II. The Cosmic Origin of the Elements|
A number of different processes and events in the Universe contribute to the formation of the elements. Figure 1 links the elements with their predominant formation mechanisms. The following sections discuss the different ways the elements arise.
A. The Big Bang
Most astronomers today theorize that the Universe as we know it started from a massive "explosion" called the Big Bang. Evidence leading to this unusual theory was first discovered in 1929, when Dr. Edwin Hubble had made a startling announcement that he had found that all of the distant galaxies in the universe were moving away from us. In addition, their speed was directly proportional to their distance from us - the further away they were, the faster they were moving from us. Dr. Hubble's data implied that every galaxy was, on average, moving away from every other galaxy because the Universe itself was expanding and carrying the galaxies with it.
An expanding Universe also suggested that earlier in time the Universe was smaller and denser. With Dr. Edwin Hubble's data, scientists could measure how fast the Universe was expanding. Turning that around, they could calculate how much smaller the Universe was long ago. Scientists have traced the expansion back to a time when the entire Universe was smaller than an atom.
The early Universe contained what would become all the matter and energy we see today. However, since it all existed in such a small space, the Universe was very, very dense. This meant that the temperature was also incredibly high - over 1032 Kelvin. The familiar matter we know today didn't exist, because the atoms, protons, neutrons, and electrons all would have been crushed by the incredible density and temperature. The Universe was a "soup" of matter and energy. The Big Bang theory describes how the Universe expanded from this tiny dot, and how the first elements formed. The "Big Bang" is the moment the expansion of the Universe began.
Within the first second after the Big Bang, the temperature had fallen considerably, but was still very hot - about 100 billion Kelvin (1011 K). At this temperature, protons, electrons and neutrons had formed, but they moved with too much energy to form atoms. Even protons and neutrons had so much energy that they bounced off each other. However, neutrons were being created and destroyed as a result of interactions between protons and electrons. There was enough energy that the protons and the much lighter electrons combined together with enough force to form neutrons. But some neutrons "decayed" back into a positive proton and a negative electron1.
As the Universe expanded, the temperature fell. At this point the protons and electrons no longer had enough energy to collide to form neutrons. Thus, the number of protons and neutrons in the Universe stabilized, with protons outnumbering neutrons by 7:1. At about 100 seconds after the Big Bang, the temperature had fallen to one billion degrees Kelvin (109 K). At this temperature the neutrons and protons could now hit each other and stick together. The first atomic nuclei formed at this point. These neutron-proton pairs formed the nuclei of deuterium, a type of hydrogen with an extra neutron. Deuterium nuclei occasionally collided at great speed to form a helium nucleus. On rare occasions there were enough collisions of the deuterium to form lithium. Due to the ongoing expansion of the Universe, the temperature continued to fall rapidly, and soon it was too cool for further nuclei to form. At this point, the Universe was a little more than a few minutes old, and consisted of three elements: hydrogen, helium, and lithium. The high number of protons in the early Universe made hydrogen by far the dominant element: 95% percent of the atoms in the Universe were hydrogen, 5% were helium, and trace amounts were lithium. These were the only elements formed within the first minutes after the Big Bang.
Recommended Activity: Kinesthetic Big Bang
1 A tiny, neutral particle called a "neutrino" is also produced, but it doesn't interact with other matter much. In our discussion of the elements, we'll generally ignore neutrinos.
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