# A Dark Matter is real

• Featured

#### sector99

For DM to aggregate around black hole it would have to lose energy while in vicinity of black hole. As DM is not interacting with anything (except gravitationally) it can't do that. So as it falls down in gravitational well it just as quickly climbs out of it except if it hits BH directly, but as Janus pointed out it's very small target to hit.
DM near a BH "Having to lose energy" due to baryonic non-interaction. To my newbie status, this is a puzzle. DM is mass of an unknown type. It may, for whimsey's sake, be a simple (as yet under-appreciated) field that is somehow modified by baryons & gravity wells.

Speaking of wells, "climbing back out [of a BH well] also is a puzzle to me. Either DM falls into a gravity well-or it doesn't. What would make DM change its inward trajectory?
**************
Protons repel each other unless a certain closeness is forced-then they attract and stick.

Is the general DM/gravity attraction (but not aggregation) as you imply-reversal back up away from BH wells similar?

Also if DM doesn't aggregate to galactic centers does this imply that DM's "normal" state is that it can't be "compressed" ... except perhaps by DE?

#### Janus

Staff Emeritus
Gold Member
DM near a BH "Having to lose energy" due to baryonic non-interaction. To my newbie status, this is a puzzle. DM is mass of an unknown type. It may, for whimsey's sake, be a simple (as yet under-appreciated) field that is somehow modified by baryons & gravity wells.

Speaking of wells, "climbing back out [of a BH well] also is a puzzle to me. Either DM falls into a gravity well-or it doesn't. What would make DM change its inward trajectory?
**************
Protons repel each other unless a certain closeness is forced-then they attract and stick.

Is the general DM/gravity attraction (but not aggregation) as you imply-reversal back up away from BH wells similar?

Also if DM doesn't aggregate to galactic centers does this imply that DM's "normal" state is that it can't be "compressed" ... except perhaps by DE?
DM falling in towards a BH is in principle no different then trying to hit the Sun with an in-falling object from, let's say, Pluto orbit distance. It doesn't take much for you to miss. You don't need much of a "sideways" component for the object to whip around the Sun rather than hit it. A sideways component of just over 114 meters/sec would put it into a trajectory that just skims above the surface of the Sun and heads back out into space, eventually returning to where it started. Shrink the Sun to a BH with a an event horizon just a few km across, and you make it even harder. Now you have to get any sideways component down to under 0.237 m/sec ( under 1/2 a mile per hr).
And this is starting with no inward component. Adding a inward starting component actually decreases the allowed sideways component (An object with an initial inwards velocity component reaches the Sun's vicinity faster, giving the Sun's gravity less time to curve the path in towards the Sun.)

In terms of gravity, BHs are no different than anything else, other than the fact that their small radius allows objects to get much closer to their center of mass without hitting a surface (in this case, the event horizon) which produces a region of extreme gravity near the BH.

#### sector99

DM falling in towards a BH is in principle no different then trying to hit the Sun with an in-falling object from, let's say, Pluto orbit distance. It doesn't take much for you to miss. You don't need much of a "sideways" component for the object to whip around the Sun rather than hit it. A sideways component of just over 114 meters/sec would put it into a trajectory that just skims above the surface of the Sun and heads back out into space, eventually returning to where it started. Shrink the Sun to a BH with a an event horizon just a few km across, and you make it even harder. Now you have to get any sideways component down to under 0.237 m/sec ( under 1/2 a mile per hr).
And this is starting with no inward component. Adding a inward starting component actually decreases the allowed sideways component (An object with an initial inwards velocity component reaches the Sun's vicinity faster, giving the Sun's gravity less time to curve the path in towards the Sun.)

In terms of gravity, BHs are no different than anything else, other than the fact that their small radius allows objects to get much closer to their center of mass without hitting a surface (in this case, the event horizon) which produces a region of extreme gravity near the BH.
Thanks for this. So...the DM/Gravity "force" is very weak leading to a highly likely BH "miss" at perigee and a conventional (if loose) "orbit" around the averaged gravitational galactic centers–all assuming DM can't be "compressed".

