Home Science and Technology Astronomy and Space Exploration Astronomy: General steady-state theory
steady-state theory
steady-state theory Cosmological theory proposed (1948) by Austrian astronomers Hermann Bondi and Thomas Gold, and further developed by Fred Hoyle and others. According to this theory, the Universe has always existed; it had no beginning and will continue forever. Although the universe is expanding, it maintains its average density – steady-state – through the continuous creation of new matter. Most cosmologists now reject the theory because it cannot explain background radiation or the observation that the appearance of the universe changes with time.
"steady-state theory." World Encyclopedia. . Encyclopedia.com. 13 Jul. 2019 <https://www.encyclopedia.com>.
"steady-state theory." World Encyclopedia. . Encyclopedia.com. (July 13, 2019). https://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/steady-state-theory
"steady-state theory." World Encyclopedia. . Retrieved July 13, 2019 from Encyclopedia.com: https://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/steady-state-theory
steady-state theory: see cosmology.
"steady-state theory." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. 13 Jul. 2019 <https://www.encyclopedia.com>.
"steady-state theory." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (July 13, 2019). https://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/steady-state-theory
"steady-state theory." The Columbia Encyclopedia, 6th ed.. . Retrieved July 13, 2019 from Encyclopedia.com: https://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/steady-state-theory
The Gale Encyclopedia of Science
Cosmological assumptions
Cosmological observations
Evolution of the universe
Steady–state theory
Arguments for and against the steady–state theory
Was there a moment of creation for the universe, or has the universe always existed? The steady-state theory (sometimes called the continuous creation theory or the infinite universe theory) is a cosmological theory for the origin of the universe that suggests the universe has always existed and did not have a moment of creation. This theory was popular during the 1950s and 1960s, but because of observations made during the 1960s, few, if any, astronomers now think that the steady–state theory is correct. The basic tenet of the steady–state theory is that the universe on a large scale does not change with time (evolve). It has always existed and will always continue to exist looking much as it does now. The universe is, however, known to be expanding. To allow for this expansion in an unchanging universe, the authors of the steady–state theory postulated that hydrogen atoms appeared out of empty space. These newly created hydrogen atoms were just enough to fill in the gaps caused by the expansion. Because hydrogen is continuously being created, the steady–state theory is sometimes called the continuous creation theory. This theory achieved great popularity for a couple of decades, but mounting observational evidence caused its demise in the late 1960s. The discovery in 1965 of the cosmic background radiation provided one of the most serious blows to the steady–state theory.
Models of the universe (cosmological models) are based on a set of assumptions. The first assumption is that physical laws are universal. Any science experiment, if performed under identical conditions, will have the same result anywhere in the universe because physical laws are the same everywhere in the universe. Second, on a sufficiently large scale the universe is homogeneous. Scientists know there is large scale structure in the universe, such as clusters of galaxies; so, they assume that the universe is homogenous only on scales large enough for even the largest structures to average out. Third, scientists assume that the universe is isotropic, meaning that there is no preferred direction in the universe. Fourth, they assume that over sufficiently long times the universe looks essentially the same at all times.
Collectively, the first three of the above assumptions are the cosmological principle (not to be confused with the perfect cosmological principle). Concisely, the cosmological principle states that the universe looks essentially the same at any location in the universe. Adding the fourth assumption—that the universe does not change on the large scale with time—gives scientists the perfect cosmological principle. Essentially, the universe looks the same at all times as well as at all locations within the universe. The perfect cosmological principle forms the philosophical foundation for the steady–state theory (i.e., with the addition of the fourth assumption that the universe does not evolve). Accordingly, observational evidence that the universe evolves would be evidence against the steady–state theory.
There are a number of observations that astronomers have made to test cosmological theories, including both the steady–state and the big bang theory. Some of these cosmological observations are described below.
When astronomers look at the most distant objects in the universe, they are looking back in time. For example if one observes a quasar that is three billion light–years away, it has taken the light three billion years to get here, because a light–year is the distance light travels in one year when in a vacuum. Astronomers are therefore seeing the quasar as it looked three billion years ago. Quasars, the most distant objects known in the universe, are thought to be very active nuclei of distant galaxies. The nearest quasar is about a billion light–years away. The fact that astronomers do not see any quasar closer than a billion light years away suggests that quasars disappeared at least a billion years ago. The universe has changed with time. Several billion years ago, quasars existed; they no longer do. This observation provides evidence that the perfect cosmological principle is untrue, and therefore that the steady–state theory is incorrect. Note, however, that when the steady–state theory and the perfect cosmological principle were first suggested, scientists had not yet discovered quasars.
