I Could dark matter consist of primordial black holes?

[Prishon]

Could dark matter consist of black holes formed shortly after the big bang? They would form the perfect development seed. If they all have Sun-like masses then they are not detectable from here (they are just 3 kilometers wide!). They have virtually no collisions with stars and could form a halo. They interact only gtavitationally.

So, could it be?

 

[Orodruin]

Yes. Primordial black holes is an active subfield of dark matter physics. There are several exclusion bounds published for different PBH mass ranges.

If I recall correctly the typical PBH mass is bounded from above by considerations such as microlensing and from below by black hole evaporation due to Hawking radiation.

 

[Prishon]

Yes. Primordial black holes is an active subfield of dark matter physics. There are several exclusion bounds published for different PBH mass ranges.

If I recall correctly the typical PBH mass is bounded from above by considerations such as microlensing and from above by black hole evaporation due to Hawking radiation.

Then what is all the fuzz about? Black matter= dark holes...Case closed... No exotic particles needed.

 

[PeterDonis]

Then what is all the fuzz about? Black matter= dark holes...Case closed...

In science, we don't "close a case" just because we have a plausible hypothesis. We only close it when we have confirmed the hypothesis by evidence (and how "closed" it is depends on how strong the evidence is). We have zero evidence for primordial black holes, and there are arguments against the hypothesis as well as in favor of it.

 

[phinds]

Then what is all the fuzz about? Black matter= dark holes...Case closed... No exotic particles needed.

Uh ...

 

[Orodruin]

Then what is all the fuzz about? Black matter= dark holes...Case closed... No exotic particles needed.

That’s not how science works. In science you actually have to find some evidence for your hypothesis and not just speculate wildly before you declare something like that. PBHs are as speculative as particle dark matter.

I have no idea how you even went from ”PBHs are being actively investigated” to ”PBHs are definitely dark matter”. There are several steps you skipped here.

 

[Orodruin]

Uh ...
View attachment 287874

Running the cannon?
Hopping the howitzer?
Skipping the artillery?

Damn, beats me ...

 

[Keith_McClary]

Wikipedia

I read somewhere that DM could be boulders the size of basketballs, except nobody can think of how these could be formed.

 

[PeterDonis]

I read somewhere

This is not a valid reference. Please give a specific reference.

 

[Keith_McClary]

This is not a valid reference. Please give a specific reference.

I think they are excluded by the considerations discussed by Sabine here.

 

[phinds]

Running the cannon?
Hopping the howitzer?
Skipping the artillery?

Damn, beats me ...

 

[Keith_McClary]

View attachment 287879

It’s believed that this expression originates from track and field racing. At the beginning of these races, it is common for a starting pistol (aka a gun) to be fired.

 

[phinds]

I always thought it was an Old West saying having to do with horse races.

 

[ohwilleke]

Could dark matter consist of primordial black holes?​

Probably not.

Primordial black holes have the virtue of having theoretically very well defined properties. They are fully described by their mass (charged primordial black holes are ruled out by other properties that observation has made clear that dark matter must have) and spin (which only every so slightly tweaks their properties for these purposes).

The most widely held view is that it has been narrowed to roughly asteroid mass black holes, but not entirely ruled out. See, e.g., Montero-Camacho, Paulo; Fang, Xiao; Vasquez, Gabriel; Silva, Makana; Hirata, Christopher M. (2019-08-23). "Revisiting constraints on asteroid-mass primordial black holes as dark matter candidates". Journal of Cosmology and Astroparticle Physics. 2019 (8): 031. arXiv:1906.05950. doi:10.1088/1475-7516/2019/08/031 which argued that there was still a possibility that primordial black holes could be the sole component of dark matter with asteroid-mass primordial black holes (3.5 × 10⁻¹⁷ – 4 × 10⁻¹² solar masses, or 7.0 × 10¹³ – 8 × 10¹⁸ kg).

Montero-Camacho (2019) does not claim, however, that there is an affirmative evidence whatsoever for primordial black holes in this mass range. It merely notes that this is the parameter space that hasn't yet been ruled out.

But there is a recent paper suggesting that the potential parameter space of primordial black holes has been entirely ruled out in the remaining mass window. See Amir Sirajh, Abraham Loeb, "Eliminating the Remaining Window for Primordial Black Holes as Dark Matter from the Dynamics of the Cold Kuiper Belt" arXiv (March 8, 2021) whose abstract appears below, argue that this remaining parameter space is entirely ruled out:

The nature of dark matter (DM) is unknown. One compelling possibility is DM being composed of primordial black holes (PBHs), given the tight limits on some types of elementary particles as DM. There is only one remaining window of masses available for PBHs to constitute the entire DM density, 10^17 - 10^23 g. Here, we show that the kernel population in the cold Kuiper belt rules out this window, arguing in favor of a particle nature for DM.

