# Cosmology, Quantum fluctuations,and entanglement

1. Jul 7, 2014

### kw1

I understand that 'roughness' in the universe is explained by inflation, because quantum fluctuations in density get separated farther than their Hubble sphere--far than any influence (distance greater than speed of light) between them. Some areas by quantum chance have higher density and can then aggregate to form matter and ultimately stars and galaxies and so on. At least, that's my general understanding (though I'm not expert in this area). But this explanation, of 'no influence' seems incomplete.

If particles of whatever type are separated farther than they can affect each other (based on distance vs speed of light), is there not still all sorts of entanglement that was established at the big bang and hence can affect particles no matter how far apart they are? That is, once an entangled particle interacts with anything else, which should be a sort of 'measurement', then its partners, in or out of their Hubble spheres, will also instantly be affected and take on a definite state. I must b misunderstanding something! Of course, if entanglement means complementarity then this would lead to a sort of symmetric roughness pattern, and this might be consistent with the symmetric (Gaussian) density distribution of the CMB.

So what am I misunderstanding?

2. Jul 7, 2014

### WannabeNewton

Curvature perturbation modes that exit the horizon during inflation and live in superhorizon scales for some N-folds of inflation before reheating are frozen on the superhorizon scales for adiabatic matter fluctuations. More precisely, if $\zeta_k$ is a Fourier mode of the curvature perturbation then $\frac{\partial}{\partial t}\zeta_k = 0$ for $\eta k \ll 1$. This is what locks in the various perturbations in the universe, such as density and temperature perturbations, even after the inflaton decays into various fluids and reheating begins because the hubble radius grows again and eventually the modes on superhorizon scales renter the horizon and start affecting local causal physics. Entanglement, even if it did exist between particle species, is completely immaterial and inconsequential for this.

3. Jul 7, 2014

### kw1

Well, thanks very much. I'm a complete amateur at this (I'm a geneticist--wondering if we'll ever have any comparably elegant view of life), so the technical details are beyond me. I will try to check the references you suggested. Ironically, I was once a meteorologist, and knew about adiabatic processes, and might even be able to refresh my understanding if the term has the same usage in cosmology. I can't really understand why entanglement is inconsequential, but that's because my knowledge of that, too, is too rudimentary. I do appreciate your taking the time to respond!

4. Jul 7, 2014

5. Jul 7, 2014

### kw1

I'll need it (luck)! But thanks.

6. Jul 8, 2014

### Chalnoth

Nope. For two reasons. First, entanglement is lost pretty rapidly when the entangled particles interact with their environment. If, for example, the temperature is too high, there will be a lot of photons moving around that will have a tendency to destroy any entanglement (room temperature is generally enough to destroy entanglement pretty rapidly). And the temperature after inflation ended was exceedingly high, so high that we haven't yet produced temperatures very close to it in our most powerful particle accelerators.

Second, information isn't transmitted between entangled particles. Entanglement is about consistency, and doesn't involve any information transfer between particles or collapse of the state of far-away particles. What it does mean that if you have measured a particle in state A, then everything else you subsequently measure about the universe will be consistent with measuring that particle in state A, as your particular branch of the wavefunction of the universe includes that state.

7. Jul 8, 2014

### kw1

My understanding (as an amateur, I admit!) was that entanglement would mean that, wherever, when one of entangled particles was 'measured' (interacted with something) it took on a specific value (collapsed its probability function) and that this also implied a specific value for its entangled particles. My thinking was that in the very early, very very small inflating universe, such events could yield locally different states that could the seed the formation of matter etc in some places and void or other states elsewhere--all when things were very local but inflating with super-c speed. So I would have expected your firs reason to be consistent with that (interactions happen quite fast); future entanglement wouldn't be necessary.

As to your second point, I wasn't suggesting transfer of information but determination of states of particles when they interacted (realizing that which was 'first' might be relativistically not a good way to think about it). But, again as an amateur, I have to think about your final point, which is not how popularization treatments of entanglement typically seem to explain it.

This is all very interesting and, to a geneticist at least, as elusive as a greased eel.

8. Jul 8, 2014

### George Jones

Staff Emeritus
I am sure there are earllier papers, but the first paper about quantum entanglement in a cosmological setting that I came across was Wald's 1992 paper "Correlations Beyond the Horizon"

http://www.gravityresearchfoundation.org/pdf/awarded/1992/wald.pdf [Broken]