Physics research highlights

A. Azarov (1), J. García Fernández (1), J. Zhao (2), F. Djurabekova (3), H. He (3), R. He (3), Ø. Prytz1, L. Vines1, U. Bektas (4), P. Chekhonin (4), N. Klingner (4), G. Hlawacek (4) and A. Kuznetsov (1) 

(1) University of Oslo, Norway 
(2) Southern University of Science and Technology, China 
(3) Department of Physics, University of Helsinki 
(4) Helmholtz-Zentrum Dresden-Rossendorf, Germany 

Principal publication: A. Azarov, J. García Fernández, J. Zhao, F. Djurabekova, H. He, R. He, Ø. Prytz, L. Vines, U. Bektas, P. Chekhonin, N. Klingner, G. Hlawacek and A. Kuznetsov, Nat. Commun. 14, 4855 (2023), https://doi.org/10.1038/s41467-023-40588-0. 

Radiation tolerance is defined by the ability of crystalline materials to endure the accumulation of disorder induced by radiation. Depending on the magnitude of this disorder, materials are commonly categorized as either low- or high-radiation tolerance. However, when subjected to sufficiently high radiation fluences, all known materials end up with either extremely high disorder levels or amorphization. In our recent study, we demonstrate that γ/β double polymorph Ga2O3 structures exhibit remarkably high radiation tolerance. These structures can withstand disorder equivalent to hundreds of displacements per atom without significant degradation of crystallinity, as illustrated in Fig. 1 (a). 

Through atomistic simulations, we attribute this behavior to the intriguing interplay between the properties of the Ga- and O-sublattices in β and γ- Ga2O3. Specifically, the O-sublattice demonstrates a robust tendency for recrystallization, seeking to restore the face-centered-cubic stacking despite the high mobility of O atoms in collision cascades. We also explained the origin of the β-to-γ Ga2O3 transformation with increased defects, see Fig. 1 (b). Simultaneously, the characteristic structure of the Ga-sublattice shows minimal sensitivity to the accumulated disorder. As a result, we conclude that the γ/β double polymorph Ga2O3 structures represent a new class of exceptionally radiation-tolerant materials. 

Alexander Mühleip (1), Rasmus Kock Flygaard (1), Rozbeh Baradaran (1), Outi Haapanen (2), Thomas Gruhl (3), Victor Tobiasson (1), Amandine Maréchal (3), Vivek Sharma (2), and Alexey Amunts (1)

(1) Stockholm University, Sweden 
(2) Department of Physics, University of Helsinki, Finland 
(3) Birbeck College, UK 

Principal publication: Mühleip et al. Structural basis of mitochondrial membrane bending by the I–II–III2–IV2 supercomplex. Nature 615, no. 7954 (2023): 934-938. https://doi.org/10.1038/s41586-023-05817-y 

Energy production in cells is highly efficient. To keep up this efficiency, cells utilize a range of nanoscale switches, gates, and valves. Most of the energy production in cells occurs in mitochondria, which consist of five respiratory complexes I-V, embedded in its inner membrane. A complex flow of electrons and protons combined with membrane, protein and hydration dynamics, results in energy generation in the form of ATP. 

What is the role of membrane bending? One possibility is high efficiency – packing larger number of complexes in a curved membrane together with enclosed compartmentalization may provide a way for efficient energy production. 

Mykhailo Girych, Giray Enkavi, Tomasz Rog, Ilpo Vattulainen  

Biological Physics Group  
Department of Physics, University of Helsinki, Finland  

Principal publication: R. Moliner, M. Girych, V. Kovaleva, C. Biojone, G. Enkavi, C. Cannarozzo, K. Kaurinkoski, N. Seiffert, C. A. Brunello, L. V. Elsilä, A. Rubiolo, M. Kuutti, E. Meshi, E. Negaeva, T. Öhman, S. M. Fred, T. Rog, M. Varjosalo, E. R. Korpi, M. Saarma, I. Vattulainen, P. C. Casarotto, E. Castren. Psychedelics promote plasticity by directly binding to BDNF receptor TrkB. Nature Neuroscience 26, 1032-1041 (2023)

Major depressive disorder (MPP) is one of the most common, if not the most common cause of disability worldwide [1]. Among its most common treatments are selective serotonin reuptake inhibitors (SSRIs), such as fluoxetine (Prozac). Its effect is usually significant, although the development of the response is quite slow and may take a month or even longer. Recent studies showed [2] that the likely reason for the slowness of the response is that fluoxetine binds rather weakly to its receptor, in which case the concentration of the drug molecule in the cell membrane maintaining the receptor must rise high enough before the response to the drug stabilizes at a sufficiently high level. Contrary to what was previously assumed, this work revealed that fluoxetine binding targets the transmembrane part of the neurotrophic receptor tyrosine kinase (TrkB) receptor [2]. Since TrkB is a key receptor in numerous brain signaling processes such as neuroplasticity, this finding also creates a new basis for the development of new antidepressant molecules. 

It is intriguing that also psychedelics produce persistent antidepressant effects and induce neuroplasticity resembling the effects of clinically approved antidepressants. However, due to their hallucinogenic effects, psychedelics cannot be used to treat depression (although with certain exceptions in some countries [3]). 

We recently showed that these two themes are related. We predicted with biosimulations that psychedelic molecules bind to the TrkB receptor [4]. The experimental part of this extensive collaborative project confirmed these predictions and further showed that the therapeutic effect of psychedelics is linked to the TrkB receptor, while the hallucinogenic effect they produce is bridged to the serotonin receptor 5-HT2A [4]. In other words, the therapeutic and hallucinogenic functions of psychedelics work through different pathways and can be separated from each other. This breakthrough opens up the possibility that psychedelics can be used to develop drugs for diseases such as MPP, but without the hallucinogenic effect. 

[1] M. J. Friedrich. Depression is the leading cause of disability around the world. JAMA 317, 1517 (2017).  
[2] P. C. Casarotto, M. Girych, S. M. Fred, V. Kovaleva, R. Moliner, G. Enkavi, C. Biojone, C. Cannarozzo, M. P. Sahu, K. Kaurinkoski, C. A. Brunello, A. Steinzeig, F. Winkel, S. Patil, S. Vestring, T. Serchov, C. R. Diniz, L. Laukkanen, I. Cardon, H. Antila, T. Rog, T. P. Piepponen, C. R. Bramham, C. Normann, S. E. Lauri, M. Saarma, I. Vattulainen, E. Castren. Antidepressants act by directly binding to TRKB neurotrophin receptors. Cell 184, 1299-1312.e19 (2021). 
[3] See, for instance, https://www.bbc.com/news/world-australia-66072427 (June 30, 2023).  

E. Annala (1), T. Gorda (2), J. Hirvonen (1), O. Komoltsev (3), A. Kurkela (3), J. Nättilä (4), and A. Vuorinen (1) 

(1) Department of Physics, University of Helsinki, and Helsinki Institute of Physics 
(2) Technical University of Darmstadt, Germany 
(3) Faculty of Science and Technology, University of Stavanger, Norway 
(4) Center for Computational Astrophysics, Flatiron Institute and Physics Department and Columbia Astrophysics Laboratory, Columbia University, USA 

Principal publication: E. Annala, T. Gorda, J. Hirvonen, O. Komoltsev, A. Kurkela, J. Nättilä, and A. Vuorinen, “Strongly interacting matter exhibits deconfined behavior in massive neutron stars”, Nature Communications 14 (2023), https://www.nature.com/articles/s41467-023-44051-y 

The cores of massive neutron stars contain matter at the highest densities reachable in our present-day Universe. As a first approximation, they can be thought of as giant atomic nuclei, with densities in their cores exceeding those of individual protons and neutrons manyfold. This makes them extremely compact astrophysical objects with as much as two solar masses of baryonic matter compressed inside a sphere of 25 km in diameter. 

A longstanding problem in both nuclear astrophysics and the theoretical study of Quantum Chromodynamics (QCD) – the microscopic theory of the strong nuclear force – is whether a new phase of deconfined QCD matter is present in the cores of the most massive neutron stars. This phase is dubbed cold quark matter, and as the name suggests, its degrees of freedom are quarks and gluons, liberated from their usual color confinement inside nucleons. 

In a new article published in Nature Communications in late 2023, we provided for the first time a quantitative estimate for the likelihood of quark-matter cores being present inside massive neutron stars. Here, we extensively used the concept of conformal invariance – a symmetry approximately present in quark matter but explicitly broken in nuclear matter. The result, based on two independent implementations of a new Bayesian framework, arrived at results of order 80-90%, slightly depending on the technical details of analyses performed. The remaining likelihood can be attributed to an equally exotic scenario, namely a very strong first-order phase transition between the confined and deconfined phases of QCD, destabilizing neutron stars right when they start to develop minuscule quark cores. 

