Why do quantum fluctuations happen?
The Heisenberg uncertainty principle says that a small uncertainty in position implies a large uncertainty in momentum (and vice versa). So anything with a small size (i.e. a small uncertainty in position) has a large uncertainty in momentum. The effect of this is that momentum, and energy, fluctuate: they are not perfectly uniform, because perfect uniformity in energy would translate to zero uncertainty in energy, which is not allowed by quantum mechanics for objects of finite size. These energy “ripples” mean ripples in density, since mass and energy are equivalent. The ripples in density are what “seed” structure formation. A region which happens to be a little more dense will preferentially attract more matter to it… which will then attract yet more matter, and eventually stuff clumps up. What inflation does is to magnify tiny ripples into structures big enough to create galaxies and clusters of galaxies.
