I How certain is dark matter?

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ohwilleke

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Yes, I get that they are considering modified theories of gravity with that property. What I don't get is their claim that I quoted that General Relativity has that property. And the fact that they make that claim in the first sentence of their abstract as though it were obvious makes me skeptical.
My understanding is that they are simply referencing special relativity in the example that I provided when they say ""Since general relativity (GR) has already established that matter can simultaneously have two different values of mass depending on its context[.]" This language may be a bit sloppy, but the gist of that statement read in that manner is true.
 

Chronos

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I am willing to concede that collisionless DM is a less than ideal fit for current observational evidence. I consider it an effective approximation. We are still struggling to identify the players in the dark sector much less draw any conclusions about their interactions.
 
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I have been following this thread and there is a lot of in depth analysis of the relative merits of dark matter theories v alternative gravity theories. In trying to make sense of this I have constructed a concise summary which I append to my entry. I am sure some cells will provoke discussion and I am showing this in an attempt to gain some clarity and in no was as a final word on the current state of cosmology.

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Talk of dark matter and with billions of clusters, galaxies, nebulae, etc, to choose from, there is the Bullet Cluster and........nothing.

Large groups of stars act as though they are controlled by light matter only and we see no weird shapes that cannot be accounted for by said light matter.

DM was once said to be in the halo of a galaxy, but where are the ring doughnut galaxies as in DM outmasses light matter by a factor of six to one?
 
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How certain are scientists that the solution to the dark matter problem is a new, unknown particle (or more)? Theories that eliminate the need for new particle(s) and suggest modifications to the current understanding of gravity seem to get less attention. Why is that?
Alternative theories have failed verification attempts of one sort or another. Something is out there that has mass but does not interact with the electromagnetic force. We can always hope that a TOE or quantum gravity will resolve the problem, but....
 
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(I say "to existing Core Theory applied using existing methods" to recognize a third possibility that could exist instead of, or in addition to the dark matter particles and modified gravity, which is that some of all of dark matter phenomena could arise from the possibility that using a Newtonian approximation of GR in lieu of complete GR to do galaxy plus scale estimates of behavior under GR could be less negligible than previously realized due to previously unrecognized flaws in how the discrepancy between a Newtonian approximation and a GR calculation is estimated. One or two people are argued this in published papers that were largely refuted, but neither side's analysis was really rigorous and extraordinarily careful so this isn't an impossibility.)
In your first section I think that the point you made in parentheses (as quoted) about a possible third way ie one that doesn’t involve dark matter or modifying gravity was a path that I think should be pursued. I think there are likely flaws in simplistically applying Newton Law of gravity to galaxies. It works for the inner solar system with 8-9 bodies in orbit but when there are hundreds of billions of orbiting bodies it may be that we need a different perspective.


Within the dark matter particle paradigm, the evidence has ruled out a lot of the potential dark matter particle parameter space. For example, both MACHOs like red dwarfs and stellar sized or larger black holes, and the hundreds of GeV WIMPs that interact via the weak force as anticipated in electroweak scale supersymmetry theories, have been all but ruled out experimentally.
In your second section, I was pleased that someone else is saying what is surely obvious that early GeV WIMPs have virtually been ruled out by experiment: Xenon 100, LUX 2013, Darkside 50, LUX 300 day results in 2016 and now the preliminary results of Xenon 1T in 2017 have found no evidence of a Gev WIMP from the first theoretical predictions.

Future experiments are planned but it appears unlikely that the marginal extension of the parameter space will show up anything new. I am not against continuing these studies but I am not holding my breath.

The dark matter idea appears not to be a unified theory but a collection of theories where one single example fails to produce all the required results.
 
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Missing from this are two important facts:
1. Modified gravity models have so far utterly failed to explain the CMB data (here's one formal argument along these lines from 2011, which I'm pretty sure remains valid to this day: https://arxiv.org/pdf/1112.1320.pdf).
2. The above argument asks you to disregard observations of comparatively simple systems (e.g. the CMB) and instead focus on comparatively complex systems (e.g. galaxies). Systematic uncertainties are far, far more likely to muddle our understanding of complex systems.
I have had a copy of this paper by Scott Dodelson for some time, so it is not new to me. I find it highly suspect. The graph is quite spectacular, too much so to be valid; it is after all a model.

