Great Discoveries in Science: The Big Bang Theory

The ATLAS detector at CERN is used for a wide range of physics experiments, such as searching for extra dimensions.

The big bang may have taken place almost fourteen billion years ago, but as a society, we’re far from past it. The big bang theory is the standard model taught in astronomy and cosmology courses around the world. Significant funding and research time is dedicated to understanding more about the big bang and filling in the remaining questions in the cosmological model.

It’s currently impossible to look back past the time of recombination (the moment when free electrons paired up with nuclei to form neutral atoms and photons of light were finally able to travel freely) with telescopes. That means the first four hundred thousand years or so of the universe can be studied only indirectly, such as by observing and analyzing the cosmic microwave background radiation for clues to the early universe or by re-creating the conditions of the big bang.

Physicists use particle accelerators to reproduce those incredibly hot, incredibly dense early conditions of the universe. Particle accelerators are powerful instruments that produce and accelerate a beam of particles, typically protons or electrons, but occasionally entire atoms such as gold or uranium. The particles are accelerated inside a beam pipe to greater and greater energies. When the particles have reached the desired energy levels, they are collided with another beam or a fixed target, such as a thin piece of metal.

The collision produces a shower of exotic particles. Detectors record the particles and the paths they take after the collision, which gives physicists a wealth of data to sort through in the aftermath.

The most famous particle accelerator studying the early conditions of the universe is the Large Hadron Collider, which is buried underground along the French and Swiss border at CERN (the European Organization for Nuclear Research). The Large Hadron Collider (LHC) is also the world’s largest particle accelerator with a ring 17 miles (27 km) long.

The LHC has four detectors at different collision points on the ring that physicists use for different purposes. ATLAS is a general purpose detector designed to investigate new physics, such as searching for extra dimensions and dark matter. CMS looks for similar things as ATLAS using different technology. ALICE is a heavy ion detector used to study the physics of strongly interacting matter at extreme energy densities similar to those just after the big bang. LHCb investigates the differences between matter and antimatter.

In one recent experiment, the scientists at CERN used the ALICE detector to study the collision of heavy ions (such as gold and lead nuclei) at energies of a few trillion electron volts each. The resulting collision, which CERN described as a “miniscule fireball,” re-created the hot, dense soup of particles moving at extremely high energies in the early universe.

The particle mixture was primarily made up subatomic particles called quarks and gluons that moved freely. (Quarks are particles that make up matter; gluons carry the strong force that binds quarks together.) For just a few millionths of a second after the big bang, the bonds between quarks and gluons were weak and the two types of particles were able to move freely in what’s known as a quark-gluon plasma.

The quark-gluon plasma existed for a few microseconds after the universe began before cooling and condensing to form protons and neutrons.

The LHC’s man-made fireball cooled immediately, and the individual quarks and gluons recombined and created many different types of particles, from protons, neutrons, antiprotons, and antineutrons to tiny particles called pions and kaons. One early finding from analysis of the quark-gluon plasma showed scientists that the plasma behaves more like a fluid than a gas, contrary to many researchers’ expectations.

Scientists at CERN are also using the Large Hadron Collider’s LHCb detector to determine what caused the imbalance between matter and antimatter after the big bang.

Particle accelerators were invented by experimental physicists to study particle physics, but they have since been used in many useful applications.

From Splitting the Atom to the Atomic Bomb

The first particle accelerator was built in 1929 by John Cockcroft and Ernest Walton in pursuit of splitting the atom to study the nucleus. They succeeded in 1932 when they bombarded lithium with high-energy hydrogen protons. Their experiment was the first time humans split an atom, a process called fission.

The experiment also confirmed Einstein’s law E=mc². Walton and Cockcroft found that their experiment produced two atoms of helium plus energy. The mass of the helium nuclei was slightly less than the mass of the combined lithium and hydrogen nuclei, but the loss in mass was accounted for by the amount of energy released.

In 1939, German physicists discovered how to split a uranium atom. Scientists across the world feared that the Nazis would build an atomic bomb capable of terrible destruction. (When the uranium-235 isotope is split, the fission begins a chain reaction that can grow large enough to cause an enormous explosion.) At the urging of Einstein and other top physicists, in 1941 the United States government launched an atomic bomb development effort code-named the Manhattan Project.

