Dark Matter: The Situation has Changed...according to Sabine

[notFritzArgelander]

Do they sell astro shirts like that for men?

Here you go....

https://www.lightinthebox.com/en/p/men- ... ptEALw_wcB

[notFritzArgelander]

This discussion is happening on backreaction now, I am curious what Sabines answer would be to your comments nFA. She says that she is not talking about MOND and that MOND is wrong so maybe there is some misunderstanding. Why there are two theories necessary, looks like more complexity to me.

I whole heartedly agree that arguing for two theories is inconsistent, like Tycho's solar system, half copernican and half ptolemaic.

If she is saying MOND is wrong, then which alternative gravity isn't? Many are adjusted to agree with MOND on galaxy rotation curves and still have problems like 1) predicting a large delay between optical and gravitational wave signatures from neutron star mergers and 2) predicting that the Sun should collapse in two weeks.

IMO the only viable (as in not clearly falsified by observations) alternative gravity theories are:

1) ECSK gravity https://en.wikipedia.org/wiki/Einstein–Cartan_theory and

2) the f(R) family of alternative gravities: https://en.wikipedia.org/wiki/F(R)_gravity

I like ECSK because it completely agrees with GR except it has BHs without singularities. I like f(R) gravities because there is a chance to dispense with BOTH dark matter and dark energy with it.

I'll look in on Backreaction and see what's going on later.

[notFritzArgelander]

Oops. I forgot one huge falsifying set of observations for alternative gravity. The existence of under luminous galaxies that have no dark matter content is important and also omitted from Dr H’s video. If alternative gravity is good it should work out of the box with no need to adjust parameters for individual galaxies. However there are galaxies where the apparent DM to normal matter ranges from nearly zero to 1000 IIRC. This is easy to understand with particle dark matter: different formation and tidal interaction histories can and should lead to abundances of DM that differ from the universe average of 5. For alternative gravity theory to be viable the value should be 5 period with slight deviations only due to observation errors. So I’m not quite done yet.

The situation is more complex than I was thinking due to recent developments that I had missed.

Ultra diffuse galaxies like Dragonfly 44 that were thought to be almost entirely DM have had findings disputed.

https://en.wikipedia.org/wiki/Dragonfly_44

https://arxiv.org/abs/2009.07846

Due to the peculiar properties of ultra-diffuse galaxies (UDGs), understanding their origin presents a major challenge. Previous X-ray studies demonstrated that the bulk of UDGs lack substantial X-ray emission, implying that they reside in low-mass dark matter halos. This result, in concert with other observational and theoretical studies, pointed out that most UDGs belong to the class of dwarf galaxies. However, a subset of UDGs is believed to host a large population of globular clusters (GCs), which is indicative of massive dark matter halos. This, in turn, hints that some UDGs may be failed L⋆ galaxies. In this work, I present Chandra and XMM-Newton observations of two archetypal UDGs, Dragonfly 44 and DF X1, and I constrain their dark matter halo mass based on the X-ray emission originating from hot gaseous emission and from the population of low-mass X-ray binaries residing in GCs. Both Dragonfly 44 and DF X1 remain undetected in X-rays. The upper limits on the X-ray emission exclude the possibility that these galaxies reside in massive (Mvir≳5×1011 M⊙) dark matter halos, suggesting that they are not failed L⋆ galaxies. These results demonstrate that even these iconic UDGs resemble to dwarf galaxies with Mvir≲1011 M⊙, implying that UDGs represent a single galaxy population.

However the finding of galaxies with NO dark matter is still a fatal flaw for alternative gravity (ECSK and f(R) excluded).

https://en.wikipedia.org/wiki/NGC_1052-DF2

[notFritzArgelander]

This discussion is happening on backreaction now, I am curious what Sabines answer would be to your comments nFA. She says that she is not talking about MOND and that MOND is wrong so maybe there is some misunderstanding. Why there are two theories necessary, looks like more complexity to me.

