Great Discoveries in Science: The Big Bang Theory

As scientists studied the origins of the universe, their cosmological research focused on three major pillars: the expanding universe, nucleosynthesis, and the cosmic microwave background radiation. Today, all three give strong support for the big bang model while also providing clarity into how the universe evolved over time.

“The whole story of the world need not have been written down in the first quantum like a song on the disc of a phonograph. The whole matter of the world must have been present at the beginning, but the story it has to tell may be written step by step.”

In the 1920s, two academics independently worked through Einstein’s general relativity equations and found that the solutions suggested an expanding universe. One was Alexander Friedmann, whose work in the field was cut short by his untimely death from typhoid fever in 1925. The other was Georges Lemaître, who went on to become the first scientist to propose a theory of an expanding universe with a discrete beginning. Today, Lemaître is known as the father of the big bang theory.

Lemaître was an avid scholar of general relativity and studied under one of its foremost experts, Sir Arthur Eddington, at Cambridge University in England. Lemaître began writing about an expanding universe in the 1920s. In the early 1930s, he added the concept of a discrete origin to his theory.

In a 1931 letter published in the journal Nature, Lemaître began by writing, “Sir Arthur Eddington states that, philosophically, the notion of a beginning of the present order of Nature is repugnant to him …” For many scientists like Eddington any cosmology with a finite beginning had too much of a creation narrative, which harkened back to mythology and supernatural forces, to be scientifically acceptable.

To Lemaître, the notion of a beginning of the universe was not only quite acceptable, it was the logical conclusion from quantum theory. If there was a constant total amount of energy in the universe and that number of distinct quanta were increasing, as theory held, then the implication must be that there were once much fewer quanta, perhaps a single quantum, that held all of the energy in the universe.

In the letter to Nature, Lemaître suggested the possibility of a single unique radioactive atom that held all the mass in the universe before decaying into smaller and smaller atoms.

“The last two thousand million years are slow evolution: they are ashes and smoke of bright but very rapid fireworks,” he wrote in a paper called “The Evolution of the Universe.” In a later text, The Primeval Atom, he wrote: “We can compare space-time to an open, conic cup … The bottom of the cup is the origin of atomic disintegration; it is the first instant at the bottom of space-time, the now which has no yesterday because, yesterday, there was no space.”

Lemaître was the first physicist to propose a widely discussed model of cosmology with a finite beginning and expansion from a single atom. However, his theory of cosmology wasn’t the big bang we think of today, which involves an explosion of pure energy that converts into all of the matter in the known universe. Lemaître’s model was a colder, disintegrating model of the universe. The hot big bang model that we know today arrived seventeen years later.

Lemaître worked on his cosmology in the years between World War I and World War II. World War II temporarily interrupted astrophysics as it diverted many top physicists to war projects and isolated others. Lemaître himself was cut off in Belgium after the Germans invaded, and he nearly died in an Allied forces’ bombing of his apartment building.

After the war, Lemaître turned his focus to other scientific pursuits, including mathematical computing. Einstein turned his attention to finding a unified field theory that would unite general relativity with quantum mechanics. Sir Arthur Eddington died in 1944. One generation of scientists stepped away from the cosmological question; a new generation of scientists stepped up.

Roughly a decade after Lemaître proposed a primeval atom that decayed into all the matter in the universe, George Gamow and Ralph Alpher published a paper detailing the foundation of the modern big bang theory.

To be successful, big bang cosmologists had to explain how elements heavier than lithium formed in the universe.

Gamow’s early work included studying radioactivity and stellar physics. When World War II broke out, Gamow had plenty of time to focus on the implications of nuclear physics on cosmology. While other American scientists were drafted to support the war effort, Gamow was left out because he had briefly served in the Red Army before fleeing Ukraine.

Years later, as a professor at George Washington University, Gamow and his doctoral student Ralph Alpher published a paper outlining their theory of the beginning of the universe and synthesis of matter. The physicists Carl von Weizsäcker and Hans Bethe had both showed how stars convert hydrogen into helium through what is called the carbon-nitrogen- oxygen cycle, but at the time no physicist could explain how heavier elements, or even carbon, were formed within stars. Gamow believed they could have formed in the beginning of the universe.

