Is there an alternative theory to dark matter?

In summary, the conversation discusses the concept of dark matter and its alternative theories, such as modified gravity and the possibility of ordinary matter not being visible. The conversation also mentions the ongoing efforts to detect WIMPS and the debate over whether dark matter is a theory or just a placeholder name. Some propose that there may be a fourth explanation for the anomalies, but it is often overlooked due to the strong hold of the dark matter paradigm. The conversation also touches on the issue of small cross sections and the potential lack of a lower limit in the search for WIMPS. Ultimately, while the dark matter hypothesis is consistent with current experimental data, it may not be experimentally falsifiable and allows for a wide range of distribution possibilities.
  • #1
KBon
My physics teacher, who dislikes the idea of dark matter, told me that a physicist created an alternative explanation to the phenomena caused by dark matter.

Is there something I missed on the news?

What is the alternative theory to dark matter and how does it explain 'things' ?
 
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  • #2
Its called modified gravity which is actually an umbrella term for any attempt to modify general relativity at large scales. Right now experimental evidence is not enough to favor one approach over the other.
 
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  • #3
ShayanJ said:
Right now experimental evidence is not enough to favor one approach over the other.

Sure there is. Look at the number of dark matter publications vs. modified gravity publications. The fact of the matter is that there is no theory of modified gravity consistent with all the data.
 
  • #4
Vanadium 50 said:
Sure there is. Look at the number of dark matter publications vs. modified gravity publications. [...]
Publications reporting direct experimental detection of actual dark matter are rather less numerous. :confused:
 
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  • #5
strangerep said:
Publications reporting direct experimental detection of actual dark matter are rather less numerous. :confused:

Of course this is true. But, as Vanadium50 pointed out, the dark matter hypothesis is consistent with the experimental data, while there is no modified gravity theory which is consistent with the data.
 
  • #6
Nobody likes the idea of dark matter, but no point in complaining unless there is a better idea.
 
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  • #7
Yes I can: no dark matter is a better idea since it saves the embarrassment of not finding it. Having reviewed most of the supposed evidence for dark matter, not one of these is unchallenged; contrary to what the dark matter fraternity try to imply.
 
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  • #8
Dark matter isn't really a theory.
It's a placeholder name for "something which has observable gravitational effect, similar to matter, but which cannot be detected by other means".
Originally there were three plausible explanations.
1. Something is wrong with our understanding of gravity. (Modified gravity).
2. It's ordinary matter, but we don't have a way of seeing it (MACHO's).
3. It's an as yet undiscovered kind of massive particle which barely interacts at all with anything else except through gravity. (WIMPS).

While we still don't know what it is, 1 and 2 now seem less likely.
So currently the effort is on detecting WIMPS. - Looking for the vary rare interactions of those particles using detectors deep underground where background noise is mostly eliminated.
 
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  • #9
Adrian59 said:
Yes I can: no dark matter is a better idea since it saves the embarrassment of not finding it.
You prefer something in complete disagreement with observations over something with just unusually small cross sections?
Having reviewed most of the supposed evidence for dark matter, not one of these is unchallenged; contrary to what the dark matter fraternity try to imply.
Challenged, but not refuted, unlike all other approaches that can’t get a consistent picture combining all observations.

The models of dark matter are not perfect, but they are the best we have so far.
 
  • #10
rootone said:
Dark matter isn't really a theory.
It's a placeholder name for "something which has observable gravitational effect, similar to matter, but which cannot be detected by other means".
Originally there were three plausible explanations.
1. Something is wrong with our understanding of gravity. (Modified gravity).
2. It's ordinary matter, but we don't have a way of seeing it (MACHO's).
3. It's an as yet undiscovered kind of massive particle which barely interacts at all with anything else except through gravity. (WIMPS).

While we still don't know what it is, 1 and 2 now seem less likely.
So currently the effort is on detecting WIMPS. - Looking for the vary rare interactions of those particles using detectors deep underground where background noise is mostly eliminated.

There is a 4 which it is possible to use the physics we have to explain most of the anomalies, and certainly for the galaxy rotation problem. I believe we have been looking at the rotation problem the wrong way and I have found several published papers pointing to alternatives but because of the hold the dark matter paradigm has on current thinking, these alternatives are either not viewed or ignored. For example: i) Jalocha, J., Bratek, L. and Kutschera1, M. (2008). ‘Is Dark Matter Present in NGC 4736? An Iterative Spectral Method for Finding Mass Distribution in Spiral Galaxies.’ Astrophysical Journal, vol 679, pp 373–378.
ii) J. D. Carrick and F. I. Cooperstock. ‘General relativistic dynamics applied to the rotation curves of galaxies.’ Department of Physics and Astronomy, University of Victoria P.O. Box 3055, Victoria, B.C. V8W 3P6 (Canada): arXiv:1101.3224v1.
 
  • #11
mfb said:
You prefer something in complete disagreement with observations over something with just unusually small cross sections?
Definitely not something in complete disagreement with observations I class myself as a scientist after all. Though the issue of small cross sections is interesting because is there a lower limit! I know I am paraphrasing Prof Stacy McGaugh here but if there is no lower limit, then theoretically there is no end to this search for a WIMP. Good news for tenure but I am not sure about physics.
 
  • #12
phyzguy said:
Of course this is true. But, as Vanadium50 pointed out, the dark matter hypothesis is consistent with the experimental data, while there is no modified gravity theory which is consistent with the data.

I'm not sure the dark matter hypothesis is experimentally falsifiable. Once you have the observation that observable matter cannot account for observable gravitational effects, you can posit some arbitrary distribution of unobservable matter in any way you want to account for the additional gravity.

But what observation could then falsify the hypothesis?

Sure, one might envision proposing alternate hypotheses (new models of how gravity works), but all these are constrained in their distribution of matter by the matter being observable through other means. In the "dark matter" hypothesis, one is free to distribute the dark matter however one pleases.
 
