Big bang theory
Big bang theory
The big bang theory is the conceptual model that scientists use to describe the origin and subsequent evolution of the Universe. It states that the universe began as a tiny, violent explosion about 14 billion years ago. That event produced all of the matter and energy in the Universe, including its hydrogen and helium. Some of these light atoms were forged in the cores of stars, over billions of years, into atoms of the heavier elements that exist today, including the atoms of which we ourselves are made. One consequence of the big bang is that today the Universe, which is of finite size and contains a finite amount of matter, is expanding; in fact, the occurrence of the big bang was originally deduced from the fact of the universe’s expansion. In recent years astronomers have made many observations that verify predictions of the big bang theory.
Studying the Universe
People have always wondered about the origin of the Universe. Questions about how and when Earth and the heavens formed have been pondered by tribal peoples, philosophers, religious thinkers, and scientists. The modern, scientific study of the origin and structure of the universe is known as cosmology.
For many centuries, cosmological thought was limited mostly to speculation. For example, it was not obvious to the ancients that the sun and the stars are objects of the same sort. The sun is a star much like the other stars and is only brighter because it is closer, but this fact took centuries to determine because it is difficult to measure the distance to most of the objects seen in the night sky. Early astronomers of the scientific era, although they knew that the stars are also suns, assumed that all stars have the same intrinsic brightness and thus that only their distance from Earth determines their apparent brightness. This was incorrect; in fact, enormous variations in brightness among individual stars exist. Examination of binary stars (paired stars that orbit each other) demonstrated these differences. (About half of all stars in binary or other multiple systems.) When binary star systems in which the two stars did not have the same brightness were observed, it became clear that the amount of light received from any given star is dependent on more than just its distance. Until the elementary task of measuring the position of astronomical objects could be pursued systematically, larger questions about the structure and history of the universe as a whole could not even begin to be answered.
Measurement techniques
All measurements of the stars must be made in the neighborhood of the Earth, since any distance achievable by a spacecraft in less than many years of travel still qualifies as “in the neighborhood of the Earth.” The nearest star other than the sun is more than four light-years away, and most objects seen from Earth, even with the naked eye, are much farther off. (A light-year is the distance that light travels in one year: 5.88 trillion miles [9.46 trillion kilometers], or about 60,000 times the distance from the Earth to the sun.)
There are two fairly direct ways to determine the distance to the nearest stars. The first is to measure their parallax or apparent change in position during the year. As the Earth circles the sun, stars are seen from a shifting vantage point. The furthest objects do not appear to move because the Earth’s change in position is too small to affect our view, but the nearest stars seem to move back and forth slightly during the course of a year. Parallax can be seen by holding up a finger a few inches before your eyes and closing first one eye, then the other, thus repeatedly shifting your point of view by the distance between your eyes: the finger seems to jump back and forth dramatically, while objects across the room move much less. The shift in the finger’s apparent position is its parallax. By measuring the parallax of a nearby star over a six-month period (during which the Earth moves from one side of its orbit to the other), and knowing the radius of the Earth’s orbit, it is a matter of straightforward trigonometry to determine the distance to that star.
Another technique to determine stellar distances is to measure the proper motion of a star. This is the apparent motion of a star with respect to other stars caused by the star’s actual motion through the sky. (All stars are moving, including the sun.) Although the motion of distant stars is too small to detect, closer stars can be seen changing position with respect to more distant stars over the years.
