Reflections on the direct detection of particle dark matter

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R.H. Sanders –

In the context of the present cosmological paradigm, less than 5% of the world is potentially visible, and even 70% of this normal baryonic component is not detected.

The remainder of the mass-energy content of the Universe is thought to consist partly of dark matter that is unidentified, and primarily of dark energy of even more uncertain nature.

The dark matter fills the Universe, promotes structure formation and accounts for the discrepancy between the visible and dynamical mass of bound astronomical systems such as galaxies and clusters; it is the major constituent of such systems.

In order to cluster on the scale of galaxies at sufficiently early epochs, the dark matter must be essentially pressureless, i.e., non-relativistic at the time it decoupled from photons and other particles.

The dark energy, which may be identified with the zero-point energy of the vacuum, causes the present accelerated expansion of the Universe and provides the 70% contribution to the density budget for the flat Universe required by observations of CMB anisotropies.

So the two primary constituents of the world are detected only by their dynamical effects: dark energy which a affects in an observable way the expansion history of the Universe, and dark matter that clumps and dominates the mass budget of bound gravitating systems.

Some members of our scientific community are uncomfortable with the concept of dark energy.

There is a theoretical problem with its small magnitude (on the scale of particle physics). Moreover, in so far as this peculiar fluid can be represented by a cosmological constant, it does not dilute with the expansion of the Universe as does the density of ordinary matter.

Why, then, are the densities of these two substances so nearly comparable at the present epoch?

Why are we now witness to this remarkable coincidence?

If the dark energy is dynamical and identified with some new field, should we not see other manifestations of that field, such as fifth force effects, evident as a violation of the universality of free fall?

Dark matter is more comprehensible and appealing to two scientific communities: physicists and astronomers.

Theoretical physicists like it because the best bet extension of their Standard Model, Supersymmetry, provides candidate particles with, possibly, the right properties.

Many very competent experimentalists are willing to spend a significant fraction of their lives looking for it in the laboratory, even though its nature, and therefore the detection strategy, remains uncertain.

The search for dark matter has become a large industry with all the vested interests of a large industry; this is because detection would be one of the major scientific discoveries of all time.

The odds are long but the stakes are high.

However, it should be realized that non-detection is not falsification, even in the context of a particular class of dark matter candidates, superpartners for example.

And given that the range of possible candidates is limited only by the human imagination, then unsuccessful dark matter searches can always be accommodated in the context of the paradigm.

Astronomers like dark matter. If you cannot see it, you can use it to produce rotation curves of any sort, stabilize disks, make warps, promote mergers, explain anomalous lensing, make large scale structure…

It is fun to simulate because all you need is Newtonian gravity which is easy to compute.

Again, the one thing you cannot do is falsify it; almost any astronomical observation can be accommodated by dark matter particularly if one ignores the systematics of galaxy rotation curves and near perfect global scaling relations.

These result from poorly understood “gastrophysics” – gas dynamical process, star formation, feedback.

This of course is accompanied by a large leap of faith that someday these processes will be understood and all will be explained.

While these two communities, physicists and astronomers, have found a common interest in dark matter, we should remember that they are, in fact, rather distinct communities with different methodologies and different criteria for interpretation of results.

Physicists are more prepared to go beyond known physics (Supersymmetry, after all, is an extension of known physics) while astronomers are more conservative in this respect (and well they should be; interpretation of astronomical results can otherwise become quite bizarre).

Physicists know that galaxy rotation curves are at and that this constitutes a primary evidence for dark matter that should manifest itself locally.

They do not know about, and are not much interested in, the regularities of rotation curves or global scaling relations; these are details for astronomers.

Astronomers know, because physicists tell them, that particle dark matter is well-motivated and that it is proper to invoke dark matter in understanding astronomical observations.

Of course, this is simplistic; there are individuals who move easily between both communities, have a broader overview, and strongly support the concept of dark matter.

The point is that both physicists and astronomers find it useful to invoke dark matter from their own different vantage points, but I argue that from both sides the dark matter concept is fundamentally not falsifiable.

It was Karl Popper [4] who first emphasized the importance of falsification in eliminating scientific theories and progressing to new ideas.

This surely must be true, given the inherent asymmetry between falsification and verification.