I've read through the comments and the talk about MOND being wrong is because it is non relativistic. She also appears to advocate for relativistic theories that reduce to MOND in the Newtonian limit. So there's really no misunderstanding. She also dismisses folks who are "prejudiced against MOND". So the comments on her blog are a pretty mixed bag. There is nothing much interesting for me there.

[notFritzArgelander]

..............
With DM particles forming a Bose Einstein condensate (a superfluid) there is need for a MOND et al.
.............

This is a typo that I didn't catch. It should read

"With DM particles forming a Bose Einstein condensate (a superfluid) there is NO need for a MOND et al."

OK that's all for me unless there is a need to discuss.

[Gmetric]

Hey nFA, there is an incredible amount of info here and you have taken a lot of time to mount a rebuttal of Dr H’s view of DM. Much appreciated by us all.

I’m right in the middle of moving houses so it’s tricky for me to get through all this at the moment. But I will!

If it’s possible, and while I have a moment, could I just ask for a little more information about this point.

You said “ The behavior of Newtonian gravity (and therefore General Relativity) has been tested in terrestrial experiments. The result is that MOND and it's cousins are falsified. No deviation from an inverse square law is found.”

In Newtonian gravity the inverse square law ensure that gravitational flux through a closed surface is preserved i.e Gauss’ law; in GR this is held true via Birkhoff theory. It seems that what you are saying above is that MOND violates BT, is that correct? If so could you expand on how and why MOND violates BT? And indeed, does it have a fudge solution for that too?

Many thanks

[notFritzArgelander]

Hey nFA, there is an incredible amount of info here and you have taken a lot of time to mount a rebuttal of Dr H’s view of DM. Much appreciated by us all.

I’m right in the middle of moving houses so it’s tricky for me to get through all this at the moment. But I will!

If it’s possible, and while I have a moment, could I just ask for a little more information about this point.

You said “ The behavior of Newtonian gravity (and therefore General Relativity) has been tested in terrestrial experiments. The result is that MOND and it's cousins are falsified. No deviation from an inverse square law is found.”

In Newtonian gravity the inverse square law ensure that gravitational flux through a closed surface is preserved i.e Gauss’ law; in GR this is held true via Birkhoff theory. It seems that what you are saying above is that MOND violates BT, is that correct? If so could you expand on how and why MOND violates BT? And indeed, does it have a fudge solution for that too?

Many thanks

I'm also a bit pressed since I have a lot of correspondence to catch up on. My friends in the real world are feeling neglected. But I can give a brief shot at your question's answer.

It is unclear to me how to reformulate MOND so that flux conservation of the field is true in the sense you mention. In Newton the integral of the flux of the gravitational field over a sphere of radius R is given by 4πGM from which the inverse square law follows. G is the gravitational constant and M is the central mass.

In MOND this is no longer true. A naive calculation of the flux from the MOND force law gives 4πGM/μ(a/a_0)GM here μ(a/a_0) is an arbitrary interpolation function such that the (a/a_0), the ratio of the acceleration to a minimum acceleration. The only requirements for μ are that if (a/a_0)>>1 μ=1 and if (a/a_0)<<1 μ=(a/a_0).

Effectively this can be viewed as making G dependent on acceleration.

That was slightly hand wavy. A better approach is via the nonlinear (!) Poisson equation for MOND

\nabla\bullet[\mu(\nabla(||\nabla\phi||/a_0)\nabla\phi]=4πGρ

This gives the add form for the flux through a sphere of radius R, larger than the density distribution ρ,

4πGM/[\mu(\nabla(||\nabla\phi||/a_0)\nabla\phi]

which agrees (give proper identification of symbols) with what we got with handwaving.

When I spoke with Milgrom back in the day I objected to 1) MOND violating special relativity and 2) making Poisson's equation extraordinarily ugly. The "flux" is like the snake eating its tail though. The acceleration depends on the acceleration.

[Gmetric]

Cheers nFA, much appreciated.