It is my understanding that cosmology simulations largely implement general-relativity, taking quantum-effects into account merely by setting the fluctuations in the initial conditions. In the real world, however, quantum fluctuations occur all the time. I'm wondering how far back to quantum fluctuations determine star and planet formation.

Stars form around random higher-density patches in interstellar dust. Are post-big-bang quantum effects enough to randomly generate or move such density fluctuations, and thus determine which nucleus stars will accrue around? Can they determine which planets will form? Or are all these things truly set by the initial conditions, as the simulations apparently presume?

My suspicion is that quantum fluctuations do determine which stars will form and the details of their planets, but I wasn't able to find any professional sources on the issue.

  • $\begingroup$ What do you think is lacking from the conventional (classical) explanations of star formation? en.wikipedia.org/wiki/Star_formation $\endgroup$
    – D. Halsey
    Jul 13, 2019 at 23:28
  • $\begingroup$ I suspect what's missing is the amplification of quantum fluctuations due to chaos during the collapse of the gas giant. These would not matter statistically as accounting for thermal fluctuations will probably already account for them, but would matter in regards to the cause of one star/planet forming rather than another. $\endgroup$ Jul 14, 2019 at 12:36

1 Answer 1


I would say that the only quantum fluctuations that are relevant for the formation of structures (such as stars or galaxies) are those of the very early universe. In particular, they play a key role in a period called "inflation", which most cosmologists think it happened around $10^{-34}$ s after the initial Big-Bang singularity. The main idea is that the universe was filled with a substance called "inflaton". This substance was responsible for driving a very rapid acceleration of the expansion of the universe. In this way, the microscopic quantum fluctuations of the inflaton magnified to cosmic sizes and they provided the seeds for the growth of matter overdensities (when inflation ended, these fluctuations experienced a transition from "quantum" to "classical"). The evolution of the matter overdensities is well described by the simple linear theory as long as these perturbations remain small. When they become big enough, they collapse and start to form the first structures in the universe. Linear theory is not suitable to describe this complicated process, so one needs to use N-body numerical simulations. Nowdays, quantum fluctuations are still everywhere as you said, but there is no mechanism as inflation responsible of magnifying them so that they are relevant for structure formation. So in conclusion, yes, it is the primordial quantum fluctuations which are important, and indeed we think that the noise we see in the CMB map of temperature anisotropies was originated during inflation.

  • $\begingroup$ When the collapse does occur, it is as you say complicated. Is it also Chaotic? (Not the overall statistics, but rather the details of which star/planet is formed). If it is, I can see Chaos amplifying e.g. a quantum fluctuation in a proton's movement to generate different turbulence patterns, which will result in this rather-than that volume collapsing to form a star. $\endgroup$ Jul 14, 2019 at 12:47

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