How the Big Bang Theory Works

The big bang theory describes the development of the universe from the time just after it came into existence up to today. It's one of several scientific models that attempts to explain why the universe is the way it is. The theory makes several predictions, many of which have been proven through observational data. As a result, it's the most popular and accepted theory regarding our universe's development.

The most important concept to get across when talking about the big bang is expansion. Many people think that the big bang is about a moment in which all the matter and energy in the universe was concentrated in a tiny point. Then this point exploded, shooting matter across space, and the universe was born. In fact, the big bang explains the expansion of space itself, which in turn means everything contained within space is spreading apart from everything else. The illustrations below should help a little.

Today, when we look at the night sky, we see galaxies separated by what appears to be huge expanses of empty space. At the earliest moments of the big bang, all of the matter, energy and space we could observe was compressed to an area of zero volume and infinite density. Cosmologists call this a singularity.

What was the universe like at the beginning of the big bang? According to the theory, it was extremely dense and extremely hot. There was so much energy in the universe during those first few moments that matter as we know it couldn't form. But the universe expanded rapidly, which means it became less dense and cooled down. As it expanded, matter began to form and radiation began to lose energy. In only a few seconds, the universe formed out of a singularity that stretched across space.

One result of the big bang was the formation of the four basic forces in the universe. These forces are:

At the beginning of the big bang, these forces were all part of a unified force. It was only shortly after the big bang began that the forces separated into what they are today. How these forces were once part of a unified whole is a mystery to scientists. Many physicists and cosmologists are still working on forming the Grand Unified Theory, which would explain how the four forces were once united and how they relate to one another.

The big bang theory is the result of two different approaches to studying the universe: astronomy and cosmology. Astronomers use instruments to observe stars and other celestial bodies. Cosmologists study the astrophysical properties of the universe.

­In the 1800s, astronomers began to experiment with tools called spectroscopes (also known as spectrographs). A spectroscope is a device that divides light into a spectrum of its component wavelengths. Spectroscopes showed that the light from a specific material, such as a glowing tube of hydrogen, always produced the same distribution of wavelengths unique to that material. It became clear that by looking at the wavelength distribution from a spectrograph, you could figure out what kind of elements were in a light source.

Meanwhile, Austrian physicist Christian Doppler discovered that the frequency of a sound wave depended upon the relative position of the source of the sound. As a noisy object approaches you, the sound waves it generates compress. This changes the frequency of the sound, and so you perceive the sound as a different pitch. When the object moves away from you, the sound waves stretch and the pitch goes down. It's called the Doppler effect.

Light travels in waves too, and astronomers discovered that some stars had more light falling into the red side of the spectrum than they expected. They theorized that this meant the stars were moving away from Earth. As the stars move away, the wavelengths from the light they emit stretch. They shift to the red end of the spectrum because that end has longer wavelengths. Cosmologists call this phenomenon the redshift. A star's redshift is an indication of how quickly it is moving away from Earth. The further toward the red end of the spectrum the light shifts, the faster the star is moving away.

In the 1920s, an astronomer named Edwin Hubble noticed something interesting. The velocity of a star appeared to be proportional to its distance from the Earth. In other words, the further away a star was from Earth, the faster it appeared to move away from us. Hubble theorized that this meant the universe itself was expanding.

Hubble's discovery led to a lengthy debate that still rages today: what exactly is the relation between a distant celestial body's velocity and its distance from the observer? Cosmologists call this relationship the Hubble constant, but no one agrees on what that relationship is. Hubble theorized that it was 464 kilometers (km) per second (sec) per megaparsec (Mpc). A megaparsec is a unit of distance equal to more than 3.08 x 10²² meters (or 1.9 x 10¹⁹ miles).

It turns out Hubble overestimated this number. That's because in Hubble's time, astronomical instruments weren't sensitive enough to measure the distance between the Earth and celestial bodies with accuracy. As instruments improved, scientists refined the Hubble constant, but debate over the actual value of the Hubble constant rages on.

What does all this have to do with the big bang theory? Keep reading to find out.

Hubble theorized that the universe expands as time passes. That meant that billions of years ago, the universe would have been much smaller and more dense. If you go back far enough, the univers­e would collapse into an area with infinite density, containing all the matter, energy, space and time of the universe. In a way, the big bang theory came as a result of backwards engineering.

Some people had a real problem with this theory. Among them was the famous physicist Albert Einstein. Einstein subscribed to the belief that the universe was static. A static universe doesn't change. It has always been and always will be the same. Einstein hoped his theory of general relativity would give him a deeper understanding of the structure of the universe.

