Slashdot Mirror
Can Primordial Black Holes Alone Account For Dark Matter?
thomst writes: Slashdot stories have reported extensively on the LIGO experiments' initial detection of gravity waves emanating from collisions of primordial black holes, beginning, on February 11, 2016, with the first (and most widely-reported) such detection. Other Slashdot articles have chronicled the second LIGO detection event and the third one. There's even been a Slashdot report on the Synthetic Universe supercomputer model that provided support for the conclusion that the first detection event was, indeed, of a collision between two primordial black holes, rather than the more familiar stellar remnant kind that result from more recent supernovae of large-mass stars.
What interests me is the possibility that black holes of all kinds -- and particularly primordial black holes -- are so commonplace that they may be all that's required to explain the effects of "dark matter." Dark matter, which, according to current models, makes up some 26% of the mass of our Universe, has been firmly established as real, both by calculation of the gravity necessary to hold spiral galaxies like our own together, and by direct observation of gravitational lensing effects produced by the "empty" space between recently-collided galaxies. There's no question that it exists. What is unknown, at this point, is what exactly it consists of.
The leading candidate has, for decades, been something called WIMPs (Weakly-Interacting Massive Particles), a theoretical notion that there are atomic-scale particles that interact with "normal" baryonic matter only via gravity. The problem with WIMPs is that, thus far, not a single one has been detected, despite years of searching for evidence that they exist via multiple, multi-billion-dollar detectors.
With the recent publication of a study of black hole populations in our galaxy (article paywalled, more layman-friendly press release at Phys.org) that indicates there may be as many as 100 million stellar-remnant-type black holes in the Milky Way alone, the question arises, "Is the number of primordial and stellar-remnant black holes in our Universe sufficient to account for the calculated mass of dark matter, without having to invoke WIMPs at all?"
I don't personally have the mathematical knowledge to even begin to answer that question, but I'm curious to find out what the professional cosmologists here think of the idea.
What interests me is the possibility that black holes of all kinds -- and particularly primordial black holes -- are so commonplace that they may be all that's required to explain the effects of "dark matter." Dark matter, which, according to current models, makes up some 26% of the mass of our Universe, has been firmly established as real, both by calculation of the gravity necessary to hold spiral galaxies like our own together, and by direct observation of gravitational lensing effects produced by the "empty" space between recently-collided galaxies. There's no question that it exists. What is unknown, at this point, is what exactly it consists of.
The leading candidate has, for decades, been something called WIMPs (Weakly-Interacting Massive Particles), a theoretical notion that there are atomic-scale particles that interact with "normal" baryonic matter only via gravity. The problem with WIMPs is that, thus far, not a single one has been detected, despite years of searching for evidence that they exist via multiple, multi-billion-dollar detectors.
With the recent publication of a study of black hole populations in our galaxy (article paywalled, more layman-friendly press release at Phys.org) that indicates there may be as many as 100 million stellar-remnant-type black holes in the Milky Way alone, the question arises, "Is the number of primordial and stellar-remnant black holes in our Universe sufficient to account for the calculated mass of dark matter, without having to invoke WIMPs at all?"
I don't personally have the mathematical knowledge to even begin to answer that question, but I'm curious to find out what the professional cosmologists here think of the idea.
Don't count on it.
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https://phys.org/news/2015-03-... http://www.sciencemag.org/news... If dark matter were simply some existing form of baryonic matter, even if trapped in black holes, then a phenoma like this where dark matter halos separate from collided galaxies and behave under different rules to continue on their existing path should not be possible at all, because it, like all the other ordinary matter involved, it should have followed the same paths gravitationally bound.
The main alternative to WIMPs are MACHOs, and black holes have long been candidates for dark matter. The problem is that they would need to have five times more mass than all the "ordinary" matter in the universe, and there is little evidence for that. For instance the amount of gravitational lensing that is observed is way less than would be expected. Dark matter appears to be more evenly distributed in galaxies and not just in the "halo". Yet we don't observe that many black holes passing through gas and dust clouds or interacting with regular stars.
TFA says that there may be 100 million black holes in our galaxy, and that may sound like a lot, but it is actually nowhere near enough to account for all the dark matter. Even if they had 10 solar masses each (unlikely), that would still be less than 1% of the mass of the galaxy's "ordinary matter", when it should be 500%.
Quote: "Dark matter [...] has been firmly established as real [...] There's no question that it exists." There is still plenty of controversy related to the idea of dark matter, and there is no such thing in physics as proving something exists - you can only prove something to be false. I'm not saying dark matter does not exist, only that statements like the above are too assertive.
26% of the mass of the universe is made up of your simplifying assumptions: space is flat and uniform everywhere and everywhen, gravity is constant everywhere and everywhen, the speed of light is constant everywhere and everywhen, the Higgs field isn't really the luminiferous aether with a fancy new name, etc. ...
So so so much of the Standard Model (and astrophysics in general) starts out like "Given a spherical cow of uniform density at STP...".
We can basically derive ALL of chemistry from first principles involving (protons, neutrons, electrons) (and their charges), electron shell configurations, etc. Does the Standard Model provide an explanation for the mass of the electron, or any of the other 92 empirically derived "constants" that make up the current orthodoxy? Does calling the gap between reality and our understanding of it really benefit from calling it "Dark Matter", or "Dark Energy", or should we just call it "phlogiston"?
I'm not trolling, I'm serious. The Standard Model has lots of (statistical) predictive power, but absolutely no explanitory power -- back to the chemistry example, it's as though we have atomic weights and molar values, but no notion of electron shells -- we can predict, but we can't explain, at least not in a meaningful way -- yet.
There is evidence for more matter than visible in the galaxies, which is completely independent of Dark Energy. The most prominent evidence is the rotational characteristics of the outer parts of a galaxy. The stars there are circling the center of the galaxy much faster than expected from a Keplerian point of view. Instead of falling with r^2/3, as Kepler's Third law of motion predicts, the speed of stars remains roughly constant if you get to the outer parts of the galaxy. This means that the mass of the galaxy inside the respective orbits of the stars has to grow much faster than the mass from the additional stars within outer orbits.
(Be careful not to confuse the speed of stars on their orbit with their angular speed! A star twice the distance from the center of a galaxy needs twice the time to complete a circle than a star closer to the center. Thus the angular speed halves, but the linear speed on the orbit keeps the same. With Kepler's Third law, we would expect the time to complete an circle for the outer star to be 2*sqrt(2) of the time the inner star needs.)
Here's a result of a 5 second Google search: Could black holes be the dark matter?
Not really. There's areas of mass where there's little to no ordinary matter, where galaxies have collided and the gas has slowed down, but the dark matter has kept on going. This is demonstrated by gravitational lensing effects of the invisible mass. This doesn't really fit with MOND theories.
It is annoying having lazy clueless laymen's idle speculations being promoted to being a slashdot article.
Dark matter seems particularly to attract these sorts of totally uninformed wild guesses being thrown out to "solve" one of the deepest questions in modern physics and cosmology.
To all and sundry out there - if you just thought of it then the answer is "no". All possible known candidates have been thought of and eliminated. Whatever dark matter and dark energy are, it is nothing we currently understand. Even most promising theories seem to be failing at present.
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