Galaxy-Size Gravitational-Wave Detector Hints At Exotic Physics (scientificamerican.com) 43
The fabric of spacetime may be frothing with gigantic gravitational waves, and the possibility has sent physicists into a tizzy. A potential signal seen in the light from dead stellar cores known as pulsars has driven a flurry of theoretical papers speculating about exotic explanations. Scientific American reports: The most mundane, yet still quite sensational, possibility is that researchers working with the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), which uses the galaxy as a colossal gravitational-wave detector, have finally seen a long-sought background signature produced when supermassive black holes crash and merge throughout the universe. Another interpretation would have it originating from a vibrating network of high-energy cosmic strings that could provide scientists with extremely detailed information about the fundamental constituents of physical reality. A third possibility posits that the collaboration has spotted the creation of countless small black holes at the dawn of time, which could themselves account for the mysterious substance known as dark matter.
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The NANOGrav collaboration still needs to confirm that it is in fact seeing gravitational waves. And the shape of those gravitational waves' spectrum has yet to be traced out and found to conform to the cosmic string interpretation, each of which is likely to take years. Meanwhile, another contingent of the physics community has suggested that the signal could originate from entities known as primordial black holes. Unlike regular black holes, which are born when gigantic stars die, these would form in the early universe, when matter and energy were nonuniformly scattered through the cosmos as a consequence of processes that occurred at the end of inflation. Certain overdense areas could collapse under their own weight, generating black holes in a variety of sizes. Observations from LIGO and Virgo that could indicate mergers between primordial black holes have already planted the idea in many researchers' minds that these strange objects are more than speculative fictions. Certain theorists like them because, as entities that give off no light, they could account for some or even all of the dark matter in the universe.
Along with two co-authors, Riotto has written a third paper appearing in PRL showing how the NANOGrav signal could be accounted for by a multitude of black holes the size of asteroids being created shortly after the big bang, producing a gravitational wave relic that would travel to us in the modern day. According to the researchers' model, these miniature primordial black holes could comprise up to 100 percent of the dark matter in the universe. [...] Nevertheless, the burst of theoretical activity shows how seriously physicists are taking these results. NANOGrav researchers have another two and a half years of pulsar data they are combing through, which could help distinguish whether some or a combination of all these explanations might be viable.
[...]
The NANOGrav collaboration still needs to confirm that it is in fact seeing gravitational waves. And the shape of those gravitational waves' spectrum has yet to be traced out and found to conform to the cosmic string interpretation, each of which is likely to take years. Meanwhile, another contingent of the physics community has suggested that the signal could originate from entities known as primordial black holes. Unlike regular black holes, which are born when gigantic stars die, these would form in the early universe, when matter and energy were nonuniformly scattered through the cosmos as a consequence of processes that occurred at the end of inflation. Certain overdense areas could collapse under their own weight, generating black holes in a variety of sizes. Observations from LIGO and Virgo that could indicate mergers between primordial black holes have already planted the idea in many researchers' minds that these strange objects are more than speculative fictions. Certain theorists like them because, as entities that give off no light, they could account for some or even all of the dark matter in the universe.
Along with two co-authors, Riotto has written a third paper appearing in PRL showing how the NANOGrav signal could be accounted for by a multitude of black holes the size of asteroids being created shortly after the big bang, producing a gravitational wave relic that would travel to us in the modern day. According to the researchers' model, these miniature primordial black holes could comprise up to 100 percent of the dark matter in the universe. [...] Nevertheless, the burst of theoretical activity shows how seriously physicists are taking these results. NANOGrav researchers have another two and a half years of pulsar data they are combing through, which could help distinguish whether some or a combination of all these explanations might be viable.
Primordial black holes.. (Score:1)
Fascinating that we might all wobble along almost unseen gravitational waves, jiggling the mess we are made of.
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Why physicists have an age requirement. (Score:1)
The fabric of spacetime may be frothing with gigantic gravitational waves, and the possibility has sent physicists into a tizzy.
