Every year, a committee of experts sits down with a tough job to do: from among all ICREA publications, they must find a handful that stand out from all the others. This is indeed a challenge. The debates are sometimes heated and always difficult but, in the end, a shortlist of 24 publications is produced. No prize is awarded, and the only additional acknowledge is the honour of being chosen and highlighted by ICREA. Each piece has something unique about it, whether it be a particularly elegant solution, the huge impact it has in the media or the sheer fascination it generates as a truly new idea. For whatever the reason, these are the best of the best and, as such, we are proud to share them here.


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  • The heart of darkness: Scientists detect a mysterious gamma-ray heartbeat coming from a cosmic gas cloud (2020)

    Torres, Diego F. (CSIC - ICE)

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    The heart of darkness: Scientists detect a mysterious gamma-ray heartbeat coming from a cosmic gas cloud

    An inconspicuous cloud in the constellation Aquila is beating with the rhythm of a nearby precessing black hole, indicating a connection between the two objects, as the team led by DESY scientist Jian Li and ICREA Professor Diego F. Torres at the Institute of Space Sciences (CSIC) reported in the journal Nature Astronomy.

    The research team is composed of international scientists from Germany, Spain, China and America. They rigorously analysed more than ten years of data from NASA's Fermi space telescope, looking at a so-called microquasar. The system catalogued as SS 433 is located some 15 000 lightyears away and consists of a giant star with about 30 times the mass of our sun and a black hole with about 10 to 20 solar masses. The two objects are orbiting each other with a period of 13 days, while the black hole sucks matter from the giant star, generating jets.

    The high-speed particles and the ultra-strong magnetic fields in the jet produce X-rays and gamma rays. The accretion disc does not lie exactly in the plane of the orbit of the two objects. It precesses, or sways, like a spinning top that has been set up slanted on a table. As a consequence, the two jets spiral into the surrounding space, rather than just forming a straight line. The precession of the black hole's jets has a period of about 162 days.

    Meticulous analysis revealed one gamma-ray signal with the same period from a position located relatively far from the microquasar's jets. It is located at the position of gas cloud. The found timing signal provides an unambiguous connection between the microquasar and the cloud, separated by about 100 light years. This is as amazing as is intriguing, opening questions regarding how the black hole powers the cloud's heartbeat thus far.

    [Based on press releases by DESY, Arecibo, CSIC. Commented worldwide, including by The New York Times, People's Magazine, Scify Channel, Astronomy Now, and in more than 150 other newspapers and news agencies, from Argentina to Fiji Islands. According to Nature Astronomy Altmetrics the impact of this work is in the top 5% of all that have been published in the journal.]





  • Why everyone should embrace "carbon pricing" (2020)

    van den Bergh, Jeroen (UAB)

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    Why everyone should embrace "carbon pricing"

    This article responds to criticism of carbon pricing, despite its already wide application (Figure). It stresses that carbon pricing – through a tax or market – deals optimally with market failures, such as negative/positive externalities and public goods/bads, and thus represent a clear systemic perspective on problems and policies. Carbon pricing (CP) is moreover a prime example of systemic policy as it shifts simultaneously choices of consumers, producers, investors and innovators in all sectors – essential to a low-carbon transformation.

    A common confusion is that the main advantage of carbon pricing is its efficiency or cost-effectiveness. But this requires effectiveness. In fact, carbon pricing is highly effective as no decision in the economy escapes its influence. As a result, it also limits energy/carbon rebound better than other instruments.

    It is often overlooked that carbon pricing is critical to innovation. It steers innovations towards low-carbon products and processes as private investors are influenced by price expectations since these co-determine profit opportunities. Furthermore, unlike other instruments, carbon pricing stimulates among ‘clean’ technologies the cleaner ones, like solar PV panels with low-carbon lifecycles.

