Ciclo di Conferenze dei corsi di Laurea in Fisica del Dipartimento di Matematica e Fisica
Edizione 2020
Laura Lupi
Dipartimento di Matematica e Fisica, Università Roma Tre
Role of Stacking Disorder on the Barrier and Pathway of Ice Nucleation
Link identifier #identifier__109095-1Locandina – 14 gennaio 2020, ore 15:00 Aula B
The freezing of water affects the processes that determine Earth’s climate, therefore, accurate weather and climate forecasts hinge on good predictions of ice nucleation rates. Such rate predictions are based on extrapolations using classical nucleation theory, which assumes that the structure of nanometer-sized ice crystallites corresponds to that of hexagonal ice, the thermodynamically stable form of bulk ice. However, simulations with various water models find that ice nucleated and grown under atmospheric temperatures is at all sizes stacking-disordered, consisting of random sequences of cubic and hexagonal ice layers. This implies that stacking-disordered ice crystallites either are more stable than hexagonal ice crystallites or form because of non-equilibrium dynamical effects, with both scenarios challenging central tenets of classical nucleation theory. Here we use rare-event sampling and free energy calculations to show that the entropy of mixing cubic and hexagonal layers makes stacking-disordered ice the stable phase for crystallites up to a size of at least 100,000 molecules. We find that stacking-disordered critical crystallites at 230 kelvin are about 14 kilojoules per mole of crystallite more stable than hexagonal crystallites, making their ice nucleation rates more than three orders of magnitude higher than predicted by classical nucleation theory. This effect on nucleation rates is temperature dependent, being the most pronounced at the warmest conditions, and should affect the modelling of cloud formation and ice particle numbers, which are very sensitive to the temperature dependence of ice nucleation rates. We conclude that classical nucleation theory needs to be corrected to include the dependence of the crystallization driving force on the size of the ice crystallite when interpreting and extrapolating ice nucleation rates from experimental laboratory conditions to the temperatures that occur in clouds.
Julien Lesgourgues
TTK – RWTH Aachen University
Cosmic concordance, cosmic discordance, and the dark sector of particle physics
Link identifier #identifier__162811-2Locandina – 4 febbraio 2020, ore 15:00 Aula C
Data from the Planck satellite and other cosmological probes offer an overall consistent picture of the standard cosmological model. This concordance is however threatened by growing evidence for a few anomalies: the long-standing “small scale crisis” and “sigma8 anomaly”, a more controversial “Hubble tension”, and an even more controversial “Edges anomaly”. A very interesting case for theorists is that if these tensions are not due to poorly understood systematics or astrophysical effects, they could be explained by new particle physics ingredients, such as new interactions in the dark sector, that would involve at least two types of dark relic particles.
Daniele Oriti
Arnold Sommerfeld Center for Theoretical Physics and Munich Center for Mathematical Philosophy Ludwig-Maximilians – University – Munich, Germany
What is space? what is time? the quest for quantum gravity
Link identifier #identifier__97462-3Locandina – 4 marzo 2020, ore 15:00 Aula B
We introduce the problem of quantum gravity, its motivations and challenges at the conceptual as well as physical level. In particular, we emphasize how constructing a new consistent quantum theory of the gravitational field implies deepening our understanding of the nature of space and time themselves. We overview existing theoretical approaches and proposals, and some recent results. Among the most recent suggestions, we illustrate the idea that spacetime itself could be understood as an emergent notion, and that geometry and topology may find their root in quantum entanglement. Finally (if time allows), we illustrate some of the general points by discussing one specific quantum gravity formalism, so-called group field theories, a promising recent blend (and evolution) of matrix/tensor models, loop quantum gravity and lattice quantum gravity, and how effective cosmological equations can be extracted from it.
Federico Tosi
Istituto Nazionale di Astrofisica – Istituto di Astrofisica e Planetologia Spaziali (INAF-IAPS)
Past and future exploration of the icy moons of the Solar System
Link identifier #identifier__164777-4Locandina – 24 novembre 2020, ore 15:00 online
In our Solar System there are 205 known natural satellites orbiting within 6 planetary satellite systems. Most satellites revolve around the gas giants Jupiter and Saturn, and the ice giants Uranus and Neptune. Moving away from the Sun, beyond the “snow line” where surface temperatures drop below 150 K or -123°C, the chemical composition of the planetary moons is dominated by water ice, which is stable over geologic timescales. However, not all of the icy moons are just dead and cratered worlds. Close robotic exploration carried out over the last decades revealed that on some icy moons had occurred, or even still occurs, geologic activity. Underneath their visible surface, some moons host substantial liquid layers (“oceans”) mixed with biogenic elements, which provides a potentially suitable habitat for the development of elementary life forms.
In this seminar we give a description of some remarkable icy satellites of our Solar System, providing a rationale for their astrobiological potential as it results from previous or ongoing investigations. We clarify what the diagnostic evidences of a potential habitat are, and we show what scientific goals are expected to be achieved on these icy worlds through a targeted exploration carried out by future space exploration.
Roberto Buizza
Istituto Scienze della Vita – Scuola Universitaria Superiore Sant’Anna
Uncertainty estimation in weather and climate
Link identifier #identifier__26452-5Locandina – 11 dicembre 2020 ore 15:00 online
One of the major advances in weather prediction of the past three decades has been the provision of reliable uncertainty estimations. This has been achieved by shifting from issuing a single to an ensemble of forecasts. This paradigm shift allows us not only to forecast of the most likely future state, but also to have reliable estimates of its accuracy, for example expressed in terms of a range of possible future scenario, or in probabilities. Similarly, today we have ensembles of analyses and reanalyses, that allows us to estimate the full probability distribution function of current and past states. In this talk, I will review how we developed the earlier ensembles, and the key characteristics of today’s operational ensembles. I will also discuss how work is progressing towards the development of more reliable, coupled Earth-system ensembles of data assimilations and forecasts, which should help us further advancing forecast skill.