(a) shows a history of the primordial helium mass fraction (YP) determinations as a function of time. The same symbols/colors correspond to determinations from collaborations involving many of the same participants and/or the same observational data (blue circle [44], blue squares [45–48], red triangles [49–51], purple circles [52, 53], and purple pentagon [54]). (b) shows the corresponding chronology of , determined from BBN using the current best estimate of the primordial deuterium abundance. The dashed line shows the SM result, .