    Carbon pricing represents a consistent economy-wide approach, which is better than sector-specific approaches that tend to be ad hoc, costly and susceptible to lobbying, while causing inter-sectoral carbon leakage. Moreover, policy harmonization among sectors and jurisdictions is needed to weaken freeriding and avoid leakage. A carbon price performs well as it facilitates comparison and harmonization of policies in distinct jurisdictions.

    While CP has been criticized as inequitable, this is not the case if complemented by appropriate revenue recycling. In fact, no other instrument generates revenues for compensation, while adoption subsidies use scarce funds and are inequitable as they disproportionately go to well-off households.

  • Holographic imaging inside cells (2020)

    van Hulst, Niek F. (ICFO)
    Álvarez Puebla, Ramón (URV)

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    Holographic imaging inside cells

    Holography is known for its ability to produce 3D images (holograms) by recording the amplitude as well as the normally invisible phase of coherent light. In biology though holography is less common, as the most suitable technique, combining sensitivity, resolution and specificity, is fluorescence imaging which is widely used in live cell imaging. It would be fantastic if one could combine fluorescence microscopy with holography, yet fluorescence is incoherent, with a very short path-length and phase memory.

    Now ICFO researchers Matz Liebel and Jaime Ortega-Arroyo implemented an interferometry scheme that measures the position-dependent phase change in wide-field fluorescence detection, to enable 3D imaging of individual molecules and nanoparticles with a resolution of 15 nm, over 8 micrometer depth. Teaming up with Hakho Lee at the Massachusetts General Hospital in Boston, they showed how fluorescence holography can track the 3D motion of extracellular vesicles inside live cells.

    While fluorescence is preferred for its brightness, Raman response has the advantage of a label-free specific contrast, distinguishing different cellular and tissue contents. Spontaneous Raman scattering is very weak, but luckily can be enhanced dramatically on metal surfaces or in metallic nanogaps: surface enhanced Raman scattering (SERS).

    Now, in collaboration with Nicolas Pazos-Perez and Ramon Alvarez-Puebla, of Univ. Rovira i Virgili in Tarragona, Matz Liebel has realized Raman holography for the first time. They synthesized plasmonic superclusters from small nanoparticle building blocks, to generate very strong electric fields in 50 nm cluster size, yielding extremely bright SERS nanoprobes. Next, making the incoherent Raman scattering to “self-interfere”, they achieved 3D holographic imaging, to localize single-SERS-particles in a 3D volume from one single-shot image. Expanding on the fluorescence holography, they now managed to identify and track single SERS nanoparticles inside living cells, in all three dimensions. The SERS signal is bright, photostable and very specific, thus highly suitable for multiplexed single-shot 3D concentration mapping in many scenarios.

  • Flexible quantum reference frames (2020)

    Winter, Andreas (UAB)

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    Flexible quantum reference frames

    A reference frame is commonly understood as a convention to give objective meaning to relative information such as time and distance. In modern physics, however, it is a concrete, rigid external structure to allow for the unambigous definition of such quantities as angles or lengths in terms of measurements. Thus, Einstein imagined a universe full of clocks and rods, but the role of a frame is not reducible to merely giving operational sense to observables through measurement. Indeed, in quantum mechanics, not all quantities can be measured simultaneously, yet frames exist for all sets of quantum numbers, regardless of whether they are jointly measureable or not (in mathematical terms: do or do not commute). In [1] we gave a simple and universal construction of frames for arbitrary quantum numbers. Previously, frames were known based on abstract representation-theoretic principles. Their crucial feature is that they enable arbitrary transformations on a system (including measurements), even ones that do not conserve the observables of interest, offsetting each change by an opposite change in the reference frame. The latter thus necessarily acts as a battery, a very useful point of view in thermodynamics. 

    It turns out that there is much freedom in constructing frames, and that this freedom can be exploited to impose additional desirable features. In thermodynamics the question had arisen whether there are frames with physically distinct parts serving as batteries for the different quantum numbers in question.