Gamow and Alpher proposed that the universe did not begin as a single super atom but as hot, highly compressed neutron gas that underwent a rapid expansion and cooling. The initial primordial matter decayed into protons and electrons as the gas pressure dropped due to the expansion. (Gas pressure is a function of molecular collisions. As the gas density of the early universe decreased, the particles collided less frequently and the pressure dropped.)

This began a process called big bang nucleosynthesis in which protons “captured” neutrons to form deuterium (an isotope of hydrogen). Neutron capture continued and formed heavier and heavier elements by adding one neutron and one proton at a time.

The relative abundance of elements was determined by the time allowed by the universe’s expansion (that is, the time in which the universe had the right conditions for nucleosynthesis to proceed). This capped window of time, Gamow and Alpher believed, explained why light hydrogen was so prevalent and heavy elements like gold so rare. Their 1948 paper contained a model for only the abundances of hydrogen and helium, but as these two elements account for 99 percent of the atoms in the universe, it was enough to make their paper credible.

In Alpher’s PhD thesis, he wrote that the nucleosynthesis of hydrogen and helium took just three hundred seconds.

Over time, large stars fuse successively heavier elements, creating “shells” of different elements that are eventually released into the universe.

Alpher’s calculations showed that there should be about ten hydrogen nuclei for every helium nucleus at the end of the big bang, which matches modern observed abundances and lent further support to the model.

In another 1948 paper, Alpher and his coauthor Robert Herman calculated that the radiation from the beginning of the universe should today be about 5 degrees K. This prediction provided a way to test the theory and provide strong supporting evidence for its validity.

Gamow and Alpher’s work created a buzz in the scientific community because it explained the origin of the most abundant elements and provided a compelling narrative of the big bang. Their work created the basic model of the big bang theory we know today.

There were problems with Gamow and Alpher’s work, however, namely that elements cannot be formed through neutron capture because there is no stable nucleus with five neutrons. Big bang nucleosynthesis cannot proceed past lithium, and the heavier elements must be created through some other process.

In just a few decades, however, this issue was resolved. In 1957, Fred Hoyle and the American physicist Willy Fowler at Caltech showed that hydrogen nuclei could fuse into helium nuclei and subsequently into beryllium, carbon, oxygen, and so on inside of stars. In Gamow and Alpher’s big bang nucleosynthesis model, this chain had broken down. In Hoyle and Fowler’s stellar nucleosynthesis model, however, it worked—so long as a special state of carbon-12 (the most common isotope of carbon) existed. Fowler was able to create this excited state of carbon-12 in the lab, proving that the chain worked.

Hoyle and Fowler’s results showed that the lightest elements could have been created in the big bang and subsequent element formation could have continued inside of stars. With this new, stronger model of nucleosynthesis, the big bang model gained strength.

One other roadblock that had to be cleared before the big bang model was embraced by the scientific community was the age of the universe.

After Hubble’s discovery of the expanding universe, astronomers used his measurements to calculate the age of the universe. Galaxies move away from each other at a velocity represented by v = H₀ x D. V is the observed velocity of the galaxy as it moves away from us, D is the distance to the galaxy, and H is the Hubble constant. The Hubble constant represents the expansion rate of the universe, and Hubble’s 1929 estimate of this value was about 500 kilometers per second per megaparsec (Mpc). (Parsecs are measurements of distance in astronomy. One parsec is 3.26 light-years long, and one megaparsec is 3.26 million light-years long.)

The Hubble constant can be used to infer the age of the galaxy. If the universe had been expanding at a rate of 500 km/s/Mpc to present day, the universe was about 1.8 billion years old. However, geologists had shown through examinations of radioactive rocks that Earth was older than 1.8 billion years, and it was assumed that stars were even older than our planet. This timescale difficulty, as it was called, was a major flaw in the big bang models proposed by Lemaître and Gamow.

It turned out, however, that Hubble’s measurements weren’t entirely accurate. The German astronomer Walter Baade discovered that there were two major types of Cepheid variable stars, which Hubble didn’t know when he used Cepheid variables to calculate the distance to the Andromeda galaxy. The younger Population I stars are hotter, brighter, and bluer than the older Population II stars. Hubble had observed Population I Cepheid variable stars in Andromeda but mistook them for dimmer Population II stars. He saw a relatively bright star and, with the dimmer stars in mind, thought it must be much closer than it really was.