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  • #13
Adrian59 said:
Definitely not something in complete disagreement with observations I class myself as a scientist after all.
What is your alternative proposal that is internally consistent both for weak and strong gravitational fields and consistent with all measurements at the same time?
Adrian59 said:
Though the issue of small cross sections is interesting because is there a lower limit! I know I am paraphrasing Prof Stacy McGaugh here but if there is no lower limit, then theoretically there is no end to this search for a WIMP. Good news for tenure but I am not sure about physics.
I'm not aware of theoretical limits. The neutrino background will make improved experimental searches very difficult after DARWIN or maybe its successor.
Dr. Courtney said:
I'm not sure the dark matter hypothesis is experimentally falsifiable.
Of course it is. We have multiple independent measurements of its overall mass density. They wouldn't have to agree - but they do. We have multiple independent measurements of the distribution within galaxies. They wouldn't have to agree - but they do. All these tests could have ruled out dark matter but did not.
 
  • #14
mfb said:
We have multiple independent measurements of its overall mass density. They wouldn't have to agree - but they do. We have multiple independent measurements of the distribution within galaxies.

Are these measurements really independent of the stellar velocities? The same stellar velocities will give the same mass distributions if one assumes dark matter as an explanation. This does not mean the dark matter hypothesis is falsifiable, only that the stellar velocity measurements are consistent.

And if the stellar velocity measurements are consistent (the same with different methods), the assumption of dark matter will always lead to the same inferences regarding dark matter densities. But this is a circular argument, not a failure to falsify the dark matter hypothesis. It is merely a confidence boost for the different methods for determining stellar velocities.
 
  • #15
mfb said:
What is your alternative proposal that is internally consistent both for weak and strong gravitational fields and consistent with all measurements at the same time?

Yes it is - see my response to rootone above. Since my position is that existing physics can explain the galaxy rotation problem, I am by default consistent with any accepted theory. In fact since one of my references (J. D. Carrick and F. I. Cooperstock. ‘General relativistic dynamics applied to the rotation curves of galaxies.’ Department of Physics and Astronomy, University of Victoria P.O. Box 3055, Victoria, B.C. V8W 3P6 (Canada): arXiv:1101.3224v1) uses General Relativity I am entirely consistent with gravitational field theories.
 
  • #16
Dr. Courtney said:
Are these measurements really independent of the stellar velocities?
Which measurements?
CMB measurements? Sure.
Gravitational lensing measurements? Sure.
Adrian59 said:
arXiv:1101.3224v1
Submitted to arXiv 2010. Where is the publication? If that would be a solid result clearly they would have tried to publish it? Why didn't it get published?
And where do they discuss the dark matter measured with other methods? The CMB and lensing are briefly mentioned as evidence for dark matter in the introduction but then they are ignored.
Adrian59 said:
Jalocha, J., Bratek, L. and Kutschera1, M. (2008). ‘Is Dark Matter Present in NGC 4736? An Iterative Spectral Method for Finding Mass Distribution in Spiral Galaxies.’ Astrophysical Journal, vol 679, pp 373–378.
That is a measurement of a single galaxy. That doesn't explain anything else, like the amount of dark matter inferred from cosmology and so on.

That is not special about these two references, it is a general pattern. "Oh, we if do this, we can explain this single observation without dark matter" - while ignoring all the others. I asked for something compatible with all observations, exactly for this reason.

By the way: There is a high mortality rate for modified gravity due to the gravitational waves from the binary neutron star merger.
 
  • #17
mfb said:
Submitted to arXiv 2010. Where is the publication? If that would be a solid result clearly they would have tried to publish it? Why didn't it get published?

Based on my experience both in physics and another scientific area that I am professionally involved in, the problem is that if your paper does not fit the prevailing paradigm then it often doesn’t get published. So it is a little disingenuous to use this argument as absence of scientific value.

mfb said:
That is a measurement of a single galaxy. That doesn't explain anything else, like the amount of dark matter inferred from cosmology and so on.

That is not special about these two references, it is a general pattern. "Oh, we if do this, we can explain this single observation without dark matter" - while ignoring all the others. I asked for something compatible with all observations, exactly for this reason.

Yes I will concede that using a limited data set is questionable. However, there are larger data sets and published studies, questioning the dark matter paradigm:

1) Magain, P. and Chantry, V. (2013). ‘Gravitational lensing evidence against extended dark matter halos.’ Institut d’Astrophysique et de Géophysique, Université de Liège, Liège Belgium.

2) Lu et al (2010). ‘Large-scale structure and dynamics of the most X-ray luminous galaxy cluster known – RX J1347−1145’. Mon. Not. R. Astron. Soc. 403, 1787–1800.
 
  • #18
mfb said:
Sure.Submitted to arXiv 2010. Where is the publication? If that would be a solid result clearly they would have tried to publish it? Why didn't it get published?

This is not really true. A number of colleagues and I stopped trying to publish many of our papers beyond arXiv several years ago. If none of the co-authors "need" the paper on their CVs, we often just post to arXiv, because this reaches our intended audience and avoids the extra time (and politics) of submitting to the print journals and encountering editors and referees who want to change our work without even a careful reading to properly understand it.

Cooperstock and his colleagues are well established enough in their fields that I don't think it is reasonable to conclude the paper is flawed because it was only published at arXiv. Many of their other papers in the field are published in peer-reviewed journals, and the paper in question ( arXiv:1101.3224v1 ) has been cited 21 times. By the time a paper has been cited over 20 times, it is being taken seriously, and one should not knock it because it only appears at arXiv.

Adrian59 said:
Based on my experience both in physics and another scientific area that I am professionally involved in, the problem is that if your paper does not fit the prevailing paradigm then it often doesn’t get published. So it is a little disingenuous to use this argument as absence of scientific value.

Lots of disciplines are "old boy networks" where the peer-review process works pretty hard to protect the prevailing paradigm. Publishing at arXiv can be an effective work-around, establishing priority, putting the ideas before the intended audience, generating widespread discussion, etc. It is disingenuous to suggest that a paper is unworthy of discussion because it only appears at arXiv.
 
