Formation of the first atoms

The study of the relative abundances of hydrogen and heavy hydrogen (or deuterium) – the very first atoms created together with helium - provides an important way to understand the nature and density of the matter that came into existence shortly (from 1 second to a few minutes) after the Big Bang.

This knowledge is key to understanding whether the amount of matter (and energy) present in the Universe is enough for the Universe to collapse or to expand indefinitely (‘closed’ or ‘open’ Universe).