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WalrusLike
2016-Jun-05, 05:57 AM
I heard on the Spacetime podcast that there is some evidence that dark matter might be primordial black holes around 30 sol masses. A scientist at Goddard has proposed that there is a large population of primordial black holes that are as yet undetected and that provide the missing mass.

I know that most think they are WIMPS. These black holes have not yet been seen... Except perhaps by the gravity wave discovery which was a surprise in terms of the masses involved. The proposal is that the primordial black holes are around that 30 sol mass.

If there was a halo of primordial black holes wouldn't you expect a gradual reduction in number and an increase in mass as they interacted and then merged?

algore
2016-Jun-05, 08:21 AM
It was decided long ago that such black holes couldn't contribute very much DM.

The key "proof" concerns Big Bang Nucleosynthesis. Elements He, H2, Li7 were supposedly formed in the early universe around t = 1 second. These primordial concentrations were supposedly extremely sensitive to the quantity of baryonic matter present. If BH's had formed before t = 1 they would be much smaller than 30 solar masses, so those proposed by Goddard were still unformed. This analysis strictly limits how much "extra" baryonic matter - which would become DM, either in a BH or not - could be there. More evidence against the likelihood of such MACHO's (Massive Compact Halo Objects) involves gravitational lensing with distant quasars as targets. These BH's should show up as fairly strong lensing in the halo. Finally, supposedly CMB studies provide supporting evidence - I never bothered to read the story. Anyway, such studies supposedly limited the amount of baryonic DM to about 1/10 that required.

Would we "expect a gradual reduction in number and an increase in mass as they interacted and then merged"? Well, they're very small, their gravitational field is not that great, and there's a lot of space out there. Perhaps they wouldn't have merged much. Still, they've been telling us for at least a decade that DM should not "clump" at all, else the halo wouldn't have the right characteristics. These BHs should clump too much, I think.

I think what's happened is as follows. Recently the search for WIMPs has faltered, with the failure of LUX and the questions surrounding DAMA. Dr. Gerald Guralnik told me 5 years ago that if LUX failed, WIMPs couldn't be the answer. So I think the DM community is beginning to look for other explanations. They're going back to MACHOs and will soon tell us that all the brilliant cast-in-concrete reasoning outlined above was wrong.

Bottom line: as I've long suspected, they don't know what they're talking about. Recommend that for more stimulating and worthwhile intellectual discussion, ignore Dark Matter theorists and watch Hillary and Trump, or World Wrestling Federation Superstars, instead.

Cougar
2016-Jun-05, 11:42 AM
Recommend that for more stimulating and worthwhile intellectual discussion, ignore Dark Matter theorists and watch Hillary and Trump...

What I want to know is how can you avoid it? :doh:

Cougar
2016-Jun-05, 12:06 PM
A scientist at Goddard has proposed that there is a large population of primordial black holes that are as yet undetected and that provide the missing mass.

What scientist? What paper?

I thought, as algore mentions, that this possibility had been ruled out. For one thing, the CMB is essentially in thermal equilibrium. If there were primordial black holes, they must have evaporated very early -- well before the CMB was "released" -- otherwise their signature would be detectable in the CMB.

algore
2016-Jun-05, 01:06 PM
What scientist? What paper?

Here's the ref http://www.nasa.gov/feature/goddard/2016/nasa-scientist-suggests-possible-link-between-primordial-black-holes-and-dark-matter


I thought, as algore mentions, that this possibility had been ruled out. For one thing, the CMB is essentially in thermal equilibrium. If there were primordial black holes, they must have evaporated very early -- well before the CMB was "released" -- otherwise their signature would be detectable in the CMB.

To avoid leaving a mark in CMB these large BHs must have formed after surface-of-last-scattering, evidently, although the paper doesn't make that clear. The mechanism proposed is "normal" gravitational collapse. The very small BHs you're referring to, Cougar, would have been formed in early super-dense conditions. The paper does talk about other background radiation evidence. I wouldn't take it too seriously; If they continue to talk about it after a year or two, then I'll study it.

01101001
2016-Jun-05, 01:09 PM
This is a possible source: NASA Scientist Suggests Possible Link Between Primordial Black Holes and Dark Matter (http://www.nasa.gov/feature/goddard/2016/nasa-scientist-suggests-possible-link-between-primordial-black-holes-and-dark-matter) [Edit: same as above NASA source]

It's about the cosmic background, but speculation is such as:


"If this is correct, then all galaxies, including our own, are embedded within a vast sphere of black holes each about 30 times the sun's mass."

Nothing in it appears to address old reasons this conjecture has been discarded.

WalrusLike
2016-Jun-05, 08:37 PM
Thank you very much for the explanations and the links. Yes that is the name I heard.

The article indicates that future LIGO results will clarify the matter. I look forward to the next LIGO detection.

algore
2016-Jun-05, 10:17 PM
So do I! Heard a rumor of some trouble with the LIGO detection. Will be interesting to see how it pans out.

Amber Robot
2016-Jun-06, 05:14 PM
I'm pretty sure that algore is right that lensing surveys have ruled out MACHOs as contributing significantly to dark matter.