Whether DM enters a BH is thus a very low probability ie. inconsequential event. However, it seems of interest insofar as concerns DM & interaction with the BH Conversion Zone–BHCZ (Event Horizon–where baryons are converted into whatever is in BH). Is DM even "convertable"? I see other questions occupy much higher priority.

As for DM and DE: Are they like oil and water?

#### Ibix

So...the DM/Gravity "force" is very weak leading to a highly likely BH "miss"
You are missing the point. Normal matter would normally swing past a black hole and escape. Dark matter is not special in this regard. It's just that space around a black hole can be quite crowded and normal matter collides with other normal matter and loses energy to friction. So it gets trapped. Dark matter doesn't interact this way - it just passes through other matter (normal and dark) so it doesn't experience friction, doesn't slow down, and doesn't get trapped.

#### sector99

You are missing the point. Normal matter would normally swing past a black hole and escape. Dark matter is not special in this regard. It's just that space around a black hole can be quite crowded and normal matter collides with other normal matter and loses energy to friction. So it gets trapped. Dark matter doesn't interact this way - it just passes through other matter (normal and dark) so it doesn't experience friction, doesn't slow down, and doesn't get trapped.

I now appreciate the baryonic frictional drag effect that DM doesn't feel. Thanks.

#### Janus

Staff Emeritus
Gold Member
Thanks for this. So...the DM/Gravity "force" is very weak leading to a highly likely BH "miss" at perigee and a conventional (if loose) "orbit" around the averaged gravitational galactic centers–all assuming DM can't be "compressed".

Whether DM enters a BH is thus a very low probability ie. inconsequential event. However, it seems of interest insofar as concerns DM & interaction with the BH Conversion Zone–BHCZ (Event Horizon–where baryons are converted into whatever is in BH). Is DM even "convertable"? I see other questions occupy much higher priority.

As for DM and DE: Are they like oil and water?
DM doesn't interact with the BH any differently than "normal" matter does in terms of gravity. The only difference is that "regular matter" can collide with other matter on its way in and and thus give up some of its kinetic energy and velocity which, in turn gives the BH gravity more time to act on it and deflect its path towards the BH. Barring any such interaction with other matter( and assuming no charge for the BH), a proton and a particle of DM starting on the same initial trajectory will follow the same path, either swinging around or crossing the event horizon, depending on their starting trajectory.

As far as DM crossing the event horizon is concerned, mass is mass, it doesn't matter if it is from baryonic matter, DM, or the mass equivalence of photons. Passing the event horizon itself doen't mean that anything is "converted" into anything else. The event horizon just marks a boundary which limits what information we can get from the other side.

#### sector99

DM doesn't interact with the BH any differently than "normal" matter does in terms of gravity. The only difference is that "regular matter" can collide with other matter on its way in and and thus give up some of its kinetic energy and velocity which, in turn gives the BH gravity more time to act on it and deflect its path towards the BH. Barring any such interaction with other matter( and assuming no charge for the BH), a proton and a particle of DM starting on the same initial trajectory will follow the same path, either swinging around or crossing the event horizon, depending on their starting trajectory.

As far as DM crossing the event horizon is concerned, mass is mass, it doesn't matter if it is from baryonic matter, DM, or the mass equivalence of photons. Passing the event horizon itself doen't mean that anything is "converted" into anything else. The event horizon just marks a boundary which limits what information we can get from the other side.
I'm having trouble with "mass is mass". DM teaches so much by being so "Dark". Not withstanding Chandra's x-ray images of hot gas presumed to be associated with DM.

#### Orodruin

Staff Emeritus
Homework Helper
Gold Member
2018 Award
Not withstanding Chandra's x-ray images of hot gas presumed to be associated with DM.
Are you referring to galaxy mergers such as the Bullet Cluster? The entire point there is that the hot gas is not associated with dark matter.

#### sector99

Are you referring to galaxy mergers such as the Bullet Cluster? The entire point there is that the hot gas is not associated with dark matter.

Chandra's detector receives high energy photons, below claimed to be "purple" in hue. Photons aren't DM. Perhaps I've misinterpreted the text.

#### Bandersnatch

Chandra's detector receives high energy photons, below claimed to be "purple" in hue. Photons aren't DM. Perhaps I've misinterpreted the text.
The purple bits to the left and right show DM distribution - it's the lensing map mentioned in the description. The x-ray emission from gas are in red, in the centre.