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  • #19
Adrian59 said:
Based on my experience both in physics and another scientific area that I am professionally involved in, the problem is that if your paper does not fit the prevailing paradigm then it often doesn’t get published. So it is a little disingenuous to use this argument as absence of scientific value.
You know how that sounds, and how that argument can be (and is) abused.
Dr. Courtney said:
This is not really true. A number of colleagues and I stopped trying to publish many of our papers beyond arXiv several years ago. If none of the co-authors "need" the paper on their CVs, we often just post to arXiv, because this reaches our intended audience and avoids the extra time (and politics) of submitting to the print journals and encountering editors and referees who want to change our work without even a careful reading to properly understand it.
I totally understand that for typical results, but if you make extraordinary claims?
It has been cited 21 times, we would have to see how many authors agree with the conclusions and how many do not. I picked one at random:
this article compares the predictions of GR and Newtonian gravity for three cases of self-gravitating dusts for which the exact general relativistic solutions are known. These investigations reveal that GR and Newtonian gravity are in excellent agreement in the appropriate limits, thus supporting the conventional use of Newtonian physics to analyze galactic rotation curves. These analyses also reveal some sources of error in the referred to works.
 
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  • #20
Vanadium 50 said:
Sure there is. Look at the number of dark matter publications vs. modified gravity publications. The fact of the matter is that there is no theory of modified gravity consistent with all the data.

The fact of the matter is that there is no theory of dark matter consistent with all the data.

Every outstanding dark matter theory is contradicted by some very solid piece of empirical data. The following 36 citations set forth some of the highlights of the current crisis in dark matter theory.

There are lots of dark matter publications, but those publications are overwhelmingly ruling out variations of dark matter theory, sometimes huge classes of them such as pretty much all MACHOs, all WIMPS, all collisionless dark matter, all cold dark matter theories, and all self-interacting dark matter theories. Warm dark matter is very close to being over constrained (and general exclusions of collisionless dark matter are the nail in the coffin), and many forms of axion dark matter are ruled out.

One common theme is that it isn't possible to devise a dark matter model that simultaneously fits lamdaCDM model constraints and the tight fix of inferred dark matter distributions to baryonic matter distributions (something that modified gravity models do naturally).

* Exclusions from the LHC. https://arxiv.org/abs/1709.02304 and https://arxiv.org/abs/1510.01516

* Dark matter can't have any significant coupling to Standard Model matter. https://arxiv.org/abs/1501.00907 This is a problem because "collisionless" dark matter that does not interact with Standard Model matter is pretty much ruled out per other citations below.

* Exclusions from Xenon-100 https://arxiv.org/abs/1709.02222

* Exclusions of Charming Dark Matter theories. https://arxiv.org/abs/1709.01930

* Theodorus Maria Nieuwenhuizen "Subjecting dark matter candidates to the cluster test" (October 3, 2017):

Galaxy clusters, employed by Zwicky to demonstrate the existence of dark matter, pose new stringent tests. If merging clusters demonstrate that dark matter is self-interacting with cross section σ/m∼2 cm2/gr, MACHOs, primordial black holes and light axions that build MACHOs are ruled out as cluster dark matter. Recent strong lensing and X-ray gas data of the quite relaxed and quite spherical cluster A1835 allow to test the cases of dark matter with Maxwell-Boltzmann, Bose-Einstein and Fermi-Dirac distribution, next to Navarro-Frenck-White profiles. Fits to all these profiles are formally rejected at over 5σ, except in the fermionic situation. The interpretation in terms of (nearly) Dirac neutrinos with mass of 1.61+0.19−0.30 eV/c2 is consistent with results on the cluster A1689, with the WMAP, Planck and DES dark matter fractions and with the nondetection of neutrinoless double β-decay. The case will be tested in the 2018 KATRIN experiment.

A variety of searches for sterile neutrinos have also ruled out this possibility in the relevant mass range. See, e.g., https://arxiv.org/abs/1710.06488 and http://iopscience.iop.org/article/10.1088/1742-6596/718/3/032008/pdf

* Exclusions for Axion Dark Matter: Renée Hlozek, David J. E. Marsh, Daniel Grin "Using the Full Power of the Cosmic Microwave Background to Probe Axion Dark Matter" (August 18, 2017).

* Combined direct dark matter detection exclusions. https://arxiv.org/abs/1708.04630 and https://arxiv.org/abs/1707.01632

* Exclusions based on non-detection of annihilations in dwarf galaxies. https://arxiv.org/abs/1708.04858

* Primordial black hole exclusions. https://arxiv.org/abs/1301.4984

* Daniele Gaggero, et al., "Searching for Primordial Black Holes in the radio and X-ray sky" (Pre-Print December 1, 2016). Abstract:

We model the accretion of gas on to a population of massive primordial black holes in the Milky Way, and compare the predicted radio and X-ray emission with observational data. We show that under conservative assumptions on the accretion process, the possibility that O(10)M⊙ primordial black holes can account for all of the dark matter in the Milky Way is excluded at 4σ by a comparison with the VLA radio catalog at 1.4 GHz, and at more than 5σ by a comparison with the NuSTAR X-ray catalog (10 - 40 keV). We also propose a new strategy to identify such a population of primordial black holes with more sensitive future radio and X-ray surveys.

* Tight Warm Dark Matter parameter exclusions. https://arxiv.org/pdf/1704.01832.pdf

* More Warm Dark Matter parameters exclusions: Simon Birrer, Adam Amara, and Alexandre Refregier, "Lensing substructure quantification in RXJ1131-1231: A 2 keV lower bound on dark matter thermal relict mass" (January 31, 2017).

We study the substructure content of the strong gravitational lens RXJ1131-1231 through a forward modelling approach that relies on generating an extensive suite of realistic simulations. The statistics of the substructure population of halos depends on the properties of dark matter. We use a merger tree prescription that allows us to stochastically generate substructure populations whose properties depend on the dark matter particle mass. These synthetic halos are then used as lenses to produce realistic mock images that have the same features, e.g. luminous arcs, quasar positions, instrumental noise and PSF, as the data. By analysing the data and the simulations in the same way, we are able to constrain models of dark matter statistically using Approximate Bayesian Computing (ABC) techniques. This method relies on constructing summary statistics and distance measures that are sensitive to the signal being targeted. We find that using the HST data for \RXJ we are able to rule out a warm dark matter thermal relict mass below 2 keV at the 2 sigma confidence level.

