A month ago, I had a post about how the search for dark matter was proving to be frustrating with one negative result after another, prompting increased speculation that an alternative theory might be necessary. The hope had been that experiments using more sensitive detectors might prove successful. But the LUX (Large Underground Xenon) experiment in a deep underground mine in South Dakota failed to find evidence of dark matter in the form of WIMPs (Weakly Interacting Massive Particles), the theoretically favored dark matter candidate. The abstract of the paper published on January 11, 2017 in Physical Review Letters says:
With roughly fourfold improvement in sensitivity for high WIMP masses relative to our previous results, this search yields no evidence of WIMP nuclear recoils. At a WIMP mass of 50 GeVc− 2 , WIMP-nucleon spin-independent cross sections above 2.2×10− 46cm2 are excluded at the 90% confidence level.
Negative results from the two most sensitive detectors to date, LUX and PandaX-II in Sichuan, China, has increased the level of skepticism about dark matter, at least in its most common formulation.
If you lost your car keys, you might first check your coat pocket and, if you didn’t find them there, search your kitchen, the car, and so on. Similarly, the null results from LUX and PandaX-II have ruled out some of the parameters (mass, cross section) that could characterize dark matter, telling researchers at direct-detection experiments that they should look elsewhere. They have told us that dark matter interactions are much rarer than suggested by many popular hypotheses. Indeed, with these new null results and the lack of evidence for supersymmetry at the LHC, a number of physicists have started to question the WIMP hypothesis, at least in its simplest form.
Paul Kroupa is a physicist and long-time dark matter skeptic who argues that the failure of the LUX experiment to find anything is a sign that perhaps it may not exist and that we should look for alternative explanations such as MOND (Modified Newtonian Dynamics) for the phenomena whose causes were attributed to it.
What can it be replaced with? The first step is that we need to revisit the validity of Newton’s universal law of gravitation. Starting in the 1980s, Mordehai Milgrom at the Weizmann Institute in Israel showed that a small generalisation of Newton’s laws can yield the observed dynamics of matter in galaxies and in galaxy clusters without dark matter. This approach is broadly known as MOND (MOdified Newtonian Dynamics). Milgrom’s correction allows gravitational attraction to fall off with distance more slowly than expected (rather than falling off with the square of distance as per Newton) when the local gravitational acceleration falls below an extremely low threshold. This threshold could be linked to other cosmological properties such as the ‘dark energy’ that accounts for the accelerating expansion of the Universe.
These links suggest a deeper fundamental theory of space, time and matter, which has not yet been formulated. Few researchers have pursued such an alternative hypothesis, partly because it seems to question the validity of general relativity. However, this need not be the case; additional physical effects related to the quantum physics of empty space and to the nature of mass might be playing a role. MOND also faces its own challenges, both observational and theoretical. Its biggest drawback is that MOND is not yet well-anchored to general relativity. Because of the prevailing dark-matter dogma, few scientists dare to build on Milgrom’s ideas. Young researchers risk not getting a job; senior researchers face losing out on grants.
Many CWRU physicists that included some of my friends and colleagues were part of the LUX team and I am sure that they must be deeply disappointed that their hard work over so many years has failed to detect a positive signal. But that is the way with science. As the Rolling Stones song goes, “You can’t always get what you want”.