Reality Check
2016-Jun-08, 12:43 AM
I heard on the Spacetime podcast that there is some evidence that dark matter might be primordial black holes around 30 sol masses
This has essentially been ruled out by the search for MACHO (https://en.wikipedia.org/wiki/Massive_compact_halo_object)s as algore noted. All dark matter cannot be massive compact objects such as black holes (primordial or not). Some dark matter not in the halo may be these massive primordial black holes but then a mechanism excluding them from galaxy halos is needed.
There are cosmological models that match the CMB data to find that ~ 4.9% is baryonic matter, detection about half of this as visible matter which leaves only ~2% to be baryonic matter in primordial black holes. We end up missing 27% of the universe (non-baryonic dark matter). Or maybe speculating about primordial black holes made of non-baryonic matter.

Cougar
2016-Jun-08, 01:29 AM
The very small BHs you're referring to, Cougar, would have been formed in early super-dense conditions.

That's what I had understood when I heard talk about "primordial" black holes - that they were prior to recombination - but it occurs to me I must have it wrong because that can't be right - such BHs would certainly not be evaporating! They could only be growing, in which case they would leave a clear signal on the CMB.

algore
2016-Jun-08, 04:18 AM
Or maybe speculating about primordial black holes made of non-baryonic matter.

I was thinking that myself. That darn Dark Matter has got to be lurking somewhere. We can share the Nobel.

kzb
2016-Jun-08, 12:49 PM
Algore wrote:

<<Recently the search for WIMPs has faltered, with the failure of LUX and the questions surrounding DAMA. Dr. Gerald Guralnik told me 5 years ago that if LUX failed, WIMPs couldn't be the answer. So I think the DM community is beginning to look for other explanations.>>

This is interesting can you extend on this?

<<They're going back to MACHOs and will soon tell us that all the brilliant cast-in-concrete reasoning outlined above was wrong.>>

MACHOs, per se, I think it's fair to say were excluded? I say this in trepidation, but have a look at our entertaining thread which got diverted (by me, I admit) into discussion of what is causing extreme scattering events and the implications:

http://cosmoquest.org/forum/showthread.php?160646-A-Difference-of-Opinion-Makes-a-Horse-Race-Keplerian

algore
2016-Jun-08, 06:25 PM
Algore wrote:
<<Recently the search for WIMPs has faltered, with the failure of LUX and the questions surrounding DAMA. ... So I think the DM community is beginning to look for other explanations.>>

This is interesting can you extend on this?

Yes ... but best would be to google the topic, get more details. Off the top of my head,

First, forget about Dr. Guralnik; I don't want to "put words in his mouth" or cite him as an authority. The simple fact is, LUX failed; and it was a very accurate experiment. That failure has made the WIMP explanation considerably less likely - that's just common sense.

DAMA is an ongoing DM detection experiment which looks for variable DM flux as the Earth orbits the Sun. It has claimed DM detection for a few years now. But the community is very dubious. If DAMA is right it's very hard to understand why LUX (and others) failed. Consensus opinion at the moment is, DAMA is wrong.

Given all this, WIMP explanation is not as strongly believed as it once was.

From my point of view, it's been a good 4 decades since DM search began in earnest. Right from the start I felt some sort of new gravity was an explanation worth looking at. I used to call it a "Fifth Force", operating at large scales: light year on up to entire Universe. Could have attractive, repulsive, and/or torsional components. Instead the community wants to think of it as modified gravity; that's Ok. TeVeS is along these lines, but not radical enough it seems to me. Anyway from my point of view DM is looking more and more like a loser. However it's still entirely possible; it makes sense, not a far-out speculation. I think they should definitely keep at it. But it's time to seriously consider alternatives.

However cosmology community is not, really, doing that. Instead, they're looking, again, at other DM than WIMPs.


<<They're going back to MACHOs and will soon tell us that all the brilliant cast-in-concrete reasoning outlined above was wrong.>>

I'm putting this humorously but there's some truth to it.


MACHOs, per se, I think it's fair to say were excluded? I say this in trepidation, ...

In cosmology they never get to a final answer. That's my humble opinion; understand that many experts disagree. Since cosmology is not experimental science there's always room to change their minds. All the anti-MACHO story, once so convincing, is vulnerable to re-thinking if they want it to be false.


but have a look at our entertaining thread: http://cosmoquest.org/forum/showthread.php?160646-A-Difference-of-Opinion-Makes-a-Horse-Race-Keplerian

I glanced at Gnacinski-Mlynik a while ago. I just can't believe that Zwicky, Vera Rubin and thousands of others since could make such a huge mistake about galactic rotation curves, in particular Milky Way. Perhaps this paper has an interesting take on some old data but not enough to discard the mainstream view. If that paper remains an issue, someday I'll study it more.

