Dark matter variations on Earth

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Not true. The evidence is that completely collisionless DM is inconsistent with the observed distribution of DM. Therefore, there has to be either self-interaction in DM or a baryonic-DM interaction or modified gravity. See, e.g.:

* Paolo Salucci and Nicola Turini, "Evidences for Collisional Dark Matter In Galaxies?" (July 4, 2017). Abstract:


* Dark matter distributions have to closely track baryon distributions, even though there is no viable mechanism to do so: Edo van Uitert, et al., "Halo ellipticity of GAMA galaxy groups from KiDS weak lensing" (October 13, 2016).
Are all models of self-interacting dark matter proposed just to solve this problem of dark matter-baryon distribution you mentioned above? If it's not only that.. then what other observed distributions needs the self-interacting dark matter model.. And why is there a second baryonic-DM interaction model. Can't we decide based on the observations whether it is best modeled using purely self-interacting DM or baryonic-DM interaction?

Absolute no paper that explores the fifth force whether it is based on gauge field, abelian/non-abelian and viable/plausible symmetry breaking mechanism and whether it is connected at all to the electroweak mechanism?
 
  • #77
ohwilleke
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Are all models of self-interacting dark matter proposed just to solve this problem of dark matter-baryon distribution you mentioned above? If it's not only that.. then what other observed distributions needs the self-interacting dark matter model.. And why is there a second baryonic-DM interaction model. Can't we decide based on the observations whether it is best modeled using purely self-interacting DM or baryonic-DM interaction?
Lots of effort is going into improving simulations and into data gathering, but the simulations still have to have a pretty gross resolution to be computationally feasible and involve lots and lots of somewhat ad hoc assumptions. The paper cited makes an analytical argument rather than a simulation.

Absolute no paper that explores the fifth force whether it is based on gauge field, abelian/non-abelian and viable/plausible symmetry breaking mechanism and whether it is connected at all to the electroweak mechanism?
Many papers take one to five models of the same or similar type and examine them individually, to see what they imply and consider isolated bit of evidence ruling them out or allowing them in some parameter space. Few compare models of different types. And, usually they are explicitly studying "toy models" that are a general as possible.
 
  • #78
Drakkith
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Absolute no paper that explores the fifth force whether it is based on gauge field, abelian/non-abelian and viable/plausible symmetry breaking mechanism and whether it is connected at all to the electroweak mechanism?
Sure, there are papers exploring the idea of self-interacting dark matter. Here's one from 1999: https://arxiv.org/abs/astro-ph/9909386
 
  • #79
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Lots of effort is going into improving simulations and into data gathering, but the simulations still have to have a pretty gross resolution to be computationally feasible and involve lots and lots of somewhat ad hoc assumptions. The paper cited makes an analytical argument rather than a simulation.



Many papers take one to five models of the same or similar type and examine them individually, to see what they imply and consider isolated bit of evidence ruling them out or allowing them in some parameter space. Few compare models of different types. And, usually they are explicitly studying "toy models" that are a general as possible.
Thanks for the thoughts... I have 3 important questions:

1. Do you agree with Neil deGrasse that it should be called dark gravity instead of dark matter.. in the video..


I put his words into text in the following where Neil said "It's a not a matter whether dark matter exist or not... its a measurement, period. Now.. dark matter is not even what should we be calling it.. because it implies that it is matter. It implies we know something about it which we actually does.. a more precise labelling for it.. would be dark gravity... now if we call it dark gravity, you are gonna say does dark gravity truly exist.. i'd say yeah.. if 85% of the gravity has no known origin.. there it is. Let's figure out what's causing it, the fact that the matter got into that word is. #^%^@#$.. not matter.. it could be something else... we are overreacting to a label that overstates our actual insight or knowledge of what it is we are describing.. then i just joke we could just call it fred or wilma.. something where there is no reference to what we think it is.. because in fact we have no idea what it is"

what is the #^%^@#$? I can't understand his English.

my question is.. if it's not matter... do you know of any exotic field that CAN'T be quantized into particle? meaning not matter but pure energy? Or could it be some form of spacetime manifold that produce energy and not matter at all (how?)

2. If let's say 0.5% of dark matter has self-interaction with our baryonic matter yet they are not enough to produce any gravitational changes.. should it still be be called dark matter? or more like part of baryonic matter only invisible?

