Long before the clearing, about second after the universe began, the temperature was K. There were no nuclei yet: protons and neutrons flew about separately.
The expansion brings the temperature down. While light fills the universe, the temperature falls in inverse proportion to the square root of the time: about K at second, about K at seconds.
Building a nucleus needs a window of temperature. Too hot, and a nucleus is broken up as fast as it is assembled. Too cool and too thin, and the particles no longer meet at all.
That window runs from about to seconds after the beginning, which is to say around K. The light elements of the universe were made during those few minutes.
The materials are of kinds, protons and neutrons. They weigh almost the same; the neutron is only slightly heavier.
While it is hot, protons and neutrons turn into one another. The traffic runs both ways fast enough that the numbers stay nearly equal.
As it cools, making the heavier of the two stops paying, and the share of neutrons falls. On top of that, a neutron outside a nucleus decays into a proton in about minutes.
By the time the elements begin to form, the ratio had settled at . This number very nearly fixes everything that follows.
A neutron on its own decays, but inside a nucleus it is stable. So a neutron goes into helium as long as there is one to go into.
Helium is made of protons and neutrons. Every neutrons that go in take protons with them.
Start with protons and neutrons. The neutrons pair with protons to make helium nucleus, and protons are left over. Those remaining protons are simply hydrogen.
Counted by weight, that is of helium against of hydrogen, so helium is of the whole. From the single number comes this .
Measured, the matter of the universe is hydrogen and helium by weight. In old stars, in the gas of distant galaxies, the ratio barely shifts.
Helium is made inside stars too, but not nearly enough of it. If stars were the only source, older objects would hold less helium. Yet however old the object, it never falls below .
Left over from the same few minutes are traces of deuterium and lithium . Deuterium runs to about atoms in hydrogen, and lithium is orders of magnitude below that.
These amounts are all fixed at once by a single choice: the density of the material. There is very nearly density that matches all , and it tells us how much ordinary matter the universe holds.