SoylentNews is people
SoylentNews
Arthur T Knackerbracket has found the following story:
UCLA physicists have proposed new theories for how the universe's first black holes might have formed and the role they might play in the production of heavy elements such as gold, platinum and uranium.
Two papers on their work were published in the journal Physical Review Letters.
A long-standing question in astrophysics is whether the universe's very first black holes came into existence less than a second after the Big Bang or whether they formed only millions of years later during the deaths of the earliest stars.
Alexander Kusenko, a UCLA professor of physics, and Eric Cotner, a UCLA graduate student, developed a compellingly simple new theory suggesting that black holes could have formed very shortly after the Big Bang, long before stars began to shine. Astronomers have previously suggested that these so-called primordial black holes could account for all or some of the universe's mysterious dark matter and that they might have seeded the formation of supermassive black holes that exist at the centers of galaxies. The new theory proposes that primordial black holes might help create many of the heavier elements found in nature.
The researchers began by considering that a uniform field of energy pervaded the universe shortly after the Big Bang. Scientists expect that such fields existed in the distant past. After the universe rapidly expanded, this energy field would have separated into clumps. Gravity would cause these clumps to attract one another and merge together. The UCLA researchers proposed that some small fraction of these growing clumps became dense enough to become black holes.
Their hypothesis is fairly generic, Kusenko said, and it doesn't rely on what he called the "unlikely coincidences" that underpin other theories explaining primordial black holes.
The paper suggests that it's possible to search for these primordial black holes using astronomical observations. One method involves measuring the very tiny changes in a star's brightness that result from the gravitational effects of a primordial black hole passing between Earth and that star. Earlier this year, U.S. and Japanese astronomers published a paper on their discovery of one star in a nearby galaxy that brightened and dimmed precisely as if a primordial black hole was passing in front of it.
(Score: 3, Interesting) by HiThere on Sunday September 03 2017, @04:45PM (7 children)
If back holes are to explain dark matter, they'll NEED to be primordial, because formed from stars they would have affected the balance of Lithium in the universe in a manner that hasn't been seen. The problem with this is Hawking radiation. If they were small enough to produce the effects attributed to dark matter, then they'd be evaporating, and if they evaporated they'd eventually explode which would produce radiation at a particular wavelength. That signal hadn't been detected when last I heard about this.
Note that if black holes are larger they tend to combine producing heavier black holes which then tend to combine...until you end up with holed too massive to do the job required of dark matter. But if they're too light to do that (I.e., small enough to have a negligible capture radius) then they evaporate.
So I don't think black holes can explain dark matter. And unless primordial black holes were quite different from other black holes in their basic quantum interactions, I doubt that any small ones survived into the current epoch.
OTOH.... there might be a size range that would do both jobs. But unless they were all the same mass you'd expect to detect signs of them exploding as the lighter ones evaporated. (I think someone calculated that a primordial black hole about the mass of a mountain would only be exploding about now...but I forget how old the universe was thought to be when they made that calculation...and I don't know which mountain they were thinking of.) So if the theory could justify saying that all the primordial black holes were about twice the mass of a mountain, it might work. I've never worked out what the capture cross-section of that would be, and most of space is rather empty, so for a small enough black hole it would evaporate faster than it grew.
Summary: I don't think black holes account for dark matter. It requires too many fine adjustments. But if they do, it's got to be primordial black holes.
Javascript is what you use to allow unknown third parties to run software you have no idea about on your computer.
(Score: 0) by Anonymous Coward on Sunday September 03 2017, @05:39PM (4 children)
No, dark matter can't be black holes. See figure 2 here:
http://www.sciencedirect.com/science/article/pii/S1355219816301563 [sciencedirect.com]
No one has ever supposed black holes would act like that (be present in just the right ratio with visible matter).
(Score: 0) by Anonymous Coward on Sunday September 03 2017, @07:50PM (3 children)
That's a philosophy paper, not a science paper.
(Score: 1, Touché) by Anonymous Coward on Sunday September 03 2017, @10:11PM (2 children)
It's a review article, be careful about following heuristics off a cliff. Knowing when to apply various heuristics or not is basically the definition of intelligence. Anyway, :https://doi.org/10.1086/421338
(Score: 0) by Anonymous Coward on Monday September 04 2017, @05:38AM (1 child)
Well, at least this irrelevant paper is about dark matter. But it has little to say on the subject of what form dark matter might take (other than that the author seems to prefer MOND, but MOND is not plausible today as it was in 2003).
https://www.xkcd.com/1758/ [xkcd.com]
Nothing in this paper would exclude primordial or microscopic black holes. While black holes aren't technically baryonic matter, nothing prevents them from being found in the same regions of space. Most dark matter candidates today are not baryonic matter, as MACHOs are out of favor.
(Score: 0) by Anonymous Coward on Monday September 04 2017, @03:02PM
As originally noted, why is there a constant ratio of visible to black holes? You haven't offered any explanation for this and it is very strange (if dark matter is black holes) that many galaxies of different ages would have the same relationship.
(Score: 3, Interesting) by Anonymous Coward on Sunday September 03 2017, @08:18PM (1 child)
The thing about Hawking radiation is that we don't necessarily know that black holes can evaporate all the way to nothing. It might be that black holes can only evaporate down to the Planck mass (which unlike most Planck units is a human-perceivable scale: about the mass of a tiny speck of dust). In that case they make an excellent candidate for dark matter - big enough to not escape from galaxies, small enough to be hard to find, and mostly interacting only by gravity.
If you can believe Wikipedia, the mass of a black hole at its maximum brightness, during its last second of existence, is 2.28×10^5 kg, which we can assume all gets converted to energy (the Planck mass being negligible). It's only 1/10000 the amount of energy released by the sun - more in line with a very low-mass red dwarf. So finding an exploding black hole is like looking for an individual 0.1Msun red dwarf that only lives for one second. No wonder we don't see them. On the positive side, all these exploding black holes should look almost exactly the same, so if we do manage to find them, they should be easy to recognize.
If the amount of dark matter in the universe is constant, then that means that if primordial black holes are responsible, they must have either all been small enough to evaporate down to Planck mass very quickly, or else all large enough that they mostly haven't evaporated yet and reduced the amount of dark matter in the universe.
But it actually seems that the amount of dark matter may actually be increasing over time:
https://www.scientificamerican.com/article/dark-matter-did-not-dominate-early-galaxies/ [scientificamerican.com]
Overall I think high-mass primordial black holes are not very likely. Low-mass ones might have evaporated early enough on that we have no chance of spotting their emissions from so far away, and left behind huge numbers of tiny, inert, primordial black hole remnants that become dark matter.
The possible problems here include:
1) We need a whole lot of these tiny black holes to account for dark matter. What happened to all the energy they would have emitted? Did it get converted back to matter somehow? Or was their creation process so efficient that most of them were actually created near the minimum mass, so they didn't have to emit as much?
2) We don't know whether there's a minimum black hole mass at the Planck mass or not. If there's not, then so much for this idea.
3) If supermassive black holes are actually primordial black holes, why did some become so big while others became so small?
(Score: 2) by HiThere on Monday September 04 2017, @05:00PM
Is there any reason to believe quantum black holes *wouldn't* evaporate? Outside of the theoretical problem that they might leave behind a naked singularity?
I'm not sure a naked singularity is a problem. Not one the size of a quantum black hole. It's not like virtual particles aren't a standard feature of modern physics.
Javascript is what you use to allow unknown third parties to run software you have no idea about on your computer.