ow did the universe really begin? Most astronomers would say that the debate is now over: The universe started with a giant explosion, called the Big Bang. The big-bang theory got its start with the observations by Edwin Hubble that showed the universe to be expanding. If you imagine the history of the universe as a long-running movie, what happens when you show the movie in reverse? All the galaxies would move closer and closer together, until eventually they all get crushed together into one massive yet tiny sphere. It was just this sort of thinking that led to the concept of the Big Bang.

The Big Bang marks the instant at which the universe began, when space and time came into existence and all the matter in the cosmos started to expand. Amazingly, theorists have deduced the history of the universe dating back to just 10-43 second (10 million trillion trillion trillionths of a second) after the Big Bang. (Timeline) Before this time all four fundamental forces—gravity, electromagnetism, and the strong and weak nuclear forces—were unified, but physicists have yet to develop a workable theory that can describe these conditions.

During the first second or so of the universe, protons, neutrons, and electrons—the building blocks of atoms—formed when photons collided and converted their energy into mass, and the four forces split into their separate identities. The temperature of the universe also cooled during this time, from about 1032 (100 million trillion trillion) degrees to 10 billion degrees. Approximately three minutes after the Big Bang, when the temperature fell to a cool one billion degrees, protons and neutrons combined to form the nuclei of a few heavier elements, most notably helium.

The next major step didn’t take place until roughly 300,000 years after the Big Bang, when the universe had cooled to a not-quite comfortable 3000 degrees. At this temperature, electrons could combine with atomic nuclei to form neutral atoms. With no free electrons left to scatter photons of light, the universe became transparent to radiation. (It is this light that we see today as the cosmic background radiation.) Stars and galaxies began to form about one billion years following the Big Bang, and since then the universe has simply continued to grow larger and cooler, creating conditions conducive to life.

Three excellent reasons exist for believing in the big-bang theory. First, and most obvious, the universe is expanding. Second, the theory predicts that 25 percent of the total mass of the universe should be the helium that formed during the first few minutes, an amount that agrees with observations. Finally, and most convincing, is the presence of the cosmic background radiation. The big-bang theory predicted this remnant radiation, which now glows at a temperature just 3 degrees above absolute zero, well before radio astronomers chanced upon it.

The explosive beginning of our universe, the Big Bang marks the earliest time we can probe with current physical theory. Theory has to guide our understanding of the first fraction of a second, since we can’t recreate the extremely high temperatures that existed during the earliest history of the universe in any earthly laboratory. What theory tells us is that from an initial state in which matter and radiation are both in an extremely hot and dense form, the universe expands and the matter cools. At that time, it is believed that all four of the fundamental forces of nature—gravity, electromagnetism, and the strong and weak nuclear forces—were unified.

The evolution of the earliest universe is not well understood because it is not clear exactly what laws were at work. However, it is known that by the end of the first second of time, the building blocks of matter had formed. By the end of the first three minutes, helium and other light nuclei (like deuterium) had formed but for a long time, temperatures remained too high for the formation of most atoms. At around one million years following the Big Bang, nuclei and electrons were at low enough temperatures to coalesce to form atoms. But the universe didn’t start to look like it does today until small perturbations in the matter distribution were able to condense to form the stars and galaxies we know today.

History of the Big Bang Theory

After a stint as an artillery officer in the Belgian army during World War I, Georges Lemaître (1894-1966) entered a seminary and was ordained a priest in the early 1920s. Shortly after his interest in astronomy brought him to Cambridge University in England and then to the Massachusetts Institute of Technology. While there, he became captivated by the new idea of an expanding universe. He reasoned that if the universe was expanding now, then the further you go in the past, the universe’s contents must have been closer together. He envisioned that at some point in the distant past, all the matter in the universe was crushed into a single object he called the “primeval atom,” in which all the universe’s matter was crushed into a sphere only a few dozen times bigger than the Sun. This primeval atom then exploded into an incredible number of smaller pieces, which in turn kept splitting apart into ever smaller pieces until the atoms of the present universe formed. “The evolution of the world could be compared to a display of fireworks just ended—some few red wisps, ashes, and smoke. Standing on a well-cooled cinder we see the slow fading of the suns and we try to recall the vanished brilliance of the origin of the worlds.” His basic idea has become the most widely accepted model for how the universe originated, what we today call the Big Bang. Lemaître’s theory didn’t immediately win a lot of converts. Einstein, for one, remained unconvinced.

The first inkling that the “universe without beginning” might be a false concept came in the 1910s, when Albert Einstein (1879-1955) developed his general theory of relativity. The theory describes gravity not as a force acting at a distance in the sense Newton thought, but rather as a curvature in the fabric of spacetime. The equations of general relativity made a startling prediction—that the universe is in fact expanding. But hundreds of years of scientific thought had taught that the universe was static and unchanging. So Einstein modified his equations by introducing something he called a “cosmological constant,” which neatly brought the universe back into balance and left it static as he believed it to be.

