宇宙大爆炸锂问题

Cosmological lithium problem

原初大爆炸核合成(big-bang nucleosynthesis, BBN)始于宇宙大爆炸约3 min之后,随着扩张着的宇宙温度和密度的逐渐降低,核反应大约半个小时后熄灭,该过程结束.原初BBN过程的产物大部分是1H和4He,还有少量的2H(即D)、3He和7Li,其他的核素含量微乎其微.这些遗迹为人们研究早期宇宙提供了独一无二的窗口.目前,利用标准BBN理论结合威尔金森微波各向异性探测器(Wilkinson Microwave Anisotropy Probe, WMAP)精确的宇宙重子密度结果,可以对原初轻核的丰度做出严格的预言.其中, D和4He的天文观测结果与理论预言符合得很好,但是BBN+WMAP预言的原初7Li丰度要比观测的高3~4倍.这种在4σ~5σ置信度区间的差异被称为宇宙锂问题.在过去的十几年里,人们做了很多尝试,但是传统的核物理都未能在标准的大爆炸模型框架下解决该疑难问题.本文针对这一悬而未决的问题进行了系统的阐述,重点介绍了核物理科学家们近年来所做出的努力和尝试,总结了相关研究方向的具体目标,为研究者指出了未来可能的研究方向.

Firstly proposed in 1946 by George Gamow, the hot big bang theory is now the most widely accepted cosmological model of the Universe, where the Universe expanded from a very high-density state dominated by radiation. The theory has been vindicated by the observation of the cosmic microwave background, our emerging knowledge of the large-scale structure of the Universe, and the rough consistency between calculations and observations of primordial abundances of the lightest elements in nature: Hydrogen, helium, and lithium. Primordial big-bang nucleosynthesis(BBN) began when the Universe was 3 minutes old and ended less than half an hour later when nuclear reactions were quenched by the low temperature(about 109 K, i.e., ~1 GK, or kT^100 keV) and density conditions in the expanding Universe. Only the lightest nuclides(~2H,~3He,~4He, and ~7Li) were synthesized in appreciable quantities through BBN, and these relics provide us a unique window on the early Universe. Constrained by the baryon density determined by the Wilkinson Microwave Anisotropy Probe(WMAP) and PLANCK satellites, the primordial abundances of ~2H(referred to as D hereafter) and ~4He inferred from observational data are in good general agreement with predictions;however, the standard BBN theory overestimates the primordial ~7Li abundance by about a factor of three to four. This significant deviation is the so-called "cosmological lithium problem." Over about two decades, many attempts to resolve this discrepancy using conventional nuclear physics have been unsuccessful yet. The possible solutions to this unresolved problem have been suggested, which can be mainly categorized into three groups, according to which part of the preceding analysis is called into question:(1) Astrophysical solutions revise the measured primordial lithium abundance;(2) solutions beyond the standard model invoke new particle physics or nonstandard cosmological physics;(3) nuclear physics solutions alter the reaction flow into and out of mass-7.This paper systematically reviews three major aspects of this long-pending cosmological lithium problem. It includes the scientific background, motivation, possible solutions and attempts made in the past. Here, we mainly focus on the nuclear physics solutions, which might solve this problem in an elegant way. In this review, the goals of different research directions are summarized and evaluated, and the promising direction is guided for future investigations.