This is the eighth article in a series on modern cosmology.
THE The cosmological model of the Big Bang says the Universe emerged from a single event in the distant past. The model was inspired by the adventurous idea of the cosmic quantum egg, which suggested that everything in existence was initially compressed into an unstable quantum state. When this single entity exploded and decomposed into fragments, it created space and time.
Taking this fanciful notion and creating a theory of the Universe has been quite a feat of creativity. To understand cosmic infancy, it turns out, we must invoke quantum physics, the physics of the very small.
The energy that binds
It all began in the mid-1940s with Russian-American physicist George Gamow. He knew that protons and neutrons are held together in the atomic nucleus by strong nuclear force, and that electrons are held in orbit around the nucleus by electrical attraction. The fact that the strong force doesn’t care about electric charge adds an interesting twist to nuclear physics. Since neutrons are electrically neutral, it is possible for a given element to have a different number of neutrons in its nucleus. For example, a hydrogen atom consists of one proton and one electron. But it is possible to add a neutron or two to its nucleus.
These heavier cousins of hydrogen are called isotopes. Deuterium has one proton and one neutron, while tritium has one proton and two neutrons. Each element has several isotopes, each constructed by adding or extracting neutrons in the nucleus. Gamow’s idea was that matter would form from the primordial substance that filled the space in the beginning. This happened progressively, building from the smallest objects to the largest ones. Protons and neutrons come together to form nuclei, then bond electrons to form complete atoms.
How do we synthesize deuterium? By fusing a proton and a neutron. What about tritium? By fusing an extra neutron to deuterium. What about helium? By fusing two protons and two neutrons, which can be done in a variety of ways. The accumulation continues as heavier and heavier elements are synthesized within the stars.
A fusion process releases energy, at least until the element iron is formed. This is called the binding energy, and is equal to the energy we need to supply a system of bonded particles to break a bond. Any system of particles bound by a force has an associated binding energy. A hydrogen atom consists of a bonded proton and an electron and has a specific binding energy. If I disturb the atom with energy that exceeds its binding energy, I will break the bond between the proton and electron, and they will then freely drift apart. This accumulation of heavier nuclei from smaller ones is called nucleosynthesis.
Universal cooking lessons
In 1947 Gamow enlisted the help of two collaborators. Ralph Alpher was a graduate student at George Washington University, while Robert Herman worked at the Johns Hopkins Applied Physics Laboratory. Over the next six years, the three researchers would develop the physics of the Big Bang model more or less as we know it today.
Gamow’s image begins with a Universe filled with protons, neutrons, and electrons. This is the matter component of the early Universe, which Alpher called ylem. Very energetic photons have been added to the mix, the heat component of the early Universe. The Universe was so hot at this initial moment that no bonding was possible. Every time a proton tried to bond with a neutron to create a deuterium nucleus, a photon raced to strike the two away from each other. The electrons, which are bonded to the protons by the much weaker electromagnetic force, didn’t stand a chance. There can be no bonding when it’s too hot. And we’re talking about some really hot temperatures here, about 1 trillion degrees Fahrenheit.
The image of a cosmic soup tends to emerge quite naturally when we describe these very early stages in the history of the universe. The building blocks of matter roamed freely, colliding with each other and with photons but never bonding to form nuclei or atoms. They acted a bit like vegetables floating in a hot minestrone. As the Big Bang model has evolved into its accepted form, the basic ingredients of this cosmic soup have changed slightly, but the fundamental recipe hasn’t.
The structure has begun to emerge. The hierarchical grouping of matter progressed steadily as the Universe expanded and cooled. As the temperature dropped and the photons became less energetic, nuclear bonds between protons and neutrons became possible. An era known as primordial nucleosynthesis began. This time saw the formation of deuterium and tritium; helium and its isotope helium-3; and an isotope of lithium, lithium-7. The lightest nuclei were fired in the first moments of the existence of the Universe.
According to Gamow and co-workers, this all took about 45 minutes. Taking into account more modern values given at various nuclear reaction rates, it only took about three minutes. The remarkable feat of Gamow, Alpher and Herman’s theory was that they could predict the abundance of these light nuclei. Using relativistic cosmology and nuclear physics, they could tell us how much helium should have been synthesized in the early Universe: It turns out that about 24 percent of the Universe is made of helium. Their predictions could then be compared with what was produced in the stars and compared with observations.
Gamow then made a far more dramatic prediction. After the era of nucleosynthesis, the ingredients of the cosmic soup were mainly light nuclei as well as electrons, photons and neutrinos, very important particles in radioactive decay. The next step in the hierarchical grouping of matter is to create atoms. As the Universe expanded, it cooled and the photons became progressively less energetic. At some point, when the Universe was about 400,000 years old, the conditions were ripe for electrons to bond with protons and create hydrogen atoms.
Before that, every time a proton and an electron tried to bond, a photon kicked them apart, in a sort of unhappy love triangle with no resolution. When the photons cooled to about 6,000 degrees Fahrenheit, the attraction between the protons and electrons overcame the photon interference, and eventually bonding occurred. The photons were suddenly free to move, following their dance across the Universe. They no longer had to interfere with the atoms, but to exist by themselves, impervious to all this bond that seems to be so important for matter.
Gamow realized that these photons would have a special frequency distribution known as a black body spectrum. The temperature was high at the time of decoupling, that is, at a time when atoms were forming and photons were free to wander through the Universe. But since the Universe has been expanding and cooling for about 14 billion years, the current temperature of photons would be very low.
Previous predictions were not very accurate, as this temperature is sensitive to aspects of nuclear reactions that were not accurately understood in the late 1940s. However, in 1948 Alpher and Herman predicted that this cosmic photon bath would have a temperature of 5 degrees above absolute zero, or about -451 degrees Fahrenheit. The current value is 2.73 Kelvin. So, according to the Big Bang model, the Universe is a giant black body, immersed in a bath of very cold photons peaking at microwave wavelengths – the so-called fossil rays – since its warm early infancy. In 1965, this radiation was discovered by accident and cosmology would never be the same. But that story deserves a separate essay.