A noteworthy detail about the project that culminated in this publication is that despite its highly international nature, almost all members of the collaboration have a direct connection to the University of Helsinki. Besides the three Helsinki-based members (Annala, Hirvonen, Vuorinen), Aleksi Kurkela obtained his PhD in Helsinki, Tyler Gorda performed his first postdoctoral period there, and Joonas Nättilä finally recently received an ERC Starting Grant to Helsinki, where he will begin as an associate professor in May 2024. 

T. Gorda (1), R. Paatelainen (2), S. Säppi (3), and K. Seppänen (2) 

(1) Technical University of Darmstadt, Germany 
(2) Department of Physics, University of Helsinki, and Helsinki Institute of Physics 
(3) Technical University of Munich, Germany 

Principal publication: T. Gorda, R. Paatelainen, S. Säppi, and K. Seppänen, Phys. Rev. Lett. 131 (2023) 18, 181902. 

Dying stars sometimes leave small but heavy remnants behind. These remnants are known as neutron stars, and by analyzing how the fundamental building blocks of matter behave at extremely high densities, we can better understand their cold, dense cores.  

Computer simulations are useful for modeling very hot systems, such as those encountered in particle colliders. However, computers face limitations in simulating the extremely dense environments of neutron stars. Instead, understanding these dense environments requires performing challenging theoretical calculations. These calculations must be performed with great precision to approximate the densities of neutron stars.  

In this research article, recently published in Physical Review Letters, such calculations have now achieved an unprecedented level of accuracy. They are now finally on par with comparable theoretical calculations performed for collider environments at high temperatures, reaching a milestone similar to one accomplished two decades ago.  In addition, by employing modern machine learning techniques, we can hypothesize how these cold, dense systems behave with even greater accuracy.  

Cosme, C. (1), Costa, F. (2), Lebedev, O. (3) 

(1) University of Coimbra, Portugal 
(2) University of Gottingen, Germany 
(3) Department of Physics, University of Helsinki 

The standard cosmological paradigm assumes that dark matter (DM) has been in thermal equilibrium with the environment. This is justified if the interactions of DM with the Standard Model particles are significant, leading to thermalization of the system. As a result, one expects to see signatures of such interactions in direct detection experiments searching for scattering of DM against heavy nuclei. Currently, no traces of these processes have been found, which poses a puzzle and puts into question the thermal dark matter paradigm. 

The null detection results motivate one to take seriously the non-thermal dark matter option. The problematic aspect of this idea is that the coupling between DM and the Standard Model fields would normally be too small to confirm or rule out this possibility. However, our recent study [1] shows that non-thermal dark matter can have a large coupling to normal matter as long as the temperature in the Universe has never been too high, above the electroweak energy scale. Since very little is known about the maximal temperature in the early Universe, this remains a real possibility. As a result, non-thermal dark matter with substantial couplings can potentially be observed in the direct detection experiments such as XENONnT and also at the LHC via Higgs decay. The signal strength is below that of conventional thermal DM candidates, yet within the reach of current or future experiments.  This opens up exciting prospects in the race for the first laboratory evidence of the existence of dark matter. 

Akke Viitanen (1), Viola Allevato (2, 3, 1), Alexis Finoguenov (1), Francesco Shankar (4), Roberto Gilli (5), Giorgio Lanzuisi (5), Fabio Vito (5) 

(1) Department of Physics, University of Helsinki, Helsinki, Finland 
(2) INAF-Osservatorio astronomico di Capodimonte, Naples, Italy 
(3) Scuola Normale Superiore, Pisa, Italy 
(4) School of Physics & Astronomy, University of Southampton, Southampton, UK 
(5) INAF-Osservatorio di Astrofisica e Scienza delle Spazio di Bologna, OAS, Bologna, Italy 

Principal publication: A. Viitanen, V. Allevato, A. Finoguenov, F. Shankar, R. Gilli, G. Lanzuisi, F. Vito, Astronomy and Astrophysics, 674, A214 (2023)

Active galactic nuclei (AGN) are a class of extragalactic objects whose light is due to accretion of relativistic material into a supermassive black hole at the center of a galaxy. It is well-established that such black holes exist in virtually every massive galaxy, and that the black hole and galaxy growth is tightly correlated, either causally due to e.g. AGN feedback. An especially important class of AGN are Compton-thick – buried under massive amounts of neutral hydrogen with column densities exceeding 10^24 cm-2. According to population studies of the X-ray background, this hidden AGN population may constitute up to 50% of the total AGN in the universe. Moreover, there is evidence that these AGN exhibit more disturbed host galaxy morphologies, hinting at the importance of galaxy mergers as being responsible for torquing gas into the central region of the black hole. In this case, additional clues are found by studying the large-scale clustering of Compton-thick AGN, as galaxy mergers are more likely to occur in specific cosmic environments characterized by a significant number of galaxies and sufficiently low fly-by velocities e.g. in galaxy groups.

Using existing wide and deep XMM-Newton and Chandra surveys, we compiled the largest sample of Compton-thin and -thick AGN with spectroscopic redshifts and robust measurements of the neutral hydrogen column density NH, in order to study the large-scale environments of these AGN as a function of both obscuration and cosmic epoch. Using the two-point correlation function, we measured the large-scale bias i.e. how biased AGN are w.r.t. the underlying dark matter (DM) distribution, and derived the typical AGN host DM halo mass. We found that irrespective of the obscuration, even at the Compton-thick limit, these AGN reside in galaxy group environments with halo masses ~10^13 Msun/h, in-line with the prediction of the unified AGN model with different classes of obscured AGN representing intrinsically similar objects. We characterized the redshift evolution of the AGN clustering by comparing the typical halo mass evolution to theoretical models (Fig. 1), which suggests that for these AGN the halo masses evolve akin to what is expected from the mass growth rate of DM halos alone. 

E. Keihänen, K. Kiiveri, H. Kurki-Suonio, V. Lindholm, A. Viitanen & J. Väliviita (Department of Physics, University of Helsinki, and Helsinki Institute of Physics ) and the Euclid Consortium 

The European Space Agency (ESA) wide-field space telescope, Euclid, was launched on July 1st, and published its first observations on November 7th.  University of Helsinki is the Finnish partner to ESA in this mission; Department of Physics and Helsinki Institute of Physics participating in the analysis of Euclid data. This analysis is divided among nine Euclid Science Data Centers (SDC).  We operate one of them, SDC-FI, in the national CSC Kajaani Data Center.  Euclid studies the “Dark Energy Question” — why is the expansion of the Universe accelerating, and what is the nature of the dark energy causing this? To this goal, Euclid will survey in 6 years over one third of the sky, obtaining images of over a billion galaxies and tens of millions of galactic spectra.  As a wide-field telescope, Euclid can image about a hundred times larger patches of sky than the earlier space telescopes, Hubble and Webb.  A single Euclid image covers a region of sky twice as large as covered by the full moon.  As a space telescope Euclid can obtain much sharper images than ground-based wide-field telescopes, as the images are not blurred by Earth’s atmosphere. 

Euclid published its first observations, the Euclid Early Release Observations (ERO) on November 7th.  These images were taken during commissioning and performance verification of the telescope and its instruments, before the beginning of the main 6-year survey, to demonstrate the capabilities of Euclid.  Five different targets were chosen: the Horsehead Nebula (1375 light-years away), globular star cluster NGS 6397 (7800 light-years away), irregular galaxy NGC 6822 (1.6 million light years away), spiral galaxy IC 342 (11 million light years away), and the Perseus Cluster of galaxies (240 million light years away).  For Euclid, they are all relatively close-by objects, as Euclid will survey the Universe up to over 10 billion light years distance.   The research articles based on these observations will be published in 2024. 

Additional  material available through this link

Vilma Väänänen (1), Mona Christensen (1), Heikki Suhonen (2), Jukka Jernvall (1, 3)

1. Institute of Biotechnology, University of Helsinki
2. Department of Physics, University of Helsinki
3. Department of Geosciences and Geography, University of Helsinki

Original publication: Väänänen V., Christensen M.M., Suhonen H. & Jernvall J. (2023) Gene Expression Detection in Developing Mouse Tissue Using In Situ Hybridization and µCT Imaging. Proc.Natl Acad. Sci. USA 120(24):e2301876120. https://doi.org/10.1073/pnas.2301876120

Intrinsic properties of biological tissues make them challenging to image simultaneously in high resolution and deep inside the tissue. X-ray microtomography approaches allow three-dimensional imaging of fairly large specimens with high resolution, making them valuable tools in fields such as medicine, evolutionary biology, and developmental biology. X-ray imaging, however, has not been well suited to visualize gene expression patterns in tissues while detection of gene activity remains essential for many biological sciences.

