Is Dark Matter a Dead End in Physics?

[mfb]

@bbbl67: Please consider your discussion style and the situation here.

You dismiss entire peer-reviewed publications because they make assumptions. Well stated assumptions, discussed among experts, tested or at least checked with simulations and so on. You dismiss them without giving any reason, without even specifying what exactly you disagree with.
On the other hand your own posts here are full of assumptions, usually not even stated explicitly, many of them are demonstrably wrong - some so wrong that everyone with introductory astronomy classes knows better. Many of them have been pointed out. Yet you continue to argue along that line, as if it wouldn't matter that it is all based on wrong assumptions.

Do you really think this is a healthy discussion style?

 

[Orodruin]

Whatever the reason is, I would imagine this is what would also prevent PBH from actually forming below stellar masses and even if they did, it is quite possible they would simply expand back out again similar to what happens when you release a stress ball.

This is not correct and does not describe PBH formation in an accurate way. Very simplified, PBHs may form when density perturbations reenter the horizon such that the Schwarzschild radius of the mass contained within one Hubble radius exceeds the Hubble radius itself. This would happen in the very early universe and there would not be any question of compressing mass.

 

[Ian J Miller]

Suppose dark matter was a fermion, and further, suppose being electrically neutral it is its own antiparticle. When matter and antimatter self-annihilated but for an asymmetry that left us with an excess of matter, these fermions would not do that. The Stanford link above argues the mass must be >25 eV, but that is not exactly a huge barrier, so if dark matter did comprise such fermions, how would you detect them given they do not interact electromagnetically, they do not clump and they do not decay to anything? In my opinion, not easily, and not with detectors looking for much more massive particles.

 

[Orodruin]

When matter and antimatter self-annihilated but for an asymmetry that left us with an excess of matter, these fermions would not do that.

You seem to here be implying that there would be no dark matter. This is not the case. In fact, what you are describing is the standard production of thermal dark matter where the final density is set by when the interaction rate falls below the Hubble rate. This happens sooner for particles that do not interact very strongly with each other.

how would you detect them given they do not interact electromagnetically, they do not clump and they do not decay to anything? In my opinion, not easily, and not with detectors looking for much more massive particles.

This depends very much on the mass range that you are looking in. Standard WIMP scenarios where you search using direct detection experiments go down to a few GeV, which is needed to produce an appreciable recoil in the experiment. The exact search strategy would depend on the type of interactions that the DM has - you would need to specify your model further - but in general it can indeed be very hard.

 

[Buzz Bloom]

I find the discussion in this thread a bit odd. The OP seems to question the evidence for the existence of DM.

I was wondering what the Physics Forums community thinks about this lack of evidence for DM despite numerous and different approaches to find it (DAMA/LIBRA experiment claims to have found it, but no one was able to reproduce their results).

All of the discussion has been about possible/plausible/maybe answers to the question: What might DM consist of? Examples:

I wouldn’t rush to get rid of DM. It has been observed gravitationally and looks like it doesn’t interact much otherwise. So it will be inherently difficult to detect.

Sterile neutrinos are pretty much the last harrah of WIMPs, most early WIMP theories were looking at SuSy particles, and those hopes started fading away after the LHC started reducing the places where SuSy can hide.

None of the posts have mentioned the evidence for the existence of DM as non-baryon stuff based on the abundance of deuterium created during the period of primordial Nucleosynthesis.

https://en.wikipedia.org/wiki/Nucleosynthesis
The first nuclei were formed about three minutes after the Big Bang, through the process called Big Bang nucleosynthesis. Seventeen minutes later the universe had cooled to a point at which these processes ended, so only the fastest and simplest reactions occurred, leaving our universe containing about 75% hydrogen, 24% helium, and traces of other elements such as lithium and the hydrogen isotope deuterium. The universe still has approximately the same composition today.​

The role of DM during the period is as follows. If R, the ratio of DM density to baryon density, was substantially less than the estimated current value,

R < R₀ = 14.5%/85.5% = 17.0%,

https://en.wikipedia.org/wiki/Dark_matter
"Thus, dark matter constitutes 84.5% of total mass..."​

then the universe would have expanded more rapidly, and the fusion of deuterium into helium would have been less, and there would therefore be substantially more deuterium. Similarly, if R was substantially greater than R₀, then there would be substantially less deuterium.

