Cosmological Solutions to the Lithium Problem

Abstract

The abundance of primordial lithium is derived from the observed spectroscopy of metal-poor stars in the galactic halo. However, the observationally inferred abundance remains at about a factor of three below the abundance predicted by standard big bang nucleosynthesis (BBN). The resolution of this dilemma can be either astrophysical (stars destroy lithium after BBN), nuclear (reactions destroy lithium during BBN), or cosmological, i.e. new physics beyond the standard BBN is responsible for destroying lithium. Here, we overview a variety of possible cosmological solutions, and their shortcomings. On the one hand, we examine the possibility of physical processes that modify the velocity distribution of particles from the usually assumed Maxwell-Boltzmann statistics. A physical justification for this is an inhomogeneous spatial distribution of domains of primordial magnetic field strength as a means to reduce the primordial lithium abundance. Another possibility is that scattering with the mildly relativistic electrons in the background plasma alters the baryon distribution to one resembling a Fermi-Dirac distribution. We show that neither of these possibilities can adequately resolve the lithium problem. A number of alternate hybrid models are discussed including a mix of neutrino degeneracy, unified dark matter, axion cooling, and the presence of decaying and/or charged supersymmetric particles.

The First Three Seconds: a Review of Possible Expansion Histories of the Early Universe

“…[668]. In the context of this review, however, we note that nonstandard expansion histories do not seem likely of being capable of ameliorating or solving the problem as they generically alter the abundances of all primordial light elements simultaneously [667,669].…”

Light Elements in the Universe

“…While the reasons for this "meltdown" were not understood, it suggested that some Spite plateau stars may have depleted their initial lithium. The true challenge, however, occurred with the measurement of the cosmic microwave background (CMB) provided by WMAP and Planck satellites, according to which the expected primordial abundance is A(Li) 2.7, a factor of about three to four higher than the abundances of the Spite plateau (see, e.g., Cyburt et al, 2016;Mathews et al, 2020;and references therein). In other words, the stellar Li measurements are inconsistent with the CMB (and deuterium observations), and the discrepancy is larger than 5σ.…”

“…The obvious question is whether the disagreement is due to uncertainties in stellar physics, or rather due to new, non standard physics that modifies SBBN. On the cosmological side, different explanations have been proposed (see, e.g., Mathews et al, 2020); at the same time, numerous studies have instead invoked non standard mixing mechanisms that would cause Li depletion in halo stars (e.g., Tognelli et al, 2020, and references therein).…”

Direct and Indirect Measurements for a Better Understanding of the Primordial Nucleosynthesis

“…This definitely leads to the idea that the cosmological lithium problem is not imputable to systematic errors in nuclear measurements, and no nuclear solution to the cosmological lithium problem can be foreseen. Proposed alternative ideas can be found in Bertulani (2019) or Mathews et al (2020). Further measurements, such as the 7 Be(n,p) 7 Li with THM, will probably be helpful to strengthen this result.…”