Dark matter and where to look for it
It is a coincidence that Dmytro and I are speaking on September 10, on the 10th anniversary of the launching of the Large Hadron Collider (LHC), the world’s largest accelerator of elementary particles, which is particularly supposed to shed light on the nature of dark matter. In essence, this problem is being tackled, albeit by different methods, at the Niels Bohr Institute in Copenhagen, Denmark, where PhD (Physics and Mathematics) Dmytro YAKUBOVSKYI has been working for two and a half years as postdoctoral fellow. At the same time, he remains a higher doctorate seeker at the Mykola Boholiubov Institute of Theoretical Physics in Kyiv and regularly comes to Kyiv to research dark matter and deliver lectures to both students and the public. We met during one of these visits. We discussed the current information about dark matter and approaches to scientific research in Denmark and Ukraine. We began with the “birthday boy,” i.e., the LHC.
ON THE LARGE HADRON COLLIDER
“In essence, all detectors of the Large Hadron Collider probe different characteristics of various elementary particles, including the well-known ones and the Higgs boson which was first detected there. Small detectors ATLAS and CMS and the detector LHCb are used in this research, while ALICE is mostly used to explore the quark-gluon plasma [the so-called “liquid” form of matter which supposedly exists in a short period of time immediately after the Big Bang – Author].
“Almost all the LHC detectors are also searching for the traces of still unknown particles that may form dark matter. They have detected no new particles so far, which affords ground for developing alternative models, including the one I work with. If the LHC fails to find even one dark matter particle, it will be very probable that the model we are developing and watching now, as well as certain experimental consequences of it, is correct.”
ON HIS OWN RESEARCH
“Our group in Denmark explores the neutrino minimal extension of the Standard Model (often abbreviated as vMSM) of particle physics. The point is that the Standard Model has particles called neutrinos which differ from all the other by their properties. Moreover, these are in fact the only particles which the Standard Model does not explain fully. In particular, it does not explain a phenomenon discovered about 25 years ago, for the discovery of which the Nobel Prize in Physic was awarded in 2015 [to Takaaki Kajita and Arthur McDonald – Author]. It is neutrino oscillations. Imagine that one type of neutrino turns into another and vice versa just during the motion. This process can be explained, for example, by means of quantum mechanics, but, from the viewpoint of oscillation observations in particle physics, it is a very rare and unexpected phenomenon. Moreover, these phenomena of oscillation cannot be explained by the Standard Model which became the cornerstone of elementary particle physics when the Higgs boson was discovered [in 2012].
“Nobody doubts now that the Standard Model of particle physics excellently explains electromagnetic and strong interactions, i.e. how nucleons, individual particles in nuclei, interact with one another and why they keep together, as well as weak interaction which is responsible for beta-decay and thermonuclear fusion in the Sun – in fact the source of life. Even in outer space, we can see no serious deviations from the Standard Model, except for a few ones.
“The aim of the model I work with is to describe the observed deviations, introducing as few additional entities as possible. It is like Occam’s razor – the principle that the simplest solution tends to be the right one. Some models introduce a lot of additional entities, such as supersymmetry. This is a good model. It was even glorified by Vakarchuk and others [the rock group Okean Elzy produced an album, ‘Supersymmetry,’ in 2003 – Author], but the problem is that this model needs twice as many particles as we know at the present moment. We know dozens of elementary particles now, and each of them needs a ‘superpartner.’ But we can see none of them. If the Large Hadron Collider fails to find traces of supersymmetric particles, this will mean that the phenomena that occur outside the Standard Model, such as neutrino oscillations, need a different solution.”
ON NON-STANDARDS IN THE STANDARD MODEL
“The goal of elementary particle physics researchers is to construct a complete, closed, and self-consistent model. Ideally, it should describe all the phenomena we can see. All of its forecasts should be checked and all the new particles should be found. And this model should be free of internal contradictions.
“It took the Standard Model of particle physics a long time to emerge – perhaps from the early 20th century, and this created a lot of problems, such as non-self-consistency. As is known, the sum of probabilities should be equal to unity. But it turned out that the model, which described for the first time the interaction of neutrinos, beta-decay, and thermonuclear fusion in the Sun, stipulated that, oddly enough, the probability of high-energy processes should exceed unity. It’s nice that it describes processes very well at low energies, shows good coincidences with the experiment, and gives less-than-unity probabilities, so let it go on working. But it must be replaced where it gives absurd answers.