Amber Robot
2016-Jun-08, 08:29 PM
From my point of view, it's been a good 4 decades since DM search began in earnest. Right from the start I felt some sort of new gravity was an explanation worth looking at. I used to call it a "Fifth Force", operating at large scales: light year on up to entire Universe. Could have attractive, repulsive, and/or torsional components. Instead the community wants to think of it as modified gravity; that's Ok. TeVeS is along these lines, but not radical enough it seems to me. Anyway from my point of view DM is looking more and more like a loser. However it's still entirely possible; it makes sense, not a far-out speculation. I think they should definitely keep at it. But it's time to seriously consider alternatives.

Do you know how modified gravity would explain something like the Bullet cluster, where the gas mass and the lensing mass seem to be in separate locations?

Reality Check
2016-Jun-08, 11:03 PM
The simple fact is, LUX failed; and it was a very accurate experiment.
What the direct detection experiments have done is exclude some detectable WIMP dark matter candidates. But the universe need not cater for our wish to detect WIMPs :D.

Modifications of physics such as Modified Newtonian Dynamics work better than non-baryonic dark matter for galaxies. The problems come when attempts are made to apply them at larger scales.

algore
2016-Jun-09, 12:56 AM
I'm not a fan of MOND / TeVeS. If you check my posts you'll see I try to avoid the term "modified gravity" because it implies MOND / TeVeS. So I say "alternative grav theories" or similar, which includes this and any other.

However, Amber Robot I'll assume you do mean precisely MOND / TeVeS. No, I don't know how they explain B.C.

But (there had to be a "but"), somebody does!

Here http://www.astro.umd.edu/~ssm/mond/moti_bullet.html is Mordehai Milgrom's take on the Bullet Cluster. BTW, Reality Check, he agrees with you: he's off by a factor of 2 at larger scales. He has an explanation for that too. That's the great thing about cosmology, you can explain anything at all, because it's not an experimental science, it's an observational science. Gives you all the latitude in the world - or, Universe.

What about TeVeS, which can be considered a relativistic version of MOND? See this paper, by Angus, Famaey & Zhao, "Can MOND take a bullet? Analytical comparisons of three versions of MOND beyond spherical symmetry", https://arxiv.org/abs/astro-ph/0606216v1. Quote from abstract: "we can generate a multicentred baryonic system with a weak lensing signal resembling that of the merging galaxy cluster 1E 0657-56 with a bullet-like light distribution." Now we're into high-level GR-like math. If you thought cosmology allows a lot of leeway to explain things, that's nothing compared to this type of math, unchained by data. Like String Theory, they can explain anything at all. They'll also explain the exact opposite of B.C., Abell 520.

I haven't read any of these papers nor do I intend to. However - these are published peer-reviewed papers which stand unchallenged. That's good enough for me.

Let's consider Abell 520. DM just can't do much with this data. They wind up saying that for B.C. the two galaxy's DM haloes didn't interact - as expected; but in Abell 520 they did - for some unknown reason. Or else, just ignore Abell hoping that further observations will invalidate it. In my view both approaches are legitimate because this is cosmology. With experimental science you couldn't just brush it off like this, but here you can. Who knows how DM works? Maybe sometimes it doesn't interact, sometimes it does. Maybe Abell 520 (and, B.C.) is really good correctly-interpreted data, maybe something else totally unexpected is going on.

BTW I do know the details about B.C. and the others: Abell 520, MACS J0025.4-1222, a couple more. I assume you do too.

This answer may seem unsatisfactory! But remember my main point: none of this can be taken too seriously. DM and MOND / TeVeS both fail to explain all the data 100%. But either could be true given the huge uncertainties in the whole enterprise.

If I have to choose, DM definitely sounds better than MOND or TeVeS. But there's a big problem with DM, they keep not finding it. More than 40 years have gone by. So I'd put even money on DM vs. MOND / TeVeS.

But what about "alternative grav theories"? Here's my standard speech on this topic.

We're dealing with phenomena 10 orders of magnitude (let's say, loosely) beyond our miniscule experience, beyond the scope of our ability to experimentally test. What happened when we went 10 orders of magnitude down? Quantum Mechanics, a crazy theory. How about 10 orders of magnitude faster? SR, crazy by the standards of our intuition, developed in the evolutionary conditions of Earth. When we went to low temperatures - same thing, crazy non-intuitive facts arose. I have every expectation the same is true at galactic scales. The explanation will be crazy. DM and TeVeS are simply not crazy enough.

I admit, DM is rather crazy: it's crazy to expect GR to hold on that scale. This obsession with GR is ridiculous. It's been verified pretty thoroughly here in the solar system, also in distant solar systems. GR is compatible with expanding universe data, but many other explanations are possible. Yes there are gravitational waves, predicted by GR but again they're compatible with many other theories as well. GR might be correct all the way to multiverses; or it might not. We simply don't know. GR retention is meaningless and unimportant, 10 powers of 10 away. Precisely analogous to demanding Newtonian mechanics and Maxwell hold, without modification, in an atom.

I have a few sufficiently crazy (but dirt simple) ideas. I can easily account for all the data, including both B.C. and Abell 520. But you can make up such theories yourself. There's so much latitude at this scale, 10 powers of 10 above, anything's possible. Just make sure your idea is real crazy, or it doesn't stand a chance of being true.