Upon completion of his theory, Einstein was surprised to discover that according to his calculations, the universe would have to be expanding or contracting. Since that conflicted with his belief that the universe was static, he searched around for a possible explanation. He proposed a cosmological constant -- a number that, when included in his general theory of relativity, explained away the apparent necessity for the universe to expand or contract.

When confronted with Hubble's findings, Einstein admitted that he was mistaken. The universe did seem to be expanding, and Einstein's own theory supported the conclusion. The theory and observations gave rise to a few predictions, many of which have since been observed.

One of those predictions is that the universe is both homogeneous and isotropic. Essentially, that means the universe looks the same no matter what the perspective of the observer. On a localized level, this prediction seems false. After all, not every star has a solar system­ of planets like ours. Not every galaxy looks the same. But on a macroscopic level that spans millions of light years, the distribution of matter in the universe is statistically homogeneous. That means even if you were across the universe, your observations of the structure of the universe would look the same as those here on Earth.

Another prediction was that the universe would have been intensely hot during the earliest stages of the big bang. The radiation from this period would have been phenomenally large, and there would have to be some evidence of this radiation left over. Since the universe must be homogeneous and isotropic, the evidence should be evenly distributed throughout the universe. Scientists discovered evidence of this radiation as early as the 1940s, though at the time they didn't know what they had found. It wasn't until the 1960s when two separate teams of scientists discovered what we now call the cosmic microwave background radiation (CMB). The CMB is the remnants of the intense energy emitted by the primordial fireball in the big bang. It was once intensely hot, but now has cooled to a chilly 2.725 degrees Kelvin (-270.4 degrees Celsius or -454.8 degrees Fahrenheit).

These observations helped solidify the big bang theory as the predominant model for the evolution of the universe.

We'll show you what scientists think happened during the big bang next.

Because of the limitations of the laws of science, we can't make any guesses about the instant the universe came into being. Instead, we can look at the period immediately following the creation of the universe. Right now, the earliest moment scientists talk about occurs at t = 1 x 10^-43 seconds (the "t" stands for the time after the creation of the universe). In other words, take the number 1.0 and move the decimal place to the left 43 times.

Cambridge University refers to the study of these earliest moments as quantum cosmology [source: Cambridge University]. At the earliest moments of the big bang, the universe was so small that classical physics didn't apply to it. Instead, quantum physics were in play. Quantum physics deal with physics on a subatomic scale. Much of the behavior of particles on the quantum scale seems strange to us, because the particles appear to defy our understanding of classical physics. Scientists hope to discover the link between quantum and classical physics, which will give us a lot more information about how the universe works.

At t = 1 x 10^-43 seconds, the universe was incredibly small, dense and hot. This homogenous area of the universe spanned a region of only 1 x 10^-33 centimeters (3.9 x 10^-34 inches). Today, that same stretch of space spans billions of light years. During this phase, big bang theorists believe, matter and energy were inseparable. The four primary forces of the universe were also a united force. The temperature of this universe was 1 x 10³² degrees Kelvin (1 x 10³² degrees Celsius , 1.8 x 10³² degrees Fahrenheit). As tiny fractions of a second passed, the universe expanded rapidly. Cosmologists refer to the universe's expansion as inflation. The universe doubled in size several times in less than a second [source: UCLA].

As the universe expanded, it cooled. At around t = 1 x 10^-35 seconds, matter and energy decoupled. Cosmologists call this baryogenesis -- baryonic matter is the kind of matter we can observe. In contrast, we can't observe dark matter, but we know it exists by the way it affects energy and other matter. During baryogenesis, the universe filled with a nearly equal amount of matter and anti-matter. There was more matter than anti-matter, so while most particles and anti-particles annihilated each other, some particles survived. These particles would later combine to form all the matter in the universe.

A period of particle cosmology followed the quantum age. This period starts at t = 1 x 10^-11 seconds. This is a phase that scientists can recreate in lab conditions with particle accelerators. That means that we have some observational data on what the universe must have been like at this time. The unified force broke down into components. The forces of electromagnetism and weak nuclear force split off. Photons outnumbered matter particles, but the universe was too dense for light to shine within it.

Next came the period of standard cosmology, which begins .01 second after the beginning of the big bang. From this moment on, scientists feel they have a pretty good handle on how the universe evolved. The universe continued to expand and cool, and the subatomic particles formed during baryogenesis began to bond together. They formed neutrons and protons. By the time a full second had passed, these particles could form the nuclei of light elements like hydrogen (in the form of its isotope, deuterium), helium and lithium. This process is known as nucleosynthesis. But the universe was still too dense and hot for electrons to join these nuclei and form stable atoms.

