It has often been said, including by me, that one of the most intriguing aspects of dark matter is that provides us with the best current evidence for physics beyond the Core Theory (general relativity plus the Standard Model of particle physics). The basis of that claim is that we have good evidence from at least two fronts — Big Bang nucleosynthesis, and perturbations in the cosmic microwave background — that the total density of matter in the universe is much greater than the density of “ordinary” matter like we find in the Standard Model.
There is one important loophole to this idea. The Core Theory includes not only the Standard Model, but also gravity. Gravitons themselves can’t be the dark matter — they’re massless particles, moving at the speed of light, while we know from its effects on galaxies that dark matter is “cold” (moving slowly compared to light). But there are massive, slowly-moving objects that are made of “pure gravity,” namely black holes. Could black holes be the dark matter?
It depends. The constraints from nucleosynthesis, for example, imply that the dark matter was not made of ordinary particles by the time the universe was a minute old. So you can’t have a universe with just regular matter and then form black-hole-dark-matter in the conventional ways (like collapsing stars) at late times. What you can do is imagine that the black holes were there from almost the start — that they’re primordial. Having primordial black holes isn’t the most natural thing in the world, but there are ways to make it happen, such as having very strong density perturbations at relatively small length scales (as opposed to the very weak density perturbations we see at universe-sized scales).
Recently, of course, black holes were in the news, when LIGO detected gravitational waves from the inspiral of two black holes of approximately 30 solar masses each. This raises an interesting question, at least if you’re clever enough to put the pieces together: could the dark matter be made of primordial black holes of around 30 solar masses, and could two of them have come together to produce the LIGO signal? (So the question is not, “Are the black holes made of dark matter?”, it’s “Is the dark matter made of black holes?”)
This idea has just been examined in a new paper by Bird et al.:
Simeon Bird, Ilias Cholis, Julian B. Muñoz, Yacine Ali-Haïmoud, Marc Kamionkowski, Ely D. Kovetz, Alvise Raccanelli, Adam G. Riess
We consider the possibility that the black-hole (BH) binary detected by LIGO may be a signature of dark matter. Interestingly enough, there remains a window for masses 10M⊙≲Mbh≲100M⊙ where primordial black holes (PBHs) may constitute the dark matter. If two BHs in a galactic halo pass sufficiently close, they can radiate enough energy in gravitational waves to become gravitationally bound. The bound BHs will then rapidly spiral inward due to emission of gravitational radiation and ultimately merge. Uncertainties in the rate for such events arise from our imprecise knowledge of the phase-space structure of galactic halos on the smallest scales. Still, reasonable estimates span a range that overlaps the 2−53 Gpc−3 yr−1 rate estimated from GW150914, thus raising the possibility that LIGO has detected PBH dark matter. PBH mergers are likely to be distributed spatially more like dark matter than luminous matter and have no optical nor neutrino counterparts. They may be distinguished from mergers of BHs from more traditional astrophysical sources through the observed mass spectrum, their high ellipticities, or their stochastic gravitational wave background. Next generation experiments will be invaluable in performing these tests.
Given this intriguing idea, there are a couple of things you can do. First, of course, you’d like to check that it’s not ruled out by some other data. This turns out to be a very interesting question, as there are good limits on what masses are allowed for primordial-black-hole dark matter, from things like gravitational microlensing and the fact that sufficiently massive objects would disrupt the orbits of wide binary stars. The authors claim (and quote papers to the effect) that 30 solar masses fits snugly inside the range of values that are not ruled out by the data.
The other thing you’d like to do is figure out how many mergers like the one LIGO saw should be expected under such a scenario. Remember, LIGO seemed to get lucky by seeing such a big beautiful event right out of the gate — the thought was that most detectable signals would be from relatively puny neutron-star/neutron-star mergers, not ones from such gloriously massive black holes.
The expected rate of such mergers, under the assumption that the dark matter is made of such big black holes, isn’t easy to estimate, but the authors do their best and come up with a figure of about 5 mergers per cubic gigaparsec per year. You can then ask what the rate should be if LIGO didn’t actually get lucky, but simply observed something that is happening all the time; the answer, remarkably, is between about 2 and 50 per cubic gigaparsec per year. The numbers kind of make sense!
The scenario would be quite remarkable and significant, if it turns out to be right. Good news: we’ve found that dark matter! Bad news: hopes would dim considerably for finding new particles at energies accessible to particle accelerators. The Core Theory would turn out to be even more triumphant than we had believed.