In his work measuring distances to galaxies, American astronomer Edwin Hubble (1889–1953), after whom the Hubble space telescope was named, noticed an interesting correlation. The more distant a galaxy is, the faster it is moving away from the Earth. This relationship is called the Hubble law. This relationship can be used to find the distances to additional galaxies, by measuring the speed of recession. More importantly, Hubble deduced the cause of this correlation. His result: the universe is expanding. To visualize this expansion, draw some galaxies on an ordinary balloon and blow it up. Notice how the galaxies move farther apart as the balloon expands. Measuring distances between the galaxies at the rates at which they move apart, would give a relationship similar to Hubble's law.
The expanding universe can be consistent with either the big bang or the steady–state theory. However in the steady–state theory, new matter must appear to fill in the gaps left by the expansion. Normally as the universe expands, the average distance between galaxies would increase as the density of the universe decreases. These evolutionary changes with time would violate the fundamental assumption behind the steady–state theory. Therefore, in the steady–state theory, hydrogen atoms appear out of empty space and collect to form new galaxies. With these new galaxies, the average distance between galaxies remains the same even in an expanding universe.
The Hubble plot also provides evidence that the universe changes with time. The slope of the Hubble plot gives scientists the rate at which the universe is expanding. If the universe is not evolving, this slope should remain the same even for very distant galaxies. The measurements are difficult, but the Hubble plot seems to curve upward for the most distant galaxies. The universe was expanding faster in the distant past, contrary to the prediction of the steady–state theory that the universe is not evolving.
In the mid 1960s, German–born American physicist Arno Allan Penzias (1933–) and American physicist Robert Woodrow Wilson (1936–) were working on a low noise (static) microwave antenna when they made an accidental discovery of cosmic significance. After doing everything possible to eliminate sources of noise, including cleaning out nesting pigeons and their waste, there was still a small noise component left. This weak noise did not vary with direction or with the time of day or year, because it was cosmic in origin. It also corresponded to a temperature of about 3K (where K = Kelvin;–518°F;–270°C, three degrees above absolute zero). This 3K cosmic background radiation turned out to be the leftover heat from the initial Big Bang that had been predicted by proponents of the Big Bang theory as early as the 1940s.
Because this cosmic background radiation was a prior prediction of the big bang theory, it provided strong evidence to support the big bang theory. Proponents of the steady–state theory have been unable to explain in detail how this background radiation could arise in a steady–state universe. The cosmic background radiation therefore gave the steady–state theory its most serious setback. Penzias and Wilson received the 1978 Nobel Prize in physics for their work.
The steady–state theory was inspired at least in part by a 1940s movie entitled Dead of Night. The movie had four parts and a circular structure such that at the end the movie was the same as at the beginning. After seeing this movie in 1946, Austrian–born American astrophysicist Thomas Gold (1920–2004), Austrian–English mathematician and cosmologist Sir Hermann Bondi (1919–2005), and English astronomer Sir Fred Hoyle (1915–2001) wondered if the universe might not be constructed the same way. The discussion that followed led ultimately to the steady–state theory.
In 1948, Bondi and Gold proposed extending the cosmological principle to the perfect cosmological principle, so that the universe looks the same at all times as well as at all locations. They then proposed the steady–state theory based on the new perfect cosmological principle. Because Hubble had already observed that the universe is expanding, Bondi and Gold proposed the continuous creation of matter. Hydrogen atoms created from nothing combine to form galaxies. In this manner, the average density of the universe remains the same as the universe expands. In the steady–state theory, the rate at which new matter is created must exactly balance the rate at which the universe is expanding. Otherwise, the average density of the universe will change and the universe will evolve, violating the perfect cosmological principle. To maintain the steady–state, in a cubic meter of space one hydrogen atom must appear out of nothing every five billion years. In a volume of space the size of the Earth, the amount of new matter created would amount to roughly a grain of dust in a million years. In the entire observable universe, roughly one new galaxy per year will form from these atoms. Bondi and Gold recognized that a new theory must be developed to explain how the hydrogen atoms formed out of nothing, but did not suggest a new theory.