#### Orodruin

Staff Emeritus
Homework Helper
Gold Member
2018 Award

Chandra's detector receives high energy photons, below claimed to be "purple" in hue. Photons aren't DM. Perhaps I've misinterpreted the text.

View attachment 242080
If you had provided the link from the beginning, we would not have had to guess. Yes, that is the Bullet Cluster. No, the x-rays are not from dark matter. Rather, the entire point is that the mass distribution inferred from gravitational lensing (purple) does not match up with the x-ray emitting gas (pink).

#### sector99

"A purple haze shows DM flanking the Bullet Cluster".

The caption's intended impression remains that the "purple haze" is DM.
If you had provided the link from the beginning, we would not have had to guess. Yes, that is the Bullet Cluster. No, the x-rays are not from dark matter. Rather, the entire point is that the mass distribution inferred from gravitational lensing (purple) does not match up with the x-ray emitting gas (pink).

I provided the link on page 5 at the beginning.

Meanwhile, here is an excerpt clarifying the Clowe, et al finding:

The original link left the incorrect impression that DM was directly imaged as a flanking "purple haze"[1st link].

Clowe et al shows immediatly above that DM was subject to "weak lensing", while the hot red-orange baryonic "plasma distribution" was subject to strong lensing and spilt, to 8 sigma. The essentially spherical bluish "halo" is the (mostly unlensed) spherical DM. It's DM because it's mostly unlensed and didn't react as baryonic plasma.

Lesson: Don't accept an interpretation when you have the original.

#### Orodruin

Staff Emeritus
Homework Helper
Gold Member
2018 Award
The purple is not an x ray image. It is a mass distribution inferred from gravitational lensing. The x ray image is shown in pink.

The original link left the incorrect impression that DM was directly imaged as a flanking "purple haze"[1st link].
This is why you should not read popularised science for any purpose other than entertainment.

Clowe et al shows immediatly above that DM was subject to "weak lensing"
No. DM is the source of the weak lensing of the background galaxies. Not what is being subjected to gravitational lensing.

the hot red-orange baryonic "plasma distribution" was subject to strong lensing
No. The baryonic matter collided, giving rise to x ray emissions.

Lesson: Don't accept an interpretation when you have the original.
Also, don’t misrepresent the original.

#### sector99

Confusion stemmed from an official NASA release by a writer with the Chandra group who is much closer to the topic than I.

Your view is correct. I have discarded the popular DM misperception that DM can be seen and more carefully reread Clowe et al to fully agree with all points of your critique. As an admitted newbie I had no intention of misrepresenting anything.
***********
My initial DM query was simply to ask "What exactly are observers seeing when they detect DM"?

Do you have any problems with this answer?

What is seen with recent x-ray studies is an advancing, post collisional hot plasma bow shock wave consistent with being gravitationally dragged through an invisible, resistant, fluid-like substance whose presence is consistent with DM theory.

Last edited by a moderator:

#### Bandersnatch

My initial DM query was simply to ask "What exactly are observers seeing when they detect DM"?

Do you have any problems with this answer?

What is seen with recent x-ray studies is an advancing, post collisional hot plasma bow shock wave consistent with being gravitationally dragged through an invisible, resistant, fluid-like substance whose presence is consistent with DM theory
No, this is still incorrect. The x-ray image of colliding plasma has nothing to do with being dragged through DM or any resistance it may pose, gravitational or otherwise. By itself, it does not show DM in any sense.
In the Bullet cluster one observes two clusters of galaxies, which as per the traditional theory of galaxy structure should have two components accounting for its mass - the stars, and the interstellar+intergalactic gas. The majority of the mass should be in the gas component.
The gas in either aggregation collides with the gas in the other (while the stars pass through), creating the x-ray image captured by Chandra's x-ray detectors, and showing the gas to have separated from the optically luminous stellar component.
This is expected regardless of any DM content and there's nothing in the paper about DM affecting the way the gas has separated.
The additional - and completely separate from x-ray detection - observation that allows DM to be inferred is the map of gravitational lensing, that shows where most of the mass in the colliding clusters is concentrated. In the no-DM paradigm, the lensing map should show the majority of the mass to be where the gas is.
Since the map shows the mass to be mostly where the stars are, not where the gas is, one can infer that there is some additional mass component, which is - like stars and unlike gas - collisionless, and - unlike stars or gas - invisible.
That's why the image credit in the bit cited in post #109 lists all those different observations. It's wrong to pick the x-ray one and say it shows DM. If anything, it's the lensing map that is the closest to 'showing' DM, since it tracks the gravitational signature.