    We show that this is indeed the case in several interesting cases [2]. E.g. for angular momentum (Fig. 1), one needs a 3-dimensional coordinate system; a frame could be three large spins pointing in (roughly) orthogonal directions. Alternatively, each of the three directions can be given indirectly via the right-hand-rule based on tow large spins (Fig. 2). Whereas the first offsets any change of the principal spin components by an opposite change of declocalized internal degrees of freedom, the latter frame has three physically distinct parts for each of the principal spin components.

  • Last Interglacial Iberian Neandertals as Fisher-Hunter-Gatherers (2020)

    Zilhão, João (UB)

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    Last Interglacial Iberian Neandertals as Fisher-Hunter-Gatherers

    Marine food-reliant subsistence systems like those in the African Middle Stone Age (MSA) have not been known from Europe until the much later Mesolithic. Whether this difference reflects taphonomic biases or behavioral distinctions between archaic and modern humans remains much debated. Figueira Brava cave, in the Arrábida range (Portugal), provides a uniquely preserved record of Neandertal coastal resource exploitation on a comparable scale to the MSA and dated to ~86-106 thousand years ago. The breadth of the subsistence base — pine nuts, marine invertebrates, fish, marine birds and mammals, tortoise, waterfowl, hoofed game — exceeds that of regional early Holocene sites. The routine harvesting of shellfish implies knowledge of tidal regimes and, along the Portuguese littoral, awareness that, between late spring and autumn, the consumption of bivalves entails a significant risk of biotoxin poisoning. These cognitive aspects of the Figueira Brava subsistence data are consistent with the rapidly accumulating evidence for jewelry, cave art and other forms of symbolic material culture in the Middle Paleolithic of Europe. The major behavioral gap once thought to separate Neandertals from modern humans would thus seem to be just another example that “absence of evidence is not evidence of absence.” Another corollary of the Figueira Brava data is that the consumption of aquatic foods is not the differentia specifica that separated anatomically modern humans in Africa from coeval Eurasians, whose demise it would ultimately explain. Indeed, the possibility must now be entertained that the familiarity with marine resources and seascapes implied by the settlement of Southeast Asia, Sahul (Australia and New Guinea) and the Americas is deeply rooted in human history.

  • Ocean warming and acidification effects on calcareous phytoplankton communities (2020)

    Ziveri, Patrizia (UAB)

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    Ocean warming and acidification effects on calcareous phytoplankton communities

    Carbon dioxide (CO2) emissions by human activities have alarmingly increased in the past decades. A quarter of this anthropogenic CO2 has been absorbed by the ocean, changing the chemistry and ultimately lowering the pH of the seawater, a phenomenon known as ocean acidification. The extra heat trapped in the atmosphere due to greenhouse gases is also causing the seawater warming. The process hampers the supply of nutrients to the upper ocean layers, due to a sharp stratification of the surface water column. Atmospheric warming is expected to evolve in the Mediterranean area 20% faster than the global average, and marine heatwaves will occur with increasing frequency by the end of the 21st century, with serious consequences for marine biodiversity and production. Coccolithophores, a very abundant group of marine calcifying phytoplankton, play a major role in the biogeochemical cycles and in the regulation of the global climate. These tiny algae, which measure 1/1000 of a mm, are at the basis of the aquatic trophic chain, and interact with atmospheric and oceanic CO2 through calcification and photosynthesis. The study detailed the behaviour of these algae under conditions of water temperature ≥ 28°C and a pH of approximately 7.8 units. The results highlight a negative effect of thermal stress on coccolithophore cell production and calcification. Likewise, anomalous calcification in this group of phytoplankton was associated with ocean acidification.  This highlights the importance of looking at species-specific responses to climate change and addressing their specific adaptation mechanisms. Scientists believe that, due to the progressive increase of CO2 emissions in the atmosphere, the risks derived from ocean warming and acidification will become even more acute in the coming decades, thus changing the plankton communities of the Mediterranean. In the future, a clear understanding of the interactions between the different components of the plankton communities will be essential to better understand the future impact of environmental changes on their adaptation and productivity.