A second minor issue relates to the discussion of primordial black holes (PBHs). Example:

Primordial black holes are actively discussed in the dark matter community and mostly ruled out by different experiments depending on the mass range.

The discussion of this point seems to assume that PBHs (if they exist) consist entirely of DM. However, one would expect that only 85.5% of a BH's mass would be DM.

Regards,
Buzz

 

[Orodruin]

The discussion of this point seems to assume that PBHs (if they exist) consist entirely of DM. However, one would expect that only 85.5% of a BH's mass would be DM.

Huh? This is not correct. The PBHs would be the dark matter.

 

[Buzz Bloom]

Huh? This is not correct. The PBHs would be the dark matter.

Hi Orodruin:

If you are correct about this, then I must have a wrong view about quite a few topics. I would much appreciate it if you would post an explanation of how a PBH would form without any baryonic matter being captured along with the DM.

I think there is a good reason to believe that a PBH would be less than 85.5% DM, perhaps even all baryonic matter. The reason I have in mind is that baryonic particles interact with other baryonic particles in such a way that relative to a nearby random primordial region R with greater than average mass density, these particles will lose kinetic energy and tend to fall towards the center of mass of R. DM will not lose kinetic energy in this manner, and therefore will follow a trajectory less likely to accumulate with the mass that will after a while form the PBH.

ADDED
I think there might be an issue regarding the timing of the creation of PBHs. I was assuming the PBHs form after the period of nucleosynthesis. Do you know of a reason why they must form before nucleosynthesis?

MORE ADDED
My error.

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

Primordial black holes belong to the class of massive compact halo objects (MACHOs). They are naturally a good dark matter candidate: they are (nearly) collision-less and stable (if sufficiently massive), they have non-relativistic velocities, and they form very early in the history of the Universe (typically less than one second after the Big Bang).​

I still do not understand, "they are (nearly) collision-less".

Regards,
Buzz

 

[Orodruin]

If you are correct about this, then I must have a wrong view about quite a few topics. I would much appreciate it if you would post an explanation of how a PBH would form without any baryonic matter being captured along with the DM.

How they formed is completely irrelevant. The black holes are dark matter in this scenario, there is no "other" dark matter to capture.

 

[Buzz Bloom]

How they formed is completely irrelevant. The black holes are dark matter in this scenario, there is no "other" dark matter to capture.

Hi Orodruin:

I get that now. The assumption is that a PBH is formed from ordinary matter very, very early after the "big bang" (< 1 second), and subsequently during primordial nucleosynthesis does not participate in the process of combining protons and neutrons into helium, etc. which occurred about 3-20 minutes following the "big bang".

I am now wondering about my last bit of confusion.

I still do not understand, "they are (nearly) collision-less".

In what sense is a PBH collision-less or nearly so? Why does not ordinary matter constantly collide with a PBH event horizon and become additional mass of the PBH?

Regards,
Buzz

 

[Bandersnatch]

I think collision-less here means they behave like a collision-less gas. I.e., without pressure. The only interactions they participate in are gravitational in nature - similarly to WIMPs.

 

[Sanborn Chase]

Does it matter that all of our celestial observations since the beginning of time have been through a filter the substance of which we know next to nothing?

 

[mfb]

Does it matter that all of our celestial observations since the beginning of time have been through a filter the substance of which we know next to nothing?

We know it doesn't influence the light going through apart from its gravitational effect - otherwise it would be part of the regular matter.

 

[Sanborn Chase]

How would you know it doesn't affect light if you'd never seen it otherwise?

 

[weirdoguy]

Because if dark matter exists then there is plenty of it in the galaxies and if it did interact with light electromagnetically we would certainly see effects of this interaction.

 

[Sanborn Chase]

I'm extremely buoyed by your confidence.

