The dynamics of the Universe and its organization currently remain an enigma for cosmologists. In particular, the propensity of the Universe to maintain a certain structural balance against thermodynamic entropy remains an unresolved paradox. Ludwig Boltzmann proposed that this organization originated in a random quantum fluctuation of very high complexity. Such fluctuations, of less intensity, could then give rise to complex structures playing the role of observers and called "Boltzmann brains".
At the end of the 1870s, the physicist Ludwig Boltzmann, from the various observations made of our universe, pondered what seems to be a paradox:why does our Universe not follow the second law of thermodynamics? In other words, for 13.7 billion years, the entropy of the Universe should have gradually increased until today there is great disorder within it. However, this is hardly what we can see. A certain order, a balance, seems to reign and to be maintained.
To solve this problem, Boltzmann hypothesized that the structured universe as we know it, that is to say a universe endowed with a high degree of organization, would have been born from an entropic fluctuation of a larger and completely messy universe. Thus, in a universe where the entropy would be high according to the correct thermodynamics, random fluctuations would appear, and from these fluctuations would be born more or less organized structures according to the intensity of the fluctuation.
For a small fluctuation, a primitive and archaic structure would be created, whereas the more intense a fluctuation, the more the structure will be organized until reaching that of a universe for example. However, the more the intensity of a fluctuation increases, the lower its probability of appearing. Thus, the creation of an entire universe following a fluctuation, borders on the impossibility. This is why we do not observe other more organized structures in our universe, because their probability of appearing, coupled with the gigantic size of the universe, is almost nil.
We have seen that the probability of extremely elaborate structures appearing is very low, so lower levels of organization are more likely to appear randomly. Thus, less complex entities can arise from these fluctuations and behave as conscious observers of the surrounding universe, like humans:the Boltzmann brains hypothesis was born. These brains should not be compared to human brains, but to minimally organized structures which, in fact, have an influence on the Universe through their role as observers.
With the advent of quantum physics, we have learned that as long as a particle is not measured, it remains in superposition, that is to say in several different simultaneous states. It is only by performing a measurement that the wave function is reduced and the particle stabilizes. Starting from this, our universe would only be the result of our own observation vis-à-vis it.
So the observable universe would be built by the observer-observed relationship and characterized by what our instruments teach us about it. This is called the constructivist hypothesis and is supported by some theoretical physicists. Considering this, if conscious structures, that is to say sufficiently organized, appear randomly in the Universe, then they impose themselves as competing observers of Man. Currently, it is estimated that given the youth of our universe and the very low probability of the appearance of such entities, no Boltzmann brain has yet appeared.
However, over time, these structures may appear in the millions or even billions. The consequence ? Well, if billions of entities start observing the Universe, the influence of their observations will surpass ours, and they