Reality Check
2016-Jun-09, 02:39 AM
What about TeVeS, which can be considered a relativistic version of MOND?
Also see the criticisms of TeVeS (https://en.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%93scalar_gravity), e.g. "a TeVeS star is highly unstable, on the scale of approximately 106 seconds (two weeks)."
Interestingly Can MOND take a bullet? Analytical comparisons of three versions of MOND beyond spherical symmetry (https://arxiv.org/abs/astro-ph/0606216v1) submitted on 9 Jun 2006 is contradicted a couple of months later by A direct empirical proof of the existence of dark matter (https://arxiv.org/abs/astro-ph/0608407) submitted on 19 Aug 2006.

The mystery with Abell 520 (https://en.wikipedia.org/wiki/Abell_520) remains that there is dark matter in its core without no concentration of bright cluster galaxies. This is not ignored - there were several studies testing the original observation.

The problem with TeVeS is more that its youth (12 years old) and complexity means that it has not been applied to all of the lines of evidence that support dark matter (https://en.wikipedia.org/wiki/Dark_matter) as non-baryonic matter.

Galaxy rotation curves (yes)
Velocity dispersions of galaxies (yes I think)
Galaxy clusters and gravitational lensing (maybe the Bullet Cluster but what does TeVeS predict for other clusters including Abell 520?)
Power spectrum of the cosmic microwave background (TeVeS prediction?)
Sky surveys and baryon acoustic oscillations (TeVeS prediction?)
Structure formation (TeVeS prediction?)


The actual problem with DM is that over the last 40 years we have found more and more of it :eek:! Not detecting dark matter directly does not say much about its existence - more that it is stranger than we thought.

The fallacy of argument by incredibility in "it's crazy to expect GR to hold on that scale" is not good, algore. As you point out GR has been tested and passed the tests. This is why it is retained as meaningful and important until a better theory (none of which you cite) comes along. You know that dark matter alone is not an argument for GR being invalid. A theory that passes all of the tests that GR has passed and fits the dark matter data better would make GR invalid.

algore
2016-Jun-09, 03:26 AM
The fallacy of argument by incredibility in "it's crazy to expect GR to hold on that scale" is not good, algore.

You didn't catch my point. The craziness is a plus! But it's not crazy enough, I said.


As you point out GR has been tested and passed the tests.

Up to about a light-month or so, yes. After that it's vague; it fits Ok but many other possibilities do also. Putting aside, of course, all the DM-evidence, which on the face of it thoroughly invalidates GR.


This is why it is retained as meaningful and important until a better theory ... comes along.

It should be an option, a guide, but of no great importance. You didn't address my analogy. By your reasoning you would retain Newtonian dynamics and Maxwell at the atomic level. And without the detailed experimental data available at atomic level, you would never question it, never get to the truth (QM). GR soaks up 99% of effort. Much worse, anything else is attacked as heresy. That's not the scientific method - to put it mildly.

Give me one example of a theory, tested well at some level, which is still true 10 orders of magnitude above (or below). I can think of a couple ... but by far, the normal situation is that new theories take over. I don't say reject GR, but work with it. Again the analogy: QM retains features of Newton and Maxwell. Without them for guidance QM couldn't exist. So use GR for guidance. Not for a strait-jacket and handcuffs.


... a better theory (none of which you cite) ...

You don't ask me to cite one. You may do so if you like.


You know that dark matter alone is not an argument for GR being invalid.

Of course not - What makes you think that?


A theory that passes all of the tests that GR has passed and fits the dark matter data better would make GR invalid.

No; no more than Newton, Maxwell, etc are invalid. They're valid in their domain of applicability. The point is not, at all, to make GR invalid, or to assume it's invalid, whatever. Rather, when moving 10 powers of 10 away, realize there's absolutely NO reason to expect it to remain (precisely) valid. Look how many times that huge move makes a theory invalid: almost always. Realize it's probably not right at that scale. Use it for guidance, but remove those handcuffs.

Or, don't. I control only my own mind, and reject mindcuffs of any sort. I can't, and don't mean to pretend I can, tell you what to do.

Reality Check
2016-Jun-09, 04:18 AM
Up to about a light-month or so, yes. After that it's vague; it fits Ok but many other possibilities do also. ...
ETA: The clear measurement of stars spinning down as predicted by GR was for the Hulse–Taylor binary (https://en.wikipedia.org/wiki/Hulse%E2%80%93Taylor_binary) 21,000 light-years away.
We have just detected gravitational waves from the merger of black holes as predicted by GR (not uncited "possibilities") 1.3±0.6 billion light years away. That is not a light-month or vague.

GR makes no predictions at all about dark matter (or neutrinos, etc.) so "all the DM-evidence" does not invalidate GR.

GR has been tested from solar scales to billions of light years.

You want a question so: Cite any of the "many other possibilities" that have passed the tests that GR has passed, algore. Or match the observation of gravitational waves.
ETA: I aware that alternative theories exist that pass one or 2 tests.

A theory that passes all of the tests that GR has passed and fits the dark matter data better would make GR invalid. Just like GR makes Newtonian gravitation invalid. That does not mean that an invalid theory cannot be used. It means that an invalid theory can be used within its limits, e.g. Newtonian gravitation can be used for velocities << c and weak gravitational fields.