* Lin Wang, Da-Ming Chen, Ran Li "The total density profile of DM halos fitted from strong lensing" (July 31, 2017). Abstract:

In cosmological N-body simulations, the baryon effects on the cold dark matter (CDM) halos can be used to solve the small scale problems in ΛCDM cosmology, such as cusp-core problem and missing satellites problem. It turns out that the resultant total density profiles (baryons plus CDM), for halos with mass ranges from dwarf galaxies to galaxy clusters, can match the observations of the rotation curves better than NFW profile. In our previous work, however, we found that such density profiles fail to match the most recent strong gravitational lensing observations. In this paper, we do the converse: we fit the most recent strong lensing observations with the predicted lensing probabilities based on the so-called (α,β,γ) double power-law profile, and use the best-fit parameters (α=3.04,β=1.39,γ=1.88) to calculate the rotation curves. We find that, at outer parts for a typical galaxy, the rotation curve calculated with our fitted density profile is much lower than observations and those based on simulations, including the NFW profile. This again verifies and strengthen the conclusions in our previous works: in ΛCDM paradigm, it is difficult to reconcile the contradictions between the observations for rotation curves and strong gravitational lensing.

As the body text explains:

It is now well established that, whatever the manners the baryon effects are included in the collisionless CDM N-body cosmological simulations, if the resultant density pro- files can match the observations of rotation curves, they cannot simultaneously predict the observations of strong gravitational lensing (under- or over-predict). And for the case of typical galaxies, the reverse is also true, namely, the SIS profile preferred by strong lensing cannot be supported by the observations of rotation curves near the centers of galaxies.

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

The more we go deep into the knowledge of the dark component which embeds the stellar component of galaxies, the more we realize the profound interconnection between them. We show that the scaling laws among the structural properties of the dark and luminous matter in galaxies are too complex to derive from two inert components that just share the same gravitational field. In this paper we review the 30 years old paradigm of collisionless dark matter in galaxies. We found that their dynamical properties show strong indications that the dark and luminous components have interacted in a more direct way over a Hubble Time. The proofs for this are the presence of central cored regions with constant DM density in which their size is related with the disk length scales. Moreover we find that the quantity ρDM(r,L,RD)ρ⋆(r,L,RD) shows, in all objects, peculiarities very hardly explained in a collisionless DM scenario.

* 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).

* One of the more successful recent efforts to reproduce the baryonic Tully-Fischer relation with CDM models is L.V. Sales, et al., "The low-mass end of the baryonic Tully-Fisher relation" (February 5, 2016). It explains:

[T]he literature is littered with failed attempts to reproduce the Tully-Fisher relation in a cold dark matter-dominated universe. Direct galaxy formation simulations,for example, have for many years consistently produced galaxies so massive and compact that their rotation curves were steeply declining and, generally, a poor match to observation. Even semi-analytic models, where galaxy masses and sizes can be adjusted to match observation, have had difficulty reproducing the Tully-Fisher relation, typically predicting velocities at given mass that are significantly higher than observed unless somewhat arbitrary adjustments are made to the response of the dark halo.

The paper manages to simulate the Tully-Fisher relation only with a model that has sixteen parameters carefully "calibrated to match the observed galaxy stellar mass function and the sizes of galaxies at z = 0" and "chosen to resemble the surroundings of the Local Group of Galaxies", however, and still struggles to reproduce the one parameter fits of the MOND toy-model from three decades ago. Any data set can be described by almost any model so long as it has enough adjustable parameters.

* Dark matter can't explain bulge formation in galaxies: Alyson M. Brooks, Charlotte R. Christensen, "Bulge Formation via Mergers in Cosmological Simulations" (12 Nov 2015).

[W]e also demonstrate that it is very difficult for current stellar feedback models to reproduce the small bulges observed in more massive disk galaxies like the Milky Way. We argue that feedback models need to be improved, or an additional source of feedback such as AGN is necessary to generate the required outflows.

* Baryon effects can't save cold dark matter models. https://arxiv.org/abs/1706.03324

* Cold dark matter models don't explain the astronomy data. https://arxiv.org/pdf/1305.7452v2.pdf

Evidence that Cold Dark Matter (ΛCDM), CDM+ baryons and its proposed tailored cures do not work in galaxies is staggering, and the CDM wimps (DM particles heavier than 1 GeV) are strongly disfavoured combining theory with galaxy astronomical observations.

* As of 2014, a review article ruled out rule out pretty much all cold dark matter models except "warm dark matter" (WDM) (at a keV scale mass that is at the bottom of the range permitted by the lamdaCDM model) and "self-interacting dark matter" (SIDM) (which escapes problems that otherwise plague cold dark matter models with a fifth force that only acts between dark matter particles requiring at least a beyond the Standard Model fermion and a beyond the Standard Model force carried by a new massive boson with a mass on the order of 1-100 MeV). Alyson Brooks, "Re-Examining Astrophysical Constraints on the Dark Matter Model" (July 28, 2014). As other more recent links cited here note, collisionless WDM and pretty much all SIDM models have since been ruled out.

* Dark matter annihilation has largely been ruled out as a source of FERMI signals attributed to dark matter annihilation. Samuel K. Lee, Mariangela Lisanti, Benjamin R. Safdi, Tracy R. Slatyer, and Wei Xue. "Evidence for unresolved gamma-ray point sources in the Inner Galaxy." Phys. Rev. Lett. (February 3, 2016). Millisecond pulsars were the source.

* Proposed warm dark matter annihilation signals also turned out to be false alarms. https://arxiv.org/abs/1408.1699 and https://arxiv.org/abs/1408.4115

* The bounds on the minimum dark matter mean lifetime of 3.57*10^24 seconds. This is roughly 10^17 years. By comparison the age of the universe is roughly 1.38 * 10^9 years. This means that dark matter (if it exists) is at least as stable as anything other than a proton, which has an experimentally determined mean lifetime of at least 10^33 years. https://arxiv.org/abs/1504.01195 This means that all dark matter candidates that are not perfectly stable or at least metastable are ruled out. Decaying dark matter and dark matter with any significant annihilation cross section are inconsistent with observation.

* Torsten Bringmann, et al., "Strong constraints on self-interacting dark matter with light mediators" (December 2, 2016). Abstract:

Coupling dark matter to light new particles is an attractive way to combine thermal production with strong velocity-dependent self-interactions. Here we point out that in such models the dark matter annihilation rate is generically enhanced by the Sommerfeld effect, and we derive the resulting constraints from the Cosmic Microwave Background and other indirect detection probes. For the frequently studied case of s-wave annihilation these constraints exclude the entire parameter space where the self-interactions are large enough to address the small-scale problems of structure formation.

