Mainz, Germany

Dear Wegner,

It strikes me that the basic problem is that both dark matter and primordial black holes are theoretical conjectures--so far as I know. The evidence of dark matter is perhaps firmer or less indirect, but it is not that there are no alternative accounts of the same range of phenomena --including movements of observed galaxies. So, the problem arises from the difficulty of trying to support one unconfirmed hypothesis with another.Weaving one theoretical conjecture together with another makes the experimentalists nervous. 

On a purely theoretical basis, it is perhaps possible to calculate an expected value for primordial black holes, but without some sort of observational support, this will likely be considered a weak go at an explanation of dark matter. As I understand the current accounts, weakly interacting particles are the favorite candidate --though without much empirical support. I would look to the advocates of such theories for some reasonable estimate of the possible role of black holes--primordial or not.

H.G. Callaway

Some scientists are revisiting an old idea that black holes born at the dawn of time are a prime suspect for all that missing mass. Researchers call these speculative black holes "massive compact halo objects," or MACHOs, since dark matter lurks in a "halo" in and around big galaxies. But a new study in The Astrophysical Journal Letters offers a serious reality check to the idea that MACHOs are dark matter. Timothy Brandt, an astrophysicist at the Institute for Advanced Study, wrote the study after he took a close look at 11 dim, feeble, and weird little galaxies. The stars there are choking with dark matter, at least compared to larger galaxies like the Milky Way or Andromeda, but the little galaxies don't seem to show any obvious signs of harboring a flotilla of old black holes."These galaxies would be less dense and larger than we see," Brandt told Business Insider. Instead they're inexplicably compact. And that could represent a big threat to the MACHO hypothesis. Meanwhile, ultra-faint dwarf galaxies are roughly 99% dark matter."The dark matter is holding them together and preventing them from flying apart," Brandt told Business Insider. And that's where Brandt realized he could see if a bunch of old black holes between 20 to 100 times the mass of the sun (a size range for MACHOs that has yet to be ruled out) really exist there. If so, they'd accelerate stars as they passed nearby, causing the entire cluster or galaxy to "puff" outward over billions of years. A diffuse cloud of dark matter particles, on the other hand, would keep the cluster glued together

Article Title: Non Dark Hyperbolic Universe

Dear Todd,

Good question,

The answers is straight forward - yes there are primordial black holes in the galactic halo.

We know this because the MACHo experiments of 2000/2001 shows that that are (invisible) microlensing objects there, at masses of between 0.5-1 solar masses. Stellar black holes have to be >3 solar masses - so what LIGO is seeing are most probably stellar black holes.

So there are primordial black holes in galactic halos, but that is not the whole story you need a modification to gravity to get the full picture.

https://www.researchgate.net/publication/269689929_Advances_in_Black_Hole_Physics_and_Dark_Matter_Modelling_of_the_Galactic_Halo

Article Advances in Black Hole Physics and Dark Matter Modelling of ...

 As far as I know, searches for MACHO in the galactic halo could not find enough masses in the form  of compact astrophysical objects. The highest estimations for such objects goes for 20% of the required dark mass. Besides this, studies by gravitational lensing of the galactic hallow shows that the dark matter, if exists, should be a homogeneous distribution rather than clumpy objects. Thus black holes would not account for dark matter. 

There is a simple answer. Because total amount of dark matter is fixed, as you increase mass of candidate object, it becomes less abundant. Hence there are chances of conflicting with measured values of abundance. However, you can consider scenarios of multi-component dark matter, even though this possibility is baroque.

The masses detected by LIGO are well into the stellar mass range, primordial BH are classed as much lower masses. There's a paper giving some recent limits attached.

https://arxiv.org/abs/1307.5798

@ George

That paper is very sub-optimal. The MACHO's already discovered in the MACHO project are between 0.5-1.0 solar masses (Alcock et al 2000), not 10^-7 -10^-9 solar masses- which are in any case too small to make any contribution to the optical depth..

Objects of around 0.5 solar masses are not though to be black holes. Primordial BH should be much smaller, those formed subsequently by stellar collapse should be larger.

P.S. I've found (linked) an old paper on the IMF of PBH, it has >150 citations so you might find it useful to look for more up-to-date estimations.

https://arxiv.org/pdf/astro-ph/9709072.pdf

The primordial BHs have already evaporated due to the Hawking radiation. The evaporated radiation is negligible according to PLANCK observations. So, even if the primordial BHs existed, they are practically absent at the present epoch. Moreover, their number was too small to contribute to the overall Omega.

“…black holes… maybe candidates for being primordial and if one considers the likelihood of observing such an event [dark matter]. …”

there cannot be principally some “black holes”, that are, as that the GR claims, the holes in the spacetime. Matter’s spacetime is [5]4D Euclidian manifold [in physical theories]/ “empty container” [in the reality], where uninterruptedly changing system of material objects “Matter” exists and changes. In emptiness there cannot be holes, “holes in emptiness” is evident absurd..

“…The primordial BHs have already evaporated due to the Hawking radiation….”

And. correspondingly, there is no some “BHs evaporation”.

In the reality indeed some objects are possible, which are so dense and massive, that on/near  their surfaces the objects’ 2-th cosmic speed is equal/more to/then the speed of light, but that are rather large objects, which are composed from “usual matter”, though in very specific phase states. What these states are  - recent physics doesn’t know answers, including because of that more 100 years already corresponding official physical theory is the general relativity with its “curved spacetime”.

When Gravity is only 4-th fundamental Nature force, which indeed is created by gravitational masses and it would be much mire useful to study this force, including – what effects appear in very strong gravity fields.

Nonetheless it seems as plausible to suggest that at Beginning only some specific primary particles were made, which further interacted creating the particles that exist now.

Rather possibly that could be Planck mass particles, and their sizes are equal by chance to the Schwarzschild radius. Since their size is fundamentally minimal, [since every particle is some close-loop algorithm] in their algorithms only gravitational charge is written and  so they could interact only gravitationally. But when at Planck mass particles interactions  very large energy is released, that resulted further in appearance of existent now particles/Matter.

 As well as seems as reasonable that at these interactions not all primary particles interacted, so possibly larger part of the particles remains existent till now; and now they constitute the [possibly existent] dark matter; without any “evaporation”.

More see https://www.researchgate.net/publication/273777630_The_Informational_Conception_and_Basic_Physics , at least Sec. 3.3 “Planck mass particles”. Though, of course, to more completely understand the Sec. is necessary to read the paper  as a whole.

Cheers

Article The Informational Conception and Basic Physics