 

[Buzz Bloom]

In considering PBHs as the DM substance, how does one distinguish PBHs from non-primordial BHs? Is it nor reasonable to assume that any PBHs would during the aging of the universe get larger by capturing additional ordinary matter? Is it possible to estimate how much of a very large black hole, such as one at the center of a galaxy, consists of what might be a PBH, and how much later added ordinary matter.

Perhaps a calculation similar to that described below might be useful.

If PBHs were formed less than 1 sec following the Big Bang (BB), might one consider the temperature T at the time compared with the Harking radiation temperature T_H of a PBH of an assumed mass?

Hawking radiation temperature:

https://en.wikipedia.org/wiki/Hawking_radiation​

From the above equation,

M(T_H) = const / T_H.
​

For any mass M > M(T_H), the PBH will gain mass rather than radiate it away. The following calculation is too difficult for me to make, but it should not be too difficult for someone with some good math skills to calculate how a PBH of an assumed mass will grow between 1 sec (following BB) and a later time. If one assumes some distribution of the masses of PBHs at 1 sec, one can calculate the corresponding PBH mass distribution at 3-20 min when nucleosynthesis occurs. The total mass density of the DM PBHs grown to be larger BHs prior and during this interval needs to match the requirement for the current ratio of deuterium mass density to hydrogen mass density. From this calculation of DM=BH mass density during this interval, one should be able to calculate what the change in average PBH/BH mass density between 3-20 min and now, ignoring newly formed BHs from stars.

 

[weirdoguy]

I'm extremely buoyed by your confidence.

It's a simple and well established empirically fact about electromagnetic interactions of matter.

 

[Sanborn Chase]

You've quelled my doubts and set me back on the track to enlightenment. I can't thank you enough.

 

[Bandersnatch]

@Sanborn Chase Let's not do that. It's unnecessary.

weirdoguy's statement is as uncontroversial as saying: 'it's invisible because we think it's there but we can't see it'.
You're asking: 'how would you know it's invisible if you'd never seen it?'
The way it is stated, the question answers itself.

I'll venture a guess that you might instead be intending to ask something along the lines of: what if we are seeing it, but since it's everywhere, it affects everything equally, and we're unaware of it? As if wearing rose-tinted glasses all our lives.
And in case that's the intended meaning, then it's ruled out by the fact that any substance interacting with light would leave more of its signatures if there was more of that substance in the way of the light.
Since from gravitational signatures we expect DM to be non-uniformly spread, this would make its effects on light be directionally-dependent. One would see more of the effect if looked at similar objects through more intervening DM, like for example observing a supernova shining from the near side vs the far side of a galaxy. Or looking across the bulk of the Milky Way vs away from it. Or at spectra of galaxies in large clusters versus small ones.
In other words, one would see some as of yet unexplained effect that would correlate with the distribution of DM.

 

[mfb]

If it interacts with light it is not dark matter by definition. So yes, clearly we are 100% confident that dark matter doesn't interact with light.

The next question is "how do we know how much non-dark matter there is". We know that pretty well through a variety of methods which all agree on the regular matter content.

 

[Sanborn Chase]

Dear Mr. Bandersnatch: Your comments are restorative; thank you. But your initial admonition is misplaced. I DO NOT use that ugly stepchild of humour, sarcasm, as a tool of condescension as you implied. Besides, I'm an old Southern gentleman and wouldn't stoop to such shenanigans. That said, please consider me a thirsty young ignorant child seeking to quench my thirst from the deep pool of knowledge you provide. So often my ignorance takes refuge behind my certainty. Thank you.

 

[ohwilleke]

MOND is not a viable alternative, regardless of any perceived problems with DM.

I wouldn’t rush to get rid of DM. It has been observed gravitationally and looks like it doesn’t interact much otherwise. So it will be inherently difficult to detect.

Science isn’t room service where you can order a result to your liking to be delivered by the end of the sitcom you are watching. It is a difficult enterprise and honest science always has a real risk of null results.

This profoundly overstates the case. Some form of modified gravity that approximates MOND is very strongly supported by the evidence, and DM's problems are very serious. See for example, this comparison.