Baade recalculated the distance to Andromeda using the knowledge of both types of Cepheid variables. His new calculation showed that Andromeda was twice as far away as previously thought. It also opened up a new look at the big bang model’s timeline: if the recession speeds remained the same but the distances doubled, the age of the universe was now around 3.6 billion years. Baade formally announced his results in 1952, just four years after Gamow and Alpher published their first paper on big bang nucleosynthesis.

This was much better for the big bang model as it allowed for a universe that was older than Earth, but it wasn’t yet a complete success. There were other elements of the universe thought to be older than 3.6 billion years.

Baade’s student Allan Sandage took on the task of measuring the distances to the farthest galaxies. Previously, due to technological limitations, astronomers had to use a variety of assumptions to measure the distance to very far-off galaxies. One of those assumptions rested on finding the brightest star in a faraway galaxy. By comparing its apparent (observed) brightness to the apparent brightness of the brightest stars in a closer galaxy, astronomers could come up with a rough estimate of how far away the distant galaxy was. However, Sandage showed that what astronomers thought was the brightest star was actually often an enormous, very luminous cloud of hydrogen gas.

That meant that the actual brightest star in the distant galaxies was much dimmer than was previously known and the galaxies were much farther off then previously calculated. Sandage revised the age of the universe to first 5.5 billion years in 1954 and eventually to an age between 10 billion and 20 billion years.

The new timeline allowed for all of the planets, stars, and galaxies to form and thus made the big bang model compatible with observations of the universe.

Today, the age of the universe is estimated to be 13.8 billion years, within Sandage’s later estimated range. (The Hubble constant, H₀, is now estimated to be somewhere between 45 km/sec/Mpc to 90 km/sec/Mpc.) The current age estimate has been calculated using a variety of methods, including measurements of stellar evolution, expansion of the universe, and radioactive decay, with all three methods in agreement of the universe’s age.

Alpher and Herman predicted that the universe would contain radiation from the big bang that in modern day would have stretched and cooled to 5 degrees K. At the time, nobody went looking for this testable prediction of the big bang model. Arno Penzias and Robert Wilson discovered the relic radiation entirely by accident in 1965.

The two radio astronomers were using the Holmdel Horn Antenna in Holmdel, New Jersey, to map the radio waves coming from the Milky Way. When they began their data collection, they found that there was a persistent radio signal that came from every direction at all times of day and night. It hissed like static on a radio. And like static on the radio, it was annoying.

The noise that Penzias and Wilson picked up was not actually so significant as to prohibit them from making their desired measurements. In fact, it was weak enough that other astronomers had dismissed the noise and therefore missed its significance. Penzias and Wilson were not so easily put off, however. For months, the two astronomers tried everything they could think of to remove or reduce this interfering “noise.”

They tried pointing the antenna toward New York City to see if the cause was some sort of city-related interference. They took the antenna apart, looked for defects, and put it back together. The swept out droppings from a pair of pigeons nesting inside the antenna and, eventually, trapped the pigeons and got rid of them.

None of these efforts helped. It seemed that the sky itself was glowing with faint radio light in all directions at a temperature of 3 degrees K.

Penzias and Wilson learned through another scientist that a Princeton University physicist, Robert Dicke, had predicted (like Alpher and Herman) that the big bang would have left residual radiation. Dicke was about to design an experiment looking for the radiation left over from the big bang when Penzias called him up to discuss what he and Wilson had found.

In the early universe, when conditions were still too hot for electrons to bond with atomic nuclei, photons of light scattered off the free electrons much like rays of light scatter off drops of water in the fog. When the hot universe cooled enough for the free electrons to bond with the nuclei to form neutral atoms in a moment called recombination, the light that filled the universe was suddenly able to move freely.

When the light first broke free from the fog of electrons, a process called photon decoupling, it had a wavelength of about one-thousandth of a millimeter. Over the course of billions of years and into the twentieth century, its wavelength stretched to about a millimeter in length. At this wavelength, the radiation was now in the radio portion of the electromagnetic spectrum. More specifically, the wavelength belongs to the microwave subsection of the radio spectrum, giving it the name “cosmic microwave background,” or CMB.