In this work, a conventional protocol to visualize specific gene expression was modified to generate radio-opaque signal. Enzymatic reduction of silver overlaid with gold on sites of a gene being expressed was used and compared against conventional approach. We demonstrate that using the new method, gene expression patterns of two genes in developing murine organs can be visualized with X-ray imaging in three dimensions. As the approach is based on routinely used protocols with standard benchtop equipment, the work paves the way for more accessible methods in biological and physical sciences

K. Muinonen (1), M. Granvik (1,2), Antti Penttilä (1), Grigori Fedorets (3,1), and Lauri Siltala (4,1) 

(1) Department of Physics, University of Helsinki, Finland
(2) Department of Computer Science, Electrical and Space Engineering, Luleå University of Technology, Kiruna, Sweden
(3) Astrophysics Research Centre, Queen’s University Belfast, Northern Ireland, U.K.
(4) Karman+

Principal publication: Gaia Collaboration, L. Galluccio, … , K. Muinonen, M. Granvik, A. Penttilä, G. Fedorets, L. Siltala, et al.: Gaia Data Release 3: Reflectance spectra of Solar System small bodies. Astronomy & Astrophysics (2022)  

ESA Gaia space telescope’s Data Release 3 (DR3) in June 13, 2022 marked the first publication of asteroid spectroscopy, including a total of 60,518 spectra in visible light. This constitutes the largest ever sample of asteroid spectra, resulting in more than an order-of-magnitude increase in the number of asteroid spectra available. Furthermore, DR3 was the first extensive release of asteroid astrometry and photometry by Gaia. Data was released for a total of 158,152 small bodies---including asteroids of all populations, transneptunian objects, and planetary satellites---observed over 34 months.

In the figure, we highlight the Gaia DR3 colors in the main belt and in the Hungaria region of asteroids. The colors are descriptive of different asteroid taxonomical classes. Along with the DR3 and forthcoming Gaia data releases, a new asteroid taxonomy will be developed, comprising hundreds of thousands of objects, unveiling the physical properties and population-level characteristics of asteroids.

As to astrometry, Gaia’s extreme sub-milliarcsecond accuracy allows for the computation of orbits of unprecedentedly small uncertainties, as well as determination of asteroid masses from mutual close encounters (Tanga et al. 2022, Siltala & Granvik 2022). As to photometry, Gaia’s millimagnitude accuracy allows to constrain rotation periods, pole orientations, convex shape models, and surface scattering properties for asteroids (Muinonen et al. 2022). In both astrometry and photometry, Gaia’s accuracy requires substantial refinement of inverse methods to interpret the observations with credible theoretical models.

Along with Gaia DR3, the Data Processing and Analysis Consortium of Gaia (DPAC) was awarded the 2023 Lancelot M. Berkeley New York Community Trust Prize of the American Astronomical Society:

https://aas.org/press/gaia-collaboration-to-receive-2023-berkeley-prize

Additional references:

• P. Tanga, …, K. Muinonen, A. Penttilä, et al. Astronomy & Astrophysics, https://doi.org/10.1051/0004-6361/202243796 (2022)
• L. Siltala and M. Granvik. Astronomy & Astrophysics 658, A65, https://doi.org/10.1051/0004-6361/202141459  (2022)
• K. Muinonen, E. Uvarova, J. Martikainen, A. Penttilä, A. Cellino, and X.-B. Wang. Frontiers in Astronomy and Space Sciences 9, 821125, https://doi.org/10.3389/fspas.2022.821125 (2022)
• Gaia Collaboration, A. Vallenari, A.G.A. Brown, T. Prusti, … ,K. Muinonen, M. Granvik, A. Penttilä, G. Fedorets, L. Siltala, et al. Astronomy & Astrophysics, https://doi.org/10.1051/0004-6361/202243940  (2022)

For all Gaia DR3 publications in Astronomy & Astrophysics, see the collection at https://www.aanda.org/component/toc/?task=topic&id=1641

For Gaia constantly scanning the sky, see the animation at https://www.esa.int/Enabling_Support/Operations/Gaia_operations

Antti Penttilä (1), Karri Muinonen (1,2), Olli Ihalainen (1,3), Elizaveta Uvarova (1), Mikko Vuori (1), Guanglang Xu (1,4), Heikki Järvinen (5)

(1) Department of Physics, University of Helsinki, Finland
(2) Finnish Geospatial Research Institute FGI, National Land Survey of Finland, Kirkkonummi, Finland
(3) VTT Technical Research Centre of Finland Ltd., Espoo, Finland
(4) Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology, Germany
(5) Institute for Atmospheric and Earth System Research, University of Helsinki, Finland

Principal publication: Penttilä, A., Muinonen, K., Ihalainen, O., Uvarova, E., Vuori, M., Xu, G., Näränen, J., Wilkman, O., Peltoniemi, J., Gritsevich, M., Järvinen, H., and Marshak, A.: Temporal Variation of the Shortwave Spherical Albedo of the Earth. Frontiers in Remote Sensing 3, 790723 (2022). DOI: 10.3389/frsen.2022.790723

The Earth’s spherical albedo describes the ratio of light reflected from the Earth to that incident from the Sun, an important variable for the Earth’s radiation balance. The spherical albedo has been previously estimated from satellites in low-Earth orbits, and from light reflected from the Moon. We developed a method to derive the Earth’s spherical shortwave albedo using the images from the Earth Polychromatic Imaging Camera (EPIC) on board the National Oceanic and Atmospheric Administration’s (NOAA) Deep Space Climate Observatory (DSCOVR). The satellite resides in the proximity of the Lagrange L1 point between the Earth and the Sun and observes the complete illuminated part of the Earth at once. The method allows us to provide continuously updated spherical albedo time series data starting from 2015. The time series shows a systematic seasonal variation with the mean annual albedo estimated as 0.295±0.008 and an exceptional albedo maximum in 2020, attributed to unusually abundant cloudiness over the Southern Oceans.

Our daily shortwave albedo product is automatically updated daily and published at https://albedo.physics.helsinki.fi/.

S. Koksbang (1), Syksy Räsänen (2)

(1) CP3 -Origins, University of Southern Denmark
(2) Department of Physics and Helsinki Institute of Physics, University of Helsinki

Principal publication: Sofie Marie Koksbang and Syksy Räsänen: The effect of dark matter discreteness on light propagation, Journal of Cosmology and Astroparticle Physics JCAP04(2022)030, available also on arXiv.

Most of the matter in the universe is dark matter, which has been detected only via its gravitational interaction. The most popular possibility is that dark matter, like ordinary matter, consists of particles.

Unless the mass of these particles is small, the density of a single particle is large – its mass is localised in a small region of space. According to general relativity, mass curves spacetime, so spacetime curvature is also large.

The curvature landscape of the universe therefore consists of large widely separated high spikes. In the usual approximation used for studying light propagation, it is assumed that the curvature is small.

What effect does this this bed of nails have on how light travels in the universe? To answer this question, we developed a new approximation for studying light propagation, where the spacetime curvature can be large.

We found that the rapid rise and fall of curvature as light passes through dark matter particles can completely change the passage of light. Such effects are strongly constrained by observations, so we can put an upper limit on the dark matter mass. If the particle is too massive, the spikes distort light too much – you can see the nail marks in the light.

Our upper limit on dark matter mass would rule out most models of dark matter that have been proposed. However, more work remains to be done to check that our approximation is valid before drawing such a conclusion.

Additional reference: Syksy Räsänen: Naulavuoteella kävelemistä. (Blog entry, in Finnish.)

Oleg Lebedev (1), Jong-Hyun Yoon (1)

(1) Department of Physics, University of Helsinki

Principal publication: Oleg Lebedev and Jong-Hyun Yoon: On gravitational preheating, J. Cosmol. Astropart. Phys. JCAP 07, 001 (2022)

Stable or long lived elementary particles can be produced at various stages of the Universe evolution, starting with the period of inflation driven by a hypothetical field dubbed "inflaton". As Figure 1 illustrates, particles are created during inflation, in the  inflaton oscillation epoch, and subsequently via inflaton decay and thermal emission.

Naturally, particle production is most efficient when the Universe undergoes a violent transition to a different state. For example, this happens immediately after inflation since the Universe expansion rate drops and the inflaton field starts oscillating around the minimum of its potential. At this stage, even feeble interactions of the inflaton with other particles lead to efficient production of the latter. Interactions of this type are ubiquitous  in quantum gravity which connects otherwise disconnected sectors of the theory.  As a result, particles of all types are copiously produced in the Early Universe via gravity itself.

If these particles are very long lived or completely stable, they survive to the present day.

They may constitute part or all of dark matter. Our paper [1] shows that  the aforementioned particle production mechanism is so efficient that one would generally expect more dark matter than observed. The fact that dark matter constitutes only 27% of the energy density of our Universe tells us that stable feebly interacting particles present in any fundamental theory have to be very light. Yet, detecting such particles directly would be very challenging due to weakness of their interactions. In any case, the simple observation that our Universe is not totally dark already sets important constraints on theories of quantum gravity.

An accessible interpretation of our results can be found in the popular article "Is the origin of dark matter gravity itself?" by Paul Sutter [2].

Additional references: https://www.space.com/dark-matter-origin-gravity-theory

Shreyas Kaptan, Mykhailo Girych, Giray Enkavi, Waldemar Kulig, Vivek Sharma, Joni Vuorio, Tomasz Rog, Ilpo Vattulainen

Department of Physics, University of Helsinki

Principal publication: S. Kaptan, M. Girych, G. Enkavi, W. Kulig, V. Sharma, J. Vuorio, T. Rog, I. Vattulainen: Maturation of the SARS-CoV-2 virus is regulated by dimerization of its main protease. Computational and Structural Biotechnology Journal 20, 3336 (2022).