Such techniques are only applicable to a few of the nearest stars, however, and disclose nothing about the large-scale structure of the universe. More sophisticated methods had to be developed for this task, requiring different astronomical observations. One such method depends on the examination of a star’s (or other celestial object’s) spectrum, that is, the intensity of its radiation (including, but not limited to, its visible light) at various wavelengths. If the light from a star is divided into its component wavelengths using a prism, a continuous spread of wavelengths punctuated by a number of dark lines can be seen. (The visible part of our sun’s spectrum is the rainbow.) These absorption lines are caused by elements in the star’s outer atmosphere that absorb light at specific wavelengths. Each dark line in a star’s spectrum corresponds to a specific element; the absorption lines in a star’s spectrum thus give a catalogue of the substances in its outer layers. Furthermore, these lines can reveal how fast the star is approaching the Earth or receding from it using the Doppler effect, a fundamental property of all traveling waves (including light waves). The absorption lines in the spectra of objects moving away from the Earth are shifted to longer wavelengths, while absorption lines in the spectra of objects moving toward the Earth are shifted to shorter wavelengths. A shift to longer wavelengths is called a redshift because red light appears near the long-wavelength end of the visible spectrum, while a shift to shorter wavelengths is called a blueshift. Measurement of the Doppler shifting of spectral lines has made it possible to map the large-scale structure of the cosmos, and it is this structure that the theory of the big bang and the theory of general relativity explain.
Historical background
In 1905, Danish astronomer Ejnar Hertzsprung (1873–1967) compared the width of various stars’ absorption lines to the absolute luminosity, or brightness of the stars as determined from proper motion measurements. Hertzsprung found that that wider lines correspond to larger and brighter stars. This provided a way to determine the absolute brightness of a star from its spectrum. Knowing its absolute brightness, he could then determine its distance from Earth. This method applied to stars at any distance, as opposed to the parallax and absolute-motion methods, which applied only to stars quite near the sun, but was limited in accuracy.
In 1908, American astronomer Henrietta Swan Leavitt (1868—1921) discovered that Cepheid variables, a type of star in which the brightness changes in a regular manner, showing a well-defined relationship between period (time required for brightness to wax and wane through a full cycle) and absolute luminosity. Brighter Cepheid variable stars have longer periods while dimmer ones have shorter periods. Leavitt calculated a simple relationship between brightness and period. This discovery had a profound effect on stellar distance measurements. Now, any time a Cepheid could be found—in, say, a distant galaxy— the distance to it could be determined accurately.
The spiral nebulae
In the early twentieth century, there was a debate among astronomers over the nature of the spiral nebulae, the diffuse spiral-shaped structures visible (through telescopes) in most parts of the sky. Some believed these were nearby objects that were part of our Milky Way galaxy, while others thought that they were much further away, and in fact were “island universes,” or separate galaxies. If the distances to these objects could be measured, then the debate could be settled, and important knowledge gained about the structure of the Universe.
In 1914, American astronomer Vesto M. Slipher (1875–1969) presented figures for the velocities of 14 spiral nebulae, obtained by measuring the Doppler shifts of their spectral lines as described above. Slipher found that most of the nebulae were, surprisingly, moving away from Earth. If the nebulae’s motions were random, just as many of them would be expected to move toward Earth as were moving away. Why should the sun just happen to be at the center of an organized pattern of motion? Another puzzling finding was the large velocities at which these objects were receding. The nearby Andromeda nebula, for example, was speeding toward earth at 180 miles per second (300 km per second). Many astronomers interpreted this to mean that the nebulae must be outside our galaxy.
In 1923, American astronomer Edwin Hubble (1889–1953), using the 60-in and 100-in (152-cm and 254-cm) telescopes at Mount Wilson Observatory, succeeded in identifying Cepheid variables in the outer regions of two nebulae, M31 and M33. By measuring the periods of these Cepheids and using the formula developed by Leavitt several years earlier, he calculated that they are about 930,000 light years distant. From these distances and the observed sizes of the nebulae, their actual sizes could be calculated. These turned out to be similar to that of Earth’s galaxy, strongly supporting the idea that the nebulae are galaxies in their own right.
