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Joan Solà Peracaula
Departament de Física Quàntica i Astrofísica (FQA), Facultat de Física, Universitat de Barcelona

What is your name, affiliation, academic position, and job title?
Joan Solà Peracaula (please notice that both Solà and Peracaula are part of my family name, there are no middle names here). I am permanent member of the “Departament de Física Quàntica i Astrofísica (FQA), Facultat de Física, Universitat de Barcelona” (this is all written in Catalan, by the way, which is my mother tongue). I am full Professor of Theoretical Physics, and I am presently in charge of the group of Particles and Gravitation of the FQA department. I am also senior staff scientist at the Institut de Ciències del Cosmos, Universitat de Barcelona (ICCUB).

What is your journey?
I was born in Santa Coloma de Farners, a fairly small city in the province of Girona, about a hundred Km from Barcelona. I spent my early years there and then for my high school studies I went to the town of Girona (20 Km from the place where I was born). Subsequently I started my first year of undergraduate studies of physics also in Girona U., but in the second year I moved to the town of Cerdanyola, near Barcelona, and completed my undergraduate studies of physics in the Universitat Autònoma de Barcelona (UAB), very close to Cerdanyola. After that I did my (compulsory) military service in Santa Cruz de Tenerife (Canary Islands) for more than one year. Back to Barcelona, I did a PhD thesis in the UAB on the renormalization of the MSSM (Minimal Supersymmetric Standard Model) and upon its defence I earned a fellowship and moved to the Deutsches Elektronen Synchroton (DESY) in Hamburg for a postdoctoral period of three years. I worked under the privileged guidance of Prof. Roberto D. Peccei (one of the prominent names behind the famous Peccei-Quinn symmetry) and he put me to work on various things of particle physics and then also on cosmology. This was actually my first contact with the subject. Specifically, he put me to work on the challenging “cosmological constant problem”, a subject that has marked off my scientific life since then! Later on I visited him in UCLA in Los Angeles for a long visit where we continued our collaboration. My first permanent position was in the UAB as associate professor, which I obtained after a public contest, and later on I was appointed “Catedràtic” (full professor) at the “Universitat de Barcelona” (UB), again upon another public contest with several competing candidates. I have remained in the UB for 23 years up to now.”

What is your field of research and/or what project are you involved in?
My field of research has been Particle Physics for a long time (especially physics beyond the standard model, Higgs physics, supersymmetry, etc.).  I combined these subjects with cosmology since the time when Prof. Roberto Peccei put me to work on this subject during my postdoc years at DESY (see the previous point).  After that period, I still worked intensively on particle physics with several PhD students who defended their theses on that subject.  Later on, theoretical cosmology has been my main research line and this has also generated an equal number of PhD theses of other students of mine.  For obvious reasons, I have always focused cosmology from the point of view of a theoretical particle physicist.  This has endowed me with a rather privileged double expertise status, and in fact half of my scientific life has been equally devoted to each of these subjects, which remain indissolubly united at this point of my career.  It helped decisively to fuse these two subjects when I initiated a fruitful collaboration with Prof. Harald Fritzsch (one of the chief names behind the formulation of QCD, the fundamental theory of strong interactions), who introduced me in the world of the possible time variation of the “fundamental constants” of Nature. Quite obviously, both particle physics and cosmology were needed to deal optimally with this fascinating subject.  I am still doing research in this line, especially from the point of view of quantum field theory (QFT) since it is my profound conviction that QFT has a clue to the basic notions of vacuum energy and its relation with dark energy (DE), as well as on the possible variation of the fundamental constants.  With Harald we wrote several papers on these matters. In the last few years I have worked with my PhD students to show that this proposal about DE can be tested and that dark energy should be in fact dynamical vacuum energy, what I called “running vacuum model” (RVM) since long ago. Our recent QFT investigations have reinforced this idea and have shown that the traditional fine tuning issues with vacuum energy (in the context of the cosmological constant problem) can be disposed of in the RVM framework.  See my companion article “Quantum vacuum: the cosmological constant problem”, which I prepared for a more general audience and serves as a much more accessible version of my CosmoVerse seminar.  Several published works addressed to experts expose in utmost detail these ideas. I hope that the scientific community will be able to appreciate it more and more with time.

