cosmological inflation - 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?

Answer

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.