In 1929, Hubble plotted data on a number of galaxies on a graph. He plotted the distance to the galaxy along the horizontal axis and the velocity of the galaxy’s recession along the vertical axis. From this limited data, it was clear that a simple, linear relationship between the two quantities existed; on average, the velocity of a galaxy’s recession was proportional to the distance to the galaxy. (The Andromeda galaxy is a member of our local galactic group, which moves more or less and as a unit: other members of the Local Group do obey this general red-shift rule.) The constant of proportionality between distance and velocity, now called the Hubble’s constant (H ), was given by the slope of the line. From those data, Hubble’s constant was estimated to be 310 miles per second per megaparsec (500 km/s/Mpc)—that is, a galaxy 1 megaparsec (one million parsecs, where one parsec = 3.26 light-years) away would be moving away from our galaxy at 310 mi/s (500 km/s), while a galaxy ten times further away would be moving ten times as fast. Modern values for H are much smaller than Hubble’s estimate of 500 km/s/Mpc. The relationship between speed and distance described by Hubble’s constant is termed Hubble’s law.
Implications of Hubble’s law
Galaxies are moving away from Earth in all directions not because Earth is at the center of the universe but because all galaxies are receding from each other, that is, every galaxy is getting further away from every other galaxy. There is a simple way to visualize this effect. Imagine a partially blown up balloon, on the surface of which a number of spots (representing galaxies) are drawn. As the balloon inflates, each spot gets farther away from its neighbors; furthermore, the distance from any one spot to any other on the same side of the balloon (as measured on the surface of the balloon) grows faster the farther away from each other the two spots become. This can be seen by imagining three spots in a row (A, B, and C), with the balloon’s surface expanding evenly between them. The distance from A to B is growing at x inches per second, and so is the distance from B to C; the distance from A to C must therefore be growing at 2 x in/s (x + x = 2 x ). Thus no matter where an observer is located on the balloon, they will observe that all the other spots on the balloon’s surface obey Hubble’s law: the farther away they are, the faster they recede. There is no preferred direction, no preferred position, and no “center” of the balloon’s surface. The behavior of galaxies in our spatially three-dimensional universe is analogous to that of the spots on the two-dimensional surface of the balloon.
There is an important implication to this model. If all galaxies are moving away from each other with a velocity proportional to their separation, then at all earlier times the galaxies were closer together. If one goes back far enough, there must have been a time when all were at the same position—that is, there must have been a beginning to the universe. In fact, if the expansion has been constant for all time, the age of the universe is simply the inverse of Hubble’s constant. The situation is not quite so simple, as scientists expect Hubble’s constant to change over time (the gravitational attraction between the galaxies has a tendency to slow the rate of expansion, while recent observations show that the expansion of the universe is actually accelerating under the influence of a still-mysterious force that acts opposite to gravity).
Hubble’s original measurements gave an age of the universe of about two billion years. This immediately caused dispute, since it was known from measurements of radioactive decay that the age of the solar system is more than twice this value. How could the solar system have been formed before the universe itself? It is now known that Hubble’s original measurements were in error. Current measurements put Hubble’s constant in the range of 50–100, giving an age of 10–20 billion years. Geological and astronomical data from many sources have recently converged on a value for the age of the universe of about 13.7 billion years.
Other developments
When Einstein developed his general theory of relativity he added a term, the cosmological constant, to his equations in order to permit a static universe which was neither expanding nor contracting. He later came to regret this, calling it the worst mistake he ever made; however, the recent discovery that the expansion of the universe is actually accelerating has brought Einstein’s cosmological constant back into favor. Nevertheless, Russian mathematician Alexander Friedmann (1888–1925) and Belgian astronomer Georges LeMaître (1894–1966) found solutions (in 1922 and 1927, respectively) to Einstein’s equations that permitted an expanding universe. After Hubble’s 1929 discovery, there was a great deal of interest in these models, which could be used to explain the observations.
A big bang seemed an obvious implication of the new data—if everything was expanding, it must all once have been scrunched together—but steady-state models that avoided the embarrassment of a universe with a definite beginning had adherents for decades after Hubble’s measurements were made. One of the more promising models, constant creation, postulated that new hydrogen atoms form constantly and spontaneously throughout space, out of nothing, providing the material for new galaxies as the older galaxies move apart. On this theory, the universe has always been expanding, had no beginning, has always looked as it does now, and will always look as it does now. This theory predicted that nearby galaxies will look similar to those far away, but it was found that distant galaxies are in fact different from nearby ones, which agrees with the big bang’s claim that the universe is not in a steady state. It was one of the originators of this steady-state theory, British astronomer Fred Hoyle (1915–2001), who coined the term big bang, now used to describe the expanding-universe model based on Hubble’s observations. Hoyle chose the expression to ridicule the theory, but the name stuck.