Over 120,000 Americans worked on the Manhattan Project, and the government spent almost $2 billion on research and development. The effort was so top secret that Vice President Harry Truman didn’t learn about the Manhattan Project until President Theodore Roosevelt died in office and Truman became president.

Nuclear fission creates a chain reaction.

When Japan refused to surrender in 1945, Truman authorized two atomic bombs that were dropped on Hiroshima and Nagasaki on August 6 and August 9, respectively. The bombs effectively ended World War II, but hundreds of thousands of Japanese people were killed and many more suffered terrible health effects from the radiation.

For example, particle accelerators are used to deliver radiation therapy, which is one of the standard methods for treating cancer. In one form, high energy X-rays are generated by beaming high-energy electrons at a material such as tungsten. These X-rays are then directed at the site of the patient’s cancerous tumor to kill the cancer cells. Healthy tissue is also damaged by the radiation beam, however, and researchers are continually looking for ways to deliver the right dose of radiation to destroy the tumor while minimizing impact to healthy cells.

Particle accelerators are also used to generate X-rays for medical imaging, such as when we have our teeth X-rayed at the dentist’s office or have a full-body magnetic resonance imaging (MRI) scan.

Outside of the medical world, particle accelerators are used for industrial purposes, such as manufacturing computer chips and producing the plastic used in shrink-wrap, for security purposes, such as inspecting cargo, and in many other applications.

Physicists are able to study many aspects of the big bang using particle accelerators, but their work is by no means over. There are still many enormous questions about the beginning of the universe. The major questions include:

•How can we detect and study the neutrinos believed to have been created in the big bang, and what will they tell us if we find them?

•How can we detect and study the gravitational waves that are believed to have been created by the big bang?

The Standard Model of particle physics has been developed since the 1930s, with significant help from particle accelerators and their cataclysmic investigations into atoms and their component parts.

According to the Standard Model, everything in the universe is made of a few fundamental particles (such as the building blocks of matter, quarks and leptons), governed by four fundamental forces (the gravitational, electromagnetic, weak, and strong forces). The Standard Model explains how these particles and three of the forces relate to one another.

The electromagnetic force, which governs the propagation of light and the magnetism that allows a magnet to pick up a paper clip, reaches over great distances, as evidenced by starlight reaching Earth. The weak force governs beta decay (a form of natural radioactivity) and hydrogen fusion and acts at distances smaller than the atomic nucleus. The strong force holds together the nucleus and acts at very small distances.

The electromagnetic, weak, and strong forces result from the exchange of a force-carrying particle that belongs to a larger group of particles called bosons. Each force has its own boson: the strong force is carried by the gluon, the electromagnetic force is carried by the photon, and the weak force is carried by W and Z bosons.

The Standard Model is able to explain the forces other than gravity, all of which operate on microscopic scales. Gravity, however, operates across large distances, and as of yet, there is only a theoretical boson called the gravitron that corresponds to the gravitational force. Even without gravity, however, the Standard Model is able to explain particle physics very well because the gravitational force has little effect at the small scale of particles.

Research has shown that at very high energies, the electromagnetic and weak forces unite into a single force. Scientists believe that at even higher energies the strong force converges with the electroweak force to create a grand unified theory (GUT).

It is thought that at the extreme conditions immediately after the big bang all four forces would have been unified, but scientists do not yet understand how this could work. (Einstein died working to discover the unified field theory, continuing his work even on his deathbed.) Figuring out this unified force could help scientists understand more about the big bang and where our universe came from.

The big bang created intense amounts of radiation, which has no mass. Eventually, the radiation converted into matter, which does have mass. Where did this mass come from?

On July 4, 2012, scientists at CERN discovered a new particle called the Higgs boson. The Higgs boson is a particle that belongs to an invisible field called the Higgs field that pervades the universe. Higgs bosons are denser spots in the Higgs field that can be detected in particle accelerators.