Briefly describe your career trajectory to date. What positions have you held, when and where?
Somehow I did it in the previous points. In summary, my active scientific life started in Barcelona with my PhD at the Universitat Autònoma de Barcelona (UAB), until I moved to DESY (Hamburg) and UCLA, Los Angeles. Subsequently I came back to Barcelona at the UAB as associate professor and eventually I became full professor at the University of Barcelona (UB) and senior scientist at the Institute of Cosmos Sciences of the UB (ICCUB). I have been advisor of a dozen PhD thesis on particle physics and cosmology and a large number (around a hundred) diploma works and master theses. I still remain in the UB teaching courses to undergraduate students, currently on General Relativity and Classical Electrodynamics, as well as on Cosmology within our master program. I continue my research on the possible connection of vacuum energy with dark energy within the quantum field theory approach and partially in string theory. I have been director of the old Department of Structure and Constituents of Matter (ECM) of the UB and I am currently in charge of the group of Particles and Gravitation in the present FQA department of the Faculty of Physics at the UB.

What are your research plans?
My research plans are to continue the investigations on the aforementioned “running vacuum model (RVM)” framework. In my opinion, it is a very fruitful idea because it provides a raison d’être for the dark energy on fundamental grounds, as in fact the latter can be understood as vacuum energy (density) in the QFT context. In contrast to previous attempts, the RVM helps to fix the traditional conundrums associated with the cosmological constant problem (in particular the tough “fine tuning problem”) and predicts that the DE is dynamical, without need of postulating ad hoc scalar fields, such as quintessence or phantom fields. Most important, our reiterated confrontation of the RVM predictions with the cosmological data show that the quality fit of the overall observations is comparable to the standard ΛCDM model, if not better. In particular, the RVM framework helps to alleviate the famous cosmological tensions currently afflicting the ΛCDM model, as shown in a number of works that we have presented in the recent years. So my main focus for the future will be to further understand the theoretical quantum field/string aspects of the RVM and continue testing this framework against the latest cosmological observations. I refer once more the reader to my companion article on these matters for the general public, called “Quantum vacuum: the cosmological constant problem”.

How does CosmoVerse fit within those plans?
It certainly helps in that CosmoVerse aims primarily at finding possible solutions to the cosmological tensions. This aim is in harmony with my own work, which has a direct bearing on these cosmological problems and also on others of more theoretical nature. Therefore, I find it natural to be part of this wonderful scientific community since we share common scientific interests. At the same time, it also provides the opportunity to interact with its members and let them know about my own proposals, apart from learning from their proposals, of course.

Which of your skills are you most proud of, or find most useful?
I think that I have a very good physical intuition on the problems of physics. Once I feel this intuition for a particular problem, I can then proceed to the next practical step, which is to prepare the framework for the calculations and the possibility to confront the results with experiment. But it all starts with a genuine and powerful physical intuition, which is the main guide of all my scientific life. Until now it worked quite well…

What new skills would you like to learn in the next year?
I would like to learn more QFT in curved spacetime and also some cosmological aspects of string theory. The first one is an old subject, but in the last 40 years after some very famous books on the subject were written, I still find that not everything has been settled on this subject. For instance, I cannot see that the RVM ideas which we have been able to unearth from QFT have ever been proposed before. I find it amazing that nobody realized that! And for this reason, I want to further plunge into the subject. We are talking about fundamental physics and hence a very solid way to face important problems and maybe also to provide solutions to them based on first principles. The methods of QFT and particles physics are in my opinion a must for facing theoretical cosmology (in particular the cosmological constant problem and its connection with the quantum vacuum). I do not think that by postulating ad hoc fields such as quintessence and phantom fields we are going anywhere, except to keep on reparametrizing our ignorance again and again. On a very different vein of life activities, I’m not very satisfied with my management of time, so I would definitely like to do something about it, meaning to improve the balance in the next years. Physics is extremely absorbing and fascinating, but there is life beyond physics…