Penzias and Wilson had discovered the relic radiation that Alpher and Herman had predicted decades before and that Dicke had predicted more recently in the 1960s.

(You might have seen the CMB for yourself on an old television. On a fuzzy screen between channels, 1 percent of the static on the screen is the microwave radiation left over from the big bang.)

This discovery was the first tested prediction of the big bang model of cosmology and the most convincing evidence that the model was accurate. Penzias and Wilson’s discovery finally turned the big bang model of cosmology into a mainstream scientific theory.

However, another question remained. If the big bang model was correct, how did galaxies and stars form in the young universe? In general, the cosmic microwave background was a smooth glow across the whole universe. It was thought that the big bang explosion had created an even expanse of matter. How could structure form from such a smooth cosmic soup?

Those who supported the big bang model believed that the early universe must not have been perfectly uniform, for otherwise stars and galaxies couldn’t have formed. Instead, they imagined a universe where some areas were denser than others, creating regions where gravity would eventually attract more matter and cause the regions to collapse under their own weight.

There was no proof of these variations in density when Penzias and Wilson first discovered the CMB. The signal they picked up was uniform across time and space. The American astronomer George Smoot hoped that if he measured the CMB with more powerful instruments, he would find the predicted density variations.

Smoot worked at the University of California at Berkeley, where he participated in several 1970s experiments using giant balloons to lift radiation detectors tens of kilometers above Earth. The scientists hoped that this high altitude would remove any radiation from microwaves in Earth’s atmosphere. However, the cold temperatures at that altitude could wreak havoc on the detectors and the balloons were prone to crash-landing.

In efforts to find other means of studying the CMB from high altitudes, Smoot used a United States Air Force spy plane to take a detector up. The data gathered ended up showing that the Milky Way was moving through the universe at a speed of 600,000 kilometers per second, which was new and interesting information, but not the data Smoot intended to find.

While his 1976 spy plane experiment was underway, Smoot began working on designing a satellite detector called COBE, or the Cosmic Background Explorer. COBE contained several detectors including a Differential Microwave Radiometer (DMR) that measured the CMB radiation from two separate directions and found the difference. The DMR could thus detect whether the CMB was perfectly smooth or had small fluctuations.

COBE was scheduled to launch in 1988, but the experiment ran into a problem when the Challenger space shuttle exploded in January of 1986. NASA upended its flight schedule and called off the scheduled COBE launch.

The COBE team explored opportunities to launch on a foreign rocket, namely with the French, but NASA objected. Eventually, NASA agreed to send COBE up in a Delta rocket, which was much smaller than they had initially planned for. The team quickly redesigned COBE to be smaller and lighter so the sophisticated equipment could fit in the rocket.

COBE launched on November 18, 1989. It took about six months to complete an initial rough, full-sky survey. The initial data showed no variations, but when the first thorough full-sky map was complete in December of 1991, the data showed something more. The peak wavelength of the CMB radiation varied by 0.001 percent, a tiny variation but significant enough to show that the early universe was inhomogeneous. The variations were big enough to cause matter to clump and, eventually, galaxies to form.

Smoot’s team announced their results in April of 1992. It was one of the most significant discoveries in the history of cosmology, for the COBE results showed that the big bang model of cosmology could explain the history of the universe from its birth to the formation of galaxies to present day. Subsequent missions by the WMAP and Planck satellites confirmed and refined COBE’s measurements of the CMB.

By the 1990s, all three pillars of the big bang model were in place and the big bang became the standard cosmological model for our universe.

Over the past few decades, scientists have arrived at a sophisticated, detailed standard model of those first few moments in the infant universe.

In the first half of this chapter, we looked at how the discovery of the big bang fell into place, piece by piece. Now, let’s dig in deeper to the details of how it worked.

The standard model of the big bang today contains details of the first fractions of a second. The numbers at this time in the universe’s history are both astronomically small and astronomically large. As a quick refresher on scientific notation, 10^-43 seconds is the equivalent of a decimal place followed by 42 zeroes and a one, or 0.000000000000000000000 0000000000000000000001 seconds.