In the same year, Hoyle proposed a modification of German–American physicist Albert Einstein's (1879– 1955) general theory of relativity. Hoyle worked independently of Bondi and Gold, but they did discuss the new theories. Hoyle's modification used a mathematical device to allow the creation of matter from nothing, as implied in general relativity. No experiments or observations have been made to justify or contradict this modification of general relativity.
There are a number of problems with the steady–state theory, but at the time the theory was proposed, there were also points in its favor.
The steady–state theory rests on the foundation of the perfect cosmological principle. Hence, any evidence that the universe evolves is evidence against the steady–state theory. The existence of quasars and the change in the expansion rate of the universe a few billion years in the past, discussed earlier, are evidence against steady–state. This evidence for the evolution of the universe did not exist in 1948, when the steady–state theory originated. It became part of the cumulative weight of evidence, which had built up against the steady–state theory, by the mid 1960s. In gallant attempts to save the steady–state theory, its proponents, chiefly Hoyle and Indian astrophysicist Jayant Vishnu Narlikar (1938–), have argued that the universe can change over time periods of a few billion years without violating the perfect cosmological principle. Cosmologists (scientists that study the universe) must look at even longer time spans to see that these changes with time average out.
The cosmic background radiation is widely considered the final blow to the steady–state theory. Again, proponents of the steady–state theory have made gallant efforts to save their theory in the face of what most astronomers consider overwhelming evidence. They argue that the background radiation could be the cumulative radiation of a large number of radio sources that are too faint to detect individually. This scheme requires the existence of roughly 100 trillion (about 10,000 times the number of observable galaxies) such sources that are about one millionth as bright as the radio sources astronomers do detect. Few astronomers are willing to go to such great lengths to rescue the steady–state theory.
Another objection raised against the steady–state theory is that it violates one of the fundamental laws of physics as that law is currently understood. The law of conservation of matter and energy states that matter and energy are interchangeable and can change between forms, but the total amount of matter and energy in the universe must remain constant. It can be neither created nor destroyed. The steady–state theory requires continuous creation of matter in violation of this law. However, laws of science result from experimental evidence and are subject to change, not at mere whim, but as experimental results dictate. The rate at which matter is created in the steady–state theory is small enough that normally it would not have been noticed. Hence, scientists would not have discovered experimentally any conditions under which matter could be created or any modifications required in this law.
Were there ever any points in favor of the steady–state theory? When the steady–state model was first suggested, the best estimate of the age of the universe in the context of the big bang model was about two billion years. However, the Earth and solar system are about five billion years old. The oldest stars in the Milky Way galaxy are at least 10 to 12 billion years old. These age estimates present the obvious problem of a universe younger than the objects it contains. This problem is no longer so severe. Modern estimates for the age of the universe range from about 10 billion years to about 20 billion years, with a currently accepted average of about 13.7 billion years. In the steady–state theory the universe has always existed, so there are no problems presented by the ages of objects in the universe.
For some people there are philosophical or esthetic grounds for preferring the steady–state hypothesis over the big bang theory. The big bang theory has a moment of creation, which some people prefer for personal or theological reasons. Those who do not share this preference often favor the steady–state hypothesis. They prefer the grand sweep of a universe that has always existed to a universe that had a moment of creation and may, by inference, also have an end in some far distant future time.
Cosmic background radiation —The leftover heat radiation from the big bang.
Cosmological principle —The set of fundamental assumptions behind the big bang theory that state the universe is essentially the same at all locations.
Hubble's law —The law that states a galaxy's red–shift is directly proportional to its distance from the Earth. This observation that tells scientists that the universe is expanding: distant galaxies are receding at a speed proportional to their distance.
Perfect cosmological principle —The set of fundamental assumptions behind the steady–state theory that state the universe is essentially the same at all locations and times.
The weight of evidence against the steady–state theory has convinced most modern astronomers and cosmologists, as of 2006, that it is incorrect and that the big bang theory is correct. The steady–state theory does, however, still stand as a major intellectual achievement and as an important part of the history of the development of cosmology in the twentieth century.
See also Doppler effect.
Bacon, Dennis Henry, and Percy Seymour. A Mechanical History of the Universe. London: Philip Wilson Publishing, Ltd., 2003.