There are many things that we do not understand just because they are very complicated. We can know elementary laws of nature and describe interactions on the elementary level. But we cannot do so on the highest level, when there is a very large collective of particles. For example, chemistry has separated from physics. I still consider it part of quantum physics, but many chemists do not agree to this. And they are right in the sense that chemistry is a complex specific science, where it is very difficult to apply the laws of physics and where other, more generalized, laws work better
“Actually, why were Weinberg, Salam, and Glashow awarded the Nobel Prize [in 1979]? Because they were the first to formulate this model. It is a very complex construction. The researchers used Einstein’s idea that not only gravitation, but also other types of interaction can be connected with symmetries in a certain variety of space. This all was very well developed and resulted in the Standard Model. Now we can see that the Standard Models works almost flawlessly, but the question of neutrino oscillation, and not only this, still remains.”
ON THE PROBLEMS OF DARK MATTER AND ANTIMATTER
“There are more problems with the Standard Model in outer space. Firstly, it is the problem of dark matter. From the viewpoint of gravitational interaction or dynamics, one fourth of our Universe consists of dark matter and only 5 percent of conventional substance. There is much more dark matter. We don’t know what it is, and there is a hypothesis that it consists of particles. If so, it’s not the particles we know – not electrons, protons, photons, or even common neutrinos. And the problem is how to find these particles.
“There are dozens of hypotheses about what dark matter may consist of. Researchers are seeking for the manifestations of these particles in very diverse experiments and not only – for example, in radiation from space objects. There are even special detectors that can allegedly find dark matter particles colliding with conventional substance underground. This means we won’t see dark matter but will detect this ‘kick.’ Accordingly, there are many competing models because we don’t know the truth.
“Another problem is antimatter. For instance, you and I consist of matter, of particles, but it has been known since the 1930s that there are also antiparticles that very much resemble ‘ours.’ If you take a particle and an antiparticle and bring them closely together, they will annihilate each other and produce a lot of matter. In fact, according to the formula E = mc2, antiparticles are the most effective source of energy. There are very few antiparticles, for otherwise, in all likelihood, we would not exist – there would have only remained cosmic microwave background, i.e. photons that would have formed as a result of annihilation. But we see our world without any major traces of antimatter. Antimatter may exist in cosmic rays – one particle per hundred or thousand. This is good enough, but we can see neither clusters of antimatter, nor galaxies that consist of it. This poses a problem because the Standard Model envisions a very close quantity of matter and antimatter.
“The model I explore allows explaining all these puzzles in theory within the framework of the Standard Model – in theory because we have seen so far only a probable signal of dark matter decay which can be explained by these very particles or, maybe, by many others. We still have a long way to go.”
ON THE CHARACTERISTICS OF DARK MATTER
“It is known for sure that dark matter exists because there is a mass excess in the Universe. We observe an effect and know that it is impossible to explain all that we can see without it.
“I go on speaking in the descending order of truth. Mass is measured in astronomy by the laws of gravitation. Therefore, you can always explain mass excess not by new particles but by your attempts to change the law of gravitation. This is possible hypothetically, and there are models that use it. For example, the Erik Verlinde model, or the model of modified Newtonian dynamics, describes very well the observation of certain objects or their classes, for example, galaxies. But if you look at the whole array of the gained data, you will see that these models describe, for example, clusters of galaxies not so well and need to be at least modified. Introducing a new substance – dark matter – will explain the observation better.
“What is still less known, dark matter consists of particles outside the Standard Model – 95 percent or more of them are the particles we don’t know. Common neutrinos may account for not more than 5 percent of dark matter. It is a challenge for particle physics to search for these new particles.
“What do we know about new particles? One of the main characteristics of a particle is mass, and the difference between the lightest and the heaviest dark matter candidate particle is about 10 in the 40th power. These hypothetical particles may be very large – they are called wimpzillas (a blend of WIMP – weakly interacting massive particles – and Godzilla – Author]. They may have a mass that corresponds to a rest energy of dozens of joules. It is very much from the viewpoint of elementary particles. The decay of these particles can form cosmic rays that have the energy of tens of joules. It is approximately like throwing half a brick with the speed of 10 meters a second – this is the highest energy of a cosmic body detected on Earth, which is billions of times as high as the energy of the particles the LHC can produce. Or it may be so light particles that their de Broglie wave should be the size of the smallest galaxies.”
ON DARK ENERGY
“There is also dark energy of which we know almost nothing [hypothetically, it accounts for three fourths of the Universe – Author]. We only know that it ‘behaves’ like antigravitation. Roughly speaking, people saw that our Universe is not simply expanding but expanding with acceleration. If you toss up an apple or a stone, it will be moving away from Earth and slowing down. Conversely, the Universe expands as a whole, accelerating instead of slowing down. The interactions we know cannot explain this.
“If the well-known equation of state of dark energy is anything to go by, the Universe will, unfortunately, be expanding eternally, and galaxies, their components, and maybe even atoms will fly away.”
THE WAY RESEARCH IS CONDUCTED
“Everything depends on the model, but use is usually made of both theoretical calculation, including a computer-assisted analysis, and the analysis of a very large number of observation data. For example, the LHC generates hundreds of petabytes of data in a year (one petabyte = 2 in the 50th power bytes – Author]. It is very much even at present, but it was unprecedentedly much 10 years ago. For this reason, a system of ramified calculations was specially invented because not a single, even the largest, computer cluster cannot process these data in real time.