3. Are all star sytem or galaxies or any object in the sky connected by cosmic filaments of big and small sizes or ambient dark matter in deep space... I mean.. in the midst of deep space.. are there ambient dark matter even if there is no matter? Are say the constellations of Sagittarius or Pieces or Gemini connected to our system by some kind of small sized cosmic filaments or ambient dark matter?

Thank you!
 
  • #80
Drakkith
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what is the #^%^@#$? I can't understand his English.
Instead of using those symbols, I'd type <unintelligible> or something similar. Usually when people put symbols like this into a sentence it means that the words are curse words. To answer your question, he started to say something and then switched. I believe it was, "the fact that the matter got into that word is forcing people to-well I have another idea, I bet it's not matter it's something else."

2. If let's say 0.5% of dark matter has self-interaction with our baryonic matter yet they are not enough to produce any gravitational changes.. should it still be be called dark matter? or more like part of baryonic matter only invisible?
Self-interaction means that dark matter is interacting with itself. That's why the "self" is placed before "interaction". An interaction between dark matter and baryonic matter is just called an interaction. Also, trying to quantify this as "0.5% of dark matter interacts with baryonic matter" is meaningless without further context. Is 0.5% of dark matter composed of particles that interact with bayonic matter, while the other 99.5% don't? Is this 0.5% talking about the strength of the interaction? is it some sort of cross section? I realize you probably don't have an answer for this and are just trying to ask a question, but my point is that it would greatly benefit you to try to think of these things before asking. Otherwise you'll just get frustrated when people consistently can't answer your question or keep correcting you.

3. Are all star sytem or galaxies or any object in the sky connected by cosmic filaments of big and small sizes or ambient dark matter in deep space... I mean.. in the midst of deep space.. are there ambient dark matter even if there is no matter? Are say the constellations of Sagittarius or Pieces or Gemini connected to our system by some kind of small sized cosmic filaments or ambient dark matter?
Galaxies and galaxy clusters are distributed in such a way as to roughly mirror the distribution of dark matter, so you'll often see long filaments of dark matter "connecting" galaxies together over cosmological distances. However, nothing is connecting individual star systems to others.
 
  • #81
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Instead of using those symbols, I'd type <unintelligible> or something similar. Usually when people put symbols like this into a sentence it means that the words are curse words. To answer your question, he started to say something and then switched. I believe it was, "the fact that the matter got into that word is forcing people to-well I have another idea, I bet it's not matter it's something else."



Self-interaction means that dark matter is interacting with itself. That's why the "self" is placed before "interaction". An interaction between dark matter and baryonic matter is just called an interaction. Also, trying to quantify this as "0.5% of dark matter interacts with baryonic matter" is meaningless without further context. Is 0.5% of dark matter composed of particles that interact with bayonic matter, while the other 99.5% don't? Is this 0.5% talking about the strength of the interaction? is it some sort of cross section? I realize you probably don't have an answer for this and are just trying to ask a question, but my point is that it would greatly benefit you to try to think of these things before asking. Otherwise you'll just get frustrated when people consistently can't answer your question or keep correcting you.
I mean "interaction".. I didn't read it again or i'll catch it. I'm familiar with the distinctions. Yesterday I was watching about dark matter at youtube. And I came across this 1 hour Lisa Randall video:


When I saw her book "Dark matter and the Dinosaurs" before.. I thought she was just talking about them as part of universe and didn't know she was describing a mechanism whereby they could be connected (i'll read her book sometime next week)... this is the best illustration I found on the net:


iTj0dJ.jpg



The gist of the idea is simply there is a thin dark matter disk at plane of galaxy that can disrupt comets, etc. or as Lisa put it in https://www.scientificamerican.com/...the-dinosaurs-a-q-a-with-author-lisa-randall/

"People have debated whether dark matter has any nongravitational interactions at all. But [my colleagues and I] thought, maybe just a fraction of dark matter does. Just the way ordinary matter is only 15 percent of all the matter in the universe, maybe there’s a fraction of dark matter—even 5 percent of the matter in the universe—that has its own interactions. It’s not the usual dark matter that forms this spherical halo [around the galaxy], it’s a new type of dark matter. So you still have the ordinary halo but in addition you have this dark matter disk."

or more details at http://www.dailygalaxy.com/my_weblo...entral-supermassive-black-hole-weekend-m.html

"The extinction of the dinosaurs, however, is just one theory that will have to be re-examined if Randall and Reece’s theory proves true.”

