Primordial black holes need to be collisionless....

[fhenryco]

Primordial Black holes (PBH) need to be collisionless to be considered a serious Dark Matter candidate. Indeed a population of PBH with masses about 30 solar masses and uniformly distributed in the galaxy have very low capture and direct collision rates.

But these PBHs should also behave as DM and not as baryons in the radiative era: how can it be that those PBHs don't "load" the acoustic fluctuations as baryonic matter does which should result in these PBHs directly contributing to the fraction of baryonic matter rather than DM when analyzing the CMB spectrum second and third peaks?
i would have expected that PBHs be also sensitive to the pressure of the surrounding fluid (radiation plus baryons) and then been dragged and participate to the oscillations of this fluid in contrast to DM.
So i understand clearly why PBHs are said to be collisionless as for their mutual interactions (low collision rate) but what about the interaction between PBHs and the baryon/photon fluid ?

 

[Chronos]

While collisionless black holes as DM can alleviate the tension with CMB power spectrum data, they must also be largely accretionless to avoid conflict with radio and xray surveys as discussed here; https://arxiv.org/abs/1612.00457, Searching for Primordial Black Holes in the radio and X-ray sky. The combined constraints of collisionless and accretionless would be consistent with our understanding of DM properties, but, there is little doubt they also strain credibility. It'd not unlike infering the absence of hoof prints and droppings does not diminish the possibility s robust unicorn popuulation accounts for low grass cover in a forest.

 

[fhenryco]

Thanks but actually I'm not considering very seriously the hypothesis of PBHs as DM because I'm aware that this is already almost excluded ... what I'm trying to figure out is why the PBHs would contribute to DM rather than baryons in the CMB . It seems that there is something very basic that i missed but what ...?

 

[kimbyd]

I'm not so sure that primordial black holes would interact with radiation much at all during the radiative era. They would absorb some fraction of the radiation, but wouldn't return radiation to their surroundings, and so would largely be unaffected by it. They're also vastly more massive than the energies of the individual photons, so they respond to the bulk motion of the photon/baryon gas, rather than its pressure.

This is in contrast to the baryons, where the radiation and the ionized baryon gas interact together as one, combined fluid.

 

[fhenryco]

When a black hole absorbs an incoming object momentum is conserved so there is a recoil of the black hole. So if a BH is in a fluid that behaves anisotropically relative to it , it should be dragged in the direction of the fluid motion. So it should follow the oscillations of the fluid and as any massive component following the oscillations of the fluid (in contrast to DM that is insensitive to it) it should participate to the load of the fluid just as baryons and therefore the height of the second CMB peak relative to the first which indeed measures the load of the fluid oscillating fluctuations e.g the ratio of baryons to photons density ... there is something wrong in this argument but i have no clear idea what is wrong

 

[fhenryco]

"they respond to the bulk motion of the photon/baryon gas, rather than its pressure"
may be this the good argument but I'm still not completely sure

 

[kimbyd]

When a black hole absorbs an incoming object momentum is conserved so there is a recoil of the black hole. So if a BH is in a fluid that behaves anisotropically relative to it , it should be dragged in the direction of the fluid motion. So it should follow the oscillations of the fluid and as any massive component following the oscillations of the fluid (in contrast to DM that is insensitive to it) it should participate to the load of the fluid just as baryons and therefore the height of the second CMB peak relative to the first which indeed measures the load of the fluid oscillating fluctuations e.g the ratio of baryons to photons density ... there is something wrong in this argument but i have no clear idea what is wrong

I think the key here is that the black hole has to be vastly more dense than the surrounding fluid. If the surrounding fluid is reasonably-dense compared to the black hole, then either other black holes would form or it would accrete rapidly into the black hole. So the picture is of the occasional black hole within a low-density plasma separated by large distances from other black holes. Because the surrounding plasma necessarily has to be low-density compared to the black hole, it can't influence the black hole's motion much at all.

 

[Chronos]

The second accoustical peak tells us about 15% of the infant universe consisted of baryonic matter Galactic surveys have only accounted for about 10% of the baryons necessary to meet this prediction - leading to the well known 'missing baryon' problem. Of course scientists were well aware not all matter emits photons and assumed the shortfall could be tied up in non emmiting sources. This supposition was validated when recent SZ studies confirmed about 1/2 of the total baryon budtget was in the form of cold intergalactic gas. The SZ effect is, unfortunately, not well suited for detecting black holes. That leaves open the possibility black holes could account for a substantial portion of the baryons still AWOL [~%33]. This does not seem nearly as unraasonable as holding black holes culpable for the third accoustical peak.

 

[fhenryco]

"That leaves open the possibility black holes could account for a substantial fractiont of the baryons"

but then these BHs would have formed after decoupling ... right?

 

[fhenryco]

Because the surrounding plasma necessarily has to be low-density compared to the black hole, it can't influence the black hole's motion much at all.

the speed might be small but what matters is the transmitted momentum (from medium to the BH) ...which is the same whatever the mass of the object

May be it's rather the low "cross section" (BH size) that makes it a weakly interacting object with the fluid, it's compactness (?)

 

[kimbyd]

the speed might be small but what matters is the transmitted momentum (from medium to the BH) ...which is the same whatever the mass of the object

May be it's rather the low "cross section" (BH size) that makes it a weakly interacting object with the fluid, it's compactness (?)

Yes, I think that's right.