“The LHC is not important for our group, as far as searching for dark matter is concerned, but we need to examine X-ray spectrums of the galaxies observed by space telescopes – it is a couple of terabytes of data on the whole. Although these telescopes cost hundreds of millions of dollars, most of their observation data are available free of charge to anybody who wishes to process them. Of particular interest for us are, above all, observations of the Andromeda Galaxy or our galaxy. There is a lot of dark matter there because a galaxy needs it in order to form. We know the way dark matter is distributed across the sky, examine various areas, estimate how much dark matter should be there, compare observations, and check whether there are interesting signals.
“One of the signals we are looking for is the so-called emission line. In 2014 our team, concurrently with a US group, detected for the first time a signal from this emission line with the energy of 3.5 kiloelectronvolts, which may be an explanation of dark matter decay. A corresponding particle of dark matter should be approximately 70 times lighter than an electron and thousands of times heavier than a common neutrino – it is called sterile or right-handed neutrino, or heavy neutral lepton, depending on the field you work in.
“As our research requires huge groups, we hope we will be the best-shooting battalion. It is a maxim that battles are won by best-shooting, not best-manned, battalions.”
ON THE NIELS BOHR INSTITUTE
“The Niels Bohr Institute emerged thanks to support from the beer giant Carlsberg. A company representative suggested to Niels Bohr that an institute named after him be established. They funded the construction of several blocks. Later, Rockefeller and other sponsors also took part in funding the institute.
“I work in Denmark at present also at the expense of Carlsberg. They sponsor my group’s research into dark matter. I am not calling on you to drink beer (smiles), but they have a budget comparable with that of the National Academy of Sciences, which they use for various fundamental research that has nothing to do with beer-brewing, chemistry, biotechnology, and so on.
“Denmark is a stable country, life is improving as time goes by, and citizens respect scientific research very much in contrast to Ukrainians. Maybe, all politicians tend to allot money for something that quickly brings in profit, but Denmark has a powerful lobby of the industrial complex. The backbone of their economy is not oil, grain or chicken meat, but items with a large percentage of added value. Among the best-known companies are Maersk, the world’s largest container transportation operator; Lego; Novo Nordisk [a pharmaceutical producer – Author]; and the abovementioned Carlsberg. Unlike Ukraine, Denmark has a well-developed innovational economy.”
ON THE INSTITUTE OF THEORETICAL PHYSICS OF UKRAINE
“The Mykola Boholiubov Institute of Theoretical Physics has very strong research teams. It was established in the 1960s as an elite research center to be headed by the son of Petro Shelest, the Communist Party boss of Ukraine. Of course, there were very good conditions for work there, progressive and young scientists from all over the Soviet Union were invited to the institute. The intention was to set up a large international center, like in the Russian town of Dubna or in Trieste, Italy. When Shelest was transferred to Moscow, he lost interest in supporting the institute. But the initial impetus was so strong that the institute stayed afloat and is still working on a high scientific level in line with good European standards. Of course, there are problems: it is, above all, an acute shortage of funds, failure of the powers that be to understand the needs of science, and excessive bureaucracy.”
ON WORKING IN UKRAINE
“My lectures in Ukraine are, in a way, volunteerism. When I was a school and college student and took interest in physics, a lot of people helped me without any financial motive – they just saw it as their mission. This is why I spend a part of my time putting across to various audiences – undergraduate and graduate students, the general public – what scientific research is and why it is important. I am trying to explain complicated things in a simple way. I have the experience of teaching senior school pupils, and it is perhaps the reason why I manage to do this much better than most of the researchers. On the whole, this occupies 3 to 5 percent of my working time, and it is not a burden at all.
ALMOST ALL DETECTORS OF THE LARGE HADRON COLLIDER ARE SEARCHING FOR THE TRACES OF STILL UNKNOWN PARTICLES THAT MAY FORM DARK MATTER. THEY HAVE DETECTED NO NEW PARTICLES SO FAR, WHICH AFFORDS GROUND FOR DEVELOPING ALTERNATIVE MODELS
“Besides, when in Kyiv, I conduct research. For example, I was at the institute today, where my colleague and I were finishing an article to be published. My work in Ukraine focuses on the same as in Denmark – I am searching for hypothetical particles of dark matter. Incidentally, it is in Ukraine that I wrote my best articles so far.”
ON THE LABORATORY FOR YOUNG SCIENTISTS
“My coauthors from the Institute of Theoretical Physics and I recently submitted to the National Academy of Sciences the project of a laboratory for young scientists. It calls for essential, by Ukrainian standards, funding which will allow paying competitive salaries, buying the equipment, and making business trips abroad. If we win the competition, this will help essentially improve the motivation of our team’s members who work in Ukraine.”