#### sector99

No, this is still incorrect. The x-ray image of colliding plasma has nothing to do with being dragged through DM or any resistance it may pose, gravitational or otherwise. By itself, it does not show DM in any sense.
In the Bullet cluster one observes two clusters of galaxies, which as per the traditional theory of galaxy structure should have two components accounting for its mass - the stars, and the interstellar+intergalactic gas. The majority of the mass should be in the gas component.
The gas in either aggregation collides with the gas in the other (while the stars pass through), creating the x-ray image captured by Chandra's x-ray detectors, and showing the gas to have separated from the optically luminous stellar component.
This is expected regardless of any DM content and there's nothing in the paper about DM affecting the way the gas has separated.
The additional - and completely separate from x-ray detection - observation that allows DM to be inferred is the map of gravitational lensing, that shows where most of the mass in the colliding clusters is concentrated. In the no-DM paradigm, the lensing map should show the majority of the mass to be where the gas is.
Since the map shows the mass to be mostly where the stars are, not where the gas is, one can infer that there is some additional mass component, which is - like stars and unlike gas - collisionless, and - unlike stars or gas - invisible.
That's why the image credit in the bit cited in post #109 lists all those different observations. It's wrong to pick the x-ray one and say it shows DM. If anything, it's the lensing map that is the closest to 'showing' DM, since it tracks the gravitational signature.
Let me take another attempt to answer my 1st query: "What exactly are observers seeing when they detect DM"?

(1) Galactic baryons showing anomalously high relative speed infers DM. (Normal stellar velocities were recently asserted to reveal an absence of DM)
(2) Detectable gas displaced from galactic stellar components infers DM. (This is the 2nd sentence and 1st assertion in the Clowe, et al paper) The definitial problem is widely varying detectable gas morphologies.

If it's possible to make this answer simpler I'll be happy to keep listening.

#### mfb

Mentor
(Normal stellar velocities were recently asserted to reveal an absence of DM)
In two galaxies. Meanwhile they show the presence of dark matter in thousands of others and on many different scales. It will be interesting to figure out how these two galaxies formed with nearly no matter.
(3) gravitational lensing shows that there is additional matter
(4) the cosmic microwave background shows that there is additional matter

#### sector99

Thank you for this.

(assembled & quoted from above)
Let me take another attempt to answer my 1st query: "What exactly are observers seeing when they detect DM"?

(1) Galactic baryons showing anomalously high relative speed infers DM. (Normal stellar velocities were recently asserted to reveal an absence of DM)
(2) Detectable gas displaced from galactic stellar components infers DM. (This is the 2nd sentence and 1st assertion in the Clowe, et al paper) The definitial problem is widely varying detectable gas morphologies.
(3) gravitational lensing shows that there is additional matter
(4) the cosmic microwave background shows that there is additional matter

Provisional (5*)
Galaxy spiral kinematics. Asked another way: Could spirals have formed without DM?

Off Topic FYI:

#### Janus

Staff Emeritus
Gold Member
Provisional (5*)
Galaxy spiral kinematics. Asked another way: Could spirals have formed without DM?
The galactic spirals are not due to there being significantly more material in the arms than between them, they are regions where there are more hot younger stars. They are basically caused by density waves traveling through the galaxy. This increases star formation. This includes massive bright stars with short lifetimes. This makes the spiral arms look brighter. But these bright stars burn out quickly, and so by the time the wave passes, only small dim stars are left and these are what make the large population of stars in the "gaps". There is still a lot of material in there, it's just dimmer. So spiral arms aren't structures of which stars are permanent residents of, but rather, stars move in and out of the spiral arms.

