The Big Bang theory is the currently accepted scientific theory of the beginning of the universe. According to the classic Big Bang theory (newer, more exotic ideas can be found in Theoretical Cosmology), the universe was once condensed into a single ball of matter that had a size of zero, defined to be a point, and infinite density and temperature. Because of these properties, the currently known laws of the physical universe break down at this point, called a singularity because no known laws explain it. For unknown reasons, this singularity began to expand some 10-20 billion years ago. The groundwork for the Big Bang theory was laid by Einstein's theory of general relativity. The greatest evidence for it was revealed by Edwin Hubble, who discovered in 1929 that all galaxies have a red shift (See The Doppler Effect) and are moving away from each other. Moreover, the galaxies' motion indicated that the universe was expanding uniformly. From this idea came the thought that all the matter currently in the universe was once condensed into a small hot ball of matter and has since expanded at an exponential rate. Though scientists have not yet worked out all the details of the theory, its basic tenets are generally accepted.
According to the classic Big Bang theory, the universe began, as stated above, as a small, hot, dense ball of primordial matter. Some event then caused this ball of matter to suddenly expand at an incredible rate of speed. The four fundamental forces at that time were merged into one (the theory of which is still being researched) and later separated into the familiar gravity, electromagnetic force, strong nuclear force, and weak nuclear force (See The Four Fundamental Forces). As the universe expanded and cooled, gravity was able to exert its force, causing particles to be attracted to one another. These "clumps" of matter were the predecessors of today's galaxies, which later subdivided into the traditional cosmological structures of stars, planets, and the like. It is worth noting that there is another, smaller school of thought that believes smaller cosmological structures developed first and then congregated into the larger structures of galaxies. This is called the bottom-up school; the more generally accepted first idea is called the top-down school. There are many variations on the Big Bang theory other than those listed here, and scientists are still divided about which if any correctly explains the universe. The most likely of these variations will be discussed in Theoretical Cosmology.
Controversy rages about the exact nature of the universe in the first few seconds after the Big Bang. However, certain aspects are agreed upon by most physicists. The "stuff" of the Big Bang is thought to be mainly elementary particles. One second after the Big Bang, the universe is thought to have cooled from infinite temperature to about ten thousand million degrees K. At this point it would have contained mainly photons, electrons, neutrinos, and their antiparticles, along with some protons and neutrons. In addition, these particles would have been so energetic that they could avoid attraction to each other due to the four fundamental forces. They are also likely to have collided with such force that particle/antiparticle pairs were produced more rapidly than they would be annihilated.
Once the universe cooled off more, however, changes would occur rapidly. Particle/antiparticle pairs would be created much less often due to lower collision energies, and eventually the rate of annihilation would be greater than the rate of creation. Also, as previously mentioned, the four forces would "crystallize" into their familiar forms and give rise to ever-increasing attractions between particles, which were then moving slowly enough to be affected by such forces. Nevertheless, virtually massless neutrinos would not have annihilated each other because of their very weak interactions. The detection of these neutrinos, left over from the Big Bang, would not only reconfirm the Big Bang theory but also provide important insight into the exact mass of neutrinos and their contribution to the total density of the universe.
Basically there are three possibilities predicted by the Big Bang theory. The first and oldest is actually the one of the first solutions (calculated by Alexander Friedmann) to Einstein's equations that predicted a Big Bang. In this model, the universe is expanding at a slow enough rate to allow gravity to overcome the expansion and eventually move the universe in the other direction, causing what has been termed the Big Crunch, or a return to the state of singularity. In a graph of this solution, the distance between two galaxies starts at zero (the Big Bang), increases to some unknown maximum, and then decreases parabolically to zero again in the Big Crunch (return to the singularity). In this model, space is shaped like the surface of the earth (but in four-dimensional spacetime) and is therefore finite. |
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In the second solution, the universe is expanding so quickly that gravity can check it, but never stop or reverse it. This, the so-called "entropy death" model, implies that the universe will expand forever and will reach a temperature of absolute zero. In a graph of this solution, the distance between two galaxies starts at zero (the Big Bang) and continues to increase forever. In this model, space is bent backwards in a saddle shape and is therefore infinite. |
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In the third solution, the universe is just barely expanding fast enough to avoid the Big Crunch. In this model, gravity has an ever-increasing role in the fate of the universe, but can never exert enough force to stop the expansion. In a graph of this solution, the distance between two galaxies starts at zero (the Big Bang) and increases forever; however, the speed at which galaxies move away from each other decreases all the time but never reaches zero. (Click here for a more advanced explanation.) In this model, space is flat and therefore also infinite. |
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The two variables to be measured in answering this question are the present rate of expansion and the average density of the universe. (The critical density is about 10-29 grams per cubic centimeter, or 5 hydrogen atoms per cubic meter - a seemingly infinitesimal value until the vast regions of vacuum separating massive cosmic structures are considered.) If the density is above the critical value, the universe will contract; if it is below the critical value, the universe will expand forever. The rate of expansion can be measured by calculating the velocities with which the other galaxies are moving away from us. However, uncertainty enters into the measurements due to the fact that the distances from earth to the galaxies are not very well known. Therefore, the best calculations available say that the universe expands by about 5-10 percent every thousand million years. The average density of the universe is even more uncertain. By adding up the known masses of all the currently cataloged cosmic structures, a value less than one hundredth of the critical value is obtained, even for the lowest estimate of the critical value itself. Even adding all currently known "dark matter" does not give an answer close to the critical value. Explanations of this fact run the gamut from uncataloged dark matter to the resurrection of Einstein's cosmological constant, a factor of curious repellent gravity that he added to his equations after they implied an expanding universe but later retracted. Many of these theories are more clearly delineated in Theoretical Cosmology.
The Big Bang theory implies that time has a definite beginning - the point at which the universe began to expand. Many people instantly opposed the idea because it seemed to imply a creator. (In fact, the Catholic Church proclaimed it consistent with the Christian Bible in 1951.) The most famous attempt to refute the Big Bang was the steady-state theory, which stated that new matter was constantly being created, resulting in a universe that did not expand and looked the roughly same from every vantage point. Even Einstein refused to accept a the idea of an expanding universe, adding a "cosmological constant" to his equations. This constant provided for "anti-gravity," built into the fabric of spacetime, that repelled matter at a rate that exactly overcame the gravitational attraction. Only Alexander Friedmann, a Russian physicist and mathematician, attempted to explain the expanding universe and thus predicted Edwin Hubble's discovery of the red shift of galaxies. Hubble's results, along with the discovery of radio waves whose patterns implied that the universe had been denser in the past and the discovery of microwave radiation supposedly left over from the Big Bang itself, destroyed the steady-state theory.
Note: if you found this page difficult to understand, review The Four Fundamental Forces and The Elementary Particles.
Created by Dan Corbett, Kate Stafford, and Patrick Wright for ThinkQuest.