Due to several requests, I am giving this post its own thread. Originally meant as a final response in The Bakhos Theory of Dark Energy and Matter, here I explain why it is not surprising that in light of the astrophysics evidence we have, scientists prefer to posit the existence of dark matter rather than modify gravitational laws like or even MOND does.
This shouldn’t be taken as an authoritative opinion universal to all physicists, but merely one physicist’s take on why the idea doesn’t seem far-fetched. In the course of writing this post I came across this excellent recent review article (Bertone & Hooper 2018, History of Dark Matter) which has a more complete picture of how this paradigm came to be developed.
Does positing dark matter conform to the scientific method?
The earliest evidence for dark matter was Fritz Zwicky’s observations of anomalies in the masses of galaxies within the Coma Cluster in 1933. However, more famous are the galaxy rotation curves which were observed by Vera Rubin and others in the 1970s. An iconic plot is the following:
As we have discussed above, far from the center of the galaxy, the observations indicate that the velocity of the stars far from the center are flat (or even increase a little), in contradiction to what is known from Newtonian mechanics. Now, there have since been other evidence for dark matter which have since come forth, but imagine for a moment that all we have is this. There are two options: either change Newtonian mechanics, or posit new, exotic dark matter (DM) particles that are causing this issue. Why is the latter option the one that ended up being much more popular in the scientific community, such that now the existence of DM is taken to be a fact, even though we don’t know exactly what it is?
To understand this, I think we have to consider the larger picture. In my opinion, one can group the larger picture into four main factors which make DM a more attractive option.
The Evidence from Particle Physics
The first factor is successes in particle physics. In the 1950s-80s, while this astrophysical evidence was just starting to get off the ground, particle physicists were excitedly discovering new particles and particle phenomena every few years. For example, the J/psi meson was discovered in 1974, which solidified the evidence for the existence of quarks. The W and Z bosons (carriers of the weak force) were discovered in 1983, which was predicted by the electroweak theory developed by Glashow, Salam, and Weinberg. This theory unified the EM and weak forces into one single force. There are many other monumental discoveries happening in these decades (such as the discovery of CP violation in kaons in 1964), such that one could call this the golden age of particle physics. In light of this, who was in a position to say that there were no more discoveries to be made? Perhaps DM consists of some particles which are just hiding around the corner, waiting for us to discover them.
The second factor is unresolved problems in particle physics. As a result of the discoveries in particle physics, the Standard Model (SM) was born. It is a theoretical framework that explains three of the four known forces (strong, weak, and EM), positing 17 fundamental particles, most of which had been observed in the laboratory. The final one, the Higgs boson, was only discovered in 2012. There is a lot of experimental evidence for the SM, and it continues to hold up extremely well even today. However, the SM also has multiple shortcomings, and particle physicists immediately started looking further to resolve them. Here are some of the major examples:
- The hierarchy problem: why do the four forces have very different strengths to each other?
- The strong CP problem: why we don’t find as much violation of time-symmetry in the strong force as we do in the weak force?
- Despite the fact that the strong force exists in the SM, it is not fully unified with the EM and weak forces. This would require a so-called Grand Unified Theory, which couldn’t happen without the introduction of new, exotic supersymmetric particles. Thus supersymmetry (SUSY) was born.
In response to these problems, extensions and modifications to the SM were proposed. Most of them posited the existence of new, exotic particles beyond the SM, which also turned out to be viable candidate particles for DM. For example:
- The WIMP miracle: in one cosmological model where DM is assumed to have been produced thermally in the early Universe, it turns out that to get the correct abundance of observed DM, it must consist of a particle that interacts weakly enough such that it does not annihilate too often with its antiparticle. The strength of the interaction turns out to correspond to a new massive particle that interacts via the weak force. Incredibly, a similar particle is predicted by SUSY (a completely separate theory of particle physics), leading this to be called a miracle. Hence the hypothesis of the Weakly Interacting Massive Particle (WIMP) was born, one of the leading candidates for DM.
- The strong CP problem could be solved by positing the existence of a new particle called the axion, which could also be a candidate for DM.
In short, there is an extraordinary confluence between the evidence for dark matter in astrophysics and the search for new particles in particle physics. If DM particles actually exist and are found, not only could they explain the mystery of DM - they could help us in our quest to unify all the forces!
The Wide Range of Unexplored Experimental Possibilities
The third factor is the lack of sufficient experimental data to rule out dark matter. The range of possibilities for DM is huge - based on observational constraints, DM could be any particle with a mass ranging from 10^{-33} to 10^{48} GeV (Safranova et al. 2018) - 80 orders of magnitude! Today, there are multiple experiments looking for all forms of dark matter, such as WIMPs and axions. Examples are LUX, XENON, ADMX, DAMA/LIBRA, CDMS, just to mention a few collaborations. Besides the large parameter range for the mass of the particle, there are also many ways in which hypothetical DM particles could be invisible to light but still interact through other forces. Many of these methods of interactions had simply not been explored, and it would be irresponsible to not pursue this avenue of research. Even today, with over two decades of searches and finding nothing, a lot of possibilities have just not been covered yet.
