Dark Matter does not exist, what do you think?

[Chronos]

The trouble with dark matter detection efforts is not knowing what it is. Some studies have focused on detection of dark matter decay products, despite the fact we do not know if it actually decays. Other studies look for interaction effects, despite the fact its ability to interact is unknown. Maybe it both decays and interacts, but, coaxing a signal out of the error bar level has thus far eluded us. So long as we don't know what we are looking for, knowing when we find it is a problem. We have a hay stack that appears to weigh far more than it should. We don't know if that extra weight is in the form of needles, noodles, or whatever. We are, however, pretty sure its not all cow pie. Keep in mind it took us 25 years to experimentally detect neutrinos, despite the fact we knew what we were looking for.

 

[ohwilleke]

The trouble with dark matter detection efforts is not knowing what it is."

True enough. In the entirely plausible scenario in which dark matter has no coupling to Standard Model bosons, and no interactions with Standard Model fermions other than Fermi contact forces (i.e. the rule that no two fermions can be in the same place at the same time), and gravity is the only connection between a dark sector and a SM sector, then direct detection of dark matter ought to be impossible.

At best we can hope to rule out dark matter that interacts with the SM sector by a means other than gravity with direct detection experiments (or to detect dark matter that interacts with the SM sector, although that parameter space grows ever smaller).

 

[CKH]

In lieu of direct detection, I wonder if the Gaia mission can tells us anything about the amounts and distribution of dark matter in the Galaxy?

 

[TumblingDice]

In lieu of direct detection, I wonder if the Gaia mission can tells us anything about the amounts and distribution of dark matter in the Galaxy?

Interesting thought. I wonder if creating the 3D model they hope for would find any 'problems' resolving the data due to DM. We already know about galaxy rotational speed issues, so there may be little to add to that. But if there are varying densities of DM within galaxies, might that cause measurable lensing anomalies? Anomalies large enough to result in problems justifying/explaining the complete 3D modeling of apparent relative motions between all stars measured in the same galaxy?

Could Gaia see curveballs like these in the data it collects from its solar L2 vantage point? That's a lot of number crunching, assuming the data contains important information within the detection limits of the mission. From wiki:

Each celestial object will be observed on average about 70 times during the mission, which is expected to last five years. These measurements will help determine the astrometric parameters of stars: two corresponding to the angular position of a given star on the sky, two for the derivatives of the star's position over time (motion) and lastly, the star's parallax from which distance can be calculated.

 

[Torbjorn_L]

FWIW, DM has survived yet another test and MOND has been blown out of the water again.

A new measurement of dark matter in the Milky Way has revealed there is half as much of the mysterious substance as previously thought. ...

"When you use our measurement of the mass of the dark matter the theory predicts that there should only be three satellite galaxies out there, which is exactly what we see; the Large Magellanic Cloud, the Small Magellanic Cloud and the Sagittarius Dwarf Galaxy."

University of Sydney astrophysicist Professor Geraint Lewis, who was also involved in the research, said the missing satellite problem had been "a thorn in the cosmological side for almost 15 years."

[ http://www.sciencedaily.com/releases/2014/10/141009091600.htm ]

If these fits were that trivial and unrelated to the issue of dark matter halos, there would be no interest in them. You seem to be unaware that these fits are important to mainstream scientists.

They may have been as I described earlier, but not now. And in retrospect I, at least, wonder why as they are trivial.

They require explanations in the context of mainstream theory. Many mainstream scientists are interested and seeking explanations in the context of CDM.

Now you are confusing the peculiar behavior of the rotation curves with their existence.[/QUOTE][/QUOTE]

 

[Torbjorn_L]

DM first:

At scales larger than central galaxies with satellite galaxies, lamda CDM does a good job.

That description is wrong, see my previous comment. The satellite galaxies was the only structural problem, now solved. Remaining observational tension is with the cores of galaxies. But they have exotic physics (SMBHs), so are not well understood.

Historical theory:

It necessarily follows that if MOND is a good fit over the entire range of dark matter phenomena up to elliptical galaxies with a single parameter, ... Your complaint is not really that it is an extraordinary claim without evidence.