The Covid-19 pandemic caused a global crisis. It is clear that we will face similar pandemics in the future as well, so we should start preparing for them well in advance. The question is, what can we learn from the pandemic we are experiencing now.

In terms of the function of the SARS-Cov-2 virus, the spike proteins S1 and S2 on its surface play a key role, thanks to which the virus is able to penetrate host cells and cause infection. For this reason, the development of vaccines that prevent the activity of the virus has focused on these spike proteins, with the aim of producing harmless pieces of the spike proteins in a body that would stimulate the body to produce antibodies that would prevent SARS-CoV-2 from sticking to the surface of host cells and thus getting inside them. The problem, however, is that a large part of the mutations of the SARS-CoV-2 virus occur precisely in the spike proteins, which is why the effectiveness of vaccines decreases as new variants of the virus develop.

Our research group considered an alternative way to prevent the infection produced by SARS-Cov-2. During viral replication, SARS-CoV-2 synthesizes long polypeptides that must be cleaved into its constituent viral proteins. The main protease of the virus, Mpro (see the attached image), carries out most of this cleavage. Inhibiting the Mpro enzyme thus prevents the virus from generating the proteins it needs to replicate.

We unveiled the activation mechanism of Mpro in full detail and revealed that it is active only in its dimeric state.  We found that while there is a huge number of different mutations observed for this enzyme, none of the mutations are at the dimerization interface of Mpro, indicating that the dimerization interface and the dimerization of Mpro are the Achilles heel of this enzyme. Based on this, the findings of our work lead to new drug-based strategies to block Mpro function and SARS-CoV-2 replication. In a broader sense, our results have an impact that extends beyond the current pandemic: we demonstrated that the activity of the main proteases of other coronaviruses is also based on their dimerization. The strategy outlined in our work to develop drug-based means to block the function of main proteases is thus a valid method for preventing the replication of other coronaviruses, too, thus suggesting a new research line for the development of methods for the treatment of future pandemics.

F. de Gasperin (1,2), L. Rudnick (3), A. Finoguenov (4), D. Wittor (1,5), H. Akamatsu (6), M. Bruggen (1) , J. O. Chibueze (7,8), T. E. Clarke (9), W. Cotton (10,11), V. Cuciti (1), P. Domínguez-Fernandez (12,1), K. Knowles (13,11), S. P. O’Sullivan (14), and L. Sebokolodi (13)

(1) Hamburger Sternwarte, Universität Hamburg, Germany
(2) INAF – Istituto di Radioastronomia, Bologna, Italy
(3) Minnesota Institute for Astrophysics, University of Minnesota, Minneapolis, USA
(4) Department of Physics, University of Helsinki, Finland
(5) Dipartimento di Fisica e Astronomia, Università di Bologna, Bologna, Italy
(6) RON Netherlands Institute for Space Research, Leiden, The Netherlands
(7) Centre for Space Research, North-West University, Potchefstroom, South Africa
(8) Department of Physics and Astronomy, University of Nigeria, Nigeria
(9) US Naval Research Laboratory, Washington DC, USA
(10) NRAO – National Radio Astronomy Observatory, Charlottesville VA, USA
(11) SARAO – South African Radio Astronomy Observatory, Observatory, South Africa
(12) Department of Physics, School of Natural Sciences UNIST, Ulsan, Republic of Korea
(13) Centre for Radio Astronomy Techniques and Technologies, Department of Physics and Electronics, Rhodes University, Makhanda, South Africa
(14) School of Physical Sciences and Centre for Astrophysics & Relativity, Dublin City University, Glasnevin, Ireland

During their lifetimes, galaxy clusters grow through the accretion of matter from the filaments of the large-scale structure and from mergers with other clusters. These mergers release a large amount of energy into the intracluster medium (ICM) through merger shocks and turbulence. These phenomena are associated with the formation of radio sources known as radio relics and radio halos, respectively. Radio relics and halos are unique proxies for studying the complex properties of these dynamically active regions of clusters and the microphysics of the ICM more generally.

Abell 3667 is a spectacular example of a merging system that hosts a large pair of radio relics. Due to its proximity (z = 0.0553) and large mass, the system enables the study of these sources to a uniquely high level of detail. However, being located at Dec = −56.8°, the cluster could only be observed with a limited number of radio facilities.

We observed Abell 3667 with MeerKAT as part of the MeerKAT Galaxy Cluster Legacy Survey. We used these data to study the large-scale emission of the cluster, including its polarisation and spectral properties. The results were then compared with simulations.

We present the most detailed view of the radio relic system in Abell 3667 to date, with a resolution reaching 3 kpc. The relics are filled with a network of filaments with different spectral and polarisation properties that are likely associated with multiple regions of particle acceleration and local enhancements of the magnetic field. Conversely, the magnetic field in the space between filaments has strengths close to what would be expected in unperturbed regions at the same cluster-centric distance. Comparisons with magnetohydrodynamic cosmological and Lagrangian simulations support the idea of filaments as multiple acceleration sites. Our observations also confirm the presence of an elongated radio halo, developed in the wake of the bullet-like sub-cluster that merged from the south-east. Finally, we associate the process of magnetic draping with a thin polarised radio source surrounding the remnant of the bullet's cool core.

Our observations have unveiled the complexity of the interplay between the thermal and non-thermal components in the most active regions of a merging cluster. Both the intricate internal structure of radio relics and the direct detection of magnetic draping around the merging bullet are powerful examples of the non-trivial magnetic properties of the ICM. Thanks to its sensitivity to polarised radiation, MeerKAT will be transformational in the study of these complex phenomena.

Principal publication: F. de Gasperin et al., MeerKAT view of the diffuse radio sources in Abell 3667 and their interactions with the thermal plasma. Astronomy & Astrophysics 659, A146 (2022).

Kristoffer Simula (1), Jake Muff (1), Ilja Makkonen (1), Neil Drummond (2)

(1) Department of Physics, University of Helsinki
(2) Department of Physics, Lancaster University, United Kingdom

Principal publication: K. A. Simula, J. E. Muff, I. Makkonen, and N. D. Drummond: Quantum Monte Carlo Study of Positron Lifetimes in Solids, Phys. Rev. Lett. 129, 166403 (2022), published as Editors’ Suggestion.

In solid state matter, positrons thermalize rapidly and are effectively trapped by open-volume-defects such as vacancies and vacancy complexes. Their annihilation radiation contains information on the defects' concentrations, average sizes, and chemical environments. This is the basis of an experimental technique called positron annihilation spectroscopy, which is a powerful method for defect studies and understanding the role atomic-scale defects play from the point of view of the macroscopic properties and utility of technologically relevant semiconductors, optoelectronic materials, and novel metallic alloys [1].

The best way to understand the indirect information contained in the spectra measured in experiments is to perform a series first-principles calculations for candidate defects potentially responsible for the measured data, and directly compare experimental and theory results [1]. This involves determining the defect’s ionic structure, electronic/positronic ground state and annihilation characteristics such as positron annihilation rate and momentum density of annihilation radiation. Since the interaction of positrons with the lattice electrons involves complex many-body correlations that are difficult to parametrize and play a role in positron annihilation, Quantum Monte Carlo (QMC) methods combined with today’s computing resources are an obvious next step when going towards completely parameter-free modeling beyond conventional density-functional-based techniques.

During the past years we have developed and validated a QMC technique for determining positron annihilation rates and the momentum density of annihilating pairs in real periodic solids. The method is practical and robust and potentially applicable in the future in defect and surface studies together with experimentalists. Our QMC analysis of positron lifetimes and electron-positron correlations in defect-free solids was recently published in Physical Review Letters [2].

References:
[1] F. Tuomisto and I. Makkonen, Rev. Mod. Phys. 85, 1583 (2013). https://doi.org/10.1103/RevModPhys.85.1583
[2] K. A. Simula, J. E. Muff, I. Makkonen, and N. D. Drummond, Phys. Rev. Lett. 129, 166403 (2022), https://doi.org/10.1103/PhysRevLett.129.166403

Yongchan Lee (1), Outi Haapanen (2), Anton Altmeyer (3), Werner Kühlbrandt (1), Vivek Sharma (2), and Volker Zickermann (3)

(1) Max Planck Institute for Biophysics, Frankfurt, Germany
(2) Department of Physics, University of Helsinki, Finland
(3) Goethe University, Frankfurt, Germany

Principal publication: Lee, Haapanen et al.: Ion transfer mechanisms in Mrp-type antiporters from high resolution cryoEM and molecular dynamics simulations. Nature Communications, 13, 2022, 6091.