The measurements of fluctuations in the CMB became increasingly precise with data from WMAP and then Planck.

Conversely, 10³² degrees K is 10 with 32 zeroes after it, or 1,000,000,000,000,000,000,000,000,000,000,000 degrees K. The scale of these numbers shows the rapid speed at which the universe was changing and the extreme conditions present in that early period.

As the universe expanded and cooled, the changes began to happen within more comprehensible time scales and environmental conditions.

The standard model of big bang cosmology states that the universe began 13.8 billion years ago as a very small, dense region at a temperature of 10³² degrees K.

The universe underwent a period of rapid expansion after 10^-43 seconds, a period known as Planck time. After 10^-33 seconds, just a short time after it began, the inflation ended and the universe had grown by a factor greater than 10³⁵. Quantum fluctuations that occurred during inflation left very small density fluctuations, or inconsistencies in the composition of the infant universe.

At first the universe was pure energy, but after the big bang, that energy converted into matter. For some reason, that conversion resulted in slightly more matter than matter’s counterpart, antimatter. (Antimatter is nearly identical to matter except antimatter particles have the opposite charge and spin of their corresponding matter particles. The two annihilate when they meet.)

The antimatter annihilated with the matter, but for some reason, for every billion antiparticles there were a billion and one particles. As a result, the antimatter was destroyed and some matter remained. The asymmetry between matter and antimatter at the beginning of the universe is a mystery to scientists and a topic of current research.

This NASA diagram shows the initial rapid expansion of the universe and slower, continued expansion over time.

Dark Matter

Dark matter was first predicted in 1933 by the astronomer Fritz Zwicky. Zwicky discovered that the mass of the stars in the Coma cluster of galaxies accounted for only about 1 percent of the mass necessary to keep the galaxies from escaping the cluster’s gravitational pull. Zwicky called this the “missing mass” problem and hypothesized that something else made up the remaining 99 percent.

In the 1970s, astronomers Vera Rubin and W. Kent Ford confirmed that Zwicky’s missing mass exists when they measured the mass and speeds of stars in typical galaxies. They found that the mass of the stars in a typical galaxy accounts for just 10 percent of the mass required to keep those stars in orbit around the galactic center. In addition, the stars at the outer edge of a typical galaxy orbit the galactic center just as fast as—or faster than—the stars nearer the center.

Standard physics tells us that if stars at the edge of a spiral galaxy are only experiencing the gravitational effects of the dense galactic center, the stars at the edge of the galaxy should travel slower than stars near the center. Additionally, the visible mass of a galaxy had too little of a gravitational effect to keep the rapidly moving stars in orbit. Rubin and Ford’s measurements showed that the galaxies contained a significant amount of unseen mass that accounted for the discrepancies.

Abell 1689, a cluster of galaxies, helps astrophysicists study dark matter because it behaves as a gravitational lens.

Dark matter can also be found by its gravitational lensing effects. Just as the mass of visible objects like the sun distorts space-time and bends light, dark matter bends light passing near it. Today, scientists estimate that 25 to 27 percent of the universe is dark matter, while 5 percent of the universe is baryonic matter (ordinary matter made up of protons, neutrons, and atomic nuclei).

By the first second after the big bang, the universe consisted of a 10-billion-degree K soup of neutrons, protons, electrons, anti-electrons, photons, and neutrinos. At this time, the region of the known universe was at least 10^19.5 cm across.

The universe cooled for another one hundred seconds and eventually big bang nucleosynthesis convened. In this process, the neutrons and protons formed to create hydrogen nuclei and eventually helium nuclei and traces of other light elements including lithium and beryllium. At this point, there were also about two billion photons per proton or neutron.

After about fifty-six thousand years, the mass density of the universe was equal to the radiation density. At this point in the timeline, the small amounts of density fluctuations mentioned earlier resulted in uneven distribution of dark matter. (Dark matter is a mysterious type of matter that accounts for about 27 percent of the universe.)

Dark matter, though invisible, permeates and shapes the visible universe. The denser regions of dark matter, due to their mass and the effects of gravity, began to attract more dark matter to form dark matter clusters.