Yes, that is what would be expected with a steady-state fit.

Lots of MOND papers are produced every year. Probably as many as Loop Quantum Gravity or any particular inflation theory. It is not merely a matter of historical interest,

In what world would mentioning LQG be support for other fringe theories? It isn't even a physical theory, it is math (lack dynamics). Something that it shares with MOND by the way.

Your claim that MOND isn't rejected by the consensus and thus has played out its role historically is not supported by remaining activity, which I have already noted (fringe). For an analogous situation, there is also a lot of group selection papers produced despite the fact that it has met a similar destiny. The frantic actrivity of the few supporters tell of a) lack of peer review (I suspect most would be arxiv papers) and b) lack of quality. You would want to cite number of researchers and number of quotes outside the MOND fringe,

 

[Jonathan Scott]

The Tully-Fisher relation and Faber-Jackson relation are empirical observational results which apply to galaxies. The MOND formula is effectively another related and somewhat more specific empirical observation - that galaxies apparently obey a rule such their rotation curves work as if there were an additional gravitational force as given by the MOND formula, which is derived purely from the visible matter (in the same way as the other relations).

Neither the original MOND theory nor the closely-related TeVeS make much sense from a physics point of view. STVG (MOG) seems better put together, but still appears to be a set of arbitrary mathematical rules rather than an explanation.

I'm not aware of any independent statistical analysis of whether the MOND curve fit could be explained by some other systematic effect, but from the examples I've seen over the years, the predicted and observed rotation curves match spectacularly well for such a simple idea, including handling types of galaxies that had not yet been observed when MOND was invented.

If these results are anywhere near as striking as they appear to be, any theory of how galaxies work needs to include an explanation of why the rotation curves appear to match the MOND formula (which also implies the other relations), and should certainly not be in conflict with this result. As the MOND results are based purely on the visible matter (with no free parameters in principle apart from the universal MOND acceleration parameter), this seems to be quite a challenge for DM to explain. There are of course some parameters involved in for example deciding the distance of a galaxy and the mass to luminosity ratio, but those apply for all theories, and the MOND formula doesn't require any special adjustments to give good results.

 

[CKH]

Does anyone know how the "missing satellite problem" has been reduced from hundreds missing to just a few and has now been solved by making our Galaxy less massive?

Torbjorn_L,

This "blows MOND out of the water" is an odd conclusion since MOND also claims much less total mass than usually assumed in DM halos.

There are many qualifications noted in this paper: ON THE SHOULDERS OF GIANTS: PROPERTIES OF THE STELLAR HALO AND THE MILKY WAY MASS DISTRIBUTION which is the basis for that news blurb. The words "assume", "assuming" and "assumption" appear 42 times in the paper. The word "seem" or "seems" appear seven times. See the last few paragraphs of the conclusion for some qualifications. The paper assumes a DM halo, of course.

LCDM is having significant problems in the local universe. Papers claiming to solve one or more (usually by appealing to baryons) are becoming common. Whether real progress has been made is unclear, since no one has gathered all of the new assumptions together into an extended theory that can be scrutinized.

Take for example the cusp problem. Lately we have some papers claiming that SNs couple to dark matter and flatten out the cusps while others say even with 100% efficient coupling (which is absurd) there is insufficient energy to do so.

Before you jump on a bandwagon claiming all is solved, bear in mind that supporters of LCDM are writing papers nearly daily theorizing about variations on CDM in an attempt to solve local problems. They wouldn't be doing this if the problems were so easily solved. There is huge human investment in LCDM so in the face of contradictions it's not surprising to see many claims of solutions.

No number of failed attempts at direct detection will disprove dominant non-baryonic matter. As long as it remains undetected, theorist can freely speculate about its properties in an attempt to support the theory.

Hopefully Gaia will have much to add to the observations on which this paper is based. It would be really nice to have some hard evidence one way or another about local non-baryonic dark matter. Unfortunately we will just have to wait.

 

[Drakkith]

Thread closed for the moment. Mentors, please see the report.

 

[Drakkith]

After review by the mentors, the decision is that this thread stay locked.