Our Sun orbits around the Galactic center, taking approximately 250 million years to make a complete revolution. However, this trajectory is not a perfect circle. The Solar System weaves up and down, crossing the plane of the Milky Way approximately every 32 million years, which coincides with the presumed periodicity of the impact variations. This bobbing motion, which extends about 250 light years above and below the plane, is determined by the concentration of gas and stars in the disk of our Galaxy.

This ordinary “baryonic” matter is concentrated within about 1000 light years of the plane. Because the density drops off in the vertical direction, there is a gravitational gradient, or tide, that may perturb the orbits of comets in the Oort cloud, causing some comets to fly into the inner Solar System and periodically raise the chances of collision with Earth. However, the problem with this idea is that the estimated galactic tide is too weak to cause many waves in the Oort cloud.

In their study, Randall and Reece focus on this second hypothesis and suggest that the galactic tide could be made stronger with a thin disk of dark matter. Dark disks are a possible outcome of dark matter physics, as the authors and their colleagues recently showed. Here, the researchers consider a specific model, in which our Galaxy hosts a dark disk with a thickness of 30 light years and a surface density of around 1 solar mass per square light year (the surface density of ordinary baryonic matter is roughly 5 times that, but it’s less concentrated near the plane).

Although one has to stretch the observational constraints to make room, their thin disk of dark matter is consistent with astronomical data on our Galaxy. Focusing their analysis on large (>20km) craters created in the last 250 million years, Randall and Reece argue that their dark disk scenario can produce the observed pattern in crater frequency with a fair amount of statistical uncertainty."

For those already familiar with Lisa proposal. It's nothing new.. but I just learnt about this last night... now to get in the mood. Lisa commented in the same url "“If you were to look at our world and assume there was only one type of particle, you’d be pretty wrong,” said Randall. “I think it’s definitely a worthwhile theory to explore, because even if this is only a small fraction of dark matter, there is six times more dark matter in the universe than ordinary matter. We care a lot about ordinary matter, and that’s precisely because it has interactions. So if there is a small portion of dark matter that has those interactions, that may be what we should pay attention to, perhaps even more so than other dark matter.”

Now my question.

What if 0.5% of dark matter has interactions with matter yet doesn't affect the cosmos gravitationally. For example.. supposed, just for sake of discussion (note this is just for sake of discussion, ok?)... life has evolved in the dark matter sector and these produced the jinns of myth and legends (said to be made of "smokeless fire" and different from matter)...

YSei57.jpg


And let's say these jinns don't affect the gravitational behavior of the galaxy or solar system or even planets.. can these beings still be referred as dark matter? If not.. then these can be referred to as normal baryonic matter but only invisible? Supposed one needs to address this to the world physicists as intel briefing.. must dark matter be used or invisible normal baryonic matter to describe them? Again this is just an example because I can't think of one right now although i'll watch more dark matter videos in the youtube to give more accurate descriptions or examples.




Galaxies and galaxy clusters are distributed in such a way as to roughly mirror the distribution of dark matter, so you'll often see long filaments of dark matter "connecting" galaxies together over cosmological distances. However, nothing is connecting individual star systems to others.
 

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  • #82
Drakkith
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I mean "interaction".. I didn't read it again or i'll catch it. I'm familiar with the distinctions.
My apologies then. I thought there might have been a translation issue, hence my explanation.

The gist of the idea is simply there is a thin dark matter disk at plane of galaxy that can disrupt comets, etc. or as Lisa put it in https://www.scientificamerican.com/...the-dinosaurs-a-q-a-with-author-lisa-randall/

"People have debated whether dark matter has any nongravitational interactions at all. But [my colleagues and I] thought, maybe just a fraction of dark matter does. Just the way ordinary matter is only 15 percent of all the matter in the universe, maybe there’s a fraction of dark matter—even 5 percent of the matter in the universe—that has its own interactions. It’s not the usual dark matter that forms this spherical halo [around the galaxy], it’s a new type of dark matter. So you still have the ordinary halo but in addition you have this dark matter disk."
Okay, so your question involves 0.5% of dark matter being composed of particles which do interact with regular matter. Got it.

Now my question.

What if 0.5% of dark matter has interactions with matter yet doesn't affect the cosmos gravitationally.
That would be something unseen before, unless you mean that it still interacts through gravity, it just doesn't create enough of a disturbance to be readily observable.

