Think of the Big Bang, and you probably imagine a moment in time when matter, energy and space itself all burst into existence at once. Yet many astrophysicists now believe that the “Big Bang” was actually two distinct events: first the inaugural instant of space and time, and second the generation of most of the “stuff” that populates that space. So, which really deserves to be called the Big Bang?
Ambiguity has plagued the expression “Big Bang” since its origin. When British astronomer Fred Hoyle coined it during a radio interview in 1948, he meant it as the ultimate put down. Hoyle refused to believe that the universe had a beginning, a first moment of time and a genesis of all matter and energy. Rather, he thought that the cosmos maintained itself in a “steady state” through a slow trickle of particles into reality. He hypothesized a “creation field” that would gradually generate new matter to fill the gaps between galaxies moving away from each other, keeping the overall density of the universe the same.
Hoyle had a point: Science rightly eschews processes without clear mechanisms, and at the time, no one could explain how the whole cosmos could emerge instantly out of nothing. Nevertheless, his alternative steady state approach could not match up with mounting evidence that the observable universe was once extremely hot and dense. In 1964, Bell Laboratory researchers Arno Penzias and Robert Wilson found the smoking gun: cooled-down radiation from the early universe, known as the cosmic microwave background (CMB), which convinced almost everyone in the scientific community that the universe started with a bang.
The discovery of the CMB was a victory for the Big Bang theory, yet it also presented a puzzle: The radiation was strikingly uniform across the sky. While later studies identified tiny variations of less than one part in 10,000, the standard Big Bang model couldn’t justify such uniformity. There simply wasn’t enough time in early cosmic history, when the universe was small, for energy to have traveled across space and evened out its temperature.
Enter the brilliant concept of inflation, proposed in 1981 by Alan Guth, and later modified by Andrei Linde, Paul Steinhardt, Andreas Albrecht, and others. Guth realized that a sudden, ultra-rapid stretching of the universe could take a tiny uniform patch and expand it to a size where it ultimately would grow and become the observable universe. During the fleeting instant of inflation, any irregularities in the primordial cosmos would be propelled beyond detection, offering a kind of blank slate. It is like taking a crinkled tablecloth and stretching it out so quickly that it appears flat on a tabletop and any wrinkles left are off the table and out of view. Only tiny, jiggling quantum fluctuations would disturb the uniformity; these fluctuations would be the seeds of the galaxies and galaxy clusters we see today.
Inflation solved critical problems in cosmology, but it also split the Big Bang into distinct phases: In the inflationary portrait, the creation of almost all of the matter and energy in the universe takes place at the close of the inflationary period, through a process called “reheating,” rather than before inflation. Reheating involves a massive release of energy from inflation’s driving engine: an entity called the “inflaton,” thought to be a fluctuating energy field that ignited ultra-rapid cosmic expansion. Theorists think that at the end of inflation, the inflaton field released an enormous reservoir of potential energy into space—which, following Einstein’s famous equivalence between energy and mass, converted into a deluge of particles. Before then, because stretching causes cooling, the universe was actually relatively cold. As the cosmos rapidly expanded, its hot initial temperature dropped by a factor of many thousand (the precise amount depends on the particular model), becoming extraordinarily hot only after reheating. If you feel that an event should be fiery if it’s going to be called the “Big Bang,” then reheating, not the cosmic dawn, was the true “bang.” (Max Tegmark has made this case in a recent blog post.)
So, science is faced with a dilemma. Which moment should the term “Big Bang” refer to: cosmic genesis, when space emerged, or the time of reheating, when most of the matter and energy was created? If the Big Bang denotes post-inflationary reheating, then we’ll have to talk about pre-Big Bang cosmology, which makes cosmologists squirm. Also, reheating occurred at slightly different times in various parts of the universe, making it more of a process than a sudden burst. On the other hand, if the Big Bang refers to the first moment of time, it was not much of a “bang.” That’s because the energy fields created then wouldn’t have been very hot.
Furthermore, in some versions of inflation, the seeds of rapid growth developed from the loam of an eternal cosmos. In that case, there could have been no initial moment of creation.
Perhaps it is time to replace the expression “Big Bang” with more precise nomenclature. Could there be a better way of distinguishing the idea of an initial cosmic moment from the concept that a field caused sudden expansion that tapered off and led to the generation of matter and structure?
Unfortunately, past attempts to find a replacement term for “Big Bang” have met with failure. Most famously, in 1993 Sky and Telescope magazine sponsored a contest to identify a new name. Astronomer and science writer Timothy Ferris, who called for a better designation, served as one of the judges. Ferris said in an interview that the term was “inappropriately bellicose” and unsuitable for conveying “the event thought to have spawned the starry skies.”
Despite more than 13,000 submissions from participants in 41 countries, including colorful suggestions such as “The Grand Expansion,” “The Hubble Bubble,” and “The First Fireball,” the contest was declared a bust. The judges did not find any of the entries as simple and evocative as “Big Bang.” Consequently, they announced that nobody had won and that the old name would remain.
More than 20 years later, “Big Bang” still doesn’t seem quite right, especially as cosmology has declared the nascent history of the cosmos more complex than once believed. Perhaps as our picture of the early universe continues to become more refined, new names will emerge to mark the complexity of its initial development. Just as we talk about an ovum, zygote, embryo and fetus in the process of human development, maybe we will someday have precise terms for early phases in the emergence of space, time, energy and matter.
Editor’s picks for further reading
The Inflationary Universe
Alan Guth’s accessible “diary” of the birth of inflation theory.
Wikipedia: Chronology of the universe
10150 years of cosmic history, from the inflationary epoch to the heat death of the universe, all on one Wikipedia page.