system
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human
Quantum fluctuations after big bang?

I was just wondering what the actual mechanism of these "quantum fluctuations" that ive read about are. As i am understood they are some how responsible for fluctuations in particle density in the very early universe which caused stars, galaxies etc. to form, and i was just curious what they actually are. Thank you.
gpt
Have you heard of the Heisenberg uncertainty principle? This is the idea that, on extremely small scales, smaller even than atoms, you can't measure things like distance and velocity with exact precision. Actually, the statement is you can't measure *both* of those precisely - the more accurately you measure some particle's position, the less accurately you know how fast it was moving, and vice versa.

You'll usually hear this phrased in terms of position and velocity, but it's a much more general principle of *quantum mechanics*. In quantum mechanics, all sorts of things are subject to uncertainty. When we talk about quantum fluctuations, we're referring to the randomness inherent in that.

In the case of quantum fluctuations after the Big Bang, we mean fluctuations in the *density* of the Universe. In particular, we mean in the density of a type of matter called the "inflaton" which dominated the Universe shortly after the Big Bang, during cosmic inflation. "Density" here means the energy density, how much energy inflatons have in some region of space. It turns out that energy has an uncertainty relation with time. That sounds a bit weird, but it means that the less time you use to make a measurement, the more uncertainty you'll have in your measurement of the energy.

The thing about cosmic inflation is that back then, the Universe was expanding on steroids. The expansion rate was *accelerating*, so that very small things quickly blew up to enormous sizes. Scales which were ruled by the strange laws of quantum mechanics became larger than the observable Universe in an instant.

So let's take some small region of space during inflation. After a very short period of time, it's suddenly of a cosmic size. The density of the patch of Universe contained in that cosmic size depends on the density of the inflaton when it blew up. But because this blow-up happened so quickly, the uncertainty principle tells us that there's uncertainty in what that density was. The actual value that nature picks is then up to random chance.

What you're left with, as you can imagine, is a bunch of regions of the Universe all with different values of the density. Inflation pushes these regions apart faster than light can reach them, so they lose contact. But after inflation ends (still only a fraction of a second after the Big Bang), they slow down and start to come back into contact with each other.

After a while, the observable Universe is made up mostly of gas, and contains a whole bunch of these regions, all with (slightly) differing densities. Eventually, in the denser regions, the gas is pulled together by gravity until it starts to collapse on itself, forming stars and galaxies. This is how quantum uncertainty in the very early Universe is thought to have lead to the cosmic structure we see today.