Why the hell did my childish mind translate this into "physicists get really excited over the discovery of...Space Farts."?
I need coffee, especially since my Hollywood childish mind translated that further to "FARRTS IIN SPAAAACEE"...
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Welp, it is written in a tabloid way.
Physicists are excited, since it measures something that we had not measured previously.
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Welp, it is written in a tabloid way.
Physicists are excited, since it measures something that we had not measured previously.
(Scientists) "We've determined that further research into space flatulence is necessary. Any ideas for candidates for this mission?"
(Me) "Whatever you do, don't send Shatner. Things could escalate quickly.."
Re: Why physicists have an age requirement. (Score:5, Interesting)
If you think scientists aren't the silliest of 'em all, you are badly mistaken, good Sir!
Especially theoretical physicists.
Think nerds, but extremely skilled at thinking outside the box of conventional rules. Basically, you have to be free to think ridiculous things, to find new sensible ones.
I mean quantum field theory and relativity are both like that kind of humor, mixed with an LSD trip. You have to have fallen into the LSD pot of the clown kitchen. How else could anyone come up with something that out there? ;)
And yes, that is a good thing. :)
Acting mature is for immature people.
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This is quite possibly the best endorsement and justification for legalizing drugs I've ever heard. Hilarious. Thanks for that.
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That's hilarious and awesome. Thanks, Barefoot.
Exotic meant as pretty standard (Score:1)
The gravitational-wave background (from mergers of super-massive black holes) is expected and pretty standard. While the other possibilities are not excluded, nothing hints to them.
It is exotic like you can consider some far-away countries to be exotic, yet they are pretty standard. We do not live close to a super-massive black hole, thus we feel it as exotic.
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We do not live close to a super-massive black hole, thus we feel it as exotic.
I live close to the dancers at Spearmint Rhino and yet I still feel them as exotic...
Re: Exotic meant as pretty standard (Score:2)
That's because you are light years away from getting some, even if you could pay them money. ;)
Re: Exotic meant as pretty standard (Score:4, Funny)
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The black hole mergers aren't the "exotic physics", that's the other possibilities such as strings.
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There is something I don't buy. (Score:5, Informative)
According to the researchers' model, these miniature primordial black holes could comprise up to 100 percent of the dark matter in the universe.
The problem with that is not the amount of mass those primordial black holes. The problem is that they wouldn't behave as Dark Matter should according to the models. Black holes can merge. They would (like baryonic (normal) matter) form a disc rotating around a bigger black hole. Dark Matter does not clump together, as it has no way to release the energy from an inelastic collision. Thus Dark Matter forms a halo around a gravitational center.
Re:There is something I don't buy. (Score:5, Interesting)
We have different observation methods that rule out many ranges of mass but there are some gaps.
* Large numbers of microscopic black holes largely gets ruled out by an absence of Hawking Radiation and presence of other compact objects like Neutron stars which shouldn't survive.
* Abundant small and mid sized black holes should be visible in sky surveys with microlensing events. The ongoing sky surveys haven't shown much evidence for this.
* Very large mass objects with up to thousands of solar masses would be visible with their gravitational influence on nearby stars. We have now started surveys of star velocities so I expect researchers are looking for evidence there.
Having said this, I don't think that the absence of mergers is great evidence against them as black hole candidates. Accretion disks and other ways to lose momentum all require some merger to occur or significant gravitational radiation from another nearby compact object. But if primordial black holes haven't collided with many objects, they'd be largely uniformly scattered still. That makes the probability of collision extremely low even if they outnumber stars 10:1 or 100:1, it would be lower than a star and star collision. If the masses of the objects are too much smaller, it'll be out of the frequency range that instruments like LIGO can detect.