The Higgs field theory, proposed in 1964 by British physicist Peter Higgs, as well as by François Englert and Robert Brout, explains how particles gain mass. The theory states that when a particle moves through the Higgs field, a small amount of distortion (as if something was being dragged, creating resistance) is created that gives the particle mass. The more a particle interacts with the field, the more mass it acquires. Other particles don’t interact with the Higgs field at all and have no mass.

One analogy that is commonly used to illustrate the effect of the Higgs field is to imagine a large party filled with people. When an accountant (stereotypically a dull job) walks through the room, nobody interacts with the accountant, much like certain particles such as photons don’t interact with the Higgs field. The accountant (and the photon) move freely.

On the other hand, what if a celebrity walks through the party? People near the door see the celebrity first and begin spreading the news to their neighbors, and the news spreads through the crowd. The crowd converges on the celebrity, slowing down her progression through the crowd.

The celebrity has become a massive particle through her interaction with the crowd. The Higgs boson can be thought of as a dense clump in the crowd created by an exciting piece of gossip, and as the gossip spreads through the crowd, the clump moves through the crowd.

The Higgs boson had been a cornerstone of the Standard Model of particle physics since the 1970s, but the 2012 discovery at CERN gave significant support to its existence.

More recently, CERN physicists have determined the mass of the Higgs boson and measured other characteristics. Active research into the Higgs boson and Higgs field is still underway as there is still much to discover.

Matter and antimatter particles are created in pairs, which means that the big bang should have created equal amounts of matter and antimatter. Matter and antimatter annihilate one another upon contact, and in the first fractions of a second, the universe was filled with particle and antiparticle pairs popping in and out of existence. At the end of this process, when all the annihilations were complete, the universe should have been filled with pure energy—and nothing else.

This is clearly not the case. We are made of matter, and we inhabit a world and universe made of matter. What, then, happened such that matter survived?

Scientists calculate that about one particle per billion particles of matter survived. It’s unknown why this is the case, but observations of particles at the LHC give one potential explanation:

Due to a weak interaction process, particles can oscillate between their particle and antiparticle state before decaying into other particles of matter or antimatter. It could be that in the early universe an unknown mechanism caused oscillating particles to decay into matter slightly more often than they decayed into antimatter.

The survival of matter over antimatter is a topic of ongoing investigation at physics institutions across the world.

Scientists across the world are searching for big bang neutrinos in hopes that these ancient particles could reveal more about the early universe.

Neutrinos were first hypothesized in 1930 by Wolfgang Pauli as particles that carried away the observed difference in energy between the initial particles and final particles when a neutron decayed into a proton and an electron. The first neutrino was detected twenty-six years later.

Neutrinos are a fundamental, subatomic particle produced by the decay of radioactive elements. They’re produced within particle accelerators, nuclear power stations, nuclear bombs, stars, and the big bang itself. Neutrinos have no electric charge and are therefore not affected by the electromagnetic force. They hardly interact with anything, in fact, which allows neutrinos to move easily through matter and across the universe without interference. (Nearly one hundred trillion neutrinos zip through your body every second!)

There are five thousand of these optical detectors buried in the ice at the South Pole, ready to detect neutrino collisions.

Neutrinos are extremely desirable particles to study because they travel largely unaffected across space and bring information to us from deep within the cosmos. A strongly interacting particle, in contrast, would be absorbed by the matter and radiation in between. But because neutrinos are so weakly interacting, they are also very hard to detect. Therefore, neutrino detectors must be very large in order to maximize the chance that a passing neutrino will interact. They are typically built inside of transparent material such as water so the instruments can detect the light given off by particles produced by neutrino interactions.

The world’s largest neutrino detector is Ice Cube, buried within a cubic kilometer of ultra-transparent ice in Antarctica. The detectors are buried about .9 miles (1,500 m) beneath the surface of the ice to shield them from the radiation at Earth’s surface. When a neutrino collides with a molecule of ice, the collision will create an array of subatomic particles and give off light, which is picked up by Ice Cube’s detectors.

While detectors like Ice Cube have detected neutrinos from cosmic sources, no detector yet has picked up big bang neutrinos. These ancient particles are predicted to be billions of times colder than neutrinos from the sun, and their collisions would be much less energetic.