The conclusion of the paper notes that:

Models of DM with velocity-dependent self-interactions have recently received a great deal of attention for their potential to produce a number of interesting effects on astrophysical scales. We have shown in this Letter that these models face very strong constraints from the CMB and DM indirect detection. In the most natural realization of this scenario with a light vector mediator with kinetic mixing, these constraints rule out the entire parameter space where the self-scattering cross section can be relevant for astrophysical systems. These bounds remain highly relevant for a number of generalizations of the scenario, such as a different dark sector temperature and different mediator branching ratios. Clearly, future efforts to develop particle physics models for SIDM need to address these issues in order to arrive at models that provide a picture consistent with all observations in cosmology, astrophysics and particle physics.

* Dark photon parameter space (the carrier boson of the SIDM models) is also tightly constrained and all but ruled out. Yet, the properties a dark photon has to have, if there is one, are tightly experimentally established based upon cluster dynamics. https://arxiv.org/abs/1504.06576

* The Bullet Cluster is a huge problem for DM. Jounghun Lee, Eiichiro Komatsu, "Bullet Cluster: A Challenge to LCDM Cosmology" (May 22, 2010). Later published in Astrophysical Journal 718 (2010) 60-65. Abstract:

To quantify how rare the bullet-cluster-like high-velocity merging systems are in the standard LCDM cosmology, we use a large-volume 27 (Gpc/h)^3 MICE simulation to calculate the distribution of infall velocities of subclusters around massive main clusters. The infall-velocity distribution is given at (1-3)R_{200} of the main cluster (where R_{200} is similar to the virial radius), and thus it gives the distribution of realistic initial velocities of subclusters just before collision. These velocities can be compared with the initial velocities used by the non-cosmological hydrodynamical simulations of 1E0657-56 in the literature. The latest parameter search carried out recently by Mastropietro and Burkert showed that the initial velocity of 3000 km/s at about 2R_{200} is required to explain the observed shock velocity, X-ray brightness ratio of the main and subcluster, and displacement of the X-ray peaks from the mass peaks. We show that such a high infall velocity at 2R_{200} is incompatible with the prediction of a LCDM model: the probability of finding 3000 km/s in (2-3)R_{200} is between 3.3X10^{-11} and 3.6X10^{-9}. It is concluded that the existence of 1E0657-56 is incompatible with the prediction of a LCDM model, unless a lower infall velocity solution for 1E0657-56 with < 1800 km/s at 2R_{200} is found.

and also

Garry W. Angus and Stacy S. McGaugh, "The collision velocity of the bullet cluster in conventional and modified dynamics" (September 2, 2007) published at MNRAS.

We consider the orbit of the bullet cluster 1E 0657-56 in both CDM and MOND using accurate mass models appropriate to each case in order to ascertain the maximum plausible collision velocity. Impact velocities consistent with the shock velocity (~ 4700km/s) occur naturally in MOND. CDM can generate collision velocities of at most ~ 3800km/s, and is only consistent with the data provided that the shock velocity has been substantially enhanced by hydrodynamical effects.

* El Gordo poses similar problems for dark matter models. Sandor M. Molnar, Tom Broadhurst. "A HYDRODYNAMICAL SOLUTION FOR THE “TWIN-TAILED” COLLIDING GALAXY CLUSTER “EL GORDO”. The Astrophysical Journal, 2015; 800 (1): 37 DOI: 10.1088/0004-637X/800/1/37

* Axion fuzzy dark matter ruled out: Vid Iršič, Matteo Viel, Martin G. Haehnelt, James S. Bolton, George D. Becker. "First Constraints on Fuzzy Dark Matter from Lyman-α Forest Data and Hydrodynamical Simulations." Physical Review Letters, 2017; 119 (3) DOI: 10.1103/PhysRevLett.119.031302
 
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  • #21
Dr. Courtney said:
Cooperstock and his colleagues are well established enough in their fields that I don't think it is reasonable to conclude the paper is flawed because it was only published at arXiv. Many of their other papers in the field are published in peer-reviewed journals, and the paper in question ( arXiv:1101.3224v1 ) has been cited 21 times. By the time a paper has been cited over 20 times, it is being taken seriously, and one should not knock it because it only appears at arXiv.

Cooperstock (2011) was published, but the arXiv entry was never updated. https://link.springer.com/article/10.1007/s10509-011-0854-z

A previous pre-print by Cooperstock was published and reached the same conclusion as https://arxiv.org/abs/1101.3224

General relativistic velocity: the alternative to dark matter
F.I. Cooperstock, S. Tieu
(Submitted on 30 Nov 2007)
We consider the gravitational collapse of a spherically symmetric ball of dust in the general relativistic weak gravity regime. The velocity of the matter as viewed by external observers is compared to the velocity gauged by local observers. While the comparison in the case of very strong gravity is seen to follow the pattern familiar from studies of test particles falling towards a concentrated mass, the case of weak gravity is very different. The velocity of the dust that is witnessed by external observers is derived for the critically open case and is seen to differ markedly from the expectations based upon Newtonian gravity theory. Viewed as an idealized model for a cluster of galaxies, we find that with the general relativistic velocity expression, the higher-than-expected constituent velocities observed can be readily correlated with the solely baryonic measure of the mass, obviating the need to introduce extraneous dark matter. Hitherto unexplained and subject-to-reinterpretation astrophysical phenomena could also be considered within this context. It is suggested that an attempt be made to formulate an experimental design at smaller scales simulating or realizing a collapse with the aim of implementing a new test of general relativity.
Comments: 12 pages, 2 figures
Subjects: Astrophysics (astro-ph); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)
Journal reference: Mod.Phys.Lett.A23:1745-1755,2008
DOI: 10.1142/S0217732308027163

There have also been at least three published papers subsequent to the 2011 paper for which pre-prints have not been posted:

N. S. Magalhaes, F. I. Cooperstock. (2017) Mass density and size estimates for spiral galaxies using general relativity. Astrophysics and Space Science 362:11. [Crossref]

