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In the course of this very enjoyable press announcement, it is mentioned that inflation can create gravity waves by amplifying gravity fluctuations.

I do not properly understand this statement. I always thought that quantum fluctuations (fluctuation of the metric in this case?) should occur pretty randomly whereas waves are rather coherent motions.

So I understand somehow that inflation can amplify quantum fluctuations, but I don't see how it can convert these random "microscopic" processes into "macroscopically" observable coherent phenomena called gravitational waves. For example, how are the sources of these gravitational waves distributed? Does each point in space-time behave as some kind of a point source? And what is observed at a specific point as the superposition of all these excitations?

In addition, in the same video it is said that not all inflation models produce gravitational waves, the ones that have them are favoured by the data now, etc.

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    $\begingroup$ This question may get attention over at physics.stackexchange.com $\endgroup$
    – David H
    Commented Mar 24, 2014 at 15:00
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    $\begingroup$ I don't think the title of this question makes much sense. Inflation doesn't cause gravitational waves, inflation blows them up to macroscopic scales. $\endgroup$
    – astromax
    Commented Mar 31, 2014 at 0:59
  • $\begingroup$ I think the questioner is expecting the waves to be regularly periodic like waves coming in on a shore (the expectation that there is some coherence) but I can't see any evidence of a claim that the waves are coherent. A single pulse travelling is a wave, but it isn't coherent. $\endgroup$
    – Jeremy
    Commented Apr 7, 2014 at 10:48
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    $\begingroup$ Surely what it means is that waves seen at the very earliest moment are, in a sense, "frozen" into the universe's "shape" by inflation? $\endgroup$ Commented Apr 17, 2014 at 11:35
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    $\begingroup$ I'm sure there are people here who could answer it, but this is more of a question for the Physics stack. $\endgroup$ Commented Apr 23, 2014 at 16:30

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When inflation occurs the stretching of the gravitational field is dependent on the perturbations within that field originally. These perturbations are caused by quantum fluctuations. This leads to fluctuations in the magnitude of the inflation itself at a given point in space, causing gravitational waves due to the gravitational potential between neighboring points. The waves are fed by the inflationary energy, but are seeded by these quantum fluctuations.

This site gives a good explanation, as well as this particular quote:

Gravitational-wave perturbations are different. They are not modulated by some unknown potential; they are produced by inflation, and we observe them directly. In straightforward models of inflation, the amplitude of the gravitational waves is directly proportional to the inflationary energy scale.

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