Maintaining ionic balance is crucial for energy generation in biological cells. Mrp (multiple resistance and pH)-type antiporters are employed by alkaliphilic/halophilic bacteria to control the pH and ion concentrations, which are necessary to produce energy in extreme environments. While these Mrp-type antiporters are prevalent in many bacteria, they are evolutionarily related to the redox-driven ion pumps of the complex I superfamily, a group of energy converting enzymes including mitochondrial complex I. Since respiratory complex I and Mrp-type antiporters have homologous subunits responsible for proton transfer, elucidating the ion transfer mechanism in Mrp-type antiporter also boosts complex I research, which has a strong biomedical significance.

Using single-particle electron cryo-microscopy, the structure of the Mrp antiporter from Bacillus pseudofirmus was resolved at a high resolution of 2.2 Å. This high-resolution structure resulted in detection of 360 water molecules out of which roughly 70 were found in potential proton and ion transfer pathways. We performed classical molecular dynamics simulations on the new high-resolution structure utilizing high-performance supercomputing resources from Center for Scientific Computing (CSC), Finland.

Based on structure and simulations, we found a Y-shaped pathway that could catalyze proton transfers (Fig. 1). The pathway is regulated by a histidine switch that undergoes protonation-dependent conformational changes and gates proton transfers to right directions. Such microscopic valves and gates are necessary to enhance efficiency of enzymes. The simulation data also revealed the potential ion (sodium) binding sites as well as ion translocation pathways (Fig. 1).

In conclusion, the combination of structural, biochemical and computational research shed light on both the proton and the ion translocation mechanisms in Mrp-type antiporters. These results also enhance our understanding of medically relevant respiratory complex I and related enzymes.

Pierre Auclair (1), Chiara Caprini (2,3), Daniel Cutting (4), Mark Hindmarsh (4,5), Kari Rummukainen (4), Danièle A. Steer (6), David J. Weir (4)

(1) Institute of Mathematics and Physics, Louvain University (Belgium)
(2) Département de Physique Théorique, Université de Genève (Switzerland)
(3) CERN (Switzerland)
(4) University of Helsinki (Finland)
(5) University of Sussex (UK)
(6) Laboratoire APC, Université Paris Cité (France)

Principal publication: P. Auclair, C. Caprini, D. Cutting, M. Hindmarsh, K. Rummukainen, D.A. Steer and David J. Weir, Generation of gravitational waves from freely decaying turbulence, J. Cosmol. Astropart. Phys. 09 (2022) 029

Link to supplementary video material

The early universe may have gone through a number of first-order phase transitions, in which bubbles of the new phase nucleated, expanded and merged. These are analogous to the condensation of steam into water, or water in turn into ice. Such phase transitions can help to answer unsolved puzzles, such as why there is (much) more matter than antimatter in the present-day universe.

These bubbles would expand in a hot plasma of all the other fundamental particles in the Standard Model, heat the plasma around them as they expanded. After the bubbles collided, there would be sound waves and other perturbations left behind in the plasma. The overlapping of these sound waves is known (from earlier studies by our group [https://doi.org/10.1103/PhysRevLett.112.041301] and others) to be an efficient source of gravitational waves. Depending on when the phase transition took place, such gravitational waves could be seen at future space-based gravitational wave detectors like LISA.

However, the formation of sound waves is only part of the story. If the transition is strong enough, such that a lot of energy ends up in the plasma, then nonlinear things can happen in the plasma before the waves decay away. These phenomena include turbulence: the transfer of power from chaotic fluid motion to ever-smaller scales, which then gets dissipated by microscopic viscous processes.

In this work we looked at the development of turbulence in a system set up to resemble the aftermath of an early universe phase transition, and the resulting gravitational wave production. One difference from typical studies of turbulence is that there is no 'forcing' - no energy is being supplied to the system after the bubbles have finished merging. The resulting phenomenon is known as 'freely decaying turbulence' and has not been widely studied before.

We compared large-scale simulations of a relativistic plasma to a simpler model involving numerical integration, and then both in turn to much simpler constant source approximation. The good agreement we have seen means we believe we have a better understanding of how turbulent motion sources gravitational waves, which will allow us to make better predictions of what future missions like LISA will see.

T. Willamo (1), J. Lehtinen (1, 2), T. Hackman (1)

(1) Department of Physics, University of Helsinki
(2) Finnish Center for Astronomy with ESO, University of Turku

Principal publications:

Lehtinen, J.J., Käpylä, M.J., Hackman, T., Kochukhov, O., Willamo, T., Marsden, S.C., Jeffers, S.V., Henry, G.W., Jetsu, L.: Topological changes in the magnetic field of LQ Hya during an activity minimum, Astronomy & Astrophysics 660, A141 (2022)

Willamo, T., Lehtinen, J.J., Hackman, T., Käpylä, M. J., Kochukhov, O., Jeffers, S.V., Korhonen, H., Marsden, S.C.: Zeeman-Doppler imaging of five young solar-type stars, Astronomy & Astrophysics 659, A71 (2022)

The surface magnetic field of a star can be mapped by the Zeeman-Doppler imaging (ZDI) inversion technique, where spectropolarimetric observations are used to reconstruct the surface magnetic field vector. The method utilises the fact, that when a star with surface spots rotates, its spectral lines will change. In particular, magnetic field structures will rotate in and out of view and the projection of the field vector will change.

The stellar astrophysics group at the University of Helsinki, together with collaborators from several other research institutes, have used Stokes I&V (intensity and circular polarisation) spectra obtained with the 3.6 m telescope at the European Southern Observatory (La Silla, Chile) and the Canadian-French-Hawaii Telescope (Mauna Kea, USA) to reconstruct the surface magnetic fields of young solar analogues. By comparing ZDI images from different epochs, we have identified reversals of the large-scale magnetic field in several stars (Fig. 1; Willamo et al. 2022; Lehtinen et al. 2022). In the Sun, such reversals happen approximately every 11 years. Because of the limited time range of stellar observations, we cannot confirm any similar regularity in our sample of active young solar type stars. However, by comparing photometric observations with ZDI maps, we have found indications of a link between the stellar activity cycle and polarity reversal of the young solar-type star LQ Hya (Lehtinen et al. 2022).

Acknowledgement. This research was supported by the Finnish Academy through the SOLSTICE project (decision no 324163).

E. Keihänen (1), V. Lindholm (2), P. Monaco (3,4,5,6), L. Blot (7), C. Carbone (8), K. Kiiveri (2), A.G. Sánchez (9), A. Viitanen (2), J. Valiviita (10), et al (112 authors)

(1) Department of Physics, University of Helsinki
(2) Department of Physics and Helsinki Institute of Physics
(3) Dipartimento di Fisica - Sezione di Astronomia, Universitá di Trieste
(4) IFPU, Institute for Fundamental Physics of the Universe
(5) INAF-Osservatorio Astronomico di Trieste
(6) INFN, Sezione di Trieste
(7) Max-Planck-Institut für Astrophysik
(8) INAF-IASF Milano
(9) Max Planck Institute for Extraterrestrial Physics
(10) Helsinki Institute of Physics

Principal publication: E. Keihänen et al., Euclid: Fast two-point correlation function covariance through linear construction, Astronomy & Astrophysics 666, A129 (2022)

The two-point correlation function (2PCF) of the galaxy distribution is one of the main observables of the Euclid satellite, the cosmology mission of the European Space Agency scheduled for launch in 2023.  The 2PCF describes the statistical properties of the distribution of visible matter in our universe, and contains information on expansion and structure formation history of the universe. This information will help to solve the mystery of dark energy.

The standard 2PCF estimators take as input the observed galaxy catalogue, and a much larger random catalogue that represents a uniform galaxy distribution. The size of the random catalog is parametrized as the random-data ratio M. For Euclid, M=50 or larger is required.

The analysis of the observed 2PCF requires a description of its statistical uncertainty in the form of a noise covariance matrix (NCVM).  The standard way of constructing the NCVM consists of running the 2PCF estimation procedure on thousands of simulated mock catalogues, and computing their sample covariance. This is a computationally heavy task, and one of the most expensive analysis steps in the Euclid pipeline.

We present a novel method for the estimation of the NCVM, called Linear Construction (LC).  The 2PCF of each mock catalogue, and the related NCVM, are first computed with small random catalogues with M=1 and M=2. While these component covariances are biased estimates of the actual NCVM due to the smallness of the random catalog, we show that a linear combination of them provides an unbiased estimate of the true NCVM for an arbitrarily large M.

The theoretical speed-up of the LC method, compared to the brute-force computation, is a factor of 14 for M=50, and increases further with increasing M.