One experiment dedicated to detecting just these cold relic neutrinos is a prototype lab, PTOLEMY, at the US Department of Energy’s Princeton Plasma Physics Laboratory. PTOLEMY’s instruments were designed to have the coldest, darkest conditions possible so that the tiny amount of energy that a big bang neutrino contains can, upon collision, stand out and be observed.

The PTOLEMY scientists hope to show that they can measure the mass of a big bang neutrino and then ramp up from a prototype laboratory to a full lab that can uncover even more information from big bang neutrinos.

Scientists believe that the initial cosmic inflation should have magnified quantum fluctuations in the early universe’s gravitational field, resulting in gravitational waves.

Gravitational waves were first predicted by Albert Einstein in his 1915 general theory of relativity. Gravitational waves, from the collision of two black holes, were first detected by LIGO (the Laser Interferometer Gravitational-Wave Observatory) in 2015.

The gravitational waves LIGO picked up confirmed Einstein’s prediction and also showed that black holes do collide. However, scientists are still on the hunt for gravitational waves specifically from cosmic inflation. Inflation-caused gravitational waves would be too weak for the LIGO detector to pick up, but they would slightly twist the orientation of light, creating an effect called polarization.

In 2012, a group behind the BICEP2 radio telescope at the South Pole thought they had detected big bang gravitational waves. Their data had shown a curlicue pattern in the polarization of the CMB, which greatly excited the science community and made news headlines around the world. However, the pattern turned out to be from dust in the Milky Way, which emits polarized light with the same curling pattern.

Research is still underway using an upgraded version of the original technology called BICEP3 whose observation period ran through much of 2016. BICEP3 includes more detectors and a finer resolution but also a broader spectrum of light that will help the team discern any signals from inflation from galactic dust.

One of the major questions around the history of our universe is whether our universe is the sole universe. Are there other universes that have undergone a similar cosmology? Are there other universes that have undergone very different cosmologies?

If you currently have a basic assumption that our universe is the only universe, it can be challenging to imagine what it means for other universes to exist. But at one time, people thought our planet was the only planet, and then that the sun was the only sun, and then that our galaxy was the only galaxy. One by one, these notions have been shattered by theoretical and observational science. It follows that we should then seriously consider whether our universe is the only universe or if it is one of many.

According to Brian Greene, one of the most prominent theoretical physicists who studies and speaks on the idea of multiple universes:

An illustration of the gravitational waves (disruptions in space-time) caused by two black holes orbiting one another

What we have found in research … is that our mathematical investigations are suggesting that what we have thought to be everything may actually be a tiny part of a much grander cosmos. And that grander cosmos can contain other realms that seem to rightly be called universe just as our realm has been called universe.

One relatively simple example Greene gives begins by considering whether the universe is finite or infinite. Currently, physicists do not know which is true. Thus, an infinite universe can be considered a viable option.

Next, consider shuffling a deck of cards an infinite number of times. Eventually, the order of the cards will begin to repeat. So, too, would the configuration of particles in an infinite universe. If space goes on for infinity, there would inevitably be repeating configurations of matter just as there are repeating configurations of cards.

While these other universes have not yet been observed, there have been other successful theories that started in a similar way. Einstein’s general theory of relativity, for example, started as a theoretical set of equations and was later tested in various ways before becoming a well-supported, accepted theory.

The collection of multiple universes is called a multiverse. It is also sometimes called a bubble universe because the term describes how physicists imagine multiple universes forming.

To imagine a bubble universe, picture a boiling pot of water. The pot has bubbles of varying sizes. Some appear and pop immediately, others grow larger and last longer. (This is not to say that the energy fueling a multiverse is thermal energy, but the image is helpful for imagining the appearance and disappearance of universes with varying sizes and lifespans.)

In this model, our region of space underwent its early cosmic expansion, which ended 13.8 billion years ago. While inflation in our region ended, inflation continued in other regions or “bubbles.” The different inflation regions separated, creating an infinite number of universes. As they inflate, the bubbles grow apart and make room for more inflating bubbles.

The idea of a multiverse or bubble universe is controversial, in part because initially scientists had no way to prove or disprove that a multiverse exists. A core component of the scientific method is the ability to test a hypothesis, and without that component, a hypothesis essentially becomes a question of philosophy, not of science.