Of course, to be falsified a theory must be in practice falsifiable; this would seem to be a hallmark of good theory (for Popper, a theory that is not falsifiable is not scientific).

Dark matter as a theory misses this attribute (which is not to say that it is wrong).

In my opinion the most serious challenge for the dark matter hypothesis is the existence of an algorithm – MOND (Modified Newtonian Dynamics) – that can predict the form of rotation curves from the observed distribution of detectable matter.

This is something that dark matter does not naturally permit because it is a different sort of fluid and not subject to all of the physical effects that in influence baryonic matter and its distribution.

Moreover, MOND, as a theory, is inherently falsifiable.

If particles with the right properties to constitute the cold dark matter are found tomorrow, then MOND is out of the window. In that sense it is a better theory (which is not to say that it is right).

With respect to progress through falsification, the reality is never so simple and certainly is not in this case. In the issue of dark matter vs. MOND we are not dealing just with two theories but with two competing paradigms in the sense meant by Thomas Kuhn [5].

Thirty-five years ago it was becoming generally recognized that something was missing in large astronomical systems like galaxies and clusters.

This recognition was not an instantaneous process.

When an observation runs counter to our expectation, we do not always recognize the anomaly;

recall the early attempts to fit observed galaxy rotation curves exhibiting no evidence for a decreasing rotation velocity by models having a built-in Keplerian decline.

But by 1980, it was no longer in doubt that something had been discovered; but what that something is, in fact, has never been so certain.

Dark matter was the initial, and natural, first attempt at a solution to this astronomical anomaly.

With respect to galaxies, the concept of dark halos was already in place as a means of taming the instability of rotationally supported systems.

At the same time, it became appreciated that the difficulty of forming the observed structure in an expanding Universe with a finite lifetime could be overcome by adding a universal non-baryonic matter component.

And at the same time, particle physics seemed to be providing a host of particle candidates.

The astronomical anomaly, the cosmological necessity, and the particle physics possibility combined to give the dark matter hypothesis the status of a paradigm: a framework, a set of assumptions that are not questioned, a list of problems that are to be addressed as well as problems that are not to be addressed.

Not long after the discovery of the anomaly (1983), the hypothesis of modified Newtonian dynamics (MOND) emerged, and this proposal can be clearly associated with a single individual – Moti Milgrom [6] (there were other such ideas in circulation, but none of them successfully addressed so many aspects of the phenomena).

At the time MOND was what Kuhn would call an anticipation and not a response to a crisis with dark matter – there was no such crisis.

MOND was truly an alternative to the dark matter hypothesis – in fact, the only viable alternative explanation for the observed anomaly.

But while the dark matter hypothesis attracted a large following early on – thanks primarily to its range of application, from galaxies to cosmology – MOND languished for some years with only a handful of advocates (myself included).

But now due to its proven predictive power, at least on the scale of galaxies, the development of a reasonable relativistic extension (thanks primarily to Jacob Bekenstein [7]), and simple frustration with the absence of dark matter particle detection, MOND has also achieved the status of a competing paradigm, although one still supported by a small minority of the relevant communities.

Supporters of different paradigms give different weight to different experimental or observational facts, and this makes the issue of falsification rather murky.

For example, supporters of the dark matter paradigm tend to emphasize cosmological aspects.

They would argue that on a cosmological scale General Relativity with dark matter (and dark energy) presents a coherent picture.

The observed phenomenology of the CMB fluctuations and the formation and distribution of galaxies on large scale is explained in the context of the Concordance Cosmology.

They tend to dismiss galaxy scale phenomenology and its systematics as being essentially due to messy baryonic physics which will someday be understood in the context of the larger picture.

MOND supporters, on the other hand, emphasize the regularities in galaxy phenomena: the predicted appearance of a discrepancy in low surface brightness systems, the near perfect Tully-Fisher law (the baryonic mass-rotation velocity relation), the ability of the algorithm to predict the amplitude and form of rotation curves (including details).

They are rather dismissive of the cosmological evidence, at least until the recent development of the relativistic extension.

The point is (and this is essentially Kuhn’s point) arguments between supporters of different paradigms are somewhat akin to arguments about religion.

The assumptions and the criteria for truth are different.

Most scientists do not feel the need to adopt a new paradigm unless the old one is in crisis, so we may ask: Is dark matter in crisis?