You are apparently more in awe of simple solutions that convoluted arguments. Well if simple relationships from direct observation are what you are looking for the the Tully Fisher graph produced by Stacy S. McGaugh, Federico Lelli, and James M. Schombert, in 2016 in their paper ‘The Radial Acceleration Relation in Rotationally Supported Galaxies’ should be of interest to you! The mechanics of rotating galaxies may be complex but the fact that there is a tight relationship between galactic baryonic matter and rotational velocity is a serious challenge to dark matter theories.

If the only thing dark matter theories can do better that other theories is to explain esoteric signals from the relic of the big bang, then it has failed. I think it is far better to get a good theory for current (ie t=0) astronomical phenomenon than minor (1/100,000) fluctuations in the CMBR.
 

kimbyd

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I have been following this thread and there is a lot of in depth analysis of the relative merits of dark matter theories v alternative gravity theories. In trying to make sense of this I have constructed a concise summary which I append to my entry. I am sure some cells will provoke discussion and I am showing this in an attempt to gain some clarity and in no was as a final word on the current state of cosmology.

View attachment 229165
The Tully-Fisher relationship is definitely predicted by LCDM (https://arxiv.org/abs/1204.1497). Whether the low scatter is fully-explained is somewhat less certain, but this definitely warrants at worst a "maybe" rather than a "no".
 

kimbyd

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I have had a copy of this paper by Scott Dodelson for some time, so it is not new to me. I find it highly suspect. The graph is quite spectacular, too much so to be valid; it is after all a model.
The CMB is an incredibly rich source of data. There's nothing to be suspicious about. This is why I always draw these discussions back to the CMB data.
 
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The CMB is an incredibly rich source of data. There's nothing to be suspicious about. This is why I always draw these discussions back to the CMB data.
We are slightly at crossed purposes here. It wasn't the CMBR data I was saying was suspect it was the Scott Dodelson paper you referenced I regard as suspect. You I am sure will disagree but let me put the argument differently - if the graph that Scott Dodelson draws is so correct and it does show a massive disparity between CDM and modified theories then why was that not the end of the argument?
 
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The Tully-Fisher relationship is definitely predicted by LCDM (https://arxiv.org/abs/1204.1497). Whether the low scatter is fully-explained is somewhat less certain, but this definitely warrants at worst a "maybe" rather than a "no".
I had a look at your new reference but slightly got disillusioned at part 2, Method as I quote, 'The basic semi-analytic methodology employed in this work is
adapted from M98 (Sect. 2). As the starting point, I take a spiral galaxy with a particular stellar mass. The Mvir-Mstar relation from the Halo Abundance Matching performed in RP11 (eq. 5) is then used to calculate the virial mass of the surrounding dark halo.

Surely, if your using an analytical method to calculate the dark halo from the stellar mass then is it any wonder that the Tully-Fisher relationship remains intact?
 

kimbyd

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I had a look at your new reference but slightly got disillusioned at part 2, Method as I quote, 'The basic semi-analytic methodology employed in this work is
adapted from M98 (Sect. 2). As the starting point, I take a spiral galaxy with a particular stellar mass. The Mvir-Mstar relation from the Halo Abundance Matching performed in RP11 (eq. 5) is then used to calculate the virial mass of the surrounding dark halo.

Surely, if your using an analytical method to calculate the dark halo from the stellar mass then is it any wonder that the Tully-Fisher relationship remains intact?
The BTFR compares the baryonic mass estimated from luminosity to the gravitational mass determined from stellar velocities.
 

kimbyd

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We are slightly at crossed purposes here. It wasn't the CMBR data I was saying was suspect it was the Scott Dodelson paper you referenced I regard as suspect. You I am sure will disagree but let me put the argument differently - if the graph that Scott Dodelson draws is so correct and it does show a massive disparity between CDM and modified theories then why was that not the end of the argument?
You mean the matter power spectrum estimate? Eh. It might be possible to do a little bit better than is presented there. But really, this is why very, very few physicists are still on the "no dark matter" train.

