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.
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.
Astronomers have observed the gravitational lens effect of dark matter. Dark matter normally surrounds normal matter, but is sometimes found separated. It appears that during galactic collisions, the dark matter can be separated from the normal matter, gas and dust of a galaxy. To do that, dark matter would have to interact with itself in a manner that does not involve gravity. A bunch of black holes would not interact in this way, so it is unlikely that dark matter consists solely of black holes.
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.
Starships were meant to fly, Hands up and touch the sky - Nicky Minaj
But why would the black holes be distributed significantly differently than stars, and have "outer" orbits?
Most visible stars are caused by relatively recent compressions and/or concentrations of gas, and the concentration is heavier toward the center of a galaxy. Thus, relatively speaking, there could be more black holes outside the visible disk of a galaxy than stars.
But this would also imply the majority of black holes did not form from stars, at least not in the way we see them form now. If the black-holes formed from typical stars, their distribution (galactic orbits) would be roughly the same as visible stars.
The theory they formed in the primordial ooze that was common before most galaxies formed may account for this: their galactic orbits would then be further out than most galaxy-born stars. Such holes would eventually be captured by galaxies but would on average have outer orbits, as gravity-captured objects tend to. This could give the "hidden heavy halo" effect seen in Hubble gravity lensing and star-orbit patterns.
Table-ized A.I.