What are the most exciting open questions in your research area?
Certainly the “cosmological constant problem”. The problem has been there since Yakov Zeldovich realized that the fluctuations of the quantum vacuum induce an extremely large value of the cosmological constant. This was 50 years after Einstein had introduced the cosmological term in his field equations in 1917. Since Zeldovich opened Pandora ’s Box of the quantum vacuum, we are surrounded by all kind of quantum genies fiddling around with us and refusing to be put back in the bottle. We cannot solve the problem by just closing our eyes to the quantum theory or putting our heads in the sand like ostriches and believe that everything will be alright, that is, that we can simply replace the vacuum as a fundamental concept in QFT with some ersatz fields taken out of our pocket. We cannot put the vacuum on equal footing with the hodgepodge of ideas thrown in the big bag of “dark energy” (DE). The vacuum must be faced as quantum vacuum if we believe that quantum theory is a fundamental theory of physics. There cannot be a solution to the cosmological constant problem by just changing its name. So I will continue trying to show that there is a possible solution to the DE problem in the very context of QFT, for QFT is after all the quantum theory of vacuum! The RVM framework is already a good start.

What advances or new results are you excited about or looking forward to?
The current tensions in cosmology, e.g. on the inconsistent values obtained of the Hubble parameter from local physics and CMB physics, as well as the discrepancy with the data on structure formation at low redshift, seem to point to a crisis within the ΛCDM model. To clarify, this is actually the raison d’être of CosmoVerse, of course. However, even if these phenomenological discrepancies could be attributed to systematic errors and would suddenly disappear into thin air, the theoretical status of the ΛCDM model is extremely poor. We don’t know anything about what is the parameter Λ in it, and we do not even know what is the nature of the missing matter (dark matter) in the universe, nor if dark matter is real at all, namely with some physical substrate of particles. We still have a lot to learn on fundamental grounds. It is not just a matter of keeping on fitting toy models again and again. We need some new “Weltanschauung” to approach cosmology in the light of quantum physics. If it is there already, the scientific community has been unable to recognize it yet! So, I would like to help in formulating some form of new paradigm enabling us to go beyond the usual conceptions. Maybe something of it is already there.

What is your view on cosmic tensions? How does your work connect with this open question in the community?
I believe I have already answered this question in previous points. These tensions are important and must be faced and understood from new perspectives. There is much more in the cosmological Pandora’s Box than meets the eye!

In your career so far, at what point were you the most excited, and what were you excited about?
I was highly excited mainly on two occasions in my scientific life: one long ago, in the context of particle physics, specifically when, in some computations we did in the context of Supersymmetry, we were able to explain a serious double discrepancy (having opposite signs) in the data of Z-boson decays to bottom-antibottom pairs and the data of Z-boson decays into charm-anticharm final states. This was part of the PhD thesis of one student of mine. At some point, however, the unexpected discovery of some systematic errors at LEP (CERN) killed this story altogether after so many efforts from many people, including us. Fortunately, however, my student had already defended his thesis! I would not be surprised if the same kind of similar thing would occur now with the cosmological tensions, especially because particle physics is still much more accurate than precision cosmology. Therefore, the probability that the cosmological tensions can be subsumed in systematic errors is not negligible at all. But there is one thing that has brought me to the maximum excitement at present, which does not depend on any experiment: the realization that the quantum vacuum is dynamical. This is a result that we found recently (2020) in fundamental QFT. Even more, we found later that the equation of state of the quantum vacuum is not exactly -1 as it has been always thought. The quantum vacuum in the RVM context can actually mimic quintessence and phantom dark energy without need of having any real quintessence or phantom (ad hoc) field. This is truly exciting to me!