Edit: And if you doubt it, why not see if any people pushing modified gravity models have results for the matter power spectrum in their model?
 

kimbyd

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Incidentally, McGaugh has presented just that as recently as 2014:
https://arxiv.org/abs/1404.7525

Even in MOND, apparently, the issue pointed out by Dodelson is very clear. McGaugh only compares the models without reference to the data, and hand-waves away the discrepancy claiming that at low redshift the discrepancy should disappear in MOND (without explaining why or modeling it). To me, this is a giant cop-out: structure formation will only impact the small-scale behavior. There are large discrepancies between MOND and dark matter models even on pretty large scales.
 
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Incidentally, McGaugh has presented just that as recently as 2014:
https://arxiv.org/abs/1404.7525

Even in MOND, apparently, the issue pointed out by Dodelson is very clear. McGaugh only compares the models without reference to the data, and hand-waves away the discrepancy claiming that at low redshift the discrepancy should disappear in MOND (without explaining why or modeling it). To me, this is a giant cop-out: structure formation will only impact the small-scale behavior. There are large discrepancies between MOND and dark matter models even on pretty large scales.
That is all very well but I am not a supporter of MOND and frequently when I question the dark matter paradigm the response is what about MOND? I not sure if this is a ploy to shift the argument to a safer place for a dark matter proponent rather than meet the question head on.
 
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You mean the matter power spectrum estimate? Eh. It might be possible to do a little bit better than is presented there. But really, this is why very, very few physicists are still on the "no dark matter" train.

Edit: And if you doubt it, why not see if any people pushing modified gravity models have results for the matter power spectrum in their model?
How about, 'Comment on “The Real Problem with MOND” by Scott Dodelson', arXiv:1112.1320 by J. W. Moffat and V. T. Toth ref arXiv:1112.4386 [astro-ph.CO].
 

kimbyd

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How about, 'Comment on “The Real Problem with MOND” by Scott Dodelson', arXiv:1112.1320 by J. W. Moffat and V. T. Toth ref arXiv:1112.4386 [astro-ph.CO].
Their argument appears to be that the data set doesn't have the resolution required to distinguish the baryonic oscillations which occur in the modified gravity model.

That's fine, but that just argues for a reanalysis of the data which is designed to focus in on the baryonic oscillations themselves, and to include more than one old data set (2006). I don't buy Moffat and Toth's argument that we don't yet have enough data. I do buy that the way in which data is often processed might hide this effect, but the raw galaxy data sets are quite large. I'm quite sure that they could bin the data in such a way that these oscillations would be made more apparent, and there's a ton more data that could be included.
 

ohwilleke

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I have been following this thread and there is a lot of in depth analysis of the relative merits of dark matter theories v alternative gravity theories. In trying to make sense of this I have constructed a concise summary which I append to my entry. I am sure some cells will provoke discussion and I am showing this in an attempt to gain some clarity and in no was as a final word on the current state of cosmology.

View attachment 229165
I would call unmodified GR a "no" for galaxy rotation and Tully-Fisher. I would say that TeVeS and MOG are both compatible with the Standard Model of Particle Physics, in the sense of not adding new particles although that is really untrue for all five including unmodified GR. Not sure why you think TeVeS fails in terms of gravitational redshift. On cluster dynamics DM is a "maybe" not a "yes". MOG would be a maybe or yes on a reasonable cosmological simulation.

Also MOG really refers to a specific modified gravity theory of Moffat, while f(R) is a different modified gravity theory. MG is the commonly used abbreviation for "modified gravity" theory, in general.
 

ohwilleke

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The dark matter idea appears not to be a unified theory but a collection of theories where one single example fails to produce all the required results.
Just so.
 

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