The evolution of the Universe
The current picture of the big bang can be described briefly as follows. Because current formulations of the laws of physics break down very close to the big bang itself, the account will start one second after the event occurs. At this time, the temperature was 10,000,000,000K. This was too hot for atoms to exist, so their elementary particle constituents (electrons, protons, and neutrons) existed separately, along with photons (particles of light), and various exotic particles. Over the next 100 seconds, the temperature dropped by a factor of 10, enough to allow the nuclei of light elements such as deuterium (an isotope of hydrogen) and helium to form. As further cooling took place, these nuclei combined with electrons to form atoms.
At this point it should be stressed that the expansion of the Universe means that space itself is expanding. This differs fundamentally from an ordinary explosion, in which matter expands into a surrounding volume of space. The expansion of space itself can be compared to the increase of the surface area of the inflating balloon mentioned earlier; as the balloon expands, its surface area grows, but not by expanding into any larger, surrounding surface, as a circular ripple expands across the surface of a pond. Similarly, our universe is not expanding into any larger, surrounding volume of space.
The expansion of space has important cosmological implications. One is that as space expands, the average temperature of the Universe drops with it. This cooling has an important effect on the cosmic background radiation.
Early in the history of the universe, when its density was extremely high, particles and radiation were in equilibrium, meaning that there was a very uniform temperature distribution. Such a distribution gives rise to radiation with a particular spectrum, a blackbody spectrum, which has a well-defined peak wavelength. Radiation of this type currently pervades all space in the form of microwave radiation, the afterglow of the big bang. Due to expansion of the Universe, the peak of this radiation’s spectrum—its temperature—has by now been shifted to below 3K (–454°F [–270°C]), or three degrees above absolute zero, despite its initial high temperature. The cosmic background radiation was first detected by U.S. astrophysicist Arno Penzias (1933–) and U.S. radio astronomer Robert Wilson (1936–) in 1965. Measurements from the COBE spacecraft have shown that the spectrum is nearly perfect blackbody radiation at 2.73K (–454.5°F [–270.27°C]), as predicted by the big bang theory.
As described above, only the lightest elements were created in the big bang itself. As the Universe expanded, lumpiness developed, and regions of more-dense and less-dense gas formed. Gravity eventually caused the high-density areas to coalesce into stars and galaxies, which became luminous due to nuclear reactions in their cores. These reactions, which still power the stars today, take hydrogen and helium and create some of the heavier elements. Once its light-element nuclear fuels are exhausted, a star may explode in a supernova, creating still heavier elements in the process. It is these heavier elements from which the solar system, Earth, and humans are made. Every atom in the human body (of every element other than hydrogen; there is little or no helium in the body) was created in the core of an exploding star billions of years ago.
What will be the ultimate fate of the Universe? Will it continue to expand forever, or will it eventually contract in a “big crunch?” To understand this question, the analogy of a projectile being launched from the surface of Earth can be used. If a projectile is launched with enough velocity, it will escape the Earth’s gravity and travel on forever. If it is too slow, however, gravity will pull it back to the ground. This same effect is at work in the universe today. If there is enough mass in the universe, the force of gravity acting between all matter will eventually cause the expansion to slow, stop, and reverse, and the universe will become smaller and smaller until it ultimately collapses.
This does not seem likely. Astronomers have made estimates of the mass in the Universe based on the luminous objects they see, and calculated a total mass much less than that required to close the universe, that is, to keep it from expanding forever. From other measurements, they know that there is a large amount of unseen mass in the Universe as well, called dark matter. The amount of this dark matter is not known precisely, but greatly exceeds that of all the stars in the Universe. The nature of dark matter is being intensely researched and debated by astronomers. It should not be confused with “dark energy,” a separate phenomenon.