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But there's a question in my mind as to whether a black hole of minimum size could emit ANY radiation. It's too small to swallow half of a virtual particle pair. And it's unlikely to grow, as it doesn't have much mass, and hence no gravitational field. Once it becomes electrically neutral, all it would have would be gravity, and it wouldn't have much of that.
I think that may BE dark matter. It would have needed to have been created in the extremely early time period so as to not disrupt the element dist
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The leftmost exclusion area (red) is limits on distribution from Hawking radiation background as measured by extragalactic gamma rays; the rightmost (blue) is CMB background distortions, and the zones at the top are various microlensing surveys. The blue line is the paper's hypothesized mass distribution for 100% dark matter.
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I'm assuming that that image came from https://journals.aps.org/prl/a... [aps.org] "article text subscription required". The image without the accompanying text doesn't mean much to me. (It might not mean much *with* the accompanying text, I'm no astrophysicist.)
Still, that's a pretty sharp cut off in the frequency, that *ought* to be detectable. But do any other theories predict the same thing?
I'm not sure what the mass diagram refers to.
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Left: mass function resulting from a flat power spectrum such that it peaks at 10 14 M , with A 5.8 × 10 3 and k s = 10 9 k l 1.6 Hz , and PBHs comprise the totality of DM, i.e., f PBH = 1 . In the tail of the population, around M , one can notice the bump in the PBH production due to the decrease of the threshold by QCD epoch equation of state [24,51]. Shown are the most stringent constraints in the mass range of phenomenological interest coming from the Hawking evaporation producing extragalactic gamma ray (EG ) [54], e ± observations by Voyager 1 ( V e ± ) [55], positron annihilations in the Galactic Center (GC e + ) [56], gamma-ray observations by INTEGRAL (INT) [57] (for other constraints in this mass range, see also [58–63]), microlensing searches by Subaru HSC [64,65], MACHO-EROS [66,67], Ogle [68], and Icarus [69], and those coming from CMB distortions by spherical or disk accretion (Planck S and Planck D , respectively) [70,71]. LVC stands for the constraint coming from LIGO-Virgo Collaboration measurements [72–74]. We neglect the role of accretion, which has been shown to affect constraints on masses larger than O ( 10 ) M [75,76]. See Ref. [4] for a comprehensive review on constraints on the PBH abundance. Notice that there are no stringent constraints in the PBH mass range of interest [52,53]. Right: the abundance of GWs according to our scenario. In black the 95% C.I. from the NANOGrav 12.5 yr experiment is shown. For more details about the projected sensitivities, see the main text.
The right-side plot is a predicted gravitational wave observability, and the colored ranges are the various existing and (mostly) proposed detectors. The paper's survey is the small black area.
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I think that may BE dark matter. It would have needed to have been created in the extremely early time period so as to not disrupt the element distribution, particularly of Lithium (though, IIUC, there are some unresolved problems with that).
The problem there is that when two blackholes enter each others gravity wells, they combine.
They don't explode into an expanding disk of new mass and matter, where the event horizon up and disappears
This is the behavior black holes would require in order to be dark matter and explain the bullet cluster observations, and we just don't observe that behavior from any other black holes
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I don't get it. Why should normal matter accumulate in accretion disks but not black holes? Is the argument that normal matter interacts with both EM and gravity, and the EM interactions make all the difference? I believe that is the standard explanation for the bullet cluster.
Also, shouldn't some black hole and matter interactions still occur and produce rare but spectacular explosions etc? I would imagine that if a small black hole actually hit the earth or the sun it would result in some characteristic s
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Yes, basically. Normal matter clumps and forms discs because of friction, and friction is an electromagnetic phenomenon.
A small black hole hitting something that's not a neutron star would be unlikely to do much of anything. Normal matter is almost entirely empty space, which is why anything uncharged will go through like there's nothing there. Micro black holes are similar to neutrinos in that way. Neutron stars are denser, and some of the limits on the number and mass of micro black holes floating around
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Accretion disks are the product collisions releasing energy as heat, which then can be radiated away. This is a feature all dark matter candidates can't have and it is ultimately required to form any large structure quickly. There's still going to be some flatenening like stars in a spiral galaxy since that structure is largely determined by gravitational interactions.