#### sector99

Thanks. This raises a query about DM density waves. I'm guessing that since DM somehow can't be "compressed" there can't be any DM density waves thus the spiral appearance (which doesn't arise from kinematic rotation-including especially the barred spirals) is due solely to baryonic matter?

#### mfb

Mentor
Dark matter can be compressed easily. What would stop it? It is collisionless (at least to a good approximation).

#### Ken G

Gold Member
Dark matter would be hard to compress externally by some kind of wall, but there aren't walls like that in any astrophysical context (other than perhaps magnetic fields, but those aren't active in, say, spiral density waves either). Collisionless does not mean low pressure, so can have high pressure and exhibit the same resistance to compression as any high pressure gas. Indeed, spiral density waves don't separate the stars from the gas, even though the former is collisionless and the latter is collisional. So something a little different than a gravitationally induced density wave must going on in the Bullet Cluster.

Gas pressure is the momentum flux density of the particles, so only requires velocities that are high and isotropic. In short, the ideal gas law. It's true that collisions are useful for maintaining the isotropic velocities (that's really all collisions do in regard to gas pressure), but there must be some other way that dark matter does it because dark matter is normally assumed to be at a temperature. I think it must be its history of obeying the cosmological principle which must ultimately be the source of the isotropy of its velocities. Honestly I am not sure why dark matter is always assumed to be thermalized, and even often treated as isothermal. Clearly the dark matter particles interacting in the Bullet Cluster has velocities that "remember" which cluster they came from, so are not thermalized, yet the Milky Way dark matter is generally assumed to be thermalized. It must have to do with the history.

The confusion about pressure and compressibility in collisionless gases probably traces to the fact that introductory sources often confuse pressure with forces on boundaries next to the gas, which requires collisions with the boundary (though still not between the particles themselves). But the force from/on a wall is actually nothing more than the action/reaction involved in the mundane "normal force" seen in so many other contexts, so is not pressure any more than standing on a bathroom scale is gravity, but it is a good way to measure pressure of gas that isn't dark matter. I don't know why so many sources like to think of gas pressure as a force on/from a wall, as it is more usefully and more flexibly thought of as a force on the gas itself, stemming from the gas itself, stemming from the momentum flux density within the gas. And even though the fluid approximation is made much easier to assume by collisions, it is used for dark matter also. But yes, the Bullet Cluster dark matter is more easily understood not in a fluid picture!

Last edited:

#### sector99

DM not being "compressed" or, collected by galactic center gravitation was what I meant to say.

On the characteristic of being collisionless–does this feature reduce exotic bosonic/hadronic matter as a DM candidate?

#### jocarren

I have a methodological objection about DM. Such objection doesn't mean that i reject the DM hypothesis nor all of the observations that support it, but to express concern about the following:

1.- The difficulty for detection.
2.- Reluctancy to postulate new physics.

Since the existence of DM is inferred from it's gravitational effect on galactic motion and there is no other interaction (internal nor external) it is pretty much defined by this two aspects.

This raises the question: how can DM be observed if not by it's defining qualities?

Up until this point, most of the observation efforts are focused on mapping it's gravitational interaction (mainly through lensing). The remaining effort is focused on finding correlations between expected behavior of particles both hypothetical and within the standard model, with no success as far as I know.

Problem is, as difficult as proving it's existence is, maybe it's even more difficult to disprove it. Since the only thing that can be observed is the very thing that defines the subject of observation: Gravity and non interaction.

The way i see it, the unfalsifiability issue is not inherent to the claim that something is causing the phenomena observed, but emerges from an excessively broad definition of DM. Efforts should be made not only to observe it, but to narrow it's definition so the verification can be methodologically simpler.

About the reluctancy towards new physics, i don't mean to say that GR is wrong and the subsequent cosmological models are too, everything seems to point out that they are correct (and i personally subscribe to that notion), but this sort of discrepancy between the observed phenomena and the proven theory claims for a paradigm shift or at the very least (which i believe is the case here) a fresh interpretation of mainstream physics.

But, as i said before, this is not about disproving DM, it's about the concern that a methodological issue can undermine it's verification.

"Dark Matter is real"

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving