The Biggest Bang of Them All

The Mother of All Bangs

Can science even hope to answer these questions? Most cosmologists think so. Over the past 70 years, they have constructed and tested a theory that seems to explain the major properties of the universe: the Big Bang theory. The theory is based on Albert Einstein's general theory of relativity, one of the advances in physics during the early part of the century which provided the intellectual basis for modern cosmology.

Einstein's theory involves equations that scientists can solve to describe the evolution of the universe. One of the possible solutions indicates that the universe was born. At the instant of birth, everything was concentrated in one tiny point ­ and that means everything: all the matter, all the radiation, all the energy we see today. Not surprisingly, the temperature at that point was extremely high. The universe started to expand rapidly, scattering its contents equally in all directions over larger and larger distances. Because this sounds a lot like an explosion, it has been dubbed the Big Bang.

The theory has two basic ideas: The universe started out infinitely small and hot several billion years ago, and it has been expanding and cooling ever since. But the theory does not say what caused this expansion. It does not say how stars and galaxies formed. And it does not predict how much matter the universe contains or what form it is in.

This is an important point: The Big Bang theory allows for many different scenarios of the detailed evolution and composition of the universe. It is simply the foundation on which specific cosmological models are built. You could compare it to making vegetable soup. You know what you have to do: cook vegetables, crush them, add water, and warm up the whole thing. But different people use different vegetables, or different amounts of water, or different cooking times. They end up with soups that taste different, but more or less match the one pictured in their cookbook.

Similarly, cosmologists use different recipes based on the Big Bang. They adjust their equations to create models of the universe, which they then compare to what they see in their guidebook: the sky. If the comparison fails, it does not mean that the whole Big Bang theory is wrong ­ merely that they did not use the correct recipe, in which case they must change their recipe and see whether the new one tastes any better. Cosmologists may argue about their specific models, but few these days question the Big Bang itself.

For the Big Bang to be proved wrong, astronomers would have to observe a phenomenon that contradicts one of the truly basic ideas. That would happen if, for example, the distribution of galaxies were found not to be homogeneous, or if a star were confirmed to be older than the universe. Such problems have been raised, but never confirmed. Over the years, the three key pieces of evidence for the Big Bang have only grown more compelling. One explains how the chemical elements were created; one explains how fast the universe is growing; and one lets us see the bang itself.

How to Make an Element

Nucleosynthesis describes how the cores of atoms (nuclei) are formed (synthesized) in the universe. There are two types of nucleosynthesis. One took place very early in the history of the universe (during the first three minutes) and is therefore called primordial nucleosynthesis. The other type, stellar nucleosynthesis, is an ongoing process inside stars such as our Sun.

It was the theory of primordial nucleosynthesis that first put the Big Bang theory on solid footing. Primordial nucleosynthesis is a merger of Big Bang theory and high-energy particle physics. The Big Bang theory tells us the conditions that existed in the early universe and how those conditions changed with time. Particle accelerators can reproduce those conditions, or at least come close. It turns out that certain nuclear reactions occurred at different stages in the evolution of the universe.

Initially the universe was a dense soup of the most elementary subatomic particles, known as quarks. There were no atomic nuclei yet, not even the building blocks of nuclei, protons and neutrons. As the universe cooled down, the quarks clumped together to form protons and neutrons. Because the proton is the only component of the nucleus of the hydrogen atom, hydrogen was the first element created in the universe. Later nuclear reactions mixed protons and neutrons, producing helium and a smidgen of lithium.

These were the three primordial elements, and they are the lightest in the periodic table. When you drink a glass of water, you are swallowing hydrogen atoms that are as old as the universe itself. In addition, primordial nucleosynthesis produced a fourth atomic nucleus: deuterium, a form of hydrogen which contains a neutron in addition to the proton.

All the other elements ­ from beryllium to uranium ­ did not exist until a few billion years later, when stars got into the nucleosynthesis act. Stars do not produce any deuterium, but they do create some additional helium by burning hydrogen. This means that all the deuterium and most of the helium we see today comes from the birth of the universe.

The theory makes specific predictions for the amount of the elements we should see in the universe. Moreover, the theory predicts those amounts for all the elements at once, whereas the observations for each element are independent. If all the observed amounts agreed with theory except for one, the whole theory would have to be rejected. Yet all those independent measurements agree ­ a very strong case that the theory is correct.

You can think of this as a mini jigsaw puzzle. If you make even one mistake as you assemble the puzzle, you will end up with at least one piece that does not fit, and you'll have to start all over again. Conversely, if all the pieces fit together and none are left over, you can be pretty confident that you got it right. Here, the pieces are the measurements of element abundances. They all fit together.

What do cosmologists mean by "the universe"? Obviously, the universe consists of everything there is. But to study "everything" would be far too complicated, so cosmologists study the universe as a unit ­ just as a doctor studies your body as a unit without thinking about all the atoms inside it. If you stand on a hill and look out at the view, you can see things at various distances: grass, trees, buildings, planets, stars, galaxies. By drawing your attention to different distances, you can place things into groups and treat each group as a single unit: lawn, forest, city, solar system, galaxy, cluster of galaxies. The clusters of galaxies comprise the largest unit of all: the universe. The most distant clusters are so far away than it has taken almost the age of the universe for their light to reach us. Diagram by Kathleen L. Blakeslee for the ASP.

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