A cosmic recipe with a missing ingredient
In the first few minutes after the Big Bang, a process called Big Bang Nucleosynthesis (BBN) forged the first atomic nuclei. For about 17 minutes, the entire universe was a nuclear furnace, producing nearly all of the hydrogen and helium that exists today, along with trace amounts of lithium. The standard model of cosmology makes sharp predictions about the abundances of these light elements.
These predictions are powerfully constrained by measurements of the Cosmic Microwave Background (CMB), the afterglow of the Big Bang. Satellites like WMAP and Planck have measured the CMB's properties, which in turn fix the cosmic baryon density, a measure of the amount of ordinary matter; this value dictates the predicted output of the BBN furnace. The predictions for hydrogen and helium isotopes match observations with remarkable precision. Lithium-7, however, is an exception. Standard BBN models predict an abundance of primordial lithium that is three to four times greater than what astronomers actually observe. This failure of an otherwise successful theory is known as the Cosmological Lithium Problem.
Searching for the lost lithium
The search for the missing lithium focuses on three main areas: errors in observation, flaws in the nuclear physics, or the existence of entirely new physics.
Astronomers measure primordial lithium by observing the atmospheres of the oldest, most metal-poor stars in our galaxy's halo. These stars preserve the chemical composition of the early universe. The first explanation, an astrophysical one, suggests that these old stars have destroyed their original lithium over their long lifetimes. Lithium is a fragile element, destroyed by nuclear reactions at temperatures around 2.5 million Kelvin, relatively cool for a stellar interior. The difficulty with this idea is that these ancient stars show a surprisingly uniform lithium abundance, known as the "Spite plateau". This uniformity suggests less destruction has occurred than is needed to explain the entire discrepancy.
A second possibility is that the nuclear physics inputs to the BBN models are wrong. Most primordial lithium-7 was formed from the decay of beryllium-7, which has a half-life of about 53 days. If the rates of the nuclear reactions that create and destroy beryllium-7 were incorrect, the prediction would change. Laboratories on Earth, including experiments at CERN, have re-measured these critical reaction rates with high precision. The updated measurements have failed to resolve the problem, making a purely nuclear physics solution less likely.
This leaves the most likely possibility: new physics beyond the Standard Model. Some theories propose that the decay of hypothetical particles, such as supersymmetric dark matter candidates, after BBN could have destroyed the beryllium-7 before it decayed into lithium. Other ideas include modifications to the laws of physics in the early universe, such as changes in fundamental constants. While no single theory has been proven, the persistent lithium problem indicates that our understanding of the universe's first moments may be incomplete.