. For example.. supposed, just for sake of discussion (note this is just for sake of discussion, ok?)... life has evolved in the dark matter sector and these produced the jinns of myth and legends (said to be made of "smokeless fire" and different from matter)...

And let's say these jinns don't affect the gravitational behavior of the galaxy or solar system or even planets.. can these beings still be referred as dark matter? If not.. then these can be referred to as normal baryonic matter but only invisible? Supposed one needs to address this to the world physicists as intel briefing.. must dark matter be used or invisible normal baryonic matter to describe them? Again this is just an example because I can't think of one right now although i'll watch more dark matter videos in the youtube to give more accurate descriptions or examples.
What's the importance behind the name? Whether we call it "dark matter" or "nearly dark matter" doesn't change anything. It's just a way to classify it for our own convenience. Non-baryonic matter is another perfectly good name you could give it. Scientists working in the relevant field are (usually) not confused by a name.
 
  • #83
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My apologies then. I thought there might have been a translation issue, hence my explanation.



Okay, so your question involves 0.5% of dark matter being composed of particles which do interact with regular matter. Got it.



That would be something unseen before, unless you mean that it still interacts through gravity, it just doesn't create enough of a disturbance to be readily observable.



What's the importance behind the name? Whether we call it "dark matter" or "nearly dark matter" doesn't change anything. It's just a way to classify it for our own convenience. Non-baryonic matter is another perfectly good name you could give it. Scientists working in the relevant field are (usually) not confused by a name.
I know all that has mass or energy interacts gravitationally.. some may just not be enough to affect on cosmos scale gravitational dynamics. This was what i meant. .

Now I wanna focus on dark matter that can interact with matter. Are there models where dark matter is part of normal matter. I'll explain. Let's say we define baryonic matter as those that has interaction with the higgs field.. then let's say we hadn't discovered relativity yet where the self energy of the strong force could give rise to mass.. then we could refer to the strong force and gluons as dark matter.. . could there be a similar scenario happening? that dark matter is part of normal matter whose extra dynamics is not yet discovered.. this is the thing I wanna explore now.. any papers about this.. ping ohwilleke for his tons of more known papers & references which I may not be aware even after arxiv search. Thanks.
 
  • #84
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What is your thought about mirror matter or shadow matter or alice matter.. https://en.wikipedia.org/wiki/Mirror_matter

"If mirror matter is present in the universe with sufficient abundance then its gravitational effects can be detected. Because mirror matter is analogous to ordinary matter, it is then to be expected that a fraction of the mirror matter exists in the form of mirror galaxies, mirror stars, mirror planets etc. These objects can be detected using gravitational microlensing.[31] One would also expect that some fraction of stars have mirror objects as their companion. In such cases one should be able to detect periodic Doppler shifts in the spectrum of the star.[14][dead link] There are some hints that such effects may already have been observed.[32][33]"

This paper was written in 1991 when superstring was still a craze.

http://adsbit.harvard.edu/cgi-bin/n...=0&data_type=GIF&type=SCREEN_VIEW&classic=YES

Not much come out in arxiv search for entry "mirror matter" maybe they use other words now....

Questions.

1. Is mirror matter a concept that only comes about in superstring theory? Without superstring theory.. would there still be a concept of mirror matter?

2. What are the experimental constrains for the detection of these? Note these are even more radical than simple dark matter because they propose ordinary objects have mirror matter.. but this seems simple to refute owing to if supposed earth had mirror earth (or all stars have mirror stars).. we could easily refute it by gravitational null effect.. so why the heck so they still propose mirror matter.. Perhaps they are saying mirror matter is only presence in some matter and not all matter?

3. If there is a 10 mile wide mirror matter satellite orbiting the earth.. can they detect this?

4. To make this not off topic.. how come no one mentioned DAMA or the detection of dark matter variations on earth.. there is not yet a full null results.. https://www.technologyreview.com/s/418687/first-evidence-that-mirror-matter-may-fill-the-universe/
 
  • #85
Drakkith
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Not much come out in arxiv search for entry "mirror matter" maybe they use other words now....
Here's a thesis on mirror matter and how it relates to cosmology: https://arxiv.org/abs/astro-ph/0312607

2. What are the experimental constrains for the detection of these? Note these are even more radical than simple dark matter because they propose ordinary objects have mirror matter.. but this seems simple to refute owing to if supposed earth had mirror earth (or all stars have mirror stars).. we could easily refute it by gravitational null effect.. so why the heck so they still propose mirror matter.. Perhaps they are saying mirror matter is only presence in some matter and not all matter?
It doesn't mean that there would be a mirror copy of every single particle in existence, it means that every type of particle has a corresponding mirror type that all interact via right-handed interactions. These mirror particles would be free to interact with themselves and form structures just like normal matter does.

