<p>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.</p>

<p><a href="http://www.preposterousuniverse.com/blog/2014/03/16/gravitational-waves-in-the-cosmic-microwave-background/" rel="nofollow">This site</a> gives a good explanation, as well as this particular quote:</p>

<blockquote>
  <p>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.</p>
</blockquote>

CC BY-SA 3.0
2014-05-26T20:29:35.397Z
2014-05-26T20:29:35.397Z
christopherlovell
<p>In the course of <a href="https://www.youtube.com/watch?v=Iasqtm1prlI">this</a> very enjoyable press announcement, it is mentioned that inflation can create gravity waves by amplifying gravity fluctuations.</p>

<p>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.</p>

<p>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?</p>

<p>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.</p>

CC BY-SA 3.0
2014-03-24T15:00:15.343Z
This question may get attention over at http://physics.stackexchange.com/
David H
CC BY-SA 3.0
2014-03-31T00:59:50.310Z
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.
astromax
CC BY-SA 3.0
2014-04-07T10:48:54.120Z
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.
Jeremy
CC BY-SA 3.0
2014-04-17T11:35:57.933Z
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?
adrianmcmenamin
CC BY-SA 3.0
2014-04-23T16:30:27.963Z
I'm sure there are people here who could answer it, but this is more of a question for the Physics stack.
Florin Andrei
CC BY-SA 3.0
2014-04-23T19:35:19.277Z
1) the most likely waves have a typical scale ($\pi/50$ or so) just like the way likely waves on the sea would have a typical scale (set by e.g. the strength of the wind). It does not mean all ways have exactly that scale, but visually that scale dominates; 2) the waves were minuscule to start with, but have been blown up to gigantic scales by inflation (hence the name!).
chris
<cosmology><general-relativity><cosmological-inflation><gravitational-waves><quantum-field-theory>
How exactly does inflation convert random gravity fluctuations into coherent gravitational waves?
Dilaton
# How exactly does inflation convert random gravity fluctuations into coherent gravitational waves?

(Asked by: username_0 on 2014-03-24)

In the course of [this](https://www.youtube.com/watch?v=Iasqtm1prlI) 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.

**Tags:** cosmology, general-relativity, cosmological-inflation, gravitational-waves, quantum-field-theory
> **Comments:**
> **username_1**: This question may get attention over at http://physics.stackexchange.com/  
> **username_2**: 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.  
> **username_3**: 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.  
> **username_4**: 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?  
> **username_5**: I'm sure there are people here who could answer it, but this is more of a question for the Physics stack.  
> **username_6**: 1) the most likely waves have a typical scale ($\pi/50$ or so) just like the way likely waves on the sea would have a typical scale (set by e.g. the strength of the wind). It does not mean all ways have exactly that scale, but visually that scale dominates; 2) the waves were minuscule to start with, but have been blown up to gigantic scales by inflation (hence the name!).  

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## 1 answers

### Answer by username_7 on 2014-05-26. Score: 2

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](http://www.preposterousuniverse.com/blog/2014/03/16/gravitational-waves-in-the-cosmic-microwave-background/) 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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