Big bang nucleosynthesis (BBN) describes the production of the lightest nuclides–D, 3He, 4He, and 7Li–at times ∼ 1 sec to ∼ 3 min after the big bang. Theoretical predictions of the light element abundances are well-understood and rest on the secure microphysics of nuclear cross sections and Standard Model weak and electromagnetic interactions (1, 2). These predictions are in broad quantitative agreement with measured primordial light element abundances derived from observations in the local and high-redshift universe. This concordance represents a great success of the hot big bang cosmology, and makes BBN our earliest reliable probe of the universe.
BBN has dramatically changed over the past decade in response to the cosmological revolution sparked by the recent flood of new observations. Notably, WMAP measurements of the cosmic microwave background (CMB) radiation have precisely determined the cosmological baryon and total matter contents (3, 4), while high-redshift supernova observations reveal that the universe recently entered a phase of accelerated expansion (5, 6, 7, 8). These and other observations reveal the existence and tightly determine the abundance of both dark matter and dark energy on cosmic scales where they dominate the mass-energy budget of the universe today. Yet the nature of both dark matter and dark energy remains unknown. Thus 21st-century cosmology finds itself in a peculiar state of “precision ignorance;” this situation is particularly exciting for particle physics because both dark matter and dark energy demand physics beyond the Standard Model.
BBN has played a central role in the development of this new cosmology. CMB data now measure of the cosmic baryon density, independently of BBN, and with high precision. This casts BBN in a new light: a comparison of these two measures of cosmic baryons provides a strong new test of the basic hot big bang framework (9). Moreover, this test can now be performed in a new way, using the CMB baryon density as an input to the BBN calculation, which outputs predictions for each light element abundance; these can then each be directly compared to light element observations (10). The result is that deuterium shows spectacular agreement between BBN+CMB predictions and high-redshift observations, and 4He shows good agreement. However, using the first-year WMAP data, 7Li showed a discrepancy of a factor 2−3, representing a 2−3σ disagreement between observations and theory (11, 12, 13, 14). This disagreement has worsened over time, now standing at a factor 3−4 in abundance or 4−5σ: this is the “lithium problem.”
In this paper we present an overview of the lithium problem, accessible to nuclear and particle physicists and astrophysicists. Broader reviews of primordial nucleosynthesis and its relation to cosmology and particle physics are available (2, 15, 16, 17).
In Section 2, we trace the origin of the lithium problem, with a focus on the physics of BBN 7Li production, the nature and precision of light element abundance measurements, and the state of light element concordance in view of the CMB-measured cosmic baryon density. We review possible solutions to the lithium problem in Section 3: (i) astrophysical systematic uncertainties in lithium abundances and/or their interpretation; or (ii) new or revised nuclear physics inputs to the BBN calculation, in the form of increased mass-7 destruction via novel reaction pathways or by resonant enhancement of otherwise minor channels; or (iii) new physics – either particle processes beyond the Standard Model occurring during or soon after BBN, or large changes to the cosmological framework used to interpret light element (and other) data. We close by summarizing the near-future outlook in Section 4.