# B Is Dark matter fact or fantasy

#### wolram

Gold Member
Not sure who this is he may be a mondian but it is an alternative view.

arXiv:1903.11217 [pdf, other]
Is dark matter fact or fantasy? -- clues from the data
Philip D. Mannheim
Comments: Essay written for the Gravity Research Foundation 2019 Awards for Essays on Gravitation
Subjects: Astrophysics of Galaxies (astro-ph.GA); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Phenomenology (hep-ph)
We discuss arguments both in favor of and against dark matter. With the repeated failure of experiment to date to detect dark matter we discuss what could be done instead, and to this end look for clues in the data themselves. We identify various regularities in galactic rotation curve data that correlate the total gravitational potential with luminous matter rather than dark matter. We identify a contribution to galactic rotation curves coming from the rest of the visible Universe, and suggest that dark matter is just an attempt to describe this global effect in terms of standard local Newtonian gravity within

#### rootone

isn't this just a rerun of of MOND?

My understanding is that this idea says the dark matter is actually just regular matter,
however it is not detectable for some reason, perhaps inadequacies of our instruments.
The problem here is that the amount of missing normal matter would be huge.
The interstellar medium would not be a near vacuum as it apparantly is.
Is would need to be fairly dense on average, roughly like the Earths atmosphere.

#### PeterDonis

Mentor
The interstellar medium would not be a near vacuum as it apparantly is.
Is would need to be fairly dense on average, roughly like the Earths atmosphere.
Where are you getting this from? The average density of dark matter in the universe is about 26 orders of magnitude smaller than the density of the Earth's atmosphere.

#### rootone

Where are you getting this from?
OK, I really shouldn't have tried to quantify the problem as a comparison with something else,
especially since my something else is a wildly innacurate thing to compare with.

However the point basically is that there more dark matter in the universe than normal matter.
It seems highly unlikely that it is there but we are simply failing to observe it, and while not as dense as atmosphere, the interstellar medium would still need to be a lot denser than almost vacuum.

#### George Jones

Staff Emeritus
Gold Member
Not sure who this is he may be a mondian but it is an alternative view.
For many years, Philip Mannheim has been pushing conformal gravity as an alternative to standard general relativity.

#### PeterDonis

Mentor
the point basically is that there more dark matter in the universe than normal matter
Yes, but it doesn't clump the way normal matter does; it's much more spread out.

#### PeterDonis

Mentor
It seems highly unlikely that it is there but we are simply failing to observe it
Why not? If it does not interact with normal matter other than gravitationally, it would be basically undetectable on the scale of the solar system, since its gravitational effect would be way too small (in fact, if it were basically spread out evenly on the scale of the solar system, its net gravitational effect would be zero, since it would be "pulling" the same in all directions).

#### wolram

Gold Member
For many years, Philip Mannheim has been pushing conformal gravity as an alternative to standard general relativity.
That is one reason I suppose, but he does give a good argument against ,What is this dark matter particle?
all the other particles in the zoo react to some thing other than gravity.

So what?

Gold Member

#### strangerep

Not sure who [Philip Mannheim] is he may be a mondian but it is an alternative view. [...]
He is not a MONDian. See this thread for some discussion on recent Conformal Gravity papers, and confrontation with galactic rotation curve data.

rootone said:
isn't this just a rerun of of MOND?
I've no idea how anyone who has actually read the paper could think that Conformal Gravity ("CG") is MOND.

Anyway,... the puzzle I'm currently stuck on is how/whether both the CG fits to the SPARC data, and Stacy McGaugh's fits of MOND to the same data can co-exist...

The basic tangential velocity profile prediction in CG has terms of order $1/r$, $r$, and $r^2$ (see the other thread linked above for details). In contrast, the effective leading term in MOND is $\ln r$ (leading to a constant $v_{\tan}$ profile). The CG fit has 1 free parameter per galaxy (the mass-to-light ratio), and 2 universal parameters. The MOND fit is obtained (iiuc) by numerically solving the (Newtonian) Poisson equation for each galaxy's mass profile, and then applying the MOND formula.

If one plays around with curves involving $1/r, r, r^2$ terms, it's possible to find regions where they go flat for a while, like the MONDian case. Mannheim wrote a paper about that a while ago, posing the question "Are outer galactic rotation profiles really flat?" (Some seem to dip and wobble around.)

I sure wish someone else besides Mannheim would try and repeat his CG fitting procedure independently. The powers of $r$ in their respective gravitational acceleration formulas don't match up where it matters, so I suspect one of them must be an accident (if the data is good, and both fits have been performed correctly).

As for DM, it's high on my list of "things I think might be bulls--t". It's too easy to make DM fit too many things. See this blog post by Stacy McGaugh about how DM can be fitted to rubbish data, whereas MOND flags a problem on the same data.

A "theory" whose free parameters can be fitted to anything explains nothing.

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#### ohwilleke

Gold Member
Not sure who this is . . . Philip D. Mannheim
He leans towards a gravitational explanation of dark matter phenomena, advancing a proposal known as conformal gravity that is one of the technically strongest, but less well known, efforts along those lines. See, e.g., https://www.researchgate.net/publication/329525401_Radial_Acceleration_and_Tully-Fisher_Relations_in_Conformal_Gravity and https://en.wikipedia.org/wiki/Conformal_gravity and http://inspirehep.net/record/1375016?ln=en

#### Herbascious J

Yes, but it doesn't clump the way normal matter does; it's much more spread out.
Do we have an idea how spread out it is? For example is it just a uniform 'disc' like cloud slightly larger than a given galaxy? Is it possible that this spreading out can reach all the way out to a neighboring galaxy? The second question would imply that the local intergalaxtic space wouldn't be as vacuous as I've assumed (at least in regards to dark matter). That makes me wonder, do we believe local intergalactic regions are as vacuous as they are because that's what we predict, or is it because we have measured a vacuum out there to precision? Would we be able to make that measurement of dark matter (perhaps galaxy cluster orbits).

#### PeterDonis

Mentor
Do we have an idea how spread out it is?
Since we can't observe it directly, our only knowledge of its distribution comes from its effect on things we can see, i.e., the visible stars in galaxies. And all we're seeing there is the effect of the average density; in other words, we have a very coarse estimate of the distribution in the visible regions of galaxies. I don't think we can say a lot outside of that at this point.

The second question would imply that the local intergalaxtic space wouldn't be as vacuous as I've assumed (at least in regards to dark matter).
We have an estimate of the average density of dark matter in the universe as a whole, which does place some limits on how much of it can be spread throughout intergalactic space. But I don't know how tight the limits are.

Would we be able to make that measurement of dark matter (perhaps galaxy cluster orbits).
I don't know if those have been studied, but yes, it seems like looking at the motions of galaxies in clusters would give some information about the dark matter distribution on those scales.

"Is Dark matter fact or fantasy"

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