Abundances of the Elements

Abundances of the Elements

An additional test of the Big Bang theory includes the abundance of elements in the universe.
We observe that matter in the universe is predominantly Hydrogen atoms. Even though
Hydrogen is not the earth’s most abundant element by mass, over 99% of our solar system’s
mass is contained in the Sun, which is 73% Hydrogen, 25% Helium, and around 2% heavier
elements by weight. We find this is approximately true of nearly all stars, except for the remnant
cores of extinct stars. And since stars form the predominant source of matter in the universe, the
universe is thus mostly Hydrogen and only one-quarter Helium. Amazingly, the remarkable
events of the early universe explain this ratio quite accurately. Early in the expansion, not even
protons and neutrons could form, due to the incredibly hot temperatures and rapid collisions of
the fundamental particles from which they are composed. But the cooling induced by the
expansion would then allow protons and neutrons to form without being subsequently destroyed.
Neutrons are unstable by themselves, decaying after a few minutes into a proton, an electron, and
a neutrino (a ghostly particle very weakly interacting with matter). Therefore, if all matter in the
early universe consisted only of protons, neutrons, and electrons, the resulting decay of neutrons
would have left only Hydrogen atoms after each proton eventually captured an electron.
What took place concurrent with the neutrons gradually decaying away was a continued cooling
until a proton could combine with a neutron long enough to form a Deuterium nucleus, which is
stable. Indeed, neutrons are quite stable inside certain nuclei. A Deuterium nucleus would then
fuse together with another Deuterium to form a Helium nucleus, which is very stable. This
process of nuclear fusion is similar to what is generating energy inside our Sun today. It is
possible to calculate how much matter should have been converted into Helium, given the
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lifetime of the neutron and the time it took for the expanding universe to cool down until it
became too cool to fuse protons and neutrons into Helium nuclei. This period ended only three
minutes after the initial explosion that began the expansion [5]. Very little elements heavier than
Helium should have formed in the Big Bang explosion since even higher temperatures are
needed, and the universe was cooling off. Leftover neutrons, not having fused into Helium
nuclei, would then have decayed into protons, eventually forming Hydrogen atoms. Only a very
small amount of Deuterium would have survived since it is very weakly held together and would
have continued to be knocked apart long after it was too cool to fuse into Helium. The resulting
expectation is that the matter in the universe should be approximately 75% Hydrogen and 25%
Helium, almost exactly what we observe today. This calculation was first carried out in 1948 by
Ralph Alpher, a colleague of Gamov [19].
If the universal expansion rate had been slightly greater, not much helium would have been
generated since the rapid cooling would have left little time for Helium nuclei to form before it
became too cool to induce fusion. In this case nearly all the matter in the universe would have
become Hydrogen atoms. If the expansion rate had been slightly less, there would have been
sufficient time for most of the protons and neutrons to fuse into heavier elements. Either way,
the universe would have been much different than it is today. Remarkably, by using the known
expansion rate along with well-established nuclear physics reactions, we find that the universe
should be mostly Hydrogen, some Helium, with a small fraction of heavier elements. It has now
been established that the 2% heavier elements, of which our earth primarily consists of, has been
primarily generated in later fusion processes inside stars, long after the initial explosion of the
universe. Our observations match precisely what is predicted. This constitutes a very strong
support for the Big Bang theory.
The Big Bang theory has remarkably predicted what we see in the universe today, from the
universal expansion to the Cosmic Microwave Background Radiation with its slight “ripples”, to
the abundance of elements we observe in the universe. While additional tests continue to be
made, the Big Bang theory has become recognized as a highly successful theory with impressive
predictive power. It meets the criteria for good science, all the more so because of the scrutiny it
has been subjected to and withstood. We now examine in the next chapter how the investigation
of supposed problems has only served to further vindicate the theory.