(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 , blue squares [45–48], red triangles [49–51], purple circles [52, 53], and purple pentagon ). (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, .
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