LaurieAG
2016-Jun-09, 10:27 AM
The actual problem with DM is that over the last 40 years we have found more and more of it :eek:! Not detecting dark matter directly does not say much about its existence - more that it is stranger than we thought.

While dark matter may be strange ratios of 2 \pi are ubiquitous.

\lambda = \bar \lambda 2 \pi
h = \hbar 2 \pi
{\omega}{\lambda} = {{v}2 \pi}
Calculated universal matter = (dark matter + ordinary matter) = (ordinary matter)2 \pi.

Cougar
2016-Jun-09, 01:42 PM
That's the great thing about cosmology, you can explain anything at all, because it's not an experimental science, it's an observational science. Gives you all the latitude in the world - or, Universe.

Maybe you can explain what you mean by this?

algore
2016-Jun-09, 04:43 PM
Up to about a light-month or so, yes. After that it's vague; it fits Ok but many other possibilities do also. ...


ETA: The clear measurement of stars spinning down as predicted by GR was for the Hulse–Taylor binary 21,000 light-years away.
We have just detected gravitational waves from the merger of black holes as predicted by GR (not uncited "possibilities") 1.3±0.6 billion light years away. That is not a light-month or vague.

There are two confusions happening here, and one non-critical mistake on my part.

First, GR includes Newtonian gravity! In other contexts, GR loosely means "the part of GR which goes beyond Newton", but that's not the case here. I realized that confusion when you mentioned Hulse-Taylor, PSR B1913+16, as an example of a system far away which has been shown to obey GR. You could simply cite any binary star system! They obey Newtonian gravity, therefore GR.

The main confusion: suppose you have two objects which are closer to each other than a light-month, but very far from us - like, 21,000 light years. Of course they can obey GR. It doesn't matter how far from us they are, but how far from each other.

The mistake: I forgot about star clusters. In the Milky Way I think they've been shown, by Virial theorem, to obey Newtonian (therefore GR) gravity. (BTW in other galaxies there are some star clusters which don't appear to). Now, all I'm wanting to say is that GR (including Newton) has been proven only for objects within a very limited distance of each other. Thinking only of Solar systems (like Sol, Sirius and other binary systems, exoplanetary systems, PSR B1913+16, etc) I said a "light-month". But remembering star clusters that's no good, should be much larger: 100 light years should do? Seems like a big mistake! - but irrelevant to my main point.

When you get out to galactic distances, more than a few thousand light years, GR (plus Newton) has never checked out correctly. Of course that's where DM, MOND etc comes in.

I hope that's clear now.

Concerning grav waves, yes, they've travelled great distances as predicted by GR. Once the grav waves are in motion GR predicts, basically, that they'll keep going, diminishing by 1/R; and this checks out. So travelling of grav waves for great distances is an exception, not critical, to my statement. I'm concentrating on the grav produced by objects; not waves. Those waves were generated by objects within 100 light-years of each other: in fact, colliding.

Bottom line, for objects at distances greater than about 100 ly - from each other, not from us - we're free to modify GR, or even invent brand-new forces, to fit the data. We don't need to insist GR is still correct and invent DM instead. Of course none of this is new.


That's the great thing about cosmology, you can explain anything at all, because it's not an experimental science, it's an observational science. Gives you all the latitude in the world - or, Universe.


Maybe you can explain what you mean by this?

Well, it's an extreme exaggeration. I've been banging away at the idea that since we can't experiment on galaxies, much less galactic clusters or the entire universe, cosmologists have a lot of latitude interpreting data. Looking at Bullet Cluster and Abell 520, both DM and MOND / TeVeS easily handle them. DM explains the first and tries to dispose of the second by showing it's faulty data (without success so far). MOND admits to being off by a factor of two, but considers it no big deal. TeVeS explains Bullet Cluster through mathematical manipulations which don't seem very convincing; I decided they could probably do the same with Abell 520. Anyway, my exaggerated statement expresses some frustration: how can you ever pin these guys down?

In fact I'm wrong, they can't explain "anything at all". But they can explain a lot more than would be possible in an experimental science. Sorry, my fingers go considerably faster than my brain. I'll avoid getting carried away like that again.


GR makes no predictions at all about dark matter (or neutrinos, etc.) so "all the DM-evidence" does not invalidate GR.

I said "on the face of it" the evidence DM was invented to explain (i.e. the DM-evidence) invalidates GR. If there's no DM (or some such explanation) then rotation curves, and galactic cluster gravity (far too much according to straightforward application of Virial theorem) invalidate GR on the large scale.


GR has been tested from solar scales to billions of light years.

Universe expansion fits GR, and accelerated expansion fits also with a cosmological constant. True but that's not a very strict test. That data can fit many theories. Compared to, say, Mercury perihelion this test is extremely imprecise. There's also gravitational lensing, limited to light bending, and quite imprecise. But alright, let's say it's been tested "with any precision, for the predicted gravitational potential (or spacetime curvature) generated by individual objects" only under 100 light years or so. Hopefully that dots all the i's and crosses all the t's.