Probably the biggest problem with a DM hypothesis is explaining why its distributions are so intimately related to the distribution of luminous matter, which is completely explained for all systems of galaxy size by MOND, over many orders of magnitude in galaxies of many different types, with a single experimentally measured constant, as tested in thousands of galaxies, with outliers appearing in magnitude and frequency at almost exactly the rate that would be predicted from measurement error. There is almost no conceivable way that a DM theory can reproduce this relationship.

MOND effects are also observed in systems that simply can't be DM driven, such as wide binary stars.

There is also pretty good evidence that the "external field effect" particular to MOND and absent from DM theories, really exists.

Now, naive MOND is just a toy model. It's domain of applicability does not extent to relativistic strong gravitational fields, where full general relativity is required, and it underestimates the phenomena observed in galactic clusters. It needs to be generalized to accommodate GR effects and it needs to be tweaked in very massive galactic cluster systems. But, its successes are pretty much inconsistent with anything but the most baroque DM theories (which also add 5th forces that allow it to interact with other dark matter and with ordinary matter). The case that some sort of modification of gravity, rather than DM particles is the source of the observed phenomena is very great, and examples such as the Bullet Cluster which have been offered to disprove modified gravity theories don't actually do that.

DM theories provide one simple explanation for the observed patterns of Cosmic Background Radiation. But, that simple theory isn't the only possible way to get that effect. No one was ruled out the possibility that a modified gravity theory could replicate that effect and indeed it has been shown that it is possible to have a modified gravity theory that produces the same CMB signature.

 

[ohwilleke]

So to answer your question about whether we've found evidence of PBH's? I think it's nearly irrefutable that all supermassive BH's are PBH's. But we haven't found evidence for sub-stellar mass PBH's yet though.

There is virtually no evidence whatsoever that supermassive BH's are PBH's and indeed, in most models of supermassive BH formation and galaxy formation, PBH's are not involved at all.

 

[ohwilleke]

@bbbl67My point is that almost every one of these quantities has a degree of uncertainty to it. So, couldn't part of the issue be compounded issues of uncertainty when we look at galactic distances? In order to determine the rotation curves, we had to determine the mass of the visible matter, we had to determine the velocities stars etc, none of which can be directly observed.

There are uncertainties, but they are well quantified. And, we have some very good methods of determining the mass of visible matter and the velocity of stars (which is directly observed).

Maybe there are WIMPS, maybe there are PBH, maybe our understanding of gravity is complete etc. or maybe it is a combination of more than one of those things. At the very least, an AI algorithm could be used to determine the most likely candidates.

Both of these possibilities are pretty much ruled out by the data, and there is nothing magic about AI algorithms. Plain old natural intelligence from human beings has been quite sufficient to rules out a whole host of DM candidates.

 

[Dale]

This profoundly overstates the case

Perhaps. I admittedly have not followed any recent developments of MOND, having examined them and lost interest in them quite some time ago. It could be that something new has overcome previous problems.

What I have seen from MOND theories at best is capable of explaining galaxies, but fails at both cosmological scales and local scales. I have yet to see a MOND theory which is not contradicted by already existing evidence at cosmological scales and at local scales.

If you know of a MOND theory which is consistent with all currently available evidence at all scales then I would be glad for a reference.

 

[ohwilleke]

Why would you think that MOND fails at local scales?

 

[Dale]

Why would you think that MOND fails at local scales?

It doesn’t predict gravitational time dilation or the correct light deflection or the precession of Mercury or the Shapiro effect or frame dragging.

 

[Buzz Bloom]

Do we need to give up on DM completely and find a new explanation for the observed gravitational behavior of the galaxies (MOND?)?

Hi BillKet:

There is another thread that also discusses MOND which you may find of interest.

https://www.physicsforums.com/threads/why-does-mond-fit-rotation-curves-so-exactly.960221/​

Regards,
Buzz

 

[Alain_BXL]

MOND is not a viable alternative, regardless of any perceived problems with DM.

I wouldn’t rush to get rid of DM. It has been observed gravitationally and looks like it doesn’t interact much otherwise. So it will be inherently difficult to detect.

