In October 2016, disappointment darkened the faces of many astrophysicists. The results published in August and September by three teams of scientists revealed the latest failure in more than 70 years of searching for any clue that proves beyond doubt that dark matter is out there . Three experiments operated over three years with a construction cost of around 30 million euros -to which maintenance would have to be added-, and three multidisciplinary teams of scientists: the Large Underground Xenon, or LUX experiment, located near the city of Lead, in South Dakota (USA), the PandaX-II in Sichuan, China; and the XENON100, located at the Gran Sasso National Laboratory in Italy. The three experiments, which supposedly can detect particles with masses 40 or 50 times greater than that of a proton, were not able to obtain even the slightest direct hint of the existence of dark matter . At least, the kind that the vast majority of scientists working in this field hope to find, particles called WIMPs.
Regardless, astronomers have found hints of dark matter in every corner of the universe, and it is now believed to be responsible for more than 90% of the total mass of galaxy clusters. The problem arises when a simple question is asked: what is dark matter?
It is already known that the imagination is free and the paper holds what you write, and in this field the feverish minds of researchers have run amok: from low-mass stars to black holes, passing through grains of cold dust or exotic particles produced by the wildest dreams of theoretical physicists, the hunt for that missing matter is one of the most important problems in astrophysics today. Now, not knowing what it is doesn’t stop cosmologists from classifying it into two types: hot and cold. They are two cosmological terms that refer to the speed at which particles move . If they have a mass of the order of a proton then they will move slowly: it is cold matter. But if they are lighter, they will travel at speeds close to the speed of light: it is hot matter. The most popular candidate for hot dark matter is the neutrino, although it is currently not very popular. Experts now tend to side with cold dark matter because the model of the universe that emerges from it is consistent with fluctuations in the cosmic microwave background radiation. And, of course, the million dollar question is what is that cold dark matter made of? Because it cannot be matter as we know it…
This is where the WIMPs (Weakly Interacting Massive Particles) come in. Under this name is grouped a whole collection of high-mass particles that are born from the equations of experts in supersymmetry, one of the attempts to unify all the constituents of matter and the four forces of nature under a single mathematical expression. To make sense, supersymmetry theories predict the existence of at least twice as many subatomic particles as we know, since each particle has what they call its supersymmetric partner : thus, the photon is accompanied by the photon , the neutrino, the neutralino . or to the electron the s-electron… The only and insignificant problem is that none have been discovered yet. What’s more, everyone expected to find some indication of its existence at the LHC, the great accelerator at CERN in Geneva, but after the Higgs physicists have nothing supersymmetric to eat. Some justify themselves by saying that it is too early; others that supersymmetry is in the doldrums.
Be that as it may, the WIMPs, the great white hope of dark matter, are becoming less and less so. That they have not been observed and that supersymmetry continues to play crazy and no proof of its reality appears makes many begin to get impatient.
Of course, the theory is still susceptible to being stretched to fit the experimental disappointments, so we’re sure to have supersymmetry for a while. But there are others who are not so optimistic. Theoretical astrophysicist, Dan Hooper, from Fermilab in Chicago has made it clear: “We have been looking all these years for a type of dark matter according to our best guess, and now we are beginning to wonder if maybe we were wrong.” Apparently, 30 years of zero results have not discouraged his pursuers. Now some new calculations -the most complex to date- published in 2016 in the journal Nature by Zoltan Fodor and his collaborators have determined that -under certain assumptions- the mass of the axion could be up to ten times greater than expected. This makes the ADMX experiment have been searching in vain, as it was designed to detect a lighter particle. Of course, this article is a blank check to fund new detectors for axions.
Dark matter hunters are incapable of discouragement and failures serve them to design new and increasingly sensitive detectors. Now, there are some researchers who care about their health and think that dark matter, even if it is there, we will never be able to detect it: “It is possible that the only way we can see dark matter is by its gravitational effects,” he says. the particle astrophysicist at the University of Albany (USA) Matthew Szydagis. This makes her a ghost greater than Hamlet’s father, which makes some, very few, dare to formulate the unthinkable: that dark matter simply does not exist.
This small group of physicists find it absurd to believe that there is a type of matter that neither sees nor interacts with ordinary matter (except gravitationally). One of these skeptics was the Pakistani and Nobel laureate in Physics Abdus Salam , who compared it to the invisible djinns of The Arabian Nights. So how to explain the observations that say it exists? They simply state that a new physics is needed, that Einstein’s general relativity (the basis of modern cosmology) should be modified when talking about large distances . So we must accept that we do not have a complete theory of gravity.
The truth is that both possibilities are quite radical. On the one hand we must assume that on the scales of galaxies and clusters gravity does not work as we think; on the other hand, if we want to keep our theories intact, we must postulate the existence of a totally unknown type of matter that has properties as incredible as being absolutely undetectable, since it neither absorbs nor emits energy; a type of matter that after more than 80 years we still do not know what it is and there is no direct proof that it is really there.
References:
Borsanyi, S., Fodor, Z., Guenther, J. et al. Calculation of the axion mass based on high-temperature lattice quantum chromodynamics. Nature 539, 69–71 (2016). doi: 10.1038/nature20115