You want a question so: Cite any of the "many other possibilities" that have passed the tests that GR has passed, algore. Or match the observation of gravitational waves.

It's an assumption on your part, not proven, that I "want" a question! Ok I'll give an example but this post is too long already, leave it for another one. Also such speculation may be judged to belong in ATM. Understand I don't believe and am not promoting any model. I just want to show how easy it is to make one up, because cosmology is an "observational science" not experimental. I have nothing anywhere near solid enough for ATM and don't expect to. But, I'd rather put it separately so if anyone objects that "bad" post can be easily deleted.

kzb
2016-Jun-09, 05:20 PM
I glanced at Gnacinski-Mlynik a while ago. I just can't believe that Zwicky, Vera Rubin and thousands of others since could make such a huge mistake about galactic rotation curves, in particular Milky Way. Perhaps this paper has an interesting take on some old data but not enough to discard the mainstream view. If that paper remains an issue, someday I'll study it more.


Sorry, that paper is just the beginning of a thread I derailed into another topic. I too found it less than convincing.

What I am trying to get more interest in is Extreme Scattering Events and what is causing them. In particular we could do with some expertise in optical microlensing in the debate.

Have a look at the "Cloudy Universe" site here. It's from 1999 but I am interested in how well it still stands up in the light of later studies.

http://www.atnf.csiro.au/pasa/16_3/walker/paper/index.html

algore
2016-Jun-09, 07:59 PM
What I am trying to get more interest in is Extreme Scattering Events and what is causing them. In particular we could do with some expertise in optical microlensing in the debate. Have a look at the "Cloudy Universe" site here. It's from 1999 but I am interested in how well it still stands up in the light of later studies.http://www.atnf.csiro.au/pasa/16_3/walker/paper/index.html

It seems a good guess that ESE's are caused by AU-sized clouds of atoms, molecules, dust.

Where it gets controversial is using these clouds as DM. There's only the one pair of "Mark"'s working on it; no one else interested; this is not a good sign! The issues they address seem difficult. There have to be a lot of clouds, they formed very early, and managed to remain stable. One thing occurred to me: given their early formation, they might have no metals at all. That might be interesting. Is composition of ESE-causing clouds known?

The only thing I'm particularly curious about, forgetting DM: are there any problems explaining ESE's by such clouds? Is it well-accepted?

algore
2016-Jun-10, 05:48 AM
Cite any of the "many other possibilities" that have passed the tests that GR has passed, algore.


Ok I'll give an example but this post is too long already, leave it for another one. Also such speculation may be judged to belong in ATM. Understand I don't believe and am not promoting any model. I just want to show how easy it is to make one up, because cosmology is an "observational science" not experimental.

It turns out the ideas are so plausible they won't be allowed here, but banished to ATM subforum. No one will believe it's just an example, they'll think I'm promoting it. So I decided to bite the "Bullet" and post one of them in ATM. Please check out this thread:

http://cosmoquest.org/forum/showthread.php?161566-Black-Hole-Modified-New-Gravity-(BHMOND)&p=2358566#post2358566

Cougar
2016-Jun-11, 06:32 PM
The main confusion: suppose you have two objects which are closer to each other than a light-month, but very far from us - like, 21,000 light years. Of course they can obey GR. It doesn't matter how far from us they are, but how far from each other.

So I guess you're questioning Einstein's fundamental Equivalence Principle, which AIUI, contains the assumption that the laws of physics (or the physical constants) are the same "everywhere." Certainly scientific assumptions should be scrutinized. Apparently that's why this group of scientists (http://arxiv.org/abs/0806.3081) observed the results of this "experiment" that was naturally set up out there in the universe between us and a quasar 6 billion lightyears distant....



"In particular, they looked at the frequency of light coming from the quasar and passing through ammonia gas in a galaxy between the quasar and Earth. That gas absorbs some of the quasar light at a frequency that depends on the properties of the atom, including the ratio of the electron mass to the proton mass."

"...the protons there are 1836.15 more massive than the electrons, the same value as here on Earth now." - article on the finding (http://www.symmetrymagazine.org/breaking/2008/06/19/are-the-laws-of-physics-the-same-throughout-the-universe)



Of course, "gravity" is unique among the four known "forces," and the above finding is not about whether the gravitational constant is constant with distance, but, as any finding does, I guess, it fills in a rather interesting piece of the overall puzzle of the universe we find ourselves within.



"I began to view Nature as an intelligence test to which humanity as a whole has been subjected..." - Gerard 't Hooft...


And of course, as Timothy Ferris said:



"We live in a changing universe, and few things are changing faster than our conception of it."

algore
2016-Jun-11, 11:46 PM
AFAIK your general idea is right but details are off. You're actually thinking of the Cosmological Principle, which says laws of physics are the same "everywhere". Equivalence principle says, basically, that inertial and gravitational mass are equal. Einstein expressed it as the equivalence of gravitation and acceleration in GR, same idea in different words. This terminology can be confusing and is not very important.