Regarding the black hole and matter interactions: Really depends on the size and speed of the object. At the 200km/s orbital speed around the
Question on the Dark Matter Statement (Score:2)
As I understand it, the "dark matter" that is being used to explain what we're seeing in the spin and coherence of distant galaxies has been described as being spread through and around these galaxies, for example in a "dark halo" that surrounds them.
This article suggests that the dark matter may in fact exist in primordial black holes that have
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> that the dark matter may in fact exist in primordial black holes
More like that those black holes _are_ the "dark matter".
There are several papers to be published on the subject in the coming months, on primordial black holes, evolution and role of globular clusters and some new theories on galaxy formation/composition.
Very exciting, but have to wait some.
There have been studies (Score:5, Interesting)
I ask because I wonder if primordial black holes, left to knock around for ~ 13 billion years, would influence the observable matter. If such black holes had existed for this time, would we expect to see accretion disks by now? Would we expect to see their influence on observable matter? For example, could they also influence the movement of stars that we observe in these galaxies?
Current measurements suggest that dark matter is evenly distributed through the galaxy, forming a big diffuse ball that's centered on the galactic core.
If the mass is due to black holes, we should see slight gravitational lensing effects when such a hole passes between us and a local star or between us and a distant galaxy.
We've done/we're doing that experiment, with the results limited by the average mass of the black holes comprising dark matter, density of bright objects in our visual field, time spent observing, and so on.
So far there have been no reported effects seen. This eliminates some ranges of density and black hole mass as an explanation, but doesn't completely rule out the hypothesis. The question will be settled with more time and observations.
Right now, it is *likely* that dark matter is not composed of black holes, but further observation is required.
How does this detector work ? (Score:2)
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The first thing that stuck out to me in the original description is that the galaxy is tens of thousands of light years across; so what we're observing in pulsars scattered across the observable part of the galaxy are events widely separated from each other in time. But I guess they get around that by looking at noise? Which presumably is static (sorry for the pun) across millennia, right?
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"not even light" -- can we stop saying this? (Score:5, Interesting)
Maybe we can stop saying nothing escapes black holes, not even light, since it's becoming more and more commonplace to observe that, indeed, gravity, and waves through the gravitational field escape black holes. When the first black hole merger was observed by the LIGO team, it was estimated that 3 solar masses worth of energy were radiated. Radiated. By shaking the entirety of the universe back and forth. In terms of energy output, brighter than the rest of the observable universe. That's not "nothing escapes". Gravity escapes.
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It is meant for sort-of static black holes, meant when they are those cuddly spherical blobs. When they go into a wild dynamics, like merging with another heavy blob, they evolve and lose some weight. And yes, it is according to general relativity.
Regarding gravity itself, it is a property of space-time around those black holes (alike around other massive bodies). And it evolves whenever a black body evolves, otherwise it is static, without any need of information escaping from the black hole.
Re:"not even light" -- can we stop saying this? (Score:4, Informative)
Nothing escapes from *inside* the event horizon. The gravitational waves are generated around the black hole.
If you'd take two small black holes with you into a huge black hole and then merge them once inside, the gravitational waves would not escape.
Also, Hawking radiation escapes from just outside the black hole. It is correct to say that nothing escapes from inside the event horizon of a black hole.
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What would you expect? That they'd endorse the guy who thinks science is a plot against him to lower his ratings?
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What would you expect? That they'd endorse the guy who thinks science is a plot against him to lower his ratings?
Well, yeah, they endorsed Biden, didn’t they?
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Indeed, I'd be more inclined to believe this so-called news if it came from a Trump-supporting publication like the National Enquirer.
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Their political bent is irrelevant when reporting on science.
Fact that they’re easily tricked means I can’t trust anything they report.