Some Aspects of Cosmological Helium

Eric Switzer

January 24, 2011

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Abstract: Helium is the second-most abundant element created in the early universe, and its electronic structure lends it a richer history than hydrogen. I will review helium recombination and its relation to CMB anisotropy formation, and describe several recently-identified physical processes. Helium’s abundance and ionization history leave traces in several epochs of the universe. I will describe constraints on the primordial helium abundance derived from the small angular-scale CMB anisotropies measured by the Atacama Cosmology Telescope. It is thought that helium is not doubly ionized until quasars can produce a hard radiation background, thus pushing its second reionization closer to the present. I will describe two strongly linear (albeit challenging to observe) tracers of its second reionization in radio and near-UV wavebands.