Are there fundamentally un-resolvable anomalies within the context of dark matter?

Again, most supporters would answer in the negative (but they would then, wouldn’t they?).

I personally think that there is a crisis – not only the observational difficulties of dark matter on galaxy scales (see discussion below) but also a creeping crisis provoked by the non-detection of dark matter particles.

I have argued that this is not properly a falsification, but it surely must be a worry.

At what point will experimentalists stop searching for these elusive particles and shift to activities more likely to produce positive results?

At what point will theorists tire of more and more speculative conjectures on the nature of hypothetical undetectable matter?

And what if apparent deviations from Newtonian gravity or dynamics are seen in the Solar System?

It is certainly true that, for scientists taken as a social group, most effort goes into attempting to prove or strengthen the existing paradigm rather than to challenge it.

Consistencies are valued over anomalies, and uncomfortable facts are overlooked or pushed into the category of complicated problems for the future.

This is probably necessary because normal science is a social process and takes place in the context of a paradigm.

The social phenomenon is reinforced by external considerations: by competition for academic positions, by the necessity of obtaining research grants.

I expect that it has always been this way, but in a general sense (and this is why Kuhn emphasizes the significance of “scientific revolutions” progress is a dialectic process and due to the conflict of ideas rather than “concordance”.

By “progress” here I mean moving in a direction of increased understanding of the world around us.

Kuhn would certainly not agree with this definition nor even with the concept of progress as movement toward a goal, but I believe that it is meaningful.

With respect to MOND, I, and others, have been impressed because it explains and  unifies  aspects of galaxy phenomenology which would appear to be disconnected in the context of dark matter.

The primary reason for MOND’s staying power is its remarkable predictive power on the scale of galaxies – a predictive power that is not, or cannot be, matched by dark matter as it is presently perceived.

Herein lies a minimalist definition of MOND – a definition which is as free as possible from emotive charge of concepts such as modified inertia or modified gravity:

MOND is an algorithm that permits one to calculate the distribution of force in an object from the observed distribution of baryonic matter with only one additional universal constant having units of acceleration.

And it works! The algorithm works very well on the scale of galaxies as evidenced by the MOND determinations of the rotation curves of spiral galaxies from the observed distributions of stellar and gaseous mass.

This is quite problematic for Cold Dark Matter (CDM), because – and I repeat – it is not something that dark matter, as it is perceived to be, can naturally do.

Moreover, MOND explains or subsumes systematic aspects of galaxy photometry and kinematics as a consequence of fundamental dynamical principles.

CDM can only address tight observational correlations, such as the Tully-Fisher relation, as emerging from the process of galaxy formation – a process that by its nature is quite haphazard, each galaxy having its own unique history of formation, interaction, and evolution.

It is difficult to imagine that the ratio of baryonic to dark mass would be a constant in galaxies, or even vary systematically with galaxy mass.

And yet this is required, in a very precise way, to explain the baryonic Tully-Fisher relation [8] – an exact correlation between the baryonic mass and the asymptotic rotation velocity which supposedly is a property of the dark matter halo.

Any initial intrinsic velocity-mass relation of proto-galaxies would surely be erased in the very stochastic processes of galaxy formation.

To believe that poorly understood mechanisms such as “feedback” or “self-regulation” can restore even tighter correlations demonstrates a remarkably naive acceptance of the paradigm.

The phenomenological successes of MOND constitute a severe challenge for CDM or any non- interacting dissipationless dark matter that clusters on the scale of galaxies – in fact, some might even say that it is a falsification of dark matter as it is perceived to be.

At the very least one can say that the astronomical evidence for particle dark matter in galaxies, such as the Milky Way, is far less firm than particle physicists have been led to believe.

And where does this leave direct detection experiments.

I certainly would not encourage anyone involved to give up on such experiments; indeed, it is fortunate that a number of experiments are underway – experiments by independent groups using different techniques.

This serves as a useful cross-check on marginal or controversial claims, and the spin-off  technology may well find other applications.

But I doubt that this is the primary motive of the experimenters in spite of the positive spin placed on each new, more stringent upper limit.

Patience is not unlimited, nor is funding, and I would advise any young physicist confronted by career choices to keep this in mind.

Perhaps it is true that the emperor has no clothes.


Written by physicsgg

November 10, 2013 at 11:30 am

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