Happily, there are ways to test the idea. If events like the ones LIGO saw came from dark-matter black holes, there would be no reason for them to be closely associated with stars. They would be distributed through space like dark matter is rather than like ordinary matter is, and we wouldn’t expect to see many visible electromagnetic counterpart events (as we might if the black holes were surrounded by gas and dust).
We shall see. It’s a popular truism, especially among gravitational-wave enthusiasts, that every time we look at the universe in a new kind of way we end up seeing something we hadn’t anticipated. If the LIGO black holes are the dark matter of the universe, that would be an understatement indeed.
@Mike P.
Gotcha. Thanks for that.
Here is what puzzles me.
A graviton would surely be something which defines the shape of space.
So how can something which defines the shape of space be said to have a velocity through space?
Robin,
In terms of the particle nature of gravity it is more likely that dark matter is a sea of photons which are displaced by matter than it is that gravitons exist.
Think of ’empty’ space being filled with photons which are displaced by the particles of matter which exist in it and move through it.
The photons displaced by the Earth pushing back and exerting pressure toward the Earth is gravity.
The state of displacement of the photons *is* curved spacetime.
@Greg:
“The photons displaced by the Earth pushing back and exerting pressure toward the Earth is gravity.”
Did you come up with that yourself? I can see more than a few problems with this.
Let’s ignore for now that photons are very unlikely to be involved here at all, since there does not seem to be any EM component to gravity. So let’s assume there is some sort of fluid, or aether, permeating a flat spacetime. Then, when this aether is displaced by ordinary matter, the resulting increase in pressure pushes back on the matter in a way that we observe as gravitational attraction. Is that a fair description of your idea?
If you assume the Earth is a perfectly rigid object, then sure, the surface of the Earth will experience an inward force from the displaced aether. But place the Moon next to it, and it too will be pushed out by the increased pressure in the aether. Your model generates no gravitational attraction, only repulsion.
Then you get to the fact that most objects, like the Earth, actually mostly contain empty space — and therefore, would also be filled with your aether. All individual nuclei and electrons would be pushed apart due to the aether’s pressure differential their existence created. Assuming a high enough displacement pressure, no bound states would be possible and everything would just repulse everything else.
I want to encourage you to consider that the effects of gravity really are best understood by true spacetime curvature, in a geometric sense. You are on a GR blog, after all.
I am a PhD in Chemical Engineering but a novice so far as astrophysics is concerned. Just writing to say that Dr Kip Thorne’s “The Science of Interstellar” (2014 ) brought me here. Keep up the good work.
It’s pretty obvious what dark matter and dark energy really are (although as an amateur, I’d concede there may be principles that make the idea problematic at the very least).
The word, which in polite physics circles dare not speak it’s name, is Tachyons! 😉
I suggest that tachyonic fields are emitted and (mostly) absorbed by black holes. For a start, that would explain why dark matter seems next to impossible to interact with – As tachyons are superluminal, controllable interactions with them would violate causality.
Also, if one conjectures that near(ish) to a black hole, e.g. in the region of its hosting galaxy they travel barely superluminally, then it might be that they are “fighting a headwind” in some sense, and thus have a mass-like aspect. But as they accelerate, in the manner I gather they always do, then perhaps beyond some threshold speed they start manifesting a “dark energy” aspect.
Also if one assumes that most tachyonic material is simply exchanged between black holes, and a small residue remains, as I say, in the form of dark energy, then that would explain why dark matter is localized in clumps (albeit fuzzy) but dark energy is uniform.
It would also explain why dark energy appears to be increasing, in the sense that the universe’s expansion is accelerating – The further apart inter-galactic black holes find themselves, the more tachyonic matter emitted by one fails to be re-absorbed by any other, if one assumes what seem reasonable, distance-related laws governing absorption probabilities.
Note also that tachyons travel into the past, at an ever faster rate, and if the above doesn’t seem too absurd a flight of fancy already, might I suggest that we and everything around us is simply a manifestation of the faint and scaled down backreaction of countless generations of future black holes, and universes like ours, evolving at a fantastically speeded up rate (from our perspective).