Rotation curves of spiral galaxies reveal a physical phenomenon that has been seen to lack a satisfactory scientific explanation: velocities of stars far from the nucleus are high and approximately constant. In the context of Newtonian dynamics, the existence of a new kind of matter (dark matter) is assumed, which, when added to the usual observed matter, would account for the phenomenon; however, the nature of such dark matter is unknown and it was never detected. There are other ongoing investigations of the phenomenon, such as MOND and emergent gravity. In this work we present new results from another approach, in which general relativity is employed to approximate a galaxy by an axially-symmetric, pressure-less fluid in stationary rotation, yielding an expression for its rotation curve and mass density. We apply this model to data of four galaxies: NGC 2403, NGC 2903, NGC 5055 and the Milky Way. We obtain mass density contours of these galaxies which we compare to observational data, a procedure that could open a new window for investigating galactic structure. In our Solar neighborhood, we found a mass density and density fall-off fitting observational data satisfactorily, addressing a critique to the model by Fuchs and Phleps. Using a threshold density apparently related to the observed optical zone of a galaxy, the model had already indicated that the Milky Way could be larger than had been believed to be the case. To our knowledge, this was the only such existing theoretical indication ever presented. Recent observational results by Xu et al. have confirmed that theoretical prediction, which we fortify here using a large set of observational data. Galactic masses are seen to be higher than the baryonic mass determined from observations but lower than those deduced from the approaches relying upon dark matter in a Newtonian context. We also calculate the non-luminous fraction of matter for our sample of galaxies and present possible general relativistic explanations for this. The evidence points to general relativity playing a significant role in the explanation of the phenomenon.

F. I. Cooperstock. (2016) The power of weak-field GR gravity. International Journal of Modern Physics D 25:12. Online publication date: 1-Oct-2016. [Abstract | http://www.worldscientific.com/doi/pdf/10.1142/S021827181644017X | http://www.worldscientific.com/doi/pdfplus/10.1142/S021827181644017X]

The power of weak-field GR gravity

Received: 7 June 2016
Accepted: 5 September 2016
Published: 6 October 2016

While general relativity (GR) is our premier theory of gravity, galactic dynamics from the outset has been studied with Newtonian gravity (NG), guided by the long-held belief in the idea of the “Newtonian-limit” of GR. This maintains that when the gravitational field is weak and the velocities are nonrelativistic, NG is the appropriate theory, apart from small corrections at best (such as in GPS tracking). However, there are simple examples of phenomena where there is no NG counterpart. We present a particularly simple new example of the stark difference that NG and weak-field GR exhibit for a modified van Stockum source, which speaks to the flat galactic rotation curve problem. We note that the linear GR compatibility equation in the literature is incomplete. Its completion is vital for our case, leading to a stark contrast between GR and NG for totally flat van Stockum rotation curves.

F. I. Cooperstock. (2016) Weak-field general relativistic dynamics and the Newtonian limit. Modern Physics Letters A 31:05. Online publication date: 20-Feb-2016. [Abstract | http://www.worldscientific.com/doi/pdf/10.1142/S0217732316500371 | http://www.worldscientific.com/doi/pdfplus/10.1142/S0217732316500371]

Weak-field general relativistic dynamics and the Newtonian limit
F. I. Cooperstock1
orcid.png

1Department of Physics and Astronomy, University of Victoria, P. O. Box 3055, Victoria, B.C. V8W 3P6, Canada

Dedicated to the memory of W. B. Bonnor

Received: 27 August 2015
Revised: 16 November 2015
Accepted: 17 December 2015
Published: 25 January 2016

We show that the generally held view that the gravity of weak-field nonrelativistic-velocity sources being invariably almost equivalent to Newtonian gravity (NG) (the “Newtonian limit” approach) is in some instances misleading and in other cases incorrect. A particularly transparent example is provided by comparing the Newtonian and general relativistic analyses of a simple variant of van Stockum’s infinite rotating dust cylinder. We show that some very recent criticisms of our work that had been motivated by the Newtonian limit approach were incorrect and note that no specific errors in our work were found in the critique. In the process, we underline some problems that arise from inappropriate coordinate transformations. As further support for our methodology, we note that our weak-field general relativistic treatment of a model galaxy was vindicated recently by the observations of Xu et al. regarding our prediction that the Milky Way was 19–21 kpc in radius as opposed to the commonly held view that the radius was 15 kpc.
 

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  • #22
phyzguy said:
Of course this is true. But, as Vanadium50 pointed out, the dark matter hypothesis is consistent with the experimental data, while there is no modified gravity theory which is consistent with the data.

You are mistaken. The dark matter hypothesis is not consistent with the experimental data. See the citations at post #20 in this thread.

While the dark matter hypothesis does a decent job of explaining the cosmological evolution of the overall universe as demonstrated in CMB data, for example, it does a poor job of explaining phenomena at the galactic cluster and cluster level, there has been no direct detection of it, it has not been observed at the LHC, every hint of dark matter annihilation has subsequently been ruled out, and there is no dark matter theory that can explain why inferred dark matter quantities and distributions are so intimately and exactly correlated with baryonic matter distributions. Simulations of how dark matter should behave contradict what is observed unless models are highly tuned (with sixteen parameters!) to the data sets that it is supposed to be predicting.

Also, the best extant modified gravity theories perform as well as better as the best extant dark matter theories.
 
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  • #23
Another series of articles (the first two published, the latter just released as a pre-print last month) which like Cooperstock argue that one can fit a basically GR analysis to reproduce the data as opposed to the Newtonian one commonly used in practice, but from a quantum gravity rather than a classical GR perspective is as follows:

* A. Deur, "A possible explanation for dark matter and dark energy consistent with the Standard Model of particle physics and General Relativity" (September 7, 2017). Abstract:

Numerical calculations have shown that the increase of binding energy in massive systems due to gravity's self-interaction can account for galaxy and cluster dynamics without dark matter. Such approach is consistent with General Relativity and the Standard Model of particle physics. The increased binding implies an effective weakening of gravity outside the bound system. In this article, this suppression is modeled in the Universe's evolution equations and its consequence for dark energy is explored. Observations are well reproduced without need for dark energy. The cosmic coincidence appears naturally and the problem of having a de Sitter Universe as the final state of the Universe is eliminated.