Also, most papers analyzing primordial black hole dark matter candidates focus on exclusions due to failures of direct detection and Hawking radiation, not to the more general issues about whether properties relevant to cold dark matter particles more generally (e.g. halo shape) would rule out primordial black holes.

These issue have pushed theorists to favor dark matter particle candidates with self-interactions, with quantum behavior at very small masses that introduces coherence to entire populations of dark matter particles, or to fifth force interactions with ordinary matter.

Primordial black hole dark matter lacks these features (and also struggles with the issue that LambdaCDM dark matter is "collisionless" which primordial black hole dark matter manifestly is not).

While primordial black hole dark matter is attractive for not requiring any new beyond the Standard Model Physics or changes in General Relativity (which, in theory, permits them to exist), observational evidence and analysis strongly disfavor this dark matter candidate compared to the alternatives.

 

[elcaro]

Could dark matter consist of black holes formed shortly after the big bang? They would form the perfect development seed. If they all have Sun-like masses then they are not detectable from here (they are just 3 kilometers wide!). They have virtually no collisions with stars and could form a halo. They interact only gtavitationally.

So, could it be?

Here is a lecture by Neal Weiner about What we know and don't know about Dark Matter.

And here an article/video by Sabine Hossenfelder about Dark Matter: The situation has changed.

 

[ohwilleke]

Here is a lecture by Neal Weiner about What we know and don't know about Dark Matter.

And here an article/video by Sabine Hossenfelder about Dark Matter: The situation has changed.

Some highlights from Sabine's post:

dark matter was a simple idea that fit to a lot of observations, which is why it was such a good scientific explanation. But that was the status 20 years ago. And what’s happened since then is that observations have piled up that dark matter cannot explain.

For example, particle dark matter predicts a density in the cores of small galaxies that peaks, whereas the observations say the distribution should be flat. Dark matter also predicts too many small satellite galaxies, these are small galaxies that fly around a larger host. The Milky Way for example, should have many hundreds, but actually only has a few dozen. Also, these small satellite galaxies are often aligned in planes. Dark matter does not explain why.

We also know from observations that the mass of a galaxy is correlated to the fourth power of the rotation velocity of the outermost stars. This is called the baryonic Tully Fisher relation and it’s just an observational fact. Dark matter does not explain it. It’s a similar issue with Renzo’s rule, that says if you look at the rotation curve of a galaxy, then for every feature in the curve for the visible emission, like a wiggle or bump, there is also a feature in the rotation curve. Again, that’s an observational fact, but it makes absolutely no sense if you think that most of the matter in galaxies is dark matter. The dark matter should remove any correlation between the luminosity and the rotation curves.

Then there are collisions of galaxy clusters at high velocity, like the bullet cluster or the el gordo cluster. These are difficult to explain with particle dark matter, because dark matter creates friction and that makes such high relative velocities incredibly unlikely. Yes, you heard that correctly, the Bullet cluster is a PROBLEM for dark matter, not evidence for it.

And, yes, you can fumble with the computer simulations for dark matter and add more and more parameters to try to get it all right. But that’s no longer a simple explanation, and it’s no longer predictive.

So, if it’s not dark matter then what else could it be? The alternative explanation to particle dark matter is modified gravity. The idea of modified gravity is that we are not missing a source for gravity, but that we have the law of gravity wrong.

Modified gravity solves all the riddles that I just told you about. There’s no friction, so high relative velocities are not a problem. It predicted the Tully-Fisher relation, it explains Renzo’s rule and satellite alignments, it removes the issue with density peaks in galactic cores, and solves the missing satellites problem.

But modified gravity does not do well with the cosmic microwave background and the early universe, and it has some issues with galaxy clusters.

So that looks like a battle between competing hypotheses, and that’s certainly how it’s been portrayed and how most physicists think about it.

But here’s the thing. Purely from the perspective of data, the simplest explanation is that particle dark matter works better in some cases, and modified gravity better in others. A lot of astrophysicist reply to this, well, if you have dark matter anyway, why also have modified gravity? Answer: Because dark matter has difficulties explaining a lot of observations. On its own, it’s no longer parametrically the simplest explanation.

I would add that since she made that post, there have been successes on the modified gravity from on the cosmic microwave background and the early universe, and that there have been failures of LambdaCDM in the early universe such as the EDGES 21 cm data.

 

[ohwilleke]

I think they are excluded by the considerations discussed by Sabine here.