Chaisson, Eric. Astronomy: A Beginner's Guide to the Universe. Upper Saddle River, NJ: Pearson/Prentice Hall, 2004.
Cheng, Ta–Pei. Relativity, Gravitation, and Cosmology: A Basic Introduction. Oxford, UK, and New York: Oxford University Press, 2005.
Harland, David Michael. The Big Bang: A View from the 21st Century. New York: Springer, 2003.
Lemonick, Michael D. Echo of the Big Bang. Princeton, NJ: Princeton University Press, 2005.
Mallary, Michael. Our Improbably Universe: A Physicist Considers How we Got Here. New York: Thunder's Mouth Press, 2004.
Mitton, Simon. Conflict in the Cosmos: Fred Hoyle's Life in Science. Washington, DC: Joseph Henry Press, 2005.
Singh, Simon. Big Bang: The Origins of the Universe. New York: Harper Perennial, 2005.
Paul A. Heckert
"Steady-State Theory." The Gale Encyclopedia of Science. . Encyclopedia.com. 13 Jul. 2019 <https://www.encyclopedia.com>.
"Steady-State Theory." The Gale Encyclopedia of Science. . Encyclopedia.com. (July 13, 2019). https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/steady-state-theory-0
"Steady-State Theory." The Gale Encyclopedia of Science. . Retrieved July 13, 2019 from Encyclopedia.com: https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/steady-state-theory-0
Was there a moment of creation for the universe, or has the universe always existed? The steady-state theory is a cosmological theory for the origin of the universe that suggests the universe has always existed and did not have a moment of creation. This theory was popular during the 1950s and 1960s, but because of observations made during the 1960s, few, if any, astronomers now think that the steady-state theory is correct. The basic tenet of the steady-state theory is that the universe on a large scale does not change with time (evolve). It has always existed and will always continue to exist looking much as it does now. The universe is, however, known to be expanding. To allow for this expansion in an unchanging universe, the authors of the steady-state theory postulated that hydrogen atoms appeared out of empty space . These newly created hydrogen atoms were just enough to fill in the gaps caused by the expansion. Because hydrogen is continuously being created, the steady-state theory is sometimes called the continuous creation theory. This theory achieved great popularity for a couple of decades, but mounting observational evidence caused its demise in the late 1960s. The discovery in 1965 of the cosmic background radiation provided one of the most serious blows to the steady-state theory.
The steady-state model is based on a set of four assumptions collectively known as the perfect cosmological principle. The first assumption is that physical laws are universal. Any science experiment, if performed under identical conditions, will have the same result anywhere in the universe because physical laws are the same everywhere in the universe. Second, on a sufficiently large scale the universe is homogeneous. We know there is large scale structure in the universe, such as clusters of galaxies; so, we assume that the universe is homogenous only on scales large enough for even the largest structures to average out. Third, we assume that the universe is isotropic, meaning that there is no preferred direction in the universe. Fourth, we assume that over sufficiently long times the universe looks essentially the same at all times.
Collectively, the first three of the above assumptions are the cosmological principle (not to be confused with the perfect cosmological principle). In a nutshell, the cosmological principle states that the universe looks essentially the same at any location in the universe. This principle remains a largely untested assumption because we cannot travel to every location in the universe to perform experiments to test the assumption. Adding the fourth assumption, that the universe does not change on the large scale with time, gives us the perfect cosmological principle. Essentially, the universe looks the same at all times as well as at all locations within the universe. The perfect cosmological principle forms the philosophical foundation for the steady-state theory. With the addition of the fourth assumption, the universe does not evolve in the steady-state theory, so observational evidence that the universe evolves would be evidence against the steady-state theory.
There are a number of observations that astronomers have made to test cosmological theories, including both the steady-state and the big bang theory . Some of these cosmological observations are described below.
When we look at the most distant objects in the universe, we are looking back in time. For example if we observe a quasar that is three billion light years away, it has taken the light three billion years to get here, because a light year is the distance light travels in one year. We are therefore seeing the quasar as it looked three billion years ago. Quasars, the most distant objects known in the universe, are thought to be very active nuclei of distant galaxies. The nearest quasar is about a billion light years away. The fact that we do not see any quasar closer than a billion light years away suggests that quasars disappeared at least a billion years ago. The universe has changed with time. Several billion years ago, quasars existed; they no longer do. This observation provides evidence that the perfect cosmological principle is untrue, and therefore that the steady-state theory is incorrect. Note, however, that when the steady-state theory and the perfect cosmological principle were first suggested, we had not yet discovered quasars.
