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FAQ: What is the evidence on Big Bang versus steady state cosmologies?
Let's consider this question first under the assumption that general relativity and standard quantum mechanics are valid. (Both QM and GR have been verified to high precision by a wide variety of empirical tests.[Will]) After that we'll see what happens if this assumption is relaxed.
We have a variety of evidence that the universe's state has been changing over time:
The Hubble law is observed. If standard quantum mechanics is valid, then these redshifts cannot be intrinsic to the emitting body. If general relativity is valid, then these redshifts are to be explained by the expansion of the universe. The Hubble expansion requires that the matter in the universe become more dilute over time. If general relativity is valid, then mass-energy is locally conserved, so there is no possibility of spontaneously creating more matter to "fill in the gaps."
When we view light from the deep sky that has been traveling through space for billions of years, we observe a universe that looks different from today's. For example, quasars were common in the early universe but are uncommon today.
Most dramatically, we observe the cosmic microwave background radiation. The universe full of hot, dense gas that emitted the CMB is clearly nothing like today's universe.
Not only has the universe changed over time, but there is a great deal of evidence that it has a finite age:
In the present-day universe, stars use up deuterium nuclei, but there are no known processes that could replenish their supply. We therefore expect that the abundance of deuterium in the universe should decrease over time. If the universe had existed for an infinite time, we would expect that all its deuterium would have been lost, and yet we observe that deuterium does exist in stars and in the interstellar medium.
The second law of thermodynamics predicts that any system should approach a state of thermodynamic equilibrium, and yet our universe is very far from thermal equilibrium, as evidenced by the fact that our sun is hotter than interstellar space, or by the existence of functioning heat engines such as your body or an automobile engine.
The combination of all these observations clearly establishes that static cosmological models are not consistent with observation, provided that general relativity and quantum mechanics are valid.
Around 1948, Hoyle and others created a steady-state cosmological model by relaxing general relativity's prohibition on the spontaneous creation of matter. A detailed account of the evidence against this model, and later variations, is given by Wright. The model was falsified in the 1950's by counts of faint radio sources. It is also inconsistent with observed abundances of helium and with the discovery of the CMB in 1965. An oscillating variant called Quasi-Steady State Cosmology was proposed by Hoyle, Burbidge, and Narlikar in 1993, but it was inconsistent with preexisting observations. They later produced a modification of the model, which is also inconsistent with observation.
Will, "The confrontation between general relativity and experiment," http://relativity.livingreviews.org/Articles/lrr-2006-3/
Wright, "Errors in the Steady State and Quasi-SS Models," http://www.astro.ucla.edu/~wright/stdystat.htm
The following forum members have contributed to this FAQ:
bcrowell
George Jones
jim mcnamara
marcus
PAllen
tiny-tim
vela
Let's consider this question first under the assumption that general relativity and standard quantum mechanics are valid. (Both QM and GR have been verified to high precision by a wide variety of empirical tests.[Will]) After that we'll see what happens if this assumption is relaxed.
We have a variety of evidence that the universe's state has been changing over time:
The Hubble law is observed. If standard quantum mechanics is valid, then these redshifts cannot be intrinsic to the emitting body. If general relativity is valid, then these redshifts are to be explained by the expansion of the universe. The Hubble expansion requires that the matter in the universe become more dilute over time. If general relativity is valid, then mass-energy is locally conserved, so there is no possibility of spontaneously creating more matter to "fill in the gaps."
When we view light from the deep sky that has been traveling through space for billions of years, we observe a universe that looks different from today's. For example, quasars were common in the early universe but are uncommon today.
Most dramatically, we observe the cosmic microwave background radiation. The universe full of hot, dense gas that emitted the CMB is clearly nothing like today's universe.
Not only has the universe changed over time, but there is a great deal of evidence that it has a finite age:
In the present-day universe, stars use up deuterium nuclei, but there are no known processes that could replenish their supply. We therefore expect that the abundance of deuterium in the universe should decrease over time. If the universe had existed for an infinite time, we would expect that all its deuterium would have been lost, and yet we observe that deuterium does exist in stars and in the interstellar medium.
The second law of thermodynamics predicts that any system should approach a state of thermodynamic equilibrium, and yet our universe is very far from thermal equilibrium, as evidenced by the fact that our sun is hotter than interstellar space, or by the existence of functioning heat engines such as your body or an automobile engine.
The combination of all these observations clearly establishes that static cosmological models are not consistent with observation, provided that general relativity and quantum mechanics are valid.
Around 1948, Hoyle and others created a steady-state cosmological model by relaxing general relativity's prohibition on the spontaneous creation of matter. A detailed account of the evidence against this model, and later variations, is given by Wright. The model was falsified in the 1950's by counts of faint radio sources. It is also inconsistent with observed abundances of helium and with the discovery of the CMB in 1965. An oscillating variant called Quasi-Steady State Cosmology was proposed by Hoyle, Burbidge, and Narlikar in 1993, but it was inconsistent with preexisting observations. They later produced a modification of the model, which is also inconsistent with observation.
Will, "The confrontation between general relativity and experiment," http://relativity.livingreviews.org/Articles/lrr-2006-3/
Wright, "Errors in the Steady State and Quasi-SS Models," http://www.astro.ucla.edu/~wright/stdystat.htm
The following forum members have contributed to this FAQ:
bcrowell
George Jones
jim mcnamara
marcus
PAllen
tiny-tim
vela
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