After about three hundred thousand years, the universe cooled further and underwent the era of recombination and photon decoupling. This time is the earliest time in the history of the universe that we can directly observe using photons (i.e., via telescopes and other radiation-detecting instruments).

For a very long time, however, there were no other sources of light in the universe. Without stars, there was no visible or infrared light, creating a cosmic dark ages.

The Millennium Run simulated the evolution of ten billion particles of matter to understand how galaxies and black holes form.

Once neutral atoms were formed, the ordinary matter in the universe began to clump along with the clusters of dark matter due to the dark matter’s mass and gravitational pull. As a result, some baryonic regions of the universe were denser than others, and over time these regions attracted more matter. These massive regions eventually collapsed due to gravity and, about two hundred million to three hundred million years after the big bang, formed into stars. The cosmic dark ages ended.

The first stars are thought to have been thirty to three hundred times the size of the sun, likely due to the limited cooling ability of the primordial gas. The minimum mass a clump of gas must have in order to collapse due to gravity is called the Jeans mass, which is directly proportional to the square of the gas temperature and inversely proportional to the square root of the gas pressure. The pressure in star- forming regions then was similar to the pressure in star-forming regions today, but the temperature in the densest region of space at this time was between 200 K and 300 K, compared to 10 K in today’s star-forming regions. Therefore, the Jeans mass was much higher for the first generation of stars.

The first-generation stars were also millions of times as bright as the sun. They burned through their fuel relatively quickly, fusing elements as heavy as iron within their cores. They died in brilliant supernovas after just a few million years.

In a supernova explosion, the core of a massive star collapses inward and in a fraction of a second, the temperature of the core skyrockets. The resulting explosion produces elements as heavy as uranium and a variety of subatomic particles.

The force of the explosion expels those materials into space, where they become available source material for future stars. When the first stars died in this way, they filled the universe with elements (such as carbon, silicon, and uranium) and particles that would be incorporated into subsequent generations of stars, galaxies, and other celestial objects (thus, the popular sayings by Carl Sagan, “We are made of starstuff” and “If you wish to make an apple pie from scratch, you must first invent the universe”).

It is also thought that the supernova death of the earliest stars may have led to the growth of the supermassive black holes that exist today in the hearts of galaxies and power the massive, powerful, energy-emitting quasars.

The second generation of stars had a more complex chemical composition than the first generation of stars and tended to be smaller and longer lived. This is perhaps related to their chemical composition: scientists have found that when a star-forming cloud contains one-thousandth of the metal abundance in the sun or more, those metals help the gas cool and condense more rapidly, which leads to stars of smaller masses and a greater overall rate of star formation.

About one billion years into the universe’s timeline, dark matter, stars, and gas clumped together and collapsed under the force of gravity into protogalaxies. Gravity separated each protogalaxy into a core and a halo. The gas particles collided, heated up, radiated energy, and fell into the core while the less interactive dark matter remained in the halo. The protogalaxies became increasingly dense, and stars began to form, turning the first protogalaxies into full, primeval galaxies.

The first galaxies eventually formed into galaxy clusters and sometimes collided and merged with other galaxies. Over time, the universe expanded to be at least 10²⁹ cm across at a temperature of 2.725 degrees K. Earth-like planets formed around the second-generation stars, and life on Earth began about four billion years ago.

Today, we can study as far back as the era of recombination because we can look back in history by looking farther across the universe. As the universe expands and light from increasingly distant galaxies reaches Earth, astronomers and physicists get a look not just into the distance but into the distant past. Because of the vast amount of space between us and the edge of the observable universe, the light that reaches us from that edge has traveled billions of years to reach us. In doing so, it gives us a peek into what the universe was like billions of years ago.

As a star runs out of fuel, the core becomes so heavy that the star collapses under its own gravitational force.

In the last century, many pieces of the cosmological puzzle have come together. Through the work of many physicists, astronomers, and astrophysicists, we have a huge amount of knowledge and new technologies that are useful in space and on Earth. However, many mysteries still elude scientists. How did the laws of physics operate in those intense, initial conditions? How did matter come to prevail over antimatter? We’ll go over these questions in the next chapter.