In his work measuring distances to galaxies, Edwin Hubble, after whom the Hubble space telescope was named, noticed an interesting correlation. The more distant a galaxy is, the faster it is moving away from us. This relationship is called the Hubble law. This relationship can be used to find the distances to additional galaxies, by measuring the speed of recession. More importantly, Hubble deduced the cause of this correlation. The universe is expanding. To visualize this expansion, draw some galaxies on an ordinary balloon and blow it up. Notice how the "galaxies" move farther apart as the balloon expands. Measuring distances between the drawn in galaxies at the rates at which they move apart, would give a relationship similar to Hubble's law.
The expanding universe can be consistent with either the big bang or the steady-state theory. However in the steady-state theory, new matter must appear to fill in the gaps left by the expansion. Normally as the universe expands, the average distance between galaxies would increase as the density of the universe decreases. These evolutionary changes with time would violate the fundamental assumption behind the steady-state theory. Therefore, in the steady-state theory, hydrogen atoms appear out of empty space and collect to form new galaxies. With these new galaxies, the average distance between galaxies remains the same even in an expanding universe.
The Hubble plot also provides evidence that the universe changes with time. The slope of the Hubble plot gives us the rate at which the universe is expanding. If the universe is not evolving, this slope should remain the same even for very distant galaxies. The measurements are difficult, but the Hubble plot seems to curve upward for the most distant galaxies. The universe was expanding faster in the distant past, contrary to the prediction of the steady-state theory that the universe is not evolving.
In the mid 1960s, Arno Penzias and Robert Wilson were working on a low noise (static) microwave antenna when they made an accidental discovery of cosmic significance. After doing everything possible to eliminate sources of noise, including cleaning out nesting pigeons and their waste, there was still a small noise component left. This weak noise did not vary with direction or with the time of day or year, because it was cosmic in origin. It also corresponded to a temperature of 3K (-518°F; -270°C, three degrees above absolute zero,). This 3K cosmic background radiation turned out to be the leftover heat from the initial big bang that had been predicted by proponents of the big bang theory as early as the 1940s.
Because this cosmic background radiation was a prior prediction of the big bang theory, it provided strong evidence to support the big bang theory. Proponents of the steady-state theory have been unable to explain in detail how this background radiation could arise in a steady-state universe. The cosmic background radiation therefore gave the steady-state theory its most serious setback. Penzias and Wilson received the 1978 Nobel Prize in physics for their work.
The steady-state theory was inspired at least in part by a 1940s movie entitled Dead of Night. The movie had four parts and a circular structure such that at the end the movie was the same as at the beginning. After seeing this movie in 1946, Thomas Gold, Hermann Bondi, and Fred Hoyle wondered if the universe might not be constructed the same way. The discussion that followed led ultimately to the steady-state theory.
In 1948, Hermann Bondi and Thomas Gold proposed extending the cosmological principle to the perfect cosmological principle, so that the universe looks the same at all times as well as at all locations. They then proposed the steady-state theory based on the new perfect cosmological principle. Because Hubble had already observed that the universe is expanding, Bondi and Gold proposed the continuous creation of matter. Hydrogen atoms created from nothing combine to form galaxies. In this manner the average density of the universe remains the same as the universe expands. In the steady-state, the rate at which new matter is created must exactly balance the rate at which the universe is expanding. Otherwise, the average density of the universe will change and the universe will evolve, violating the perfect cosmological principle. To maintain the steady-state, in a cubic meter of space one hydrogen atom must appear out of nothing every five billion years. In a volume of space the size of the earth the amount of new matter created would amount to roughly a grain of dust in a million years. In the entire observable universe, roughly one new galaxy per year will form from these atoms. Bondi and Gold recognized that a new theory must be developed to explain how the hydrogen atoms formed out of nothing, but did not suggest a new theory.
In the same year, Fred Hoyle proposed a modification of Einstein's general theory of relativity. Hoyle worked independently of Bondi and Gold, but they did discuss the new theories. Hoyle's modification used a mathematical device to allow the creation of matter from nothing in general relativity. No experiments or observations have been made to justify or contradict this modification of general relativity.