4. To make this not off topic.. how come no one mentioned DAMA or the detection of dark matter variations on earth.. there is not yet a full null results.. https://www.technologyreview.com/s/418687/first-evidence-that-mirror-matter-may-fill-the-universe/

I didn't because I didn't know about it.
 
  • #86
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Here's a thesis on mirror matter and how it relates to cosmology: https://arxiv.org/abs/astro-ph/0312607



It doesn't mean that there would be a mirror copy of every single particle in existence, it means that every type of particle has a corresponding mirror type that all interact via right-handed interactions. These mirror particles would be free to interact with themselves and form structures just like normal matter does.




I didn't because I didn't know about it.
I'm reading it and these passages catch me (in page 103):

"Finally, in the interesting case where mirror baryons constitute all the dark matter, they drive the evolution of perturbations. In fact, in figure 5.21b we clearly see that the density fluctuations start growing in the mirror matter and the visible baryons are involved later, after being recombined, when they rewrite the spectrum of already developed mirror structures. This is another effect of a mirror decoupling occurring earlier than the ordinary one: the mirror matter can drive the growth of perturbations in ordinary matter and provide the rapid growth soon after recombination necessary to take into account of the evolved structures that we see today."

The paper was written in 2004.


6jsd4k.jpg


What year was the above image taken showing the dark matter separating from the gases... and shouldn't the dark matter be not self interacting with itself to form this pattern? Or can self-interacting dark matter (with itself) still produce the diffuse mass at either sides?

Or let's use definitions and note the difference between mirror baryonic matter.. and non-baryonic cold dark matter... how can mirror baryonic matter imitate non-baryonic dark matter? Let's say it's the reverse situation where the mirror baryonic world would see their galaxies with more mass as the arms.. what kind of normal baryonic matter can produce the mass in the arms? what particles.. maybe hydrogen that doesn't interact?

Also what must be the nature of normal and mirror matter so there is possibility to initiate handedness transformation so we can shift from this world to the mirror matter universe.. I mean.. if normal and mirror matter were emergence and not fundamental, could some symmetry mechanism initiate this handedness reversal? This is what would make it interesting.. plain mirror dark only accessible by gravity would make a very boring universe.
 

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  • #87
Drakkith
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What year was the above image taken showing the dark matter separating from the gases...
No idea, sorry.
and shouldn't the dark matter be not self interacting with itself to form this pattern?
I believe so, yes. Or at least self-interacting in only a weak manner.
Or let's use definitions and note the difference between mirror baryonic matter.. and non-baryonic cold dark matter... how can mirror baryonic matter imitate non-baryonic dark matter?
I don't know. I only just heard about mirror matter from you, so I haven't read much about it.
Also what must be the nature of normal and mirror matter so there is possibility to initiate handedness transformation so we can shift from this world to the mirror matter universe..
Let's leave sci-fi out of this please. :wink:
 
  • #88
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Gordon Kane said axions could be emitted from the sun.. how would the pattern look like? He mentioned in "String Theory and the Real World":

"Their properties lead to fascinating ways to detect them. The theory says the axions have an interaction that allows them to decay to two photons, a -> y y, denoting photons with y. Axions will be emitted by the sun, for example, since it has lots of particles moving around energetically. Set up a large magnet, which has a large region with a magnetic field, carried by photons. Also put up a wall that would stop any photons from the Sun or other sources. Then axions from the Sun can encounter photons of the magnetic field and generate the other photon of the ayy vertex, so suddenly photons appear on the far side of the wall where there should not be any. Innovative new axion detectors are being proposed as well, so axion detection is becoming increasingly likely".

Say, could axions from the sun also be self-interacting on way to earth (if they exist)?
 
  • #89
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Say, could axions from the sun also be self-interacting on way to earth (if they exist)?
I'm not aware of any model that predicts any relevant self-interaction for a source like the Sun where everything just streams outward at nearly the speed of light.

What you cited before is studied by helioscopes like CAST.
 
