Astronomer Fred Hoyle is responsible for the term "big bang," although he used it as a dismissive term. See more Milky Way pictures.
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For centuries, humans have gazed at the stars and wondered how the universe developed into what it is today. It's been the subject of religious, philosophical, and scientific discussion and debate. People who have tried to uncover the mysteries of the universe's development include such famous scientists as Albert Einstein, Edwin Hubble and Stephen Hawking. One of the most famous and widely accepted models for the universe's development is the big bang theory.
Although the big bang theory is famous, it's also widely misunderstood. A common misperception about the theory is that it describes the origin of the universe. That's not quite right. The big bang is an attempt to explain how the universe developed from a very tiny, dense state into what it is today. It doesn't attempt to explain what initiated the creation of the universe, or what came before the big bang or even what lies outside the universe.
Another misconception is that the big bang was a kind of explosion. That's not accurate either. The big bang describes the expansion of the universe. While some versions of the theory refer to an incredibly rapid expansion (possibly faster than the speed of light), it's still not an explosion in the classic sense.
Summing up the big bang theory is a challenge. It involves concepts that contradict the way we perceive the world. The earliest stages of the big bang focus on a moment in which all the separate forces of the universe were part of a unified force. The laws of science begin to break down the further back you look. Eventually, you can't make any scientific theories about what is happening, because science itself doesn't apply.
So what's the big bang theory in a nutshell? Find out in the next section.
WHAT IS A THEORY
In science, a theory is an attempt to explain a particular aspect of the universe. Theories can't be proven, but they can be disproven. If observations and tests support a theory, it becomes stronger and usually more scientists will accept it. If the evidence contradicts the theory, scientists must either discard the theory or revise it in light of the new evidence.
While many people believe that the big bang theory refers to an explosion, it actually refers to the expansion of the universe.
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The Short and Skinny on the Big Bang
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 asingularity.
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:
- Electromagnetism
- Strong nuclear force
- Weak nuclear force
- Gravity
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 theGrand Unified Theory, which would explain how the four forces were once united and how they relate to one another.
We'll take a look at where the big bang theory came from in the next section.
BLAME THE NAME
Confusion about the big bang is partly due to its confusing name -- it sounds like it should be an explosion. Blame that on Sir Fred Hoyle, a critic of the theory, who dismissively called the model a "big bang" as an insult. The derogatory comment caught on and the name stuck.
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Where the Big Bang Theory Came From
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 calledspectroscopes (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 fromEarth. 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 theredshift. 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 theHubble 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.
POINT ME AT THE SKY
Different teams of scientists look at different celestial bodies while trying to determine the true value of the Hubble constant. Some look at young stars called Cepheid variables. Others look at supernovae. The result is that estimates for the Hubble constant vary from 53 km/sec/Mpc to 80 km/sec/Mpc [source: Cosmology Tutorial].
Images of ancient galaxies taken by the Hubble Telescope.
Courtesy STSci and NASA
More on the Big Bang Story
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 galaxylooks 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).
This image of the cosmic microwave background radiation was taken by the Wilkinson Microwave Anisotropy Probe.
Courtesy NASA
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 on the next page.
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