Specifically, she starts from a presentation made by Glenn Starkman in June 2015 at a conference (emphasis mine) that discusses https://arxiv.org/abs/1410.2236 stating:

His talk was about “macro dark matter,” a dark matter candidate that has received little if no attention. I had only become aware of it briefly before, through a paper by Starkman together with David Jacobs and Amanda Weltman. Unlike the commonly considered particle dark matter, macro dark matter isn’t composed of single particles, but of macroscopically heavy chunks of matter with masses that are a priori anywhere between a gram and the mass of the Sun.

It is often said that observations indicate dark matter must be made of weakly interacting particles, but that is only true if the matter is thinly dispersed into light, individual particles. What we really know isn’t that the particles are weakly interacting but that are rarely interacting; you never measure a cross-section without a flux. Dark matter could be rarely interacting because it is weakly interacting. That’s the standard assumption. Or it could be rarely interacting because it is clumped together to tiny and dense blobs that are unlikely to meet each other. That’s macro dark matter.

But what is macro dark matter made of? It might for example be a type of nuclear matter that hasn’t been discovered so far, blobs of quarks and gluons that were created in the early universe and lingered around ever since. These blobs would be incredibly dense; at this density the Great Pyramid of Giza would fit inside a raindrop!

If you think nuclear matter is last-century physics, think again. The phases and properties of nuclear matter are still badly understood and certainly can’t be calculated from first principles even today. . . .

So matter of nuclear density containing some of the heavier quarks is a possibility. But Starkman and his collaborators prefer to not make specific assumptions and keep their search as model-independent as possible. They were simply looking for constraints on this type of dark matter which are summarized in the figure below

Constraints on macro dark matter. Fig 3 from arxiv:1410.2236

On the vertical axis you have the cross-section, on the horizontal axis the mass of the macros. The grey and green diagonal lines are useful references marking atomic and nuclear density. In general the macro could be made up of a mixture, and so they wanted to keep the density a variable to be constrained by experiment. The shaded regions are excluded by various experiments.

To arrive at the experimental constraints one takes into account two properties of the macros that can be inferred from existing data. The one is the total amount of dark matter which we know from a number of observations, for example gravitational lensing and the CMB power spectrum. This means if we look at a particular mass of the macro, we know how many of them there must be. The other property is the macros’ average velocity which can be estimated from the mass and the strength of the gravitational potential that the particles move in. From the mass and the density one gets the size, and together with the velocity one can then estimate how often these things hit each other – or us.

The grey-shaded left upper region is excluded because the stuff would interact too much, causing it to clump too efficiently, which runs into conflict with the observed large scale structures.

The red regions are excluded by gravitational lensing data. These would be the macros that are so heavy they’d result in frequent strong gravitational lensing which hasn’t been observed. These constraints are also the reason why neutron stars, brown dwarfs, and black holes have long been excluded as possible explanations for dark matter. There are two types of lensing constraints from two different lensing methods, and right now there is a gap between them, but it will probably close in the soon future.

The yellow shaded region excludes macros of small mass, which is possible because these would be hitting Earth quite frequently. A macro with mass 10⁹g for example would pass through Earth about once per year, the lighter ones more frequently. Searches for such particles are similar to searches for magnetic monopoles. One makes use of natural particle detectors, such as the sediment mica that forms neatly ordered layers in which a passing heavy particle would leave a mark. No such marks have been found, which rules out the lighter macros.

What about that open region in the middle? Could macros hide there? Starkman and his collaborators have some pretty cool ideas how to look for macros in that regime, and that’s what my New Scientist piece with Naomi is about.

Macro dark matter of course leaves many open questions. As long as we don’t really know what it’s made of, we have no knowing whether it can form in sufficient amounts or is stable enough. But its big advantage is that it doesn’t necessarily require us to construe up new particles.

Note that primordial black hole macro dark matter candidates have to exactly on the diagonal purple line in the bottom right hand corner of the chart. Dark matter candidates above that line can't be primordial dark matter because they don't have sufficient density, dark matter candidates below that line can't exist because they are more dense than black holes of the same mass.

But, primordial black holes, unlike generic "Macro Dark Matter" should have a very high cross section of interaction. It should interact with anything it gets close to, strongly tending to swallow it up, but the chart restricts macro dark matter candidates of those masses to very low cross-sections of interaction. (The mass itself is already used to adjust for infrequency of interaction.)

Also, a basic premise of LambdaCDM is that the aggregate amount of dark matter in the Universe is constant or very nearly so, while with Primordial Black Holes, every time a primordial black hole interacts with ordinary matter or photons, the aggregate mass of primordial black holes increases a little. You should have 13.8 billion years of growth in the dark matter fraction by accretion, which can be very slight and yet still add up over that time frame.