* A. Deur, "Self-interacting scalar fields in their strong regime" (November 17, 2016). Abstract:

We study two self-interacting scalar field theories in their high-temperature limit using path integrals on a lattice. We first discuss the formalism and recover known potentials to validate the method. We then discuss how these theories can model, in the high-temperature limit, the strong interaction and General Relativity. For the strong interaction, the model recovers the known phenomenology of the nearly static regime of heavy quarkonia. The model also exposes a possible origin for the emergence of the confinement scale from the approximately conformal Lagrangian. Aside from such possible insights, the main purpose of addressing the strong interaction here --given that more sophisticated approaches already exist-- is mostly to further verify the pertinence of the model in the more complex case of General Relativity for which non-perturbative methods are not as developed. The results have important implications on the nature of Dark Matter. In particular, non-perturbative effects naturally provide flat rotation curves for disk galaxies, without need for non-baryonic matter, and explain as well other observations involving Dark Matter such as cluster dynamics or the dark mass of elliptical galaxies.

* Alexandre Deur, "A correlation between the amount of dark matter in elliptical galaxies and their shape" (July 28, 2014). Abstract:

We discuss the correlation between the dark matter content of elliptical galaxies and their ellipticities. We then explore a mechanism for which the correlation would emerge naturally. Such mechanism leads to identifying the dark matter particles to gravitons. A similar mechanism is known in Quantum Chromodynamics (QCD) and is essential to our understanding of the mass and structure of baryonic matter.

At the back of napkin level, at least, this approach solves all dark matter and dark energy problems, although I would be the first to acknowledge that these promising theories need more rigorous and thorough investigation by other authors with more data.
 
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  • #24
mfb said:
You know how that sounds, and how that argument can be (and is) abused.

I'm not exactly sure what your point is but I agree that publication is a measure of quality but not necessarily that the reverse is true. What is true is a lot of registered but not published papers will be of less merit. However, rather than get involved in a hypothetical discourse I would be more interested in a response to my other comment in post 17 which I did reference.
 
  • #25
Dr. Courtney said:
Sure, one might envision proposing alternate hypotheses (new models of how gravity works), but all these are constrained in their distribution of matter by the matter being observable through other means. In the "dark matter" hypothesis, one is free to distribute the dark matter however one pleases.
No you can't. It is observable via gravity, specifically lensing. It is easy to imagine a scenario (that hasn't happened) whereby placement needed to explain dynamics is contradicted by lensing observations. Then you would be pretty much forced to look at modified gravity of some sort.
 
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  • #26
PAllen said:
No you can't. It is observable via gravity, specifically lensing. It is easy to imagine a scenario (that hasn't happened) whereby placement needed to explain dynamics is contradicted by lensing observations. Then you would be pretty much forced to look at modified gravity of some sort.

OK, so the distribution of dark matter can be determined both by dynamics as well as by gravitational lensing, providing two independent checks on the purported density of dark matter in a certain location.

While I agree in principle that this provides falsifiability for the hypothesis of dark matter, in practice, the degree of falsifiability really depends on the uncertainties (error bars) in the purported density of dark matter determined by the independent methods (dynamics and lensing) and the number of locations in the universe in which reasonably accurate determinations of the dark matter density can be made by independent measurements.

If the error bars on the dark matter density in a specific location determined from lensing are on the order of 50-100% of the density itself, I don't really regard that as having much power in terms of falsification when compared with the density determined from dynamics at the same location. Error bars as big (or nearly so) as the density itself really just means the density of dark matter is purportedly positive. Push the error bars down to 10% (both methods) and determine the density of dark matter (both methods) in 50-100 overlapping locations, and then you really have something in terms of falsifiability. (Meaning the absence of a contradiction provides significant support for the dark matter hypothesis.)
 
  • #27
PAllen said:
No you can't. It is observable via gravity, specifically lensing. It is easy to imagine a scenario (that hasn't happened) whereby placement needed to explain dynamics is contradicted by lensing observations. Then you would be pretty much forced to look at modified gravity of some sort.

This is precisely how pretty much all of the leading dark matter scenarios have been ruled out. Actually, not just lensing by also dynamics in various circumstances. In particular, lensing observations are, in general, inconsistent with observed dynamics in many galaxies (see post #20) and the inferred fit of dark matter to baryonic matter is to tight to happen without abandoning the property that dark matter must have for lambdaCDM which is that it is virtually collisionless (i.e. doesn't interact via Standard Model forces with baryonic matter to any significant extent).
 
  • #28
Thanks, everyone !
I thought I'd grasped the various arguments' gist, but now realize the situation is much, much more complex.
I know some of the 'missing' baryonic matter recently showed up between galaxies, but that wasn't 'Dark Matter'..
I must wonder if there's 'heavy neutrinos' or their shy equivalent from 'Beyond Standard Model' to be found here-abouts, and how they would be affected by planets' gravity...
IMHO, MOND variants have become like 'String Theory', too many degrees of freedom, too-slowly constrained by data...
 
  • #29
[QUOTE="Nik_2213, post: 5868216, member: 252015
IMHO, MOND variants have become like 'String Theory', too many degrees of freedom, too-slowly constrained by data...[/QUOTE]

Inflation theories have exactly this problem. There are literally hundreds of them, each with many free parameters, and the data isn't sufficient to strictly constrain the alternatives all that much yet.

For modified gravity theories seeking to explain phenomena attributed to dark matter, however, this is not really the case.

Modified gravity theories generally are fit to the data with far fewer degrees of freedom than dark matter theories. The original 1983 toy-model MOND theory was able to fit the dynamics of galaxies from dwarves to spirals to elliptical galaxies with a single parameter and made accurate provable predictions about galactic dynamics with that one degree of freedom. It took about four to six parameters to achieve that in most dark matter theories at the single type of galaxy level alone, and a really decent fit to the baryonic Tully-Fisher relationship (which empirically holds true very exactly) which MOND captures with one degree of freedom for all galaxies, takes about sixteen free parameters in dark matter models. The original MOND didn't capture galactic cluster behavior and wasn't relativistic, but it only takes two or three more parameters tops to add those features and dark energy as well if you are clever.

Basically, any dark matter theory has to be able to replicate this result with the same number or fewer free parameters, and to date, no dark matter theory has accomplished this economy of fit to the data with so few free parameters.

