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.

The Importance of Units

Artist’s conception of the Martian Climate Orbiter.

In December 1998, NASA launched the 125-million dollar Mars Climate Orbiter, intended as the red planet’s first weather satellite. After a 416-million mi journey, the spacecraft was supposed to go into Mars’ orbit on September 23, 1999. Instead, it entered Mars’ atmosphere about 100 km (62 mi) lower than planned and was destroyed by heat. The mission controllers said the loss of the spacecraft was due to the failure to convert English measurement units into metric units in the navigation software.

Engineers at Lockheed Martin Corporation who built the spacecraft specified its thrust in pounds, which is an English unit. Scientists at NASA’s Jet Propulsion Laboratory, on the other hand, had assumed that thrust data they received were expressed in metric units, as newtons. Normally, pound is the unit for mass. Expressed as a unit for force, however, 1 lb is the force due to gravitational attraction on an object of that mass. To carry out the conversion between pound and newton, we start with 1 lb = 0.4536 kg and from Newton’s second law of motion,

force    = mass ˣ acceleration

= 0.4536 kg ˣ 9.81 m/s²

= 4.45 kg m/s²

= 4.45 N

because 1 newton (N) = 1 kg m/s². Therefore, instead of converting one pound of force to 4.45 N, the scientists treated it as 1 N. The considerably smaller engine thrust expressed in newtons resulted in a lower orbit and the ultimate destruction of the spacecraft. Commenting on the failure of the Mars mission, one scientist said: “This is going to be the cautionary tale that will be embedded into introduction to the metric system in elementary school, high school, and college science courses till the end of time.”

Distribution of Elements on Earth and in Living Systems

The majority of elements are naturally occurring. How are these elements distributed on Earth, and which are essential to living systems?

Earth’s crust extends from the surface to a depth of about 40 km (about 25 mi). Because of technical difficulties, scientists have not been able to study the inner portions of Earth as easily as the crust. Nevertheless, it is believed that there is a solid core consisting mostly of iron at the center of Earth. Surrounding the core is a layer called the mantle, which consists of hot fluid containing iron, carbon, silicon, and sulfur.

Of the 83 elements that are found in nature, 12 make up 99.7 percent of Earth’s crust by mass. They are, in decreasing order of natural abundance, oxygen (O), silicon (Si), aluminum (Al), iron (Fe), calcium (Ca), magnesium (Mg), sodium (Na), potassium (K), titanium (Ti), hydrogen (H), phosphorus (P), and manganese (Mn). In discussing the natural abundance of the elements, we should keep in mind that (1) the elements are not evenly distributed throughout Earth’s crust, and (2) most elements occur in combined forms. These facts provide the basis for most methods of obtaining pure elements from their compounds, as we will see in later chapters.

The accompanying table lists the essential elements in the human body. Of special interest are the trace elements, such as iron (Fe), copper (Cu), zinc (Zn), iodine (I), and cobalt (Co), which together make up about 0.1 percent of the body’s mass. These elements are necessary for biological functions such as growth, transport of oxygen for metabolism, and defense against disease. There is a delicate balance in the amounts of these elements in our bodies. Too much or too little over an extended period of time can lead to serious illness, retardation, or even death.

Structure of Earth’s interior.

(a) Natural abundance of the elements in percent by mass. For example, oxygen’s abundance is 45.5 percent. This means that in a 100-g sample of Earth’s crust there are, on the average, 45.5 g of the element oxygen. (b) Abundance of elements in the human body in percent by mass.