  • #91
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I'd like to ask something about modified gravity.. let me just quote sabine a bit (see her entire article) so I'd not mistakenly misrepresent her

http://backreaction.blogspot.com/2018/02/shut-up-and-simulate-in-which-i-try-to.html
"Shut up and simulate. (In which I try to understand how dark matter forms galaxies, and end up very confused.) "......

"And then there are the low surface-brightness galaxies. These are interesting because 30 years ago they were thought to be not existent. They do exist though, they are just difficult to see. And they spelled trouble for dark matter, just that no one wants to admit it.

Low surface brightness galaxies are basically dilute types of galaxies, so that there is less brightness per surface area, hence the name. If you believe that dark matter is a type of particle, then you’d naively expect these galaxies to not obey the Tully-Fisher relation. That’s because if you stretch out the matter in a galaxy, then the orbital velocity of the outermost stars should decrease while the total luminosity doesn’t, hence the relation between them should change.

But the data don’t comply. The low surface brightness things, they obey the very same Tully-Fisher relation than all the other galaxies. This came as a surprise to the dark matter community. It did not come as a surprise to Mordehai Milgrom, the inventor of modified Newtonian dynamics, who had predicted this in 1983, long before there was any data.

You’d think this would have counted as strong evidence for modified gravity. But it barely made a difference. What happened instead is that the dark matter models were adapted.

You can explain the observations of low surface brightness galaxies with dark matter, but it comes at a cost. To make it work, you have to readjust the amount of dark matter relative to normal matter. The lower the surface-brightness, the higher the fraction of dark matter in a galaxy."

I don't know if Sabine believes in modified gravity (MOND) or dark matter as particle. But something I don't understand. Let's say modified gravity is true.
How do you explain the following?

tsuP3c.jpg


Is there a version of modified gravity (or MOND) that can explain the above too?
 

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I'd like to ask something about modified gravity.. let me just quote sabine a bit (see her entire article) so I'd not mistakenly misrepresent her

http://backreaction.blogspot.com/2018/02/shut-up-and-simulate-in-which-i-try-to.html
"Shut up and simulate. (In which I try to understand how dark matter forms galaxies, and end up very confused.) "......

"And then there are the low surface-brightness galaxies. These are interesting because 30 years ago they were thought to be not existent. They do exist though, they are just difficult to see. And they spelled trouble for dark matter, just that no one wants to admit it.

Low surface brightness galaxies are basically dilute types of galaxies, so that there is less brightness per surface area, hence the name. If you believe that dark matter is a type of particle, then you’d naively expect these galaxies to not obey the Tully-Fisher relation. That’s because if you stretch out the matter in a galaxy, then the orbital velocity of the outermost stars should decrease while the total luminosity doesn’t, hence the relation between them should change.

But the data don’t comply. The low surface brightness things, they obey the very same Tully-Fisher relation than all the other galaxies. This came as a surprise to the dark matter community. It did not come as a surprise to Mordehai Milgrom, the inventor of modified Newtonian dynamics, who had predicted this in 1983, long before there was any data.

You’d think this would have counted as strong evidence for modified gravity. But it barely made a difference. What happened instead is that the dark matter models were adapted.

You can explain the observations of low surface brightness galaxies with dark matter, but it comes at a cost. To make it work, you have to readjust the amount of dark matter relative to normal matter. The lower the surface-brightness, the higher the fraction of dark matter in a galaxy."

I don't know if Sabine believes in modified gravity (MOND) or dark matter as particle. But something I don't understand. Let's say modified gravity is true.
How do you explain the following?

View attachment 221242

Is there a version of modified gravity (or MOND) that can explain the above too?
I just learnt the modified gravity is not same as MOND because modify gravity (Tensor–vector–scalar gravity) is relativistic generalization of MOND. In the bullet cluster, modified gravity can allegedly make the fields displace. Sabine explained in http://backreaction.blogspot.de/2017/01/the-bullet-cluster-as-evidence-against.html

"But modifying gravity works by introducing additional fields that are coupled to gravity. There’s no reason that, in a dynamical system, these fields have to be focused at the same place where the normal matter is. Indeed, one would expect that modified gravity too should have a path dependence that leads to such a delocalization as is observed in this, and other, cluster collisions. And never mind that when they pointed at the image of the Bullet Cluster nobody told you how rarely such an event occurs in models with particle dark matter."

Can these additional fields also be used to block gravity? Is this a prediction of Tensor–vector–scalar gravity too?
 