Science isn’t room service where you can order a result to your liking to be delivered by the end of the sitcom you are watching. It is a difficult enterprise and honest science always has a real risk of null results.

I realize that it can take very long to obtain experimental confirmation of our hypotheses (cf. Higgs boson, which took almost 60 years to be confirmed experimentally) but with each day that passes without experimental confirmation of DM, the latter looks increasingly like the modern-day equivalent of the planet Vulcan that was once thought to explain Mercury's perihelion precession.

I also realize that recent discoveries have essentially killed-off certain modified gravity theories but the fact that we don't yet have a successful modified gravity theory doesn't lend any additional credence to DM. We didn't have a fully successful theory (in terms of the facts known at the time) before GR yet that didn't make Vulcan a thing.

I sometimes wonder if there's some perverse sociological reason why the majority of physicists prefer to attempt proving that the standard model is incomplete rather than proving GR incomplete? Maybe there's more funding to be had from the former?

 

[Dale]

but with each day that passes without experimental confirmation of DM, the latter looks increasingly like the modern-day equivalent of the planet Vulcan that was once thought to explain Mercury's perihelion precession.

The comparison is fair, but I think your timescale is way too short. It took over 100 years to detect gravitational waves, and we knew more about them than we do about dark matter. Science just isn’t a pizza delivery service with a “30 minutes or it’s free” guarantee.

With Vulcan we had previously used similar measurements to predict the existence of other planets and we had observed them where predicted. So we had experience with finding other similar objects from similar data. The scientific community therefore had a reasonable sense of how long it should take, and no major advance in instrumentation was required.

We have no experience detecting particles that interact only gravitationally. It is orders of magnitude more difficult than detecting neutrinos was, and that had many early failures too.

 

[Alain_BXL]

Sure, I take your point. However, I'm not sure that I accept the equivalence between gravitational waves and DM. The former were predictions of a broader theory, the latter seems more like an an hoc hypothesis to fix some observational anomalies (as far as I understand, Lambda CDM doesn't provide any theory of the origin or nature of DM) . Also, it seems that DM is gradually running out of places to hide, no?

 

[Buzz Bloom]

the latter seems more like an an hoc hypothesis to fix some observational anomalies

Hi Alain:

If I remember correctly, the earliest thinking about what we now call dark matter was a result of analyzing what happened during the early period of the universe, approximately 3 to 20 minutes after the big bang, when nucleosynthesis was taking place, combining protons and neutrons into nuclei: deuterium, tritium, lithium, helium3, and helium4, and also very small amounts of a few other nuclei. This period corresponds to a temperature range whose length of duration depends on the density of matter present from GR analysis. In addition, the surviving fraction of nuclei which were deuterium also depended on the density of baryonic matter. I believe this was the earliest deduction that some mysterious non-baryonic matter (not participating in nucleosynthesis) existed because the required density of matter was roughly six times the required density of baryonic matter.

Regards,
Buzz

 

[Orodruin]

Also, it seems that DM is gradually running out of places to hide, no?

This could not be further from the truth. We have not yet started to scrape off the parameter space of most DM models.

 

[Dale]

However, I'm not sure that I accept the equivalence between gravitational waves and DM. The former were predictions of a broader theory, the latter seems more like an an hoc hypothesis to fix some observational anomalies

Agreed. My intended point there wasn’t that they are the same kind of search, but that gravitational phenomena can be very difficult to detect. As far as kinds of searches I think the neutrinos are more analogous.

Also, it seems that DM is gradually running out of places to hide, no?

Definitely not. If they interact only gravitationally then we are not merely years away, but probably centuries away from being able to detect them. And there could be new interactions anywhere between the weak force scale and the gravitational scale. The room to hide is enormous. I don’t think you appreciate how weak gravity is and how far away our instruments are from detecting individual particles gravitationally.

 

[Alain_BXL]

Hi Alain:

If I remember correctly, the earliest thinking about what we now call dark matter was a result of analyzing what happened during the early period of the universe
Regards,
Buzz

Hi, I thought the 1st modern usage of the term was by Zwicky in the 1930s. See for example https://arxiv.org/pdf/1605.04909.pdf.