Published: 25 July, 2007

Big bang led to formation of most of the universe's protons

Can you create protons? And if not, where did the hydrogen nuclei required to form a star come from?

Protons can be created in several ways. One simple way would be for a pair of particles of light -- called photons -- to collide and form a proton plus an antiproton, which is the antiparticle of the proton. This process can happen provided that the photons are energetic enough.

However, this is not really how most of the protons in the Universe were created. Cosmologists believe that there were no protons until about a microsecond had passed since the Big Bang.

The key idea is that protons themselves are not elementary particles, but are made up of particles called quarks. At very early times after the Big Bang, the Universe was a soup of elementary particles including electrons and their antiparticle, the positron, photons, quarks and antiquarks, among others. Many different reactions, including creation of quarks and antiquarks from collisions of photons, maintained a mixture of these particles. As the Universe aged, it also cooled, and once it got cold enough -- about a few trillion degrees -- the quarks in the soup bound up into particles called hadrons, which include not only the proton but also the neutron, the two constituents of all atomic nuclei.

Some nuclear reactions can convert protons into neutrons and vice versa, and others can bind them up to form atomic nuclei, much as the Sun burns protons into ordinary helium, which consists of two protons and two neutrons. Such reactions acted for a very short time in the early Universe, ceasing about three minutes after the Big Bang. The end result was a mixture of protons, helium and deuterium, with about 75% in the form of protons, about 25% in the form of ordinary helium, and tiny traces of deuterium (a bound state of a proton and a neutron sometimes called "heavy hydrogen''), helium-3 (which has two protons but only one neutron), and even smaller traces of lithium. Based on numerous observations and calculations, astrophysicists have concluded that all of the deuterium in the Universe today, and most of the ordinary helium, were produced about three minutes after the Big Bang.

Thus, the protons and neutrons in stars did not always exist but have been around since very early in the history of the Universe. Moreover, experiments show that the proton is exceptionally stable. Lower limits on its lifetime exceed a million trillion trillion years!