  • #93
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Unlike light, gravity cannot be shielded or blocked any known means. To the best of our knowledge bosons, like the photon and putative graviton, completely ignore other bosons. This is evident when you shine a light through a magnetic field. the light is not deflected irrespective of magnetic field strength.
 
  • #94
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To the best of our knowledge bosons, like the photon and putative graviton, completely ignore other bosons.
Various bosons couple to each other. ATLAS recently observed light-by-light scattering, associated production and vector boson fusion as Higgs production modes (seen by ATLAS and CMS) are examples. Radiative penguin diagrams (also seen) have W/photon interactions.
And so on.

Gravitons should interact with all bosons, but we don't have experimental results for that yet of course.
Your statement is completely wrong.
This is evident when you shine a light through a magnetic field.
Delbrück scattering
 
  • #95
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I just learnt the modified gravity is not same as MOND because modify gravity (Tensor–vector–scalar gravity) is relativistic generalization of MOND. In the bullet cluster, modified gravity can allegedly make the fields displace. Sabine explained in http://backreaction.blogspot.de/2017/01/the-bullet-cluster-as-evidence-against.html

"But modifying gravity works by introducing additional fields that are coupled to gravity. There’s no reason that, in a dynamical system, these fields have to be focused at the same place where the normal matter is. Indeed, one would expect that modified gravity too should have a path dependence that leads to such a delocalization as is observed in this, and other, cluster collisions. And never mind that when they pointed at the image of the Bullet Cluster nobody told you how rarely such an event occurs in models with particle dark matter."

Can these additional fields also be used to block gravity? Is this a prediction of Tensor–vector–scalar gravity too?
I'd like to ask about modified gravity.. first.. how accurate is Sabine statements in the following found at the url above:

"Cyberax,

The idea of modifying gravity is that there's no particle dark matter. I mean, strictly speaking it could be both, but if you have particle dark matter anyway, you don't need modified gravity, so it's kinda pointless to combine them. So, you modify gravity instead of adding particle dark matter.

Then the question is what's the difference? Well, if you add particle dark matter you add quantum fields to the standard model of particle physics. If you modify gravity otoh, you add classical fields to general relativity. The main difference is (besides the one being quantized and the other not) the way that the additional fields couple. For what is relevant here, however, is only that there is a priori no reason for the focus of the additional fields in modified gravity to be located where the 'normal' dark matter is.

I say 'a priori' because to figure out where it is you have to solve the dynamical equations. Which I haven't done. On that matter I can merely tell you that people working on modified gravity claim they can fit the Bullet Cluster without too many problems. I haven't looked into this too deeply and can't say much about this. Hence, all I am saying here is that at least for what the theoretical structure is concerned the focus of gravity can be offset from the normal matter distribution in modified gravity too. Best,

B. "

To avoid having quantized particles (and hence particulate dark matter) required by QFT.. Sabine said you added classical fields to general relativity in modified gravity. So there is no particle. Is this correct? And you can shift fields even when there is no matter like in the bullet clusters where the gravitational field are at the sides while the matter is at center... is this correct?
 
  • #96
Chronos
Science Advisor
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Agreed, mfb, QFT is not my forte. A little checking corroborated boson boson interactions do sometime occur - e.g. ,https://physics.aps.org/synopsis-for/10.1103/PhysRevLett.111.080405 notes:
"
"Despite what movie lightsabers suggest, light beams pass through each other without effect. However, two photons will, on rare occasion, bounce off each other. This elastic photon-photon scattering, which occurs via intermediate particles, has never been observed directly, but a new analysis in Physical Review Letters shows that the Large Hadron Collider (LHC) at CERN could detect around 20 photon-photon events per year

Photons only interact with charged particles, so they shouldn’t interact with themselves. But quantum physics allows for a photon to temporarily fluctuate into a particle-antiparticle pair (such as an electron-positron pair), and one of these charged particles can absorb a second photon. When these intermediate particles recombine, they emit two photons. The whole process appears as two photons ricocheting off each other, but it has only been observed indirectly by its effect on the magnetic moments of the electron and muon."

I'll even cop a plea on magnetic fields bending light since technically a sufficiently powerful magnetic field could have have enough stored energy to induce gravitational lensing.. I chimed in with this because I vividly recall from years ago a professor waving a big magnet under his nose before asking if we saw it wiggle before launching a spiel about boson boson interactions. But, unless I missed a major breakthrough, gravitational shielding is still doing hard time at sci fi penitentiary.
 

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