The cosmological lithium problem is the discrepancy between the observed abundance of lithium in the universe and the amount of lithium that is predicted by the standard Big Bang nucleosynthesis (BBN) model. The BBN model predicts that the primordial lithium abundance should be around 7.5 parts per billion (ppb), while observations of metal-poor stars, which are thought to have formed shortly after the Big Bang, show a lithium abundance of only around 3 ppb.
There are a number of possible explanations for the cosmological lithium problem. One possibility is that the BBN model is incorrect, and that the primordial lithium abundance was actually lower than predicted. Another possibility is that some of the lithium that was produced in the Big Bang was destroyed by subsequent processes, such as stellar evolution or interactions with dark matter.
A third possibility is that the lithium abundance in metal-poor stars is not as low as it appears. This could be due to a number of factors, such as the effects of stellar rotation or magnetic fields, or the presence of unseen binary companions.
Recent studies have suggested that the cosmological lithium problem may be resolved if the lithium abundance in metal-poor stars is revised upwards. For example, a study by Asplund et al. (2016) found that the lithium abundance in a sample of metal-poor stars was around 4.2 ppb, which is significantly higher than the previously accepted value of 3 ppb.
If the lithium abundance in metal-poor stars is indeed higher than previously thought, then this could provide a solution to the cosmological lithium problem. This would mean that the BBN model is correct, and that the lithium that was produced in the Big Bang was not destroyed by subsequent processes.
However, it is important to note that the lithium abundance in metal-poor stars is still a matter of debate. Further studies are needed to confirm the results of Asplund et al. (2016) and to determine whether the cosmological lithium problem can be resolved.
One of the possible solutions to the cosmological lithium problem is that some of the lithium that was produced in the Big Bang was destroyed by stellar evolution. When stars burn hydrogen, they produce lithium as a byproduct. However, as stars evolve, they also destroy lithium through a process called "Li-burning".
The rate at which stars destroy lithium depends on a number of factors, such as the mass of the star and its metallicity. More massive stars destroy lithium more quickly than less massive stars, and stars with higher metallicities destroy lithium more quickly than stars with lower metallicities.
As a result of stellar evolution, the lithium abundance in the universe decreases over time. This means that the oldest stars in the universe have the lowest lithium abundances.
The effects of stellar rotation and magnetic fields can also affect the lithium abundance in stars. Rotation and magnetic fields can create mixing zones in stars, which can transport lithium from the surface of the star to the interior, where it can be destroyed.
As a result of rotation and magnetic fields, the lithium abundance in stars decreases with increasing rotation rate and magnetic field strength. This means that the youngest and most rapidly rotating stars have the lowest lithium abundances.
The presence of unseen binary companions can also affect the lithium abundance in stars. Binary stars can interact with each other through tidal forces, which can cause the stars to exchange mass. If one of the stars in a binary system is a lithium-rich star, then the transfer of mass from this star to the other star can increase the lithium abundance of the other star.
As a result of the presence of unseen binary companions, the lithium abundance in stars can be higher than expected. This means that the cosmological lithium problem may be resolved if a significant fraction of metal-poor stars have unseen binary companions.
The cosmological lithium problem is a complex problem that is not yet fully understood. There are a number of possible solutions to the problem, and it is likely that the true solution involves a combination of factors. Further studies are needed to determine the exact cause of the cosmological lithium problem and to find a solution that is consistent with all of the observational data.