ON THE NATIONAL FUND OF RESEARCH
“I favor the idea of establishing the National Fund of Research [it is to start functioning in 2019 – Author]. The key principle of funding distribution, which, unfortunately, is not observed in Ukraine, is that money ‘goes after’ the best research associates. Like in many other spheres in this country, this principle has been watered down as a result of a compromise between the groups of stakeholders.
“In my opinion, the National Academy of Sciences is a collective body. On the one hand, it is a very stable organization, but, on the other, funding is often ‘scattered’ little by little among many recipients. As a result, the allocated money is not enough at all for advanced research, and senior research associates are not paid in proportion to their contribution to research. Besides, projects are not assessed externally. It can also be that one has a certain administrative resource in Ukraine, but he or she is not known outside this country. In advanced countries, projects are reviewed by independent external experts in order to prevent the emergence of such ‘clans.’ This is the hallmark of the newly-established National Fund of Research, where this kind of assessment will be introduced.
“I hope this fund will really work because it comprises a lot of progressive things. In particular, it is planned to give up salary-leveling and, instead, give a small number of rather big grants to advanced groups. Therefore, even if you failed to win the competition, you will be sufficiently motivated to apply again.”
ON THE NON-POPULARITY OF PHYSICS
“In my view, the fact that too few university applicants chose physics and mathematics this year results from the decline of research and the related industries in the past few decades. When I was a child, most of my peers in Kryvyi Rih wanted to be racketeers. Now the situation is a bit different – the majority wants to become lawyers and economists, but the essence remains the same.
“Applicants can see that they don’t have to strain themselves to get some specialties – they will just ‘receive’ a diploma and ‘find’ a job. This problem arose because the people who apply their intellect to work and use their brains do not derive a deserved benefit from this. For an Academy of Sciences research associate is paid an almost minimal salary. This raises a question: why should I become a researcher if I might as well work as, for example, a yard cleaner? And if you enter the university to study, say, tourism or learn a foreign language, you can work, figuratively speaking, as a yard cleaner in a country like Poland.
“So, in my view, this problem is the result of the absence of a sufficient number of highly-paid competitive jobs in science and the related high-tech industries.”
ON HIS OWN MOTIVATION FOR RESEARCH
“I’ve liked reading since my childhood. My parents created such conditions that I always had a lot of interesting books, which saved me from hanging about the streets, taking drugs, and so on. I took interest in this and would apply to educational institutions of an advanced scholarly level. For example, I graduated from the Ukrainian Physics and Mathematics Lyceum at the National Taras Shevchenko University of Kyiv and then was admitted to the university itself without exams because I had take part in an international physics Olympiad.
“Incidentally, talking part in Olympiads helped me, even though I had to concentrate very much on the systemic solution of tasks that are simple enough and distant from real scientific problems, where no solution is known in advance, and to vie with a small number of my peers. Yet, for example, from the viewpoint of self-organization, this experience was useful.”
ON WHAT CANNOT BE DESCRIBED WITH THE LAWS OF PHYSICS
“Physics is a very specific science, for it describes nature on the whole and certain general regularities which can be reproduced. Such sciences as cosmology do not fall within this definition because there is only one Universe and we are unlikely to reproduce it, although there are alternative versions here as well.
“There are many things that we do not understand just because they are very complicated. We can know elementary laws of nature and describe interactions on the elementary level. But we cannot do so on the highest level, when there is a very large collective of particles. For example, chemistry has separated from physics. I still consider it part of quantum physics, but many chemists do not agree to this. And they are right in the sense that chemistry is a complex specific science, where it is very difficult to apply the laws of physics and where other, more generalized, laws work better.
“Or take biology, another part of physics. Again, biologists are not used to considering biology as part of physics because it is a complex nontrivial science with different laws. It is very difficult to describe, for example, DNA replication by means of the Schroedinger equation. For this reason, the generalized methods of biology also have the right to exist.
“Then go higher matters outside biology, such as psychology, sociology, and culturology. These sciences are still more difficult to describe on the fundamental level at present. However, the constant intersection of sciences produces, for example, sociophysics and econophysics, when the apparatus of physics is used to understand the laws of social or economic systems.
“At present, there are about eight million research associates in the world – perhaps a thousand times more than 100 years ago. Science has become quite a serious and developed industry in the sense that there can no longer be an individual like Einstein who works alone and whose flash of inspiration makes it possible to make strides in a certain field. Large collectives are often at work. For example, the LHC involves thousands of people – 2,000 at the ATLAS detector alone. These researchers really work very much, but each of them usually deals with small but important aspects. I don’t think there is a person in the world who knows exactly how all the LHC experiments work. This is so different and specialized knowledge that one individual cannot possibly grasp it.”