I'm not questioning Cosmological Principle (here). On the contrary I'm asserting that no matter where two objects are interacting, throughout the universe, they obey the same laws. Regardless how far from us they are.

Instead I'm questioning equivalence to some degree. Suggesting that gravity behaves very different far away from the gravitating object. If it's not isotropic, in particular, you can definitely violate that equivalence.

I could put it like this. If you have any other matter here where Earth is, it will follow exactly the same orbit - that's a standard fact of equivalence of inertial and gravitational mass. Similarly, if you drop two objects off the leaning tower of Pisa (as Galileo was apocryphally said to do), they hit the ground at the same time, regardless of their mass.

But if gravity of an object can be anisotropic - i.e., directional - that wouldn't be true. Suppose you have two such objects in Earth orbit. For one of them, "point" its gravitational "vector" towards the sun; the other, not. Then evidently the first would go into a tighter orbit than the second. That's the case even if they're identical, just oriented different.

Since this is only speculation one can't say what would actually happen but that seems like the obvious conclusion.

Such crazy guesses are allowed, I think, as long as we're in a regime where current experiments and observations tell us nothing. Most facts about Black Holes are like that.

I hope this clears up confusion but suspect it might just increase it!

Cougar
2016-Jun-12, 12:00 PM
You're actually thinking of the Cosmological Principle...

You're probably right. I was led to believe that the Comological Principle was a necessary assumption of the Equivalence Principle. Perhaps it is, but as you say, this distinction is not crucial to the point being made.

The problem I have with your allowance that the gravitational effect may vary with distance (but differently from Newton or Einstein's formulations) is that the strength of gravity is "well behaved" when objects are reasonably close together, but then the effect of gravity must get stronger when objects are more distant. This is clearly not the case within our solar system, not even a little. No one has even attempted to explain how or why gravity would have such a quality. This does not mean it is an impossibility. I'd just call it a bad bet.

algore
2016-Jun-12, 04:02 PM
Cougar, I believe you're talking about the idea of replacing the Dark Matter (DM) hypothesis with some sort of modified gravity hypothesis. Elsewhere I posted an idea BHMOND which postulates that extra grav force is found in Black Holes (BH), particularly Super Massive BH's (SMBH) at the center of galaxies. The first question is how this "extra" gravity potential must be shaped to account for the more-or-less flat rotation curves of galaxies. I assume that's the situation you're talking about.

Let's address this, hand-wavingly.

It's probably not true that to account for rotation curves - high velocities of stars and dust far from the galactic center - my hypothetical grav must actually get stronger as R increases. Objects farther out are attracted not only by the SMBH but also all the other mass within the sphere inside their orbit. This reduces the attraction necessary for the SMBH; gravity might be roughly constant throughout the disk.

Another factor: it's not unreasonable to suppose the extra "far field" grav has torsion, like the magnetic field in the Lorentz force equation of EM. Note, Kerr solution for BH has torsion component in the ergosphere. Admittedly it's one-way, unlike Lorentz force equation. TeVeS also has "two-way" gravitomagnetic torsion; and it's a "mainstream" model (although somewhat "fringe"). Remember that work on torsional grav fields like TeVeS was done by Einstein. He was motivated by general theoretical considerations, didn't even know about rotation curve problem. So torsion is not such a "bad bet". With torsion to help account for high rotation speeds, the centripetal grav component can be much weaker.

Admittedly, overall grav must remain much stronger than 1/R^2 all the way to 100,000 ly and more. Don't forget it also must account for high dispersion of galaxies within galactic clusters, and grav lensing around those clusters which indicates far more grav potential than straightforward application of Newtonian potential gives. That requires my new grav to be stronger out to millions of ly; but it can diminish as it goes out there, even more than within the galaxy.

Bottom line, the grav can fall off by perhaps even as much as inverse R, or square root, or at least logarithm. Work remains to be done on this issue.

The above goes to show the new grav potential doesn't have to be too strange. The other fork of my response is as follows.

At the moment there is no real justification for BHMOND type gravity being unintuitively stronger than Newton, as R increases: it's driven only by the need to fit the data. DM hypothesis is more intuitive, by retaining normal gravity. Non-baryonic matter is not intuitive but you could argue it's better than my type of modified gravity. Certainly if DM were found it would be a great solution to the problem.

But as you say "nonintuitive" doesn't make it impossible just a somewhat "bad bet". If a single "crazy idea" can fit all the data, I feel the history of science justifies exploring it. BHMOND is far less strange than either QM or SR were, before they were accepted.

So 1) BHMOND is not as strange as you make out - the grav doesn't have to increase with distance; and 2) one strange new idea in a new theory is par for the course in science.

[EDIT] BTW it occurs to me, I wish you'd asked this down in the appropriate ATM thread; this discussion really belongs there. I think I'll transfer it after giving you a chance to see it up here.

Reality Check
2016-Jun-13, 11:27 PM
It turns out the ideas are so plausible they won't be allowed here, but banished to ATM subforum.
This is the Space/Astronomy Questions and Answers section. My question is on your assertion about mainstream science. If you have a citation to the scientific literature (which contains mainstream science) then you can cite any of the "many other possibilities" that have passed the tests that GR has passed here, algore.
However your description of these "ideas" as ATM suggests that they do not exist in the scientific literature.

