Where did the matter come from that makes up the galaxies, stars, planets
and humans? Creation of the universe (the "big bang") includes
the creation of space and the matter within it, so this question leads
us to consider what kind of matter was initially created. Astronomical
observations and cosmological theory together demonstrate that the varieties
of matter created in the early universe determine the amount of helium
(He) observed in the universe today.
Today's visible matter is made up of atoms. But atoms could not exist
in the intense heat of the early universe. If time could be made to run
backwards, the universe would become so hot that an atom would break up
into a nucleus and electrons. By continuing to run the clock backwards,
causing the temperature of the universe to increase, a nucleus would break
up into its constituent protons and neutrons. A neutron would start to
undergo radioactive decay into an electron and a less commonly known particle
called a neutrino (more precisely, an anti-neutrino). Increasing the temperature
of the universe even more (traveling further back in time) causes the
break-up of neutrons and protons into quarks and the particles that ordinarily
keep the quarks bound (gluons).
Let us consider the universe at this early stage of its evolution, when
it is less than a millionth of a second old and is made up of photons
(electromagnetic radiation causing the high temperature), neutrinos, quarks,
electrons, anti-particles, and other particles that do not concern us
now. Running the clock forwards to one millionth of a second, we find
the universe cool enough so that the quarks coalesce into protons and
When the universe is older than one millionth of a second, an extremely
hot mixture (temperature of ten trillion Kelvin) of photons, neutrons
(n), protons (p), electrons (e), neutrinos, anti-electrons, and anti-neutrinos
participate in the reactions below.
More protons are created in
these reactions because they are slightly less heavy than neutrons. These
reactions occur until the expansion and cooling of the universe prevent
the last two. One second after the big bang, when the universe had cooled
to a temperature of less than 10 billion Kelvin, there were 6 times as
many protons as there were neutrons.
Two protons and two neutrons
are needed to make a helium nucleus, but the universe is still too hot
to form He. A four-particle collision is not how a He nucleus is formed,
however. He nuclei are formed from deuteron (a bound proton and neutron)
collisions, but the universe must cool down to a temperature of 1 billion
Kelvin for deuterons to form via the collision of a proton and neutron
to give a deuteron and a photon.
Helium can now form in either
one of the following two-step sequences.
While the universe was cooling
to form deuterium, the neutrons were decaying so that only 80% of the
initial number were left by the time of the deuteron collisions.
The mass fraction of helium
in the universe can now be calculated. Recall that a helium nucleus consists
of 2 protons and 2 neutrons. Therefore to get the number of total He nuclei,
divide the total number of neutrons by 2 (2 neutrons make 1 He nucleus,
4 neutrons make 2 He nuclei, etc.). And because the masses of the proton
and neutron are nearly the same, the mass of the He nucleus is approximately
4 times the mass of one neutron. Dividing this value by the mass of
all neutrons and protons gives the mass fraction of He in the universe.
This calculated value corresponds
closely to the observed value as shown in the graphs below (Izotov and Thuan, ApJ Letters, 710: L67-L71, 2010), another confirmation of the big-bang theory of the universe.