How do quantum fluctuations lead to overdense regions in the CMB? I've read a lot of work about Inflation but yet to find a cogent description where someone explains exactly what 'quantum fluctuations' are and how they led to overdense regions in the CMB. I get the part where virtual particles can spontaneously appear and annihilate, I even get the part where the effective charge on an electron can be changed. But creating pools where matter collects sounds like real work. How can virtual particles perform real work? Specifically, how do random quantum fluctuations lead to coherent over-dense regions of matter?
 A: Inflation Theory Presents - 


*

*10^{-35} seconds or less after the Planck time–the universe went through a phase of accelerated expansion.  

*Then the energy that was driving the accelerated expansion converts into ordinary matter and radiation.
Net the Big Bang begins


*The Heisenberg uncertainty principle infers there will be an irreducible minimum amount of jiggle in any quantum system, even when it’s in its lowest-energy (“vacuum”) state.

*In our inflation context this means that quantum fields of low mass will exhibit fluctuations. 

*Based on two kinds of light quantum fields we expect to see two types of perturbations the “inflaton” field and the gravitational field. The field that drove Inflation has not been determined so we just give it a name the "Inflaton" field. This field eventually converts into matter and radiation and its so called "Scalar" fluctuations produce the perturbed density of the early plasma. 

*The story doesn't end only with this Field. It is also believed that the Gravitational field also played an important impact on the energy densities of the CMB observed. Again as we look at the G-field as a quantum field the Heisenberg Uncertainty Principle implies fluctuations once again and result in gravitational waves {or gravitons} the so called "Tensor" fluctuations. 

*The observables - 
We know that the types of polarization observed in the CMB can support the fluctuations of the two proposed fields. E modes support the Scalar type of waves or "Inflaton" fluctuations and the B modes polarization supports the Tensor type of wave or the Gravity fluctuations.
"Scientists have already found E-modes in the CMB, but until now they haven’t confirmed that any B-modes exist." - See more at: http://www.skyandtelescope.com/astronomy-news/seeking-the-cosmic-dawn/#sthash.OaazSNo0.dpuf

E-mode polarization patterns look like asterisks or loops, and they don't change when mirror-imaged. B-modes, on the other hand, curl either clockwise or counterclockwise into spiral patterns.
Sky & Telescope
- See more at: http://www.skyandtelescope.com/astronomy-news/seeking-the-cosmic-dawn/#sthash.OaazSNo0.dpuf

This polarization map shows the first E-modes detected in 2002 by the Degree Angular Scale Interferometer (DASI) telescope. This false-color image shows the temperature variations (yellow is hot, red is cold) in the CMB's radiation. The lines mark the polarization at each spot in the image; the length of the line shows the polarization strength while the orientation shows the direction.
DASI / University of Chicago
- See more at: http://www.skyandtelescope.com/astronomy-news/seeking-the-cosmic-dawn/#sthash.OaazSNo0.dpuf


*In concluding I hope you see how the different proposed fields and their polarized wave fluctuations would have a impact on the energy densities observed in the CMB. Once B - modes are observed and their index determined and if they agree with the G-Field waves we can feel much more confident in the theory of inflation.

A: 
I've read a lot of work about inflation but yet to find a cogent description where someone explains exactly what 'quantum fluctuations' are and how they led to overdense regions in the CMB.

I have yet to find a cogent description of inflation! The more I learn, the more it seems to be superfluous. See this Scientific American blog piece:
Physicist Slams Cosmic Theory He Helped Conceive
..which relates to this article, sorry it's a stub:
Paul Steinhardt Disowns Inflation, the Theory He Helped Create 

I get the part where virtual particles can spontaneously appear and annihilate

Have a look at this answer by anna v : "Thus virtual particles exist only in the mathematics of the model used to describe the measurements of real particles". Virtual particles are virtual. As in not real. They aren't popping in and out of existence like magic. That's a popscience myth I'm afraid. 

I even get the part where the effective charge on an electron can be effected. But creating pools where matter collects sounds like real work. How can virtual particles perform real work? 

They can't. Virtual particles are field quanta. It's like you divide up the electron's electromagnetic field into abstract chunks, and then do the same for the proton. Then when they attract one another, they "exchange field" such that the resultant hydrogen atom has very little in the way of a field.  

Specifically, how do random quantum fluctuations lead to coherent over-dense regions of matter?

That's a hypothesis, and I have to say I don't know. But note that vacuum fluctuations aren't the same thing as virtual particles. Virtual particles are abstract mathematical things, vacuum fluctuations aren't. They are real, and the electromagnetic equivalent to the ripplets that cover the surface of the sea. The Casimir effect is a demonstration of vacuum fluctuations. However the vacuum fluctuations of the very early universe were subject to a "maelstrom"  that lasted for circa 380,000 years before the surface of last scattering associated with the CMB. IMHO we can't say with any real confidence that the very slightly overdense regions of the CMB resulted from quantum fluctuations. Or the much more dramatic variations in matter density. 
You know, whenever I hear about this, I think of Hawking saying the universe was born of a quantum fluctuation. That makes me think What fluctuated? And then I end up thinking this is turtles all the way down, and that's not much better than 'God did it'. Anyway, if you get a convincing answer to this, do let me know.      
