Uno dei problemi ancora aperti dell’astrofisica riguarda la questione del litio. Infatti, la quantità di litio prevista nelle stelle non è quella che ci aspettiamo (post1; post2). Nonostante ciò, i calcoli sono esatti, così come è stato confermato per la prima volta da un esperimento recente condotto presso i Laboratori Nazionali del Gran Sasso che ha permesso di calcolare la percentuale di litio che si produce sotto certe condizioni estreme, come quelle che hanno caratterizzato, appunto, il Big Bang. Continua a leggere LUNA riproduce la nucleosintesi del litio
Un gruppo internazionale di ricercatori sono stati in grado di verificare alcune previsioni del modello del Big Bang eliminando così una discrepanza significativa che ha tenuto i fisici col fiato sospeso per almeno due decenni. Uno dei problemi fondamentali dell’astronomia riguarda l’inconsistenza tra l’abbondanza degli isotopi del litio osservati nelle stelle più vecchie della nostra galassia che suggeriscono dei livelli di litio-6 circa 200 volte superiori e di litio-7 circa 3-5 volte inferiori rispetto a quanto calcolato in base alla nucleosintesi primordiale. Per risolvere l’enigma astrofisico, gli scienziati hanno cercato di introdurre dei processi fisici più esotici per riconciliare le differenze osservate.
The team, led by Karin Lind of the University of Cambridge, has proven the decades-old inventory relied on lower quality observational data with analysis using several simplifications that resulted in spurious detections of lithium isotopes. Using observations of ancient stars with W. M. Keck Observatory’s 10-meter telescope and state-of-the-art models of their atmospheres has shown that there is no conflict between their lithium-6 and lithium-7 content and predictions of the standard theory of Big Bang nucleosynthesis, restoring thus the order in our theory of the early Universe. The discovery that the universe was expanding by Edwin Hubble in the 1920s and subsequent observations suggest the Universe began about 13.8 billion years ago in an event called the Big Bang. The fundamental observations that corroborate the Big Bang are the cosmic microwave radiation and the chemical abundances of the light elements described in the Big Bang nucleosynthesis theory. “The predictions of Big Bang nucleosynthesis have been one of the main successes of the standard Big Bang model”, said lead author Lind. “Our findings remove much of the stark tension between 6Li and 7Li abundances in stars and standard BBN, even opening up the door for a full reconciliation. This further consolidates a model resting heavily on the pillars of the cosmic microwave background and the expanding Universe”. Taking accurate measurements of lithium-6 and lithium-7 in old stars is extremely challenging, both from a theoretical and observational perspective, in particular for lithium-6, because being the less abundant isotope of lithium, its signature is very weak. The required data can only be obtained with the largest telescopes on Earth such as the Keck Observatory on the summit of Mauna Kea, Hawaii equipped with the powerful High Resolution Echelle Spectrometer (HIRES) spectrograph to disperse the stellar light into its constituent colors and absorption features. “Back in 2004 HIRES was upgraded with CCDs having smaller pixels, allowing to see finer details in the spectrum”, University of Sao Paulo’s Jorge Meléndez said. “A high spectral resolution provided by HIRES is needed to study with exquisite detail the line profile and to estimate the presence of Lithium-6. The large light-collecting power of Keck Observatory allowed us to observe stars with a more ‘pristine’ composition than any previous study”. Even with the mighty Keck I telescope, a single star must be observed for several hours to gather enough photons for a detailed observation. The modeling of such data is also very demanding, as different processes in the atmospheres of such metal-deficient old stars may mimic the presence of lithium-6. The data must be analyzed using sophisticated model atmospheres created by the team in 3D and included complex calculations that run for weeks on powerful super computers. “We simultaneously relaxed two key physical assumptions in the modeling of stellar atmospheres; one-dimensional hydrostatic and local thermodynamic equilibrium”, Lind said. “Using more sophisticated physics and powerful super-computers, we managed to remove the systematic biases that plague traditional modeling and have previously led to false identifications of the 6Li/7Li isotopic signature”. The synergy of high quality Keck observations and detailed theoretical modeling has solved cosmological problems that haunted particle physicists and astrophysicists during the last two decades. “Understanding the birth of our Universe is pivotal for the understanding of the later formation of all its constituents, ourselves included”, Lind said. “The Big Bang model sets the initial conditions for structure formation and explains our presence in an expanding Universe dominated by dark matter and energy”. The Big Bang theory now rests on more firm footing.