Primordial Helium and the Big Bang Theory

Where did we come from? How did the universe begin? Humans have asked these questions for as long as we have been able to think. The search for answers provides an example of the scientific method.

In the 1940s the Russian-American physicist George Gamow hypothesized that our universe burst into being billions of years ago in a gigantic explosion, or Big Bang. In its earliest moments, the universe occupied a tiny volume and was unimaginably hot. This blistering fireball of radiation mixed with microscopic particles of matter gradually cooled enough for atoms to form. Under the influence of gravity, these atoms clumped together to make billions of galaxies including our own Milky Way Galaxy.

Gamow’s idea is interesting and highly provocative. It has been tested experimentally in a number of ways. First, measurements showed that the universe is expanding; that is, galaxies are all moving away from one another at high speeds. This fact is consistent with the universe’s explosive birth. By imagining the expansion running backward, like a movie in reverse, astronomers have deduced that the universe was born about 13 billion years ago. The second observation that supports Gamow’s hypothesis is the detection of cosmic background radiation. Over billions of years, the searingly hot universe has cooled down to a mere 3 K (or 2270°C)! At this temperature, most energy is in the microwave region. Because the Big Bang would have occurred simultaneously throughout the tiny volume of the forming universe, the radiation it generated should have filled the entire universe. Thus, the radiation should be the same in any direction that we observe. Indeed, the microwave signals recorded by astronomers are independent of direction.

A color photo of some distant galaxy, including the position of a quasar.

The third piece of evidence supporting Gamow’s hypothesis is the discovery of primordial helium. Scientists believe that helium and hydrogen (the lightest elements) were the first elements formed in the early stages of cosmic evolution. (The heavier elements, like carbon, nitrogen, and oxygen, are thought to have originated later via nuclear reactions involving hydrogen and helium in the center of stars.) If so, a diffuse gas of hydrogen and helium would have spread through the early universe before many of the galaxies formed. In 1995, astronomers analyzed ultraviolet light from a distant quasar (a strong source of light and radio signals that is thought to be an exploding galaxy at the edge of the universe) and found that some of the light was absorbed by helium atoms on the way to Earth. Because this particular quasar is more than 10 billion light-years away (a light-year is the distance traveled by light in a year), the light reaching Earth reveals events that took place 10 billion years ago. Why wasn’t the more abundant hydrogen detected? A hydrogen atom has only one electron, which is stripped by the light from a quasar in a process known as ionization. Ionized hydrogen atoms cannot absorb any of the quasar’s light. A helium atom, on the other hand, has two electrons. Radiation may strip a helium atom of one electron, but not always both. Singly ionized helium atoms can still absorb light and are therefore detectable.

Proponents of Gamow’s explanation rejoiced at the detection of helium in the far reaches of the universe. In recognition of all the supporting evidence, scientists now refer to Gamow’s hypothesis as the Big Bang theory.