Antti-Jussi Kallio (1), Nina S. Genz (2), Alexander Weiß (3), Ramon Oord (2) Rene Bes (1), Frank Krumeich  (4), Anuj Pokle (5), Mikko J. Heikkilä (3), Øystein Prytz (5), Unni Olsbye (6), Mikko Ritala (3), Marianna Kemell (3), Florian Meirer (2), Bert Weckhuysen(2), Simo Huotari (1)

(1) Department of Physics, University of Helsinki
(2) Inorganic Chemistry and Catalysis group, Department of Chemistry, Utrecht University
(3) Department of Chemistry, University of Helsinki
(4) Laboratory of Inorganic Chemistry, Department of Chemistry, ETH Zurich
(5) Department of Physics, Center for Materials Science and Nanotechnology, University of Oslo
(6) Department of Chemistry, University of Oslo

Principal publications:

N. S. Genz, A.-J. Kallio, R. Oord, F. Krumeich, A. Pokle, Ø. Prytz, U. Olsbye, F. Meirer, S. Huotari, B. M. Weckhuysen: Operando Laboratory-Based Multi-Edge X-ray Absorption Near Edge Spectroscopy of Solid Catalysts, Angew. Chem. Int. Ed. 61, e202209334 (2022)

A.-J. Kallio, A. Weiß, R. Bes, M.J. Heikkilä, M. Ritala, M. Kemell, S. Huotari, Laboratory-scale X-ray absorption spectroscopy of 3d transition metals in inorganic thin films, Dalton Trans. 51, 18593 (2022)

Characterization of the local chemical environment and the oxidation state of an element of interest are essential tools in various disciplines, such as biology, chemistry, materials science, environmental and Earth sciences, as well as in research of our material cultural heritage. Their evolution in time is vital in chemistry to study e.g., catalysts or thin film growth under working conditions i.e., in situ / operando. Such operando x-ray tools have not been hitherto available outside large scale facilities such as synchrotrons. We have developed new laboratory-scale x-ray spectroscopy methods to study the chemistry of a) heterogeneous catalysis and b) thin films at Helsinki Center for X-ray Spectroscopy [1].

In heterogeneous catalysis, we significantly improve the understanding of the CO2 hydrogenation catalysts by tuning the selectivity of supported mono-, bi-, and trimetallic (combinations of Ni, Fe, and Cu) nanoparticles. The study revealed the synergistic effect of metal alloy formation in the Ni, Fe, and Cu containing nanoparticles [2]. Detailed operando x-ray absorption near edge spectroscopy (XANES) of multi-elemental catalysts provided new insights into the metal-dependent differences in the reducibility and re-oxidation behavior and their influence on the catalytic performance in CO2 hydrogenation.

For thin films grown by atomic layer deposition (ALD) we applied the x-ray absorption spectroscopy for the first time in a local laboratory environment to in situ chemistry of films, in this case CuI, down to 12 nm thickness [3]. This improvement in photon collection efficiency has enabled us to collect the XAS spectrum from even thin films in minutes, which was not previously thought possible using low-brilliance x-ray sources. We demonstrate the in situ capabilities of the local laboratory-based x-ray spectroscopy by observing the oxidation of CuI into CuO upon annealing at 240°C, with both XANES and extended x-ray absorption fine structure (EXAFS).

The research studies vastly enabled new research on inorganic chemistry and materials and created tools for novel operando studies on working catalysts and thin films.

References

[1] https://www.helsinki.fi/en/infrastructures/center-for-x-ray-spectroscopy
[2] N. S. Genz, A.-J. Kallio, R. Oord, F. Krumeich, A. Pokle, Ø. Prytz, U. Olsbye, F. Meirer, S. Huotari, B. M. Weckhuysen: Operando Laboratory-Based Multi-Edge X-ray Absorption Near Edge Spectroscopy of Solid Catalysts, Angew. Chem. Int. Ed. 61, e202209334 (2022)
[3] A.-J. Kallio, A. Weiß, R. Bes, M.J. Heikkilä, M. Ritala, M. Kemell, S. Huotari, Laboratory-scale X-ray absorption spectroscopy of 3d transition metals in inorganic thin films, Dalton Trans. 51, 18593 (2022)

Eemeli Annala & Aleksi Vuorinen

Neutron stars are the remnants of old stars that have undergone a supernova explosion and a subsequent gravitational collapse, which ended just before the formation of a black hole. They contain the densest matter in our observable Universe, with one millilitre of neutron-star material weighing more than 10¹¹ kg.

A question that has been puzzling both astrophysicists and particle theorists for decades is whether the cores of heavy neutron stars might contain an entirely new phase of matter with quarks and gluons as the fundamental degrees of freedom. If confirmed, the discovery of such cold quark matter would be a breakthrough in astrophysics and would additionally shed light on the structure of the phase diagram of Quantum Chromodynamics. 

In a recent article, just accepted for publication in Physical Review X, we implemented a set of new neutron-star observations in a framework designed to build a model-independent family of viable equations of state for neutron-star matter. The new observational information included a new radius measurement by the NICER collaboration and the likely formation of a black hole in the astrophysical event that led to the first observed gravitational-wave signal from a binary neutron-star merger, GW170817. 

Our results, displayed in the figures below, indicate a dramatic reduction in the current uncertainties associated with the neutron-star matter equation of state and the neutron-star mass-radius relation. Interestingly, all the now excluded equations of state corresponded to very high sound speeds in neutron-star matter, in which case the existence of quark-matter cores in massive neutron stars would be uncertain. The new results thus significantly strengthen the case for quark-matter cores, argued for already in our previous Nature Physics publication in 2020. 

Additional material

• Eemeli Annala, Tyler Gorda, Aleksi Kurkela, Joonas Nättilä and Aleksi Vuorinen, "Evidence for quark-matter cores in massive neutron stars", Nature Physics (2020).
   
• Eemeli Annala, Tyler Gorda, Evangelia Katerini, Aleksi Kurkela, Joonas Nättilä, Vasileios Paschalidis and Aleksi Vuorinen, "Multimessenger constraints for ultra-dense matter", accepted for publication in Physical Review X, arXiv.org.

Sami Raatikainen & Syksy Räsänen

Most of the matter in the universe is dark matter, which has been detected only via its gravitational interaction. Black holes are a prominent candidate. 

Dark matter predates the first stars. Therefore, if dark matter consists of black holes, they were not formed in stellar collapse like run-of-the-mill black holes. Creating them requires matter to be strongly clumped and collapse in some rare regions already at early times. 

The most successful explanation for the existence of inhomogeneities in the universe –like stars and galaxies– is cosmic inflation. According to inflation, all structures trace their origin back to quantum fluctuations in the primordial universe. 

We studied a model of cosmic inflation where the inflationary quantum fluctuations produce not only normal structures, but also seed the right amount of dark matter in black holes with same mass as the asteroid Eros. 

The key new ingredient in our calculation is that we take consistently into account that while the quantum fluctuations affect how the universe evolves, the evolution of the universe also changes the quantum noise. 

We solved this coupled evolution by running over 100 billion simulations of cosmic inflation, using over 1 million supercomputer CPU hours at CSC. 

We found that taking the effect of evolution into account enhances the kicks, increasing the number of Eros-mass black holes by a factor of a 100 000. This shows that it is essential to treat quantum noise consistently. 

Additional material

• Daniel G. Figueroa, Sami Raatikainen, Syksy Räsänen, Eemeli Tomberg: Non-Gaussian tail of the curvature perturbation in stochastic ultra-slow-roll inflation: implications for primordial black hole production, Phys. Rev. Lett. 127, 101302 (2021), available on arXiv.
   
• Syksy Räsänen: Potkut ylöspäin. (Blog entry, in Finnish.)

Lauri Niemi & David Weir

Shortly after the Big Bang, the early universe is expected to have undergone phase transitions as it cools down from its hot initial state. Depending on the particle content that makes up the universe, these transitions can be violent first-order phase transitions that fill the universe with gravitational waves, and can produce the out-of-equilibrium conditions required to explain the lack of antimatter in the present universe. In the coming decades, stochastic gravitational waves will be probed over a broad frequency range by several new experiments. Observation of gravitational waves from cosmological phase transitions would shed light on the conditions fractions of seconds after the Big Bang.  

Unfortunately, cosmological relics from phase transitions are improbable within the Standard Model of particle physics. Both the transitions between hadronic and quark matter, as well as the electroweak phase transition associated with the Higgs mechanism, are known to occur smoothly rather than through a first-order transition, much like the liquid-gas transition in a supercritical fluid. However, many models of new physics beyond the Standard Model involve new Higgs fields and predict discontinuous phase transitions in the early universe. In particular, the electroweak Higgs mechanism could have occurred through consecutive phase transitions between Higgs fields of different types, a possibility studied in detail in our recent article (link below).  

A technical complication to studying phase transitions within quantum field theory is that the long-distance thermodynamics is dominated by strongly-interacting bosons. Making robust theoretical predictions thus requires nonperturbative methods such as numerical lattice simulations. Unfortunately, due to the relative complexity of these simulations, almost all of existing literature on cosmological phase transitions with multiple Higgs fields is based on low-order perturbative estimates that are often ambiguous because of large intrinsic uncertainties in the calculations.  

In our work, we utilized nonperturbative lattice simulations to probe the electroweak phase structure in the presence of a new Higgs field in the adjoint representation. We found that discontinuous transitions are absent in most of the allowed parameter space, whereas perturbation theory predicts a weakly first-order transition. In a relatively narrow region of parameter space the model admits a two-stage electroweak phase transition, with a different realization of the Higgs mechanism at intermediate temperatures and large latent heat available for gravitational-wave production in the latter stage.  