Reality Check
2016-Jun-13, 11:30 PM
Elsewhere I posted an idea BHMOND ....
Which should be addressed in that new ATM thread,. Here we have the scientific literature to look at which a blogger has:
Black hole dark matter by Résonaances (http://resonaances.blogspot.co.nz/2016/06/black-hole-dark-matter.html)

The idea that dark matter is made of primordial black holes is very old but has always been in the backwater of particle physics. The WIMP or asymmetric dark matter paradigms are preferred for several reasons such as calculability, observational opportunities, and a more direct connection to cherished theories beyond the Standard Model. But in the recent months there has been more interest, triggered in part by the LIGO observation of a black hole binary merger. In that event, the mass of each of the black holes was estimated at around 30 solar masses. While such a system may well be of boring astrophysical origin, it is somewhat unexpected because typical black holes we come across in everyday life are either a bit smaller (around one solar mass) or much larger (supermassive black hole in the galactic center). On the other hand, if the dark matter halo were made of black holes, scattering processes would sometimes create short-lived binary systems. Assuming a significant fraction of dark matter in the universe is made of primordial black holes, this paper estimated that the rate of merger processes is in the right ballpark to explain the LIGO event.

Cougar
2016-Jun-14, 12:59 PM
With torsion to help account for high rotation speeds, the centripetal grav component can be much weaker.

Not much. Doesn't the equation for torsion have a "c" in the denominator? I don't see this adding much to the gravitational field strength.


Bottom line, the grav can fall off by perhaps even as much as inverse R, or square root, or at least logarithm. Work remains to be done on this issue.

But nearby, it has to fall off precisely by inverse R2. Then as distance increases, it has to become stronger than that formula. If the gravitational field strength became weaker than inverse R2 with distance, I can imagine that there could be a natural explanation for that. It's much more difficult to imagine a natural explanation for a gravitational field strength that becomes stronger than inverse R2 with distance.


At the moment there is no real justification for BHMOND type gravity being unintuitively stronger than Newton, as R increases: it's driven only by the need to fit the data.

I don't know why this bugs me. It's close to what scientists do. I guess it's that word "only." Milgrom laid out the galaxy rotation data and fashioned an equation from it, so of course the equation fits the data (except for cluster dynamics). Otherwise, there is no explanation even attempted as to how this could occur. "It just does" is not very satisfying in this case.

algore
2016-Jun-15, 05:04 PM
I haven't figured out how torsion works yet. Gravity in the disk must be greater than inverse r^2, we agree on that.

I care more about "what" than "why" but, of course, want both. It's not logical to reject Milgrom just because he doesn't answer "why". That's looking a gift horse in the mouth. If the system works - fits the data - that justifies its existence. Others are working on the "Why" of MOND, TeVeS for instance is a step in that direction. If it's correct (I don't say it is; I don't know) then there must be an explanation, which will be found in due course.

Cougar
2016-Jun-15, 07:13 PM
Gravity in the disk must be greater than inverse r^2, we agree on that.

Haha. It must be if there is no dark matter, which I (contingently) disagree with, since dark matter currently offers the best explanation for these observations, IMO.


I care more about "what" than "why" but, of course, want both. It's not logical to reject Milgrom just because he doesn't answer "why". That's looking a gift horse in the mouth. If the system works - fits the data - that justifies its existence. Others are working on the "Why" of MOND, TeVeS for instance is a step in that direction. If it's correct (I don't say it is; I don't know) then there must be an explanation, which will be found in due course.

Yeah, science in general is not into the "why." But it'd be nice if there was some explanation as to the "how" of these modified gravity theories....

algore
2016-Jun-15, 07:50 PM
Haha. It must be if there is no dark matter ...

You're right, I forgot that caveat.

WalrusLike
2016-Jun-15, 09:12 PM
So now there has been another black holes merging event detected by LIGO at 8 and 14 Sol masses I wonder how that aligns (or not) with the OP.

Reality Check
2016-Jun-15, 09:53 PM
Gravity in the disk must be greater than inverse r^2, we agree on that.
'What you mean, "we," Kemo Sabe?' (http://c2.com/cgi/wiki?WhoDoYouMeanWeKemoSabe) :D.
The consensus is that gravity in the disk is the same as everywhere else - an inverse square law. Thus the need for dark matter, including the possibility of the subject of this thread (primordial black holes).

Reality Check
2016-Jun-15, 10:14 PM
So now there has been another black holes merging event detected by LIGO at 8 and 14 Sol masses I wonder how that aligns (or not) with the OP.
It is irrelevant to the OP.
The first detection was above the mass usually expected for stellar black holes but see On the Maximum Mass of Stellar Black Holes (http://adsabs.harvard.edu/abs/2010ApJ...714.1217B) which has a limit of 30 solar masses in favorable conditions. This detection is closer to what we expect for stellar black holes. Both detections emphasize that just looking at the mass of a black hole does not tell us that it is a primordial black hole - it could be the result of a merger.