#### marcus

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The notion of the big bang is familiar to everybody at PF. The idea has dominated how people think about cosmic history maybe since around 1965 with the detection of the CMB.

Widely shared myths or images about beginnings are important parts of culture. If anyone has a link to some online perspective about the big bang please post it. I want to list here what I think are the essentials in the story of this idea----which for me and probably for most of us is not just a myth but something we believe. Every popular idea, true or not, has a history. Please correct any mistakes.

BTW I think that in the future the Big Bang concept will change

for two main reasons: the neutrino background from the first second of expansion will be observed (discussed in a brief survey paper by Ringwald http://arxiv.org/abs/hep-ph/0301157) and quantizing General Relativity will eliminate the glitch at time zero (discussed by Ashtekar in understandable non-technical terms http://arxiv.org/abs/math-ph/0202008).

The expectation that this central idea or story will change is the reason I want to review the history of it.

As I see it the important events are:

1916 Einstein comes up with GR equation which seems to require for stability a (possibly very gradual) expansion or contraction. This bothers him so he puts an extra term in it as a kludge to get a steady state. This kludge does not actually produce a stable static universe---but only the semblance of a steady state.

In 1922 a Russian named Friedmann derives simplified equations (assuming a type of uniformity) from Einstein's complicated ones. The solutions of Friedmann's equations are either expanding or contracting.

In 1929 Edwin Hubble announces that distant galaxies appear to recede---the farther the faster. Hubble didn't know about the changing scale factor in the metric of general relativity that makes space either expand or contract. It was a surprise confirmation of the model Friedmann had derived from GR.

In the 1940s Gamow, Alpher, and Hermann took the expansion seriously enough to extrapolate it back to an earlier time of much higher density and temperature. In 1948 Alpher and Hermann predicted the remnant thermal glow would have been stretched out to microwave by the expansion of space and would still be detectable and would have a temperature of about 5 kelvin. (Eventually the temperature would turn out to be 2.73 kelvin but this was certainly in the ballpark.)

In the 1960s Robert Dicke took that prediction seriously enough he decided to look for the microwave background. In 1965 a postdoc of Dicke named Peebles published an estimate of about 10 kelvin. Word spread and before Dicke and Peebles could get their antenna working, some other people Penzias and Wilson reported having already detected the microwave background as unexplained "noise"---at roughly the predicted temperature.

Finding the CMB gave credibility to the expansion model of the universe partly because of serendipity. The accidental un-looked-for elements of the story helped convice people. Einstein hadn't wanted expansion but his equations predicted it (or contraction) almost to spite him. Hubble didn't know that about Einstein's model when he found the galaxies were receding. Gamow and others extrapolated back to a hot time and predicted remnant microwaves, which Penzias and Wilson then realized they had observed without knowing of the prediction!

A more complete account would have to talk about how the abundance of different chemical elements fits into the story and about what has been learned by mapping slight temperature variations in the CMB---the COBE results of the 1990s and the WMAP results coming in at present. There is the further coincidence that the angular size of the temperature variations is consistent with spatial flatness---one could see it as a continuation of the run of luck this idea has been having.

Widely shared myths or images about beginnings are important parts of culture. If anyone has a link to some online perspective about the big bang please post it. I want to list here what I think are the essentials in the story of this idea----which for me and probably for most of us is not just a myth but something we believe. Every popular idea, true or not, has a history. Please correct any mistakes.

BTW I think that in the future the Big Bang concept will change

for two main reasons: the neutrino background from the first second of expansion will be observed (discussed in a brief survey paper by Ringwald http://arxiv.org/abs/hep-ph/0301157) and quantizing General Relativity will eliminate the glitch at time zero (discussed by Ashtekar in understandable non-technical terms http://arxiv.org/abs/math-ph/0202008).

The expectation that this central idea or story will change is the reason I want to review the history of it.

As I see it the important events are:

1916 Einstein comes up with GR equation which seems to require for stability a (possibly very gradual) expansion or contraction. This bothers him so he puts an extra term in it as a kludge to get a steady state. This kludge does not actually produce a stable static universe---but only the semblance of a steady state.

In 1922 a Russian named Friedmann derives simplified equations (assuming a type of uniformity) from Einstein's complicated ones. The solutions of Friedmann's equations are either expanding or contracting.

In 1929 Edwin Hubble announces that distant galaxies appear to recede---the farther the faster. Hubble didn't know about the changing scale factor in the metric of general relativity that makes space either expand or contract. It was a surprise confirmation of the model Friedmann had derived from GR.

In the 1940s Gamow, Alpher, and Hermann took the expansion seriously enough to extrapolate it back to an earlier time of much higher density and temperature. In 1948 Alpher and Hermann predicted the remnant thermal glow would have been stretched out to microwave by the expansion of space and would still be detectable and would have a temperature of about 5 kelvin. (Eventually the temperature would turn out to be 2.73 kelvin but this was certainly in the ballpark.)

In the 1960s Robert Dicke took that prediction seriously enough he decided to look for the microwave background. In 1965 a postdoc of Dicke named Peebles published an estimate of about 10 kelvin. Word spread and before Dicke and Peebles could get their antenna working, some other people Penzias and Wilson reported having already detected the microwave background as unexplained "noise"---at roughly the predicted temperature.

Finding the CMB gave credibility to the expansion model of the universe partly because of serendipity. The accidental un-looked-for elements of the story helped convice people. Einstein hadn't wanted expansion but his equations predicted it (or contraction) almost to spite him. Hubble didn't know that about Einstein's model when he found the galaxies were receding. Gamow and others extrapolated back to a hot time and predicted remnant microwaves, which Penzias and Wilson then realized they had observed without knowing of the prediction!

A more complete account would have to talk about how the abundance of different chemical elements fits into the story and about what has been learned by mapping slight temperature variations in the CMB---the COBE results of the 1990s and the WMAP results coming in at present. There is the further coincidence that the angular size of the temperature variations is consistent with spatial flatness---one could see it as a continuation of the run of luck this idea has been having.

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