Starkman revisited this analysis in 2019 in https://arxiv.org/abs/1912.04053 the abstract of which states:

Macroscopic dark matter -- "macros"-- refers to a broad class of alternative candidates to particle dark matter with still unprobed regions of parameter space. These candidates would transfer energy primarily through elastic scattering with approximately their geometric cross-section. For sufficiently large cross-sections, the linear energy deposition could produce observable signals if a macro were to pass through compact objects such as white dwarfs or neutron stars in the form of thermonuclear runaway, leading to a type IA supernova or superburst respectively. We update the constraints from white dwarfs. These are weaker than previously inferred in important respects because of more careful treatment of the passage of a macro through the white dwarf and greater conservatism regarding the size of the region that must be heated to initiate runaway. On the other hand, we place more stringent constraints on macros at low cross-section, using new data from the Montreal White Dwarf Database. New constraints are inferred from the low mass X-ray binary 4U 1820-30, in which more than a decade passed between successive superbursts. Updated microlensing constraints are also reported.

The updated chart from the body is as follows:

The legend to that figure states (references omitted):

Objects within the region in the bottom-right corner should not exist as they would simply be denser than black holes of the same mass. The grey region is ruled out from structure formation; the yellow from mica observation; the red from superbursts in neutron stars (this work – the hatched region representing potential future constraints); the dark blue from white dwarf becoming supernovae; the purple from a lack of human injuries or deaths; the green from a lack of fast-moving bolides; the maroon from a lack of microlensing events toward the Large Magellanic Cloud and the Galactic center, and, in pink, toward M31.

The empirical exclusions rule out a lot of the primordial black hole line, but I don't really see how the low cross-sections of interaction in the part not excluded are possible even in theory for primordial black holes due to their very small cross-section of interaction.

 

[PeterDonis]

she starts from a presentation made by Glenn Starkman in June 2015 at a conference

Isn't macro dark matter, which, if it is not to require postulating new particles, would have to be made of ordinary baryons, ruled out by the limits on how many baryons can be made in Big Bang nucleosynthesis?

 

[ohwilleke]

Isn't macro dark matter, which, if it is not to require postulating new particles, would have to be made of ordinary baryons, ruled out by the limits on how many baryons can be made in Big Bang nucleosynthesis?

I don't think that it the case for the most general version of macro dark matter, in which one could imagine some candidate with an extremely low cross-section of interaction (lower than neutrinos), since the interactions are what give rise to the BBN boundaries.

This said, for any realistic model of baryonic composite particle macro dark matter, I think that is a fair limitation and I don't see how it could be overcome. You'd need some sort of stable baryon that was "hyperconfined" so that even the very short range residual strong force that binds atoms together wasn't present, and I haven't seen any hypothetical QCD consistent candidates for that.

Also, the stability requirement pretty much rules out any significant strange, charm, bottom, or top quark contribution, yet we understand composite particles composed only of up and down quarks more or less comprehensively. And, protons are the only stable hadron composed only of up and down quarks (and isn't hyperconfining) although neutrons are metastable and are stable when bound in atomic nuclei (and also aren't hyperconfining). All atomic combinations of protons and neutrons are likewise well explored and don't behave in this manner.

 

[PeterDonis]

I don't think that it the case for the most general version of macro dark matter, in which one could imagine some candidate with an extremely low cross-section of interaction

Which would be a new particle, so it removes the attractiveness of macro dark matter in the first place, that it appears to hold out the hope of not having to postulate new particles.

you'd need some sort of stable baryon that was "hyperconfined"

Even that would still be a baryon and would still have to be made of quarks. I think there are also limits on quark abundance that would not leave room for that.

 

[ohwilleke]

Which would be a new particle, so it removes the attractiveness of macro dark matter in the first place, that it appears to hold out the hope of not having to postulate new particles.Even that would still be a baryon and would still have to be made of quarks. I think there are also limits on quark abundance that would not leave room for that.

Both very fair points. Certainly, I am no fan of macro dark matter, primordial black holes, or (now ruled out MACHOs). In the case of macro dark matter and primordial black holes, some direct or nearly direct detection approaches have ruled out a lot of the parameter space, but people aren't bothering to write papers that demolish these theories on other grounds in the remaining parameters space, even though the road map for such papers would be pretty straight forward.

In the same vein, the phenomenology pre-prints at arXiv are constantly full of papers that suggest that this or another approach to dark matter mechanisms is "well motivated" when in fact there are mountains of papers outside the narrow subfield of the authors that very strongly disfavor them.