Who in ancient times, and until quite recently, would have believed that limestone is actually the result of free swimming microscopic sea creatures deposited in layers, sometimes thousands of feet thick, building up over millions of years? With that in mind, perhaps the reader will be slightly more receptive to the idea that our reality is an almost unimaginably faint echo of future universes evolving, and interacting, at a factor of say 10^1000 the speed of ours (which in turn backreacts to inperceptablly small sub-atomic transitions in many other universes in the distant past countless generations before our Big Bang).
Squinting at Feynman’s formal solution of the Schrodinger Equation, with its apparently bizarre mixture of derivative and interals, the main impression of what messy physical process might lead to it is “churn”, and I submit that the mechanism sketched above is how nature achieves this and is what the wave function represents.
Note that this churning process also gives an obvious natural interpretation of how a closed boundary can faithfully represent everything inside.
As to how a black hole could emit tachyonic material, I can only suggest that a high degree of certainty in energy (concentrated inside the black hole) could perhaps by the Uncertainty Principle lead to a correspondingly large uncertainty in time, sufficient to overcome the constraints that presumably prevent the formation of tachyons in less extreme conditions.
@John Ramsden:
I must say, I find it impossible to reason about causality-violating systems, so I have no idea if any part of your argument made sense given the assumptions. Since you apparently have less of a problem drawing conclusions about tachyons, can you state any testable predictions that your hypothesis makes? (I’m genuinely interested)
@Seb
> can you state any testable predictions that your hypothesis makes? (I’m genuinely interested)
The only thing that readily comes to mind is to test the absorption side of the “black holes as tachyon emitters and absorbers” notion I sketched.
Given that most if not all galaxies are believed to host a large black hole at their centre, one approach might be to measure the stellar velocity profile of pairs of merging galaxies (or conduct lensing studies, if dark matter density profiles can be determined that way).
Given that the galaxies’ respective black holes would be approaching each other at the same time, I would expect that the amount of dark matter in their combined vicinity would gradually decline but then rise abruptly after the black hole merger.
But of course the black hole approach happens over millions of years. So one couldn’t expect to measure changes in the same pair of merging galaxies. It would have to be a comparative study, sampling many galaxies both merging and otherwise.
Also, the many stellar mass black holes which presumably pepper any galaxy might muddy the waters somewhat if they also contribute significantly to this supposed dark matter exchange.
(BTW, where I said “inter-galactic” in my previous post I meant of course “galactic”!)
Why would such primordial black holes miss an accretion disk ? Outside the galaxies there is probably not enough gas to maintain it for long but still we should be able to see some examples in the neighborhood if the BH are so many.
Also, a different question: dark matter supposedly interacts only gravitationally with itself and normal matter. Does a BH behaves the same way?
Hi Sean. Just came across your blog after seeing one of your lectures on Youtube.
I have a question about this “ripple” that was detected by LIGO.
When I play my guitar, the movement of the plucked string makes a wave which creates a musical note. The medium for the wave is the string itself.
That note created by the string generates waves which moves air molecules and we perceive that note as a musical tone. The medium is air.
When I drop a stone in a puddle, it creates waves emanating from the point where the stone entered the water. The medium is water.
What’s the medium through which gravity waves move?
What I don’t understand about this idea is that dark matter is said to behave differently from regular matter – it surrounds galaxies in a halo, etc. Attributing dark matter simply to black holes doesn’t account for this, does it?
Or is it the point that the black holes are primordial, so somehow spread evenly throughout the universe? Still, there’s the galaxy halo arrangement and the bullet cluster observation that seem to indicate that dark matter doesn’t interact with normal gravity normally.
@Curt
> What I don’t understand about this idea is that dark matter is said to behave differently from
> regular matter – it surrounds galaxies in a halo, etc. Attributing dark matter simply to black
> holes doesn’t account for this, does it?
The properties of dark matter have been inferred hitherto only on a large, at least galactic, scale. So the idea that it comprises different kinds of particles is still conjectural, and large diffuse swarms of black holes (however and of whatever formed) is claimed to be a possible explanation, again on a large scale, as implied by Sean’s remark “(So the question is not, “Are the black holes made of dark matter?”, it’s “Is the dark matter made of black holes?”)
But wouldn’t a halo of black holes, like any other masses such as stars, flatten out over time? Or does that happen only if the central mass (the galaxy in this case) is above a certain threshold, and/or the orbital directions the objects not too random? I suppose there are such things as elliptical galaxies, and they aren’t flat.
Also, what would the effect of this halo on the many dwarf galaxies that orbit a large galaxy such as ours, or at least the ones (if any) within the halo?