Besides direct detection, physicists have also started exploring ways in which dark matter can cause small changes in otherwise regular phenomena. This is getting especially popular in my field of atomic physics. One prominent example is the effect of dark matter on atomic structure - a wave of dark matter passing through the Earth could cause atomic clocks to tick slightly differently. While again, no signal for a DM-caused anomaly has been found, several experiments are currently still investigating these hypotheses. (More detail can be found in the Safronova et al. paper linked above.)
Finally, physicists have also investigated the possibility that DM is actually not an exotic, new particle, but a form of ordinary matter that is simply difficult (or impossible) to be optically observable. Examples include MACHOs (massive astrophysical compact halo objects) and neutrino dark matter. More information on the history of this avenue of research can be found in the Bertone & Hooper historical paper. My point is, physicists are not naive - almost all hypotheses you can think of to explain the evidence for DM have been thought of and explored.
Why People Prefer Not To Modify Gravity
The next set of factors in favor of positing DM is that the second alternative hypothesis in response to the evidence for DM - modifying gravity - is undesirable. I will outline several reasons why.
First, a very important trend in physics is that science progresses by building on top of previous theories, instead of overturning them. For example, GR reduces to Newtonian mechanics at large distances and slow speeds. Quantum mechanics reduces to classical mechanics for large objects. This is also called the correspondence principle. This is why if one wants to modify gravity, it has to be done in a way which doesn’t trample on what we already know about gravity - which is Newtonian mechanics and Einstein’s general relativity (GR).
Modified Newtonian Dynamics (MOND), proposed by Milgrom in the 1980s, is the leading candidate to modify gravity in response to the evidence for DM. Initially, MOND argued that Newtonian gravity (or inertia, depending on your viewpoint) behaves differently at very low accelerations. (For a little bit more detail, see the Appendix following this post.) In one sense, this is a good feature of the theory - it is only said to apply at low accelerations, which we have not experimentally probed as much. However, MOND was problematic when it first came out because it was initially proposed as a simple modification of Newton’s second law, with no clear way to harmonize it with GR. Not until 2004, when Bekenstein and Milgrom proposed TeVeS, did a GR-compatible version of MOND emerge.
There is another flaw with MOND, in that while it is able to predict galaxy dynamics and luminosity very well, it has not been as successful in predicting the behavior of galaxy clusters, such that even with MOND, some DM is still required. This removes the allure of doing away completely with DM, which is the reason one might consider MOND in the first place.
Finally, as @PdotdQ has explained in What Parts of the Big Bang Do Scientists Dispute?, MOND is also problematic in that it conflicts with the standard cosmological paradigm, the Lambda CDM model, for which we have tons of observational evidence for, as @Patrick likes to remind us. There is yet no agreed-upon way to harmonize MOND/TeVeS with observational evidence in a new cosmological model. In addition, the viability of TeVeS has been constrained by the recent tests of GR via gravitational waves.
Thus to summarize the issues with MOND: it was hard to reconcile with GR, it cannot fully explain all the evidence for DM, it is hard to reconcile with cosmology, and finally it is also constrained by other lines of evidence. If someone wants to modify gravity in a different way than MOND, then they also have to do better than MOND with regards to these problems. Clearly, this is a difficult task.
Dark Matter Makes Sense
To sum up, we have explored four factors why DM has risen to become the dominant theory explaining the evidence from galaxy rotation curves and other evidences:
- The historical success of particle physics makes it conceivable that there are still some new particles that we haven’t detected yet
- The existence of dark matter particles meshes well with various hypothetical particles that are proposed to resolve problems in the Standard Model of particle physics
- The experimental parameter space for DM is large, and not fully explored
- The alternative of modifying gravity is not attractive for multiple reasons.
Unsurprisingly, the quest to search for DM particles continues to this day, with billions of dollars being devoted to multiple large collaborative experiments involving hundreds of physicists each. The fact that we have not found anything is very frustrating (The Frustrating Search for New Physics), but based on these reasons, one cannot fault physicists for positing the existence of DM. Physicists are not merely making up stuff in order to rescue theories, but just trying to prevent having to completely destroy and revise the powerful and astonishing edifice of work in physics which has been built up in the last 400 years. We have to be careful that in attempting to explain the existence of a single, odd rock, we do not forget about the existence of the rest of the mountain.
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