In 1998, astronomers studying a certain group of supernovas discovered that the older objects were receding at a speed about the same as the younger objects. According to the theory of a closed universe, the expansion of the universe should slow down as it ages, and older supernovas should be receding more rapidly than the younger supernovas. The fact that observations have shown the opposite has led many scientists to believe that the Universe is, in fact, open. Other theorists hold that the universe is flat—that is, that it will neither collapse nor expand forever, but will maintain a gravitational balance between the two and remain in a coasting expansion.
Starting in the late 1990s, various observations have proved—to astronomers’ astonishment—that the expansion of the universe is actually accelerating. It thus appears that the fate of the universe will be to expand without end. Eventually, all sources of energy will exhaust themselves, and after many trillions of years even the protons and neutrons of which ordinary matter is constructed will break down. If this vision is correct, the universe will end up as a diffuse, eternally expanding gas of subatomic particles at a uniform temperature.
Future work
Although the big bang model has done a good job of explaining what is seen of the universe and has been confirmed by numerous astronomical observations, there are still many unanswered questions. Until a few years ago, there was still disagreement about the exact value of the Hubble constant by approximately a
KEY TERMS
Cepheid variable star— A class of young stars that cyclically brighten and dim. From the period of its brightness variation, the absolute brightness of a Cepheid variable can be determined. Cepheid variables in distant galaxies give a measure of the absolute distance to those galaxies.
Hubble constant— The constant of proportionality in Hubble’s law which relates the recessional velocity and distance of remote objects in the universe whose motion is determined by the general expansion of the universe.
Light-year— The distance that light travels in one year, equal to 5.87 million miles 9.46 million km.
Parsec— 3.26 light-years.
factor of two. NASA’s WMAP (Wilson Microwave Anisotropy Probe) satellite began making observations in 2003 to resolve the question; as of 2006, a value for the Hubble constant of 70 km/s/Mpc (with an uncertainty of +2.4 to –3.2 km/s/Mpc) had been agreed upon by most astronomers. WMAP data has also revealed that the universe consists of only 4% ordinary matter—known particles of whatever kind. Twenty-two percent of the Universe is dark matter, the nature of which is unknown but which is known to gravitate like other matter, and 75% of the Universe consists of “dark energy,” the nature of which is also unknown but which serves to accelerate the Universe’s expansion. Thus, until breakthrough discoveries in fundamental physics reveal the nature of the dark matter and dark energy, we remain ignorant of the nature of 96% of the Universe.
Another open question is how galaxies actually formed from what was very close to a uniform, homogeneous medium in the early universe. From the uniformity of the microwave background radiation, it is known that this uniformity was better than one part in a thousand. Just by looking at the sky, however, a great deal of structure in the Universe is seen up to very large-size scales of clusters of galaxies and beyond. There must have been some type of clumping which occurred to start the process (with gravity helping the process along), but what started it? Physicists are seeking the answer in quantum mechanics, the science of very small events. The answer resides there because at the moment of the big bang, the universe was subatomic in size. Random fluctuations at the quantum level, which today appear only at the subatomic level, were then sufficient—for an extremely brief interval of time—to cause ripples throughout the entire Universe. These ripples persist today as large-scale clumpings of the galaxies, which have been observed by astronomers and found to agree with the big bang theory.
See also Blackbody radiation; Elements, formation of; Redshift.
Resources
BOOKS
Fleisher, Paul. The Big Bang. Breckenridge, CO: Twenty-First Century Books, 2005.
Lemonick, Michael D. The Echo of the Big Bang. Princeton, NJ: Princeton University Press, 2005.
Singh, Simon. Big Bank: The Origin of the Universe. New York: Harper Perennial, 2005.
PERIODICALS
Beers, Timothy C. “The First Generations of Stars.” Science. 309 (2005): 390-391.
Weinberg, David H. “Mapping the Large-Scale Structure of the Universe.” Science. 309 (2005): 564-565.
Wilczek, Frank. “Particle Physics: Did the Big Bang Boil?.” Nature. 443 (2006): 637-638.
Larry Gilman