Scientists are looking for impact marks that could show other universes have collided with ours. These marks would validate the multiverse theory.

However, in recent years, astrophysicists have thought of a way to test the multiverse theory. Consider again the pot of boiling water analogy—as water boils, some of the bubbles that rise up will collide. We don’t know how dense the theoretical multiverse would be, but it is possible that another universe could have collided with our own.

Astrophysicists think that such a collision would be observable as imprints or “bruises” on the cosmic microwave background. The collision point would be a round spot of either higher or lower radiation intensity. While it’s not a guarantee that our (hypothetical) bubble universe has collided with another bubble universe, finding such an imprint would lend significant support to the multiverse theory.

The idea of a multiverse is especially attractive for proponents of string theory. String theory is a theory that attempts to integrate general relativity with quantum mechanics.

In string theory, the fundamental components of all matter are tiny strings that vibrate in eleven dimensions. Just like a guitar string can vibrate in different ways that produce different notes, in string theory, strings have different “excitation modes,” or frequencies that produce different particles. If string theory is accurate, it explains not just the subatomic particle but also the laws of physics.

String theory is a complex theory beyond the scope of this book, but it’s worth noting briefly for two major reasons: string theory is one possible way to achieve a unified theory of physics. More significantly for the multiverse theory, string theory is another one of the mathematical models that predicts that there may be many different universes.

The big bang theory, like most scientific theories, has its opponents. While most of the opposition comes from the realm of religious people who favor a literal interpretation of their religious texts, there have been small camps of scientists who held out against the big bang model of cosmology into the twentieth century.

The big bang theory of cosmology is the standard model accepted by a majority of scientists. However, the steady state theorist Fred Hoyle continued to oppose the big bang theory throughout his life, as did his Synthesis of the Elements in Stars coauthors Margaret and Geoffrey Burbidge.

Geoffrey Burbidge created a revision of the steady state theory called the “quasi-Steady State.” The new version of the theory proposes that the universe expands and contracts over one-hundred-billion-year cycles. According to the Burbidges, if stars can eject new types of matter as their paper with Hoyle showed, perhaps galaxies could also eject huge collections of matter to create new galaxies. Margaret Burbidge spent years observing quasars, theorizing that they could be a candidate for these ejected collections of matter.

The present situation in cosmology is that most people like to believe they know what the skeleton looks like, and they’re putting flesh on the bones. And Fred [Hoyle] and I would continuously say, we don’t even know what the skeleton looks like. We don’t know whether it’s got 20 heads instead of one, or 60 arms or legs. It’s probable that the universe we live in is not the way I think it is or the way the Big Bang people think it is. In 200 years, somebody is going to say how stupid we were.

In other words, Burbidge believes that too many scientists have prematurely accepted the current big bang model of cosmology. Geoffrey Burbidge has since died, and Margaret Burbidge is in her late nineties. Few other scientists have continued to oppose the big bang theory.

The predominant opposition to the big bang comes from those who disagree due to religious reasons. The Institute for Creation Research, which bills itself as a “leader in scientific research within the context of biblical creation,” publishes articles such as “The Big Bang Theory Collapses” that characterize the big bang theory as irreparably flawed—though scientific studies show otherwise. The ICR ultimately argues against any scientific cosmology, including the quasi-steady state model, because they all contradict the ICR’s belief that the Christian god created heaven and Earth.

The ICR is just one example of literal religious thinkers who oppose the big bang theory. There are many others who dismiss the model for similar reasons.

There are also many religious people who do not dismiss the big bang theory. Georges Lemaître was himself a Catholic priest, and many religious thinkers from various faiths see no opposition between the big bang model of cosmology and their religious beliefs. Some see their creator as the creative force that sparked the big bang, while others, as Lemaître, consider the religious and scientific realms as entirely separate and able to exist separately without conflict.

Many of the major questions that exist about the big bang model of cosmology—such as what came before the big bang, do other universes exist, and how did the fundamental forces unite in those first fractions of a second—are simply curiosities to many people outside of the science world. And yet, the answers, when we find them, will fundamentally shape the way we understand how we came to be where we are today. Few questions matter more.