we're seeing even looking back this far is way after the normal matter/antimatter annihilated immediately after inflation), it's effectively the same reason dark matter doesn't clump and retains it's roughly spherical form at larger than entire visible galaxy sizes. Regular matter interacts via the electromagnetic force, which has an infinite interaction radius. Charged particles repel and attract each other from large distances. Thus, fundamental particles don't need to get that close to each other to form atoms and molecules. Dark matter only interacts via the weak force, which has a tiny interaction radius. The fundamental particles need to more or less make a direct beeline to the same point in spacetime to ever touch each other, which has an extremely low probability of ever happening. It's the same reason Earth can be bombarded nonstop with unimaginably large numbers of neutrinos every second from the sun, yet virtually all of them go straight through everything. All of space is mostly empty space, even things that look solid to us because the wavelengths we can discriminate are much larger than the spaces between atoms and molecules. It wouldn't look that way to dark matter. It would look actually empty.
And gravity, or so I'm told.
But both gravity and the weak force are fields, and so just like EM, they pervade space. Isn't that right? The weak force weakend dramatically with distance, but it doesn't disappear - I thought one property of a field is that it pervades spacetime.
Not that that makes any difference to your argument.
As for the weak force, I think that's not necessarily a known property of DM, but rather a property of hypothetical candidates for being the dark matter known as WIMPs(weakly interacting massive particles).
So most of the dark star is still ordinary matter, DM just adds a bit of extra gravity.
> If the DM particles are their own antiparticles, then their annihilation provides a heat source that stops the collapse of the clouds and in fact produces a different type of star, a Dark Star, in thermal and hydrostatic equilibrium.
> Three key ingredients are required for the formation of DSs:
> 1) sufficient DM density
> 2) DM annihilation products become trapped inside the star
> 3) the DM heating rate beats the cooling rate of the collapsing cloud.
How much if this is speculation? Also, do other particles behave like this?
I didn't realise particles could be their own antiparticles, but it transpires that e.g. photons are, because all photons are neutral, not charged somehow.
However, even though a proton is its own antiparticles, two photons do not annihilate, right?
Yes, as you note, photons are their own antiparticles.
The maths doesn't have a preferred time direction, so two photons can annihilate into an electron-positron pair.
I'm not sure if this has actually been observed given how hard it is. That said, my favourite type of supernova is caused by pair creation, though I don't know the proportion of that which comes from 2-photon interactions: https://en.wikipedia.org/wiki/Pair-instability_supernova
There's also Majorna particles, but as I understand it the only known particles that are definitely Majornas are also quasiparticles:
The wikipedia article on the Breit-Wheeler process has some history of the work on experimental observations, although I don't know how accurate or up to date it is https://en.wikipedia.org/wiki/Breit–Wheeler_process
But only if their energy is high enough. So by that reckoning, photons below 511 keV don't have antiparticles, and those above it do. That's pretty weird. So maybe it's better to say that photons aren't really their own antiparticle, but they might theoretically destroy each other in some rare circumstances.
Nobody is sure, but some people think that neutrinos are they own antiparticle https://en.wikipedia.org/wiki/Neutrino#Majorana_mass I never liked that theory, but some people that know more than me about particle physics liked it.
There were some experiment using atoms that decay ejecting two neutrinos, and hopping that in some case the two neutrinos will annihilate each other. https://en.wikipedia.org/wiki/Neutrinoless_double_beta_decay . IIRC, none of the experiments found the strange annihilation, so perhaps neutrinos are not their own antiparticle :) .
I'm a bit confused at what made you think this. If there is any way to transfer energy from it to something else (which there must be, else it would be impossible to ever interact with it), then it can heat in exactly the same way as anything else.
I think the confusing thing for me is that dark matter doesn’t interact with the electromagnetic field so it doesn’t reflect, absorb, or emit electromagnetic radiation.
But your explanation makes sense to me.
[1] P. Salati, 2014. Dark Matter Annihilation in the Universe. https://arxiv.org/abs/1403.4495
Gravitational heat transfer would, I assume, work for everything, but it would also be very very slow.
We're a little bit spoiled by EM - it makes thermal interactions happen quickly and at all energy scales.
(I don't know if this is relevant at all! Just an interesting tangent)
> If there is any way to transfer energy, then it can heat in exactly the same way as anything else.
If it could heat up, Dark Matter wouldn't be dark. So yeah, dark matter that heats isn't quite what one would expect.
I don't think this is correct at all. A system doesn't have to interact with EM to be thermodynamic. If you can define a temperature for it and there is the possibility for energy transfer, then it can heat up
I think this is where the debate is. I’m not a physicist but my understanding of the current dark matter models is that it doesn’t interact with itself in a way that could be thought of as “energy transfer” (ie. like particles that collide), but only gravitationally. This would mean there’s no real way for a dark matter “particle” to transfer momentum to another particle, and thus no real way for “heat” to exist as such.