What is the biggest obstacle that is slowing down your research field right now?
I need more helping hands. I would like to have a couple of good students sharing my passion for continuing the above mentioned investigations! There is much to do and to test yet! All my PhD students have always been very good, so I am a very lucky professor! I would still like to continue with a couple more of lucky ones…

What role do you think a community network like CosmoVerse can play in developing theoretical astroparticle physics and cosmology?
As I explained before, it provides an invaluable platform for scientific communication, hence optimizing the possible solution of the current problems in cosmology. In my opinion, it plays a fundamental role in the field.

What do you like and dislike about being a scientist?
I expressed already my likes. In summary, I like passionately discovering the truth in the mysteries of the Universe. But I deeply dislike the exceeding marketizing of science. As a result many (most) scientists only focus on publishing papers and more papers, without taking enough time to reflect on the ideas and their philosophical implications, as well as on the practical impact that the ideas might have on the future of Humanity. One should be able to understand that beyond formulas and complicated calculations there is a conceptual/philosophical formulation of these ideas and results, which can improve the spirits and souls (whatever they are) of men and women, namely that part of ourselves that makes us truly human beings. This could make us better, which is not a minor thing. But I am afraid that we do not go into this direction, unfortunately. Not surprisingly, we now find ourselves in a planet extremely deteriorated by our unsustainable policies, and with a very uncertain future.

What’s your favourite food? Why?
Papaya. I like very much this exotic fruit since I visited Brazil long ago. They call it “Mamao”. It has a lot of good nutrients and properties, and it tastes moderately sweet. I just like it very much. I eat it almost every day as part of my breakfast. Fortunately it is available in many supermarkets.

Your favourite scientist and/or science fiction film?
I know that to say Einstein does not sound very original, but I will say it: Einstein! I repeat: Einstein! His gravitational field equations are the most beautiful piece of “cosmic poetry” that a man of science can ever write. If there is a God, which we don’t know, these equations must be a pristine revelation of some of His innermost and unfathomable thoughts.
My favourite science fiction movie: “Interstellar”. But the much older one “2001: A Space Odyssey” is no less so!

How do you relax after a hard day of work?
I like music, especially jazz and classical music. But I also like to watch movies.

What non-physics interests do you have and want to share?
I like to watch stars with my binoculars, and planets and nebulae with my little telescope, when I have time. I like to read as well. I cannot avoid reading Bertrand Russel’s books quite often, for instance. I admire Russell’s writings and thoughts like no other.

If you were not a scientist, what do you think you would be doing?
I don’t know… maybe I would have been a rustic man, like my father, who cultivated the land and loved Nature in all forms, knowing the names of all plants and animals populating the woods around our place. My mother was an expert on all kind of herbs of the region. Love and knowledge of Nature is in the heart of my family.

What do you hope to see accomplished scientifically in the next 50 years?
It is very difficult to say. I prefer not to answer this speculative question. The reason is, in part, also because of the next question.

In your view, what’s the most important challenge that humanity faces currently?
Global warming and all its manifold consequences, as well as the continuous destruction of the Nature by the human being. Our beautiful blue planet is more and more in real danger, and we are responsible for this disaster. That’s why I said in a previous answer that there is a part of science that I dislike profoundly, which is when science continues blindly its way and does not lead to a reflection of the new achieved ideas in the benefit of Humanity. AI, for instance, is a real danger, if unrestricted, it can lead to our extinction.
Science must do something about all these problems, and not only scientists. We all must devote a lot of efforts to stop the continuous destruction of our planet. These are problems that go far beyond any other problem we have been talking about. Every scientific subject, no matter how fascinating is, pales as compared to the devastating magnitude of these threats , which impinge directly and urgently the future of our planet, and hence the future of the human being. The increasing war escalate in the world in many fronts (including Europe) is also extremely worrisome!

What question would you have liked us to ask you, and what would you have answered?
I think you asked already a lot, and I believe I also provided a non-negligible feedback. Thank you for offering me the opportunity to answer these questions! No need for me to add anything else.