Also, pretty much the only parameter space left in dark matter models requires more than a dark matter particle mass because sterile collisionless dark matter doesn't fit the data. You also need parameters for coupling constants and massive carrier bosons for a massive Yukawa self-interaction force and some sort of parameter to secure interactions with ordinary baryonic matter. So you need to completely new forces instead of a higher order tweak to one that already exists. In more phenomenological dark matter models, you set a lot of parameters to describe the distribution of the dark matte without even trying to explain how that distribution arose. Also, the more baroque a dark matter model gets, the more you should doubt it, because modified gravity theories have explained the same phenomena with far fewer parameters.

Lots of modified gravity theories are ruled out by data. Verlinde's Emergent Gravity was approximately right, but clearly inconsistent with the data. Toy-model MOND has always been known to be wrong because it isn't relativistic. Scalar-tensor theories explained dark energy but not dark matter and appear to have taken another blow from the neutrino merger data. There are probably half a dozen or a dozen modified gravity theories that can meet the basic test of fitting rotation curves in galaxies, of which several can also fit cluster data and at least a couple can fit cosmology data.

In general, it is easier to falsify modified gravity theories than it is to falsify dark matter theories, because in a modified gravity theory more of the model is explicit, while in the dark matter theory you are only half way there when you specify the properties of the particle itself. You still need to figure out how and why it is distributed in the way that dark matter phenomena are today.

Back when the whole dark matter modeling enterprise got started, a lot of people had faith that one could simply model a basic WIMP and the right distribution would miraculously appear, in much the same way that SUSY theories were predicting the right thermal relic density for dark matter.

This fell apart, however, when it turned out that traditional cold dark matter produced inferred distributions of dark matter different from those predicted. For example, naively, if you do something similar to the statistical mechanics that are at the root of thermodynamics, dark matter should have what is called a Navarro-Frenck-White distribution in a typical spiral galaxy. But, instead, the inferred shape of a dark matter halo in a system like that is something closer to a rugby ball which is called an "isothermal" distribution.

Cold dark matter WIMP theories took another blow when it became clear from the LHC and from direct dark matter detection experiments, that there were no weakly interacting stable particles in the right mass range for a SUSY derived theory. While simple, sterile cold dark matter does a decent job of explaining cosmology scale phenomena like the CMB, it starts to fail badly in colliding galactic clusters like the Bullet Cluster and El Gordo, and generically produces dark matter distributions other than those observed at the galaxy level as noted above.

Also, while MOND has made several accurate true predictions of galactic dynamics, and Deur's theory successfully predicted a relationship between apparent dark matter and the extent to which elliptical galaxies are spherical, dark matter theories have not made any notable predictions. They have only been fit to observation after the fact.

One can't absolutely rule out either modified gravity or dark matter theories, and neither has a perfect and well validated solution yet. But, your intuition on the number of degrees of freedom involved in the models and the easy with which they can be tested is basically wrong.
 
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  • #30
"I thought I'd grasped the various arguments' gist, but now realize the situation is much, much more complex."
And I got it back to front...
QED.
 
  • #31
ohwilleke said:
One can't absolutely rule out either modified gravity or dark matter theories, and neither has a perfect and well validated solution yet.

Thanks for your comprehensive analysis of the current status of dark matter research. It has taken me a while to get through it all. The main findings that I took to be is that the LHC and Planck do not show physics beyond the standard model; and CDM appears to be inconsistent with astronomical observations. I note you have given a published reference for the Cooperstock paper that I mentioned first in this thread and in doing so I presume mfb should be more happy

(mfb said:
Sure.Submitted to arXiv 2010. Where is the publication? If that would be a solid result clearly they would have tried to publish it? Why didn't it get published?)

but in doing so are you in any way convinced by the argument that galaxy rotation curves could be explained without recourse to exotic matter or modifying Newtonian gravity..
 
  • #32
I seem to remember that although I was quite excited by the abstract of the Cooperstock paper, the conclusion that Newtonian and GR theory could be so different was totally implausible, and on closer inspection the results seemed to be spurious artifacts related to the use of a rotating coordinate system which didn't appear to be self-consistent, which was quite disappointing. I don't remember the details, but there are probably papers out there which discuss the problems; I think it was discussed in a thread on PF at the time.
 
  • #33
I've been reading some work by Hartnett on Carmeli's treatment of SpaceTime, using the Hubble Law as an axiom, with rapid acceleration of the universe, including a bounded, possibly isotropic universe, which is interesting because, as our choice of cosmological philosophy is purely personal, we find that the regular working out of Hartnett and Carmeli's approach avoids the need for dark matter or dark energy to solve for the excess velocities of stars, or more accurately, hydrogen atoms, in distant galaxies. The number of critically selected parameters is very low, according to Hartnett, while the data-fit with actual Galaxies is very good.

As I'm not competent at that level of physics, I wonder if anyone more advanced can offer a critique of Hartnett's working?
I have no question about his choice of cosmology, only about the veracity of his physics working.
The publication I have read about Hartnett and Carmeli's work in, is ISDN 978-0-949906-68-7 'Starlight Time and the New Physics', Dr John Hartnett, PhD 2007

International Journal of Theoretical Physics carries some of the work, by JG Hartnett, issue 45, page 11.:2118-2136, 2006. It's online in the archive at arxiv.org/abs/astro-ph/0511756, I can't find it at reuters...but it must be legit enough surely? Article titled 'Spiral galaxy rotation curves determined by carmeli...'
 
  • #34
Adrian59 said:
but in doing so are you in any way convinced by the argument that galaxy rotation curves could be explained without recourse to exotic matter or modifying Newtonian gravity.

I am pretty sure neither Cooperstock (in a classical GR framework) or Deur (in a quantum gravity framework) are actually applying mainstream GR to get their results and actually subtly deviate from mainstream interpretation of GR. But, the fact that both investigators can get such impressive results with such very, very subtle tweaks to GR interpretation is in my view very promising. It is not at all obvious that conventional GR does not have a subtle flaw or two that make a big difference at large scales in weak gravitational fields.

Everyone agrees that Newtonian gravity has to be modified. GR is a Newtonian gravity modification. The question is whether GR has to be modified.
 
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  • #35
Graham P said:
[...] International Journal of Theoretical Physics [...] I can't find it at reuters...
It's on the Reuters list. (Do a search on the full journal title.)

[Edit: ... and then I looked up Hartnett on Wikipedia. His extracurricular activities send cold shivers down my spine.]
 
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