Variations in the Primordial Helium Abundance

• Published in: Monthly notices of the Royal Astronomical Society 1974-03, Vol.166 (3), p.663-671
• Main Authors: Gisler, G. R., Harrison, E. R., Rees, M. J.
• Format: Article
• Language: English
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/166.3.663

We first show that simple temperature fluctuations of constant specific entropy do not affect the production of helium in the early Universe. This result is contrary to conclusions reached in previous work. We then show that curvature fluctuations, associated with temperature fluctuations and previously ignored, affect the cosmic helium abundance Y. Positive curvature fluctuations increase the expansion time and therefore reduce Y; negative curvature fluctuations on the other hand increase Y. In the case of constant specific entropy the effect of positive curvature fluctuations dominates, thus yielding a net reduction in the cosmic helium abundance. The effect is most pronounced for perturbations of scale length approximately equal to the Hubble distance when the temperature is 1010 K. Analytical and computational methods are used to estimate the change in Y as a function of the temperature fluctuation. In general, the net reduction in Y is small (a few per cent only); but under special conditions, when specific entropy is not constant, Y can be reduced from 0.27 (the adopted unperturbed value) to approximately the value 0.21.