Do Primordial Fluctuations have a Quantum Origin?
A quick lunchtime post containing a confession and a question, both inspired by an interesting paper I found recently on the arXiv with the abstract:
We investigate the quantumness of primordial cosmological fluctuations and its detectability. The quantum discord of inflationary perturbations is calculated for an arbitrary splitting of the system, and shown to be very large on super-Hubble scales. This entails the presence of large quantum correlations, due to the entangled production of particles with opposite momentums during inflation. To determine how this is reflected at the observational level, we study whether quantum correlators can be reproduced by a non-discordant state, i.e. a state with vanishing discord that contains classical correlations only. We demonstrate that this can be done for the power spectrum, the price to pay being twofold: first, large errors in other two-point correlation functions, that cannot however be detected since hidden in the decaying mode; second, the presence of intrinsic non-Gaussianity the detectability of which remains to be determined but which could possibly rule out a non-discordant description of the Cosmic Microwave Background. If one abandons the idea that perturbations should be modeled by Quantum Mechanics and wants to use a classical stochastic formalism instead, we show that any two-point correlators on super-Hubble scales can exactly be reproduced regardless of the squeezing of the system. The later becomes important only for higher order correlation functions, that can be accurately reproduced only in the strong squeezing regime.
I won’t comment on the use of the word “quantumness” nor the plural “momentums”….
My confession is that I’ve never really followed the logic that connects the appearance of classical fluctuations to the quantum description of fields in models of the early Universe. People have pointed me to papers that claim to spell this out, but they all seem to miss the important business of what it means to “become classical” in the cosmological setting. My question, therefore, is can anyone please point me to a book or a paper that addresses this issue rigorously?
Please let me know through the comments box, which you can also use to comment on the paper itself…
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In the Dark 
October 21, 2015 at 2:39 pm
I can’t answer your question, Peter, as I@m not a cosmologist, but I can make a few comments. It is not ultimately possible to couple a classical field to a (special-relativistic) quantum description of matter in a consistent manner, so that far enough back in time a quantum theory of gravity is needed (which we do not presently have). Even farther back, it is possible that *that* breaks down and a hidden variable theory is unavoidable for accurate description.
The quantum correlations seen in Bell-test experiments are visible only if the two particles are not interacting with anything else, but in the early universe they are going to be interacting with everything else!
October 21, 2015 at 5:02 pm
The issue of classicality of inflationary perturbations is surprisingly contentious and confusing, for something so central to the field. Here is the best recent paper I know about:
http://arxiv.org/abs/1408.5002
October 21, 2015 at 5:06 pm
Many thanks. I’ll put that at the top of my reading list!
October 24, 2015 at 6:45 am
In the orthodox Copenhgaen-style interpretation of quantum mechanics, classicality emerges along with the collapse of the wave function. It is the same with the quantum fluctuations.
However, Mukhanov in his textbook “Physical Foundations of Cosmology” favours the a Many-Worlds Interpretation for why the world looks classical.