That's a busy first second. Next we'll find out what happened over the next 13 billion years.

A lot happened in that first second of the big bang. But that's just the beginning of the story. After 100 seconds, the universe's temperature cooled to 1 billion degrees Kelvin (1 billion degrees Celsius, 1.8 billion degrees Fahrenheit). Subatomic particles continued to combine. By mass, the distribution of elements was approximately 75 percent hydrogen nuclei and 24 percent helium nuclei (the other percent consisted of other light elements like lithium).

The temperature of the universe was still too high for electrons to bond with nuclei. Instead, electrons collided with other subatomic particles called positrons, creating more photons. But the universe was too dense to allow light to shine inside of it.

The universe continued to expand and cool. After about 56,000 years, the universe had cooled to 9,000 degrees Kelvin (8,726 degrees Celsius, 15,740 degrees Fahrenheit). At this time, the density of the matter distribution in the universe matched the density of radiation. After another 324,000 years, the universe had expanded enough to cool down to 3,000 degrees Kelvin (2,727 degrees Celsius, 4,940 degrees Fahrenheit). Finally, protons and electrons could combine to form neutral hydrogen atoms.

It was at this time, 380,000 years after the initial event, when the universe became transparent. Light could shine throughout the universe. The radiation that humans would later identify as cosmic microwave background radiation locked into place. When we study the CMB today, we can extrapolate a picture of what the universe looked like then.

For the next 100 million years or so, the universe continued to expand and cool. Small gravitational fluctuations caused particles of matter to cluster together. Gravity caused gases in the universe to collapse into tight pockets. As gases contract, they become more dense and hot. Some 100 to 200 million years after the initial creation of the universe, stars formed from these pockets of gas.

Stars began to cluster together to form galaxies. Eventually, some stars went supernova. As the stars exploded, they ejected matter across the universe. This matter included all the heavier elements we find in nature (everything up to uranium). Galaxies in turn formed their own clusters. Our own solar system formed around 4.6 billion years ago.

Today, the temperature of the universe is 2.725 degrees Kelvin (-270 degrees Celsius, -455 degrees Fahrenheit), which is only a couple of degrees above absolute zero. The homogenous section of the universe we can theorize about reaches 1 x 10²⁹ centimeters across (6.21 x 10²³ miles). That's larger than what we're able to physically observe using our most advanced astronomical instruments.

What does the big bang theory tell us about the universe? We'll find out next.

Some cosmologists use the big bang theory to estimate the age of the universe. But due to different measurement techniques, not all cosmologists agree on the actual age. In fact, the range spans more than a billion years!

The discovery that the universe is expanding led to another question. Will it expand forever? Will it stop? Will it reverse? According to the general theory of relativity, it all depends on how much matter is within the universe.

It boils down to gravity. Gravity is the force of attraction between particles of matter. The amount of gravitational force one body exerts on another depends upon the size of the two objects and the distance between them. If there's enough matter in the universe, the force of gravity will eventually slow the expansion and cause the universe to contract. Cosmologists would designate this as a closed universe with positive curvature. But if there isn't enough matter to reverse expansion, the universe will expand forever. Such a universe would either have no curvature or negative curvature. To learn more about curvature of the universe, read "Does space have a shape?"

If we are in a closed universe, eventually the entire universe will contract and collapse in on itself. Cosmologists call this the big crunch. Some theorize that our universe is just the latest in a series of universes generated in a cycle of space expanding and contracting.

According to the big bang theory, there's no center of the universe. Every point in the universe is the same as every other point, with no centralized location. This is difficult to imagine, but it's a requirement for a universe that is both homogeneous and isotropic. From our perspective, it seems like everything in the universe is moving away in the manner suggested by the big bang. One alternative theory is that the Earth itself is the center of the universe, which would explain why everything else is moving away. Cosmologists dismiss this theory because it's extremely unlikely that we'd occupy the central point of the entire universe.

There are also some very big questions the big bang theory doesn't address:

Not everyone subscribes to the big bang theory. Why do they disagree with the theory, and what are some of the alternate models for our universe? Read on to see what the skeptics say.

Since scientists first proposed the big bang theory, many people have questioned and criticized the model. Here's a rundown on some of the most common criticisms of the big bang theory:

There are several alternative models that attempt to explain the development of the universe, though none of them have as wide an acceptance as the big bang theory:

There are several other models as well. Could one of these theories (or other ones we haven't even thought of) one day replace the big bang theory as the accepted model of the universe? It's quite possible. As time passes and our capability to study the universe increases, we'll be able to make more accurate models of how the universe developed.

To learn more about the big bang and related topics, take a look at the links that follow.