It wasn’t until Edwin Hubble discovered that almost every galaxy was rushing away from Earth and that the farther away the galaxy, the faster it was receding that Einstein came around.  Although Lemaître’s conception of the Big Bang was prophetic in many ways, his idea about the initial state of the universe is almost the polar opposite of what we think today. Instead of a complex initial structure that broke apart to form the universe’s basic constituents, scientists now believe that the universe started out very simple and then grew more complex as it evolved

That idea first came from the Russian-born American physicist George Gamow in the 1940s. Working with his student Ralph Alpher, he envisioned the universe beginning with an extraordinarily hot Big Bang. As the universe expanded, this superhot primordial soup of protons, neutrons, electrons, and radiation grew steadily cooler, and the constituents began fusing into heavier elements. Helium formed first, followed by all of the heavier elements, with the process wrapping up within about half an hour.

Steady State Theory of the Universe

Not every scientist was happy with the idea that the universe was created in a Big Bang. To some, this suggested a creator as well as a creation; to others it simply ran counter to their beliefs in a universe that had always existed. Whatever the reason, many cosmologists searched for a theory of the universe’s formation that did not require a beginning.

Austrian-born scientists Thomas Gold and Hermann Bondi and British astrophysicist Fred Hoyle developed the most successful of these competing theories in 1948, at the same time Gamow was promoting the big-bang theory. Called the steady-state theory, it held that the universe had always existed and had always looked the way it does now. The theory was based on an extension of something called the perfect cosmological principle. This holds that the universe looks essentially the same from every spot in it and at every time. This applies only to the universe at large scales; obviously planets, stars, and galaxies are different from the space between them. Obviously, for the universe to look the same at all times, there could have been no beginning or no end. This struck a philosophical chord with a number of scientists, and the steady-state theory gained many adherents in the 1950s and 1960s. After helping to develop radar for the British effort in World War II, Fred Hoyle (1915-) turned his talents toward astrophysics and cosmology. Ironically, he coined the term “Big Bang” to describe the competing theory, while looking for a snappy, memorable phrase for a radio audience.

The three cosmologists accepted the observations that showed the universe to be expanding. They argued that matter was being created continuously to take the place of the matter in the receding galaxies. (Otherwise, the universe would become less dense as the galaxies moved away and occupied ever greater volumes).  Thus, by the time the distance between two galaxies doubled, enough material would have to be created to form a new galaxy. The rate needed was almost vanishingly small: a couple of atoms in every cubic mile every year, or about one-thousandth of an ounce in the entire volume of Earth during its whole history

For the steady-state model to work, however, it had to come up with an alternative way to make all the elements we see around us. The theory’s supporters looked to the interiors of stars for the chemical factories. And they found them. These stellar cauldrons, where temperatures soar to tens of millions of degrees, generate energy by constantly fusing hydrogen into helium. The energy produced, which eventually works its way to the surface and gets emitted as light, balances the inward pull of gravity to keep the star stable. Eventually the star runs out of hydrogen fuel, however, and gravity starts to crush the core, raising the star’s internal temperature. After a while, this triggers new fusion reactions, creating carbon, oxygen, and the other elements essential to life. For the largest stars, this process continues until iron forms in the core. But fusing iron requires more energy than it produces, so the reactions can no longer hold off the inward pull of gravity. The core then collapses, causing the star to explode and spew its heavy elements back into the galaxy, where they can be incorporated into future generations of stars, planets, and perhaps even life

The steady-state supporters had accomplished most of what they wanted. They had shown that most of the elements could be created in stars, an idea that all scientists today embrace. But they failed to account for helium—they couldn’t make anywhere near enough of it in stars to match the amount seen in the universe. Only the big-bang theory could do that. Ironically, the success of the steady-state supporters in explaining the origin of the elements ended up hurting their argument for the origin of the universe.  

The battle for the hearts and minds of cosmologists continued to rage through the 1950s and much of the 1960s. But as the 1960s wore on, observers started to pile up support for the Big Bang.  First, they noticed that far more quasars, the ultraluminous cores of young galaxies, and radio galaxies reside in the distant universe than nearby. Because light travels at a finite speed, we see objects far away as they were a long time ago, so these observations implied that the universe has changed over time. Even more significant, however, was the discovery of the cosmic background radiation by radio astronomers Arno Penzias and Robert Wilson. Predicted as a natural outgrowth of the hot Big Bang by George Gamow and his collaborators, this radiation represents the fading echo of the original “fireball” explosion. Just as important, the steady-state theory could not reasonably explain the radiation.

This was the observational proof the big-bang backers were looking for. Just a few years later,  Stephen Hawking provided a theoretical proof for the Big Bang. He showed that any expanding universe described by general relativity must begin with a singularity and, thus, a Big Bang. Fittingly, the originator of the big-bang hypothesis, Georges Lemaître, learned of the discovery of the cosmic background radiation shortly before his death in 1966.