Additional material

• L. Niemi, M.J. Ramsey-Musolf, T.V.I. Tenkanen, D.J. Weir, “Thermodynamics of a Two-Step Electroweak Phase Transition”,
  Phys.Rev.Lett. 126, 171802, DOI: 10.1103/PhysRevLett.126.171802

Minna Palmroth & the Vlasiator team

Space is the richest reachable plasma laboratory, hence many of the fundamental and universal physics discoveries of the fourth state of matter – plasma – root in space physics. The near-Earth space is the only place one can send spacecraft to study plasmas. But: Normally one can send only a few satellites, leaving gaps in observations – and demanding modelling of space. 

Modelling space plasmas has three broad categories from computationally feasible to almost impossible. The easiest is to assume that plasmas are a fluid, allowing using a coarse grid, where each cell are like pixels in a 3D camera picture. The computationally most demanding is to model electrons and protons as particles, in which case the simulation volume needs to be filled with tiny grid cells capturing electron physics. Since space is big electron-scale physics cannot extend to the entire near-Earth space. 

There is a midway, in which protons are particles and electrons are fluid. Even this hybrid method is so demanding computationally that it has been feasible only in two spatial dimensions. Until now: the Vlasiator group at UH was able to extend the world’s most accurate space environment simulation Vlasiator to cover all six dimensions.  

If you measure the temperature of plasmas in space, you do not get a nice normal distribution of particles like in air. You might get several different temperatures per location, meaning that to model the plasma temperature – and by extension almost everything that matters in plasmas – you need to model how the particles are distributed. This needs an additional three-dimensional space inside the three-dimensional position space containing the model grid pixels. Thus, six dimensions are needed. 

With the help of PRACE Tier-0 grant and the HLRS supercomputer Hawk in Stuttgart, the Vlasiator group completed the world’s first 6D simulation of one of the most mysterious questions in space physics: what causes the Earth’s magnetospheric tail to erupt plasma clouds at times? This question has not been answered by observations nor by previous fluid models, because the decisive physics occurs at ion-scales. 

Mykhailo Girych, Giray Enkavi, Tomasz Rog & Ilpo Vattulainen

The findings of a new study challenge the prevailing thinking on the primary role of serotonin and other neurotransmitters in the effects of antidepressants. 

The effects of selective serotonin reuptake inhibitors (SSRIs) and other conventional antidepressants are believed to be based on their increasing the levels of serotonin and noradrenalin in synapses, while ketamine, a new rapid-acting antidepressant, is thought to function by inhibiting receptors for the neurotransmitter glutamate. 

Neurotrophic factors regulate the development and plasticity of the nervous system. While all antidepressants increase the quantity and signalling of brain-derived neurotrophic factor (BDNF) in the brain, the drugs have so far been thought to act on BDNF indirectly, through serotonin or glutamate receptors. 

A new study combining neuroscience and computational biophysics demonstrated, however, that antidepressants bind directly to a BDNF receptor known as TrkB. This finding challenges the primary role of serotonin or glutamate receptors in the effects of antidepressants. In essence, the effects of antidepressant on plasticity do not require increases in the serotonin levels or the inhibition of glutamate receptors, as previously thought. 

The binding site of antidepressants in the transmembrane region of TrkB was identified through biomolecular simulations performed at the Department of Physics, University of Helsinki. Biochemical binding studies and mutations introduced in the TrkB receptor verified the site. Biomolecular simulations also demonstrated that the structure of TrkB is sensitive to the cholesterol concentration of the cell membrane. TrkB is displaced in cholesterol-rich membrane compartments, such as synaptic membranes. In addition to findings pertaining to the effects of antidepressants, the study produced a substantial amount of new information on the structure and function of the growth factor receptor. 

Additional material

• Casarotto P.C., et. al. Antidepressant drugs act by directly binding to TRKB neurotrophin receptors. Cell 184, 1299-1313.e19 (2021). DOI: 10.1016/j.cell.2021.01.034
   
• Read the press release

Fredric Granberg & Kai Nordlund

One of the key hurdles to designing a commercially viable fusion power plant is finding materials that can withstand the enormous, about 100 million degree, heat in the fusion plasma. While this plasma does not directly get in touch with the materials, still some fraction of the very hot hydrogen isotopes and electrons will escape the plasma and interact with the inner wall material of the reactor. These escaping particles can heat the material, erode it, or enter it. In case they do enter, they are lost from the fusion reactor fuel and degrade the material properties. Due to this, it is very important to understand the nature of plasma-material interactions. 

After decades of testing different materials, the fusion community has reached a consensus that out of all possible elements, tungsten (W) is the one material that is not prohibitively expensive, yet seems to be able to withstand well the fusion plasma environment. This is due to its high melting point, which gives the best possible tolerance against heating damage, and high cohesive energy, which makes particle-induced erosion unlikely. However, like any other material, also W does have the disadvantage that energetic hydrogen particles may enter it. This again can be a major problem at least from two points of view: the hydrogen that has entered the material is lost from the fusion fuel, and may degrade the material properties. Due to these issues, intensive research is ongoing into the behaviour of H and its isotopes Deuterium (D) and tritium (T) in W.  

One of the basic key questions to solve is how much hydrogen will be retained in W in plasma-facing conditions. In a collaboration between the Max-Planck Institute for Plasma Physics in Germany, the Culham Science Center in Oxfordshire, UK and our University, we have recently combined experimental and simulation efforts to understand the limits of D retention [1]. In these experiments, W samples were first irradiated with high-energy 20 MeV W ions to mimic neutron damage in a fusion reactor, and then exposed to a high flux of low-energy (~ 10 eV) D ions from a plasma, corresponding closely to the condition in fusion reactors like ITER under construction in France, and the future DEMOnstration power plant. In our group, we modelled the damage buildup by a combination of the simplified “CRA” (creation-relaxation algorithm) and full molecular dynamics (MD) simulations. This combination, developed in our group, was shown to lead to realistic radiation damage features even for very high damage level (measured in units of “displacements-per-atom", dpa, a special unit for energy deposited in nuclear collisions [2]) irradiations. We found that especially “cascade annealing” will solve the problem with limited CPU resources and human time, where heavily damaged structures (generated by CRA) are bombarded with full MD impacts. The concentration of D retained in the material was measured experimentally with nuclear reaction methods, and determined in simulations via an analysis of how vacancies are filled with D atoms. 

Both the experimental and simulation results show that while the D concentration initially grows rapidly, after a dose of about 0.1 dpa, the D concentration saturates at a reasonable level of about 1.6%. This is a very encouraging result for fusion reactor design, as it indicates that the level of hydrogen isotopes (D and T) lost to the wall material will not rise limitlessly. Moreover, the simulations explain the reason to the saturation: in short, it can be understood based on our earlier observation in systems without D that when neutron-induced collision cascades in metals overlap previous damage, they inherently anneal some of the pre-existing damage [3]. This causes the damage level (vacancy- and interstitial-like defects) to saturate after an irradiation dose of about 0.1 dpa. Since the D ions are retained mainly in vacancy-like defects, this damage saturation also limits the D level. The really good news is that our explanation shows that this is an inherent physical effect not strongly dependent on details of the material properties, so one can trust that it will be effective in any heavy metal placed in a fusion reactor. 

Additional material

[1] D. R. Mason, F. Granberg, M. Boleininger, T. Schwarz-Selinger, K. Nordlund and S. L. Dudarev, Parameter-free quantitative simulation of high dose microstructure and hydrogen retention in ion-irradiated tungsten, Phys. Rev. Mater. 5, 095403 (2021).

[2] K. Nordlund, S. J. Zinkle, A. E. Sand, F. Granberg, R. S. Averback, R. Stoller, T. Suzudo, L. Malerba, F. Banhart, W. J. Weber, F. Willaime, S. Dudarev, and D. Simeone, Improving atomic displacement and replacement calculations with physically realistic damage models, Nature communications 9, 1084 (2018)

[3] F. Granberg, K. Nordlund, M. W. Ullah, K. Jin, C. Lu, H. Bei, L. M. Wang, F. Djurabekova, W. J. Weber, and Y. Zhang, Mechanism of radiation damage reduction in equiatomic multicomponent single phase alloys, Phys. Rev. Lett. 116, 135504 (2016)

Laurent Forthomme, Tiina Naaroja, Fredrik Oljemark, Kenneth Österberg, Heimo Saarikko & Jan Welti

The building blocks of matter, the quarks, have a property called “colour” and form bound states, whose colour is “neutral” either by having all three colours or colours cancelling, since the antiparticles of quarks have corresponding anti-colour. Good examples of such bound states are protons and neutrons. The quarks are bound together by the exchange of gluons. The gluons can also be bound together by themselves, to form glueballs. Discovering glueballs is not easy, as one must ensure that no quarks were involved both at production and decay of the glueball. So far there exists no firm experimental evidence for glueballs. 