Arguments for and against the steady-state theory
There are a number of problems with the steady-state theory, but at the time the theory was proposed, there were also points in its favor.
The steady-state theory rests on the foundation of the perfect cosmological principle. Hence, any evidence that the universe evolves is evidence against the steady-state theory. The existence of quasars and the change in the expansion rate of the universe a few billion years in the past, discussed earlier, are evidence against the steady-state. This evidence for the evolution of the universe did not exist in 1948, when the steady-state theory originated. It became part of the cumulative weight of evidence that had built up against the steady-state theory by the mid 1960s. In gallant attempts to save the steady-state theory, its proponents, chiefly Hoyle and Jayant Narlikar, have argued that the universe can change over time periods of a few billion years without violating the perfect cosmological principle. We must look at even longer time spans to see that these changes with time average out.
The cosmic background radiation is widely considered the final blow to the steady-state theory. Again, proponents of the steady-state theory have made gallant efforts to save their theory in the face of what most astronomers consider overwhelming evidence. They argue that the background radiation could be the cumulative radiation of a large number of radio sources that are too faint to detect individually. This scheme requires the existence of roughly 100 trillion (about 10,000 times the number of observable galaxies) such sources that about are one millionth as bright as the radio sources we do detect. Few astronomers are willing to go to such great lengths to rescue the steady-state theory.
Another objection raised against the steady-state theory is that it violates one of the fundamental laws of physics as that law is currently understood. The law of conservation of matter and energy states that matter and energy are interchangeable and can change between forms, but the total amount of matter and energy in the universe must remain constant. It can be neither created nor destroyed. The steady-state theory requires continuous creation of matter in violation of this law. However, laws of science result from our experimental evidence and are subject to change, not at our whim, but as experimental results dictate. The rate at which matter is created in the steady-state theory is small enough that we would not have noticed. Hence we would not have discovered experimentally any conditions under which matter could be created or any modifications required in this law.
Were there ever any points in favor of the steady-state theory? When the steady-state model was first suggested, our best estimate of the age of the universe in the context of the big bang model was about two billion years. However, the earth and solar system are about five billion years old. The oldest stars in our galaxy are at least 10-12 billion years old. These age estimates present the obvious problem of a universe younger than the objects it contains. This problem is no longer so severe. Modern estimates for the age of the universe range from about 10 billion years to about 20 billion years. The lower end of this range still has the problem, but the upper end of the range gives a universe old enough to contain the oldest objects we have found so far. In the steady-state theory the universe has always existed, so there are no problems presented by the ages of objects in the universe.
For some people there are philosophical or esthetic grounds for preferring the steady-state hypothesis over the big bang theory. The big bang theory has a moment of creation, which some people prefer for personal or theological reasons. Those who do not share this preference often favor the steady-state hypothesis. They prefer the grand sweep of a universe that has always existed to a universe that had a moment of creation and may, by inference, also have an end in some far distant future time.
The weight of evidence against the steady-state theory has convinced most modern astronomers that it is incorrect. The steady-state theory does, however, still stand as a major intellectual achievement and as an important part of the history of the development of cosmology in the twentieth century.
Harrison, Edward R. Cosmology The Science of the Universe. Cambridge: Cambridge University Press, 1981.
Hoyle, Fred, and Jayant Narlikar. The Physics-AstronomyFrontier. San Francisco: Freeman. 1980.
Hoyle, Fred. Astronomy. London: Rathbone Books, 1962.
Narlikar, Jayant V. The Primeval Universe. Oxford: Oxford University Press, 1988.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—The leftover heat radiation from the big bang.
Cosmological principle
—The set of fundamental assumptions behind the big bang theory that state the universe is essentially the same at all locations.
Hubble's law
—The law that states a galaxy's red-shift is directly proportional to its distance from Earth.This observation that tells us the universe is expanding: distant galaxies are receding at a speed proportional to their distance.
Perfect cosmological principle
—The set of fundamental assumptions behind the steady-state theory that state the universe is essentially the same at all locations and times.
"Steady-State Theory." The Gale Encyclopedia of Science. . Encyclopedia.com. (July 13, 2019). https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/steady-state-theory
"Steady-State Theory." The Gale Encyclopedia of Science. . Retrieved July 13, 2019 from Encyclopedia.com: https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/steady-state-theory