TOTEM is an experiment at CERN’s Large Hadron Collider (LHC) focusing on elastic scattering, where the two protons only scatter (“change their direction”) slightly in the collision. The distinct feature of TOTEM is the ability to measure protons with very small scattering angles corresponding to distances of only a few mm from the outgoing beam far (> 200 m) from the collision point. 

Elastic proton-proton scattering has been described by the exchange of the “Pomeron”, a 2-gluon combination, where the gluon colours cancel each other, see Figure 1. However, already 50 years ago, the existence of an “Odderon”, corresponding to a 3-gluon combination, was predicted. Contrary to the Pomeron, the Odderon interacts differently with the proton and its antiparticle, the antiproton. 

Comparing elastic proton-proton collisions at the LHC with elastic proton-antiproton collisions at Fermilab’s Tevatron collider at the same energy, the TOTEM and D0 could observe that at some particular scattering angles the probabilities of proton-proton and proton-antiproton scattering were significantly different, see Figure 2. The only viable explanation was that instead of exchanging Pomerons, the two colliding particles were exchanging Odderons. 

The Odderon is not an ordinary particle but instead a compound of gluons sufficiently bound together to be exchanged between two protons (or a proton and an antiproton) without the gluons of the Odderon interacting individually with the building blocks of the proton, the quarks and the gluons. After the Pomeron, it is only the second object made up only of gluons ever observed. 

Additional material:

• K. Österberg: The discovery of the Odderon, Helsinki Institute of Physics blog, https://blog.hip.fi/the-discovery-of-the-odderon/
   
• D0 and TOTEM Collaboration: Odderon Exchange from Elastic Scattering Differences between and  Data at 1.96 TeV and from Forward Scattering Measurements, Physical Review Letter 127, 062003 (2021),doi: 10.1103/PhysRevLett.127.062003

Ari-Pekka Honkanen & Simo Huotari

Specialized synchrotron light based methods such as x-ray Raman scattering (XRS) spectroscopy can reveal the chemistry of elements under harsh conditions such as an operating battery or an in-situ chemical catalysis.  

We combined the power of XRS spectroscopy and X-ray diffraction (XRD) at the European Synchrotron Radiation Facility (ESRF) beamline ID20 [1] and provided insights into the chemistry of cobalt and carbon by observing the cobalt carbide (Co2C) formation during Fischer-Tropsch Synthesis (FTS) reaction. The observations were made using measuring the core-electron excitation spectra of  Co L2,3-edges and C K-edge in a Co/TiO2 catalyst. [2] 

Cobalt-based FTS catalysts are one of the most relevant catalysts for industrial applications. Controversy exists, however, regarding the role of the cobalt oxides and carbides during the reaction. Co2C is suspected to play a key role in the deactivation of the catalyst as a degradation mechanism [3]. While indeed some studies correlate Co2C with the deactivation of the catalyst, others assign it with higher selectivity toward lower olefins and as intermediate species during the reaction. This work focused on the study of the formation process of cobalt carbides at relevant conditions of temperature and pressure. 

We could clearly reveal the formation of the cobalt carbide as a function of time and local position of the capillary-based reactor bed. Since Co2C is unstable outside the chemical environment, we could observe for the first time its spectroscopic fingerprint in the cobalt and carbon spectra under these operando conditions.  

To maximise Co2C formation, a carburisation experiment was performed where the catalyst was exposed to a carbon monoxide gas. The results are shown in Figure 1. 

Additional material

[1] S. Huotari et al., A large-solid-angle X-ray Raman scattering spectrometer at ID20 of the European Synchrotron Radiation Facility, Journal of Synchrotron Radiation 24, 521 (2017);

[2] J. Moya-Cancino et al., In Situ X-ray Raman scattering spectroscopy of the formation of cobalt carbides in a Co/TiO2 Fischer-Tropsch synthesis catalyst, ACS Catalysis 11, 809-819 (2021)

[3]  I.C. ten Have and B.M. Weckhuysen, Chem  Catal. 1, 339-363 (2021).

Elina Keihänen & Hannu Kurki-Suonio and the Euclid Consortium

Euclid is a cosmology mission of the European Space Agency. Euclid will study the “Dark Energy Question” — why is the expansion of the Universe accelerating, and what is the nature of the dark energy causing this? To this goal, Euclid will survey over one third of the sky, obtaining images of over a billion galaxies and tens of millions of galactic spectra.  Euclid is a 1.2-meter wide-field space telescope with two instruments, NISP (Near Infared Spectrometer and Photometer) and VIS (imager at visible wavelengths). The Euclid Consortium will use the observations to determine the 3-dimensional distribution of galaxies and dark matter in the Universe, compare their statistics to cosmological models, and thus constrain the law of gravity and the dark energy equation of state. Euclid will be launched in 2023 and will make observations for 6 years.

The analysis of Euclid data is divided among nine Euclid Science Data Centers (SDC). We operate one of them, SDC-FI, in the national CSC Kajaani Data Center. In 2021 we participated in the Euclid Science Challenge 8 where the current version of the Euclid data analysis pipeline was tested. Science Challenge 8 represented a major upgrade in the maturity of the Euclid pipeline. We participated in the Operational Rehersal to demonstrate the ability of the SDC infrastructure to process the continuous data flow from the satellite. We contributed in the development and validation of the code to produce simulated NISP data, and in the production of the simulated VIS data. 

Together with an international team we are developing the 2PCF code, which is used to estimate one of the main cosmology products of Euclid, the 2-point correlation function of the distribution of galaxies. In addition, in the Euclid Theory Working Group we continued preparing forecasts for the constraining power of Euclid on early universe models.

Additional material:

• https://www.euclid-ec.org/

Jonathan Lasham, Amina Djurabekova, Outi Haapanen & Vivek Sharma

Energy plays a central role in our lives. Due to continuously increasing demands of energy, there is a quest to develop novel methods of energy generation and storage. In this vein, we have a lot to learn from microorganisms that generate energy by enzyme catalyzed protonic currents. One such large enzyme is respiratory complex I, which transfers electrons from substrate NADH to quinone, and couples this reaction to proton pumping across a biological membrane. In this work [1], structural biology and computer simulations were applied to obtain a deeper understanding of biological energy conversion by respiratory complex I. High-resolution structure revealed the position of water molecules in protein interior. These water molecules, together with the amino acid residues can catalyze long range proton transfer. Atomistic molecular dynamics simulations were performed by Jonathan Lasham, Amina Djurabekova and Outi Haapanen (from Vivek Sharma’s group). These simulations allowed identification of novel design features in the enzyme, that makes it highly efficient in pumping protons without failing.

Besides energy, another corner stone of life is health. Despite overall understanding of diseases and drugs used to treat those, molecular picture remains enigmatic. If we would have clearer picture of what happens at an atomic level, drugs with better efficacy and low toxicity can be designed. In this study [2], researchers discovered an important role for hydrogen bonding in one mitochondrial disease mutation. They find that a single point mutation from glycine to serine perturbs local environment of the protein resulting in enzyme dysfunction. Molecular dynamics simulations and quantum chemical calculations performed by Jonathan Lasham (from Vivek Sharma’s group) not only provided molecular explanation to experimental data, but also led to novel functional insights.

Additional material

• [1] Parey et al https://www.science.org/doi/10.1126/sciadv.abj3221, Press release
• [2] Kuleta et al. https://www.pnas.org/content/118/33/e2026169118/tab-article-info, Press release

Vesa-Matti Leino & Heikki Suhonen

As part of a multidisciplinary research project [1], we used x-ray microtomography imaging at the X-ray Laboratory [2] to study biodegradable bone implants composed of bioactive glass S53P4. The porous scaffolds of bioactive glass had been implanted into rabbit femur bones to support and enhance the regeneration of bone tissue. We assessed the rate of regeneration at the inner parts of the samples by measuring the relative areas of the gradually dissolving glass and the newly formed bone tissue. In addition, we characterized the 3D-structure of the scaffold, e.g. porosity and pore sizes. First results of the research were published in Acta Biomaterialia in May 2021 [1].

Together with results obtained from other biomedical imaging methods and measurements [1] conducted on the same samples, the x-ray microtomography measurements verified that porous scaffolds of bioactive glass S53P4, in conjunction with the single-staged induced membrane technique described in [1], enable bone regeneration when the injured bone contains large defects, i.e. gaps, in between the remaining bone. Porous, sintered scaffolds of bioactive glass S53P4 are therefore a prospective alternative to other existing bone implant materials for the treatment of critical-sized diaphyseal defects. The research on this biomaterial will continue, and we expect that the measurements conducted with x-ray microtomography could be significantly refined in the future.

Additional material

[1] Eriksson et al., S53P4 bioactive glass scaffolds induce BMP expression and integrative bone formation in a critical-sized diaphysis defect treated with a single-staged induced membrane technique, Acta Biomaterialia 126, 463-476 (2021). DOI: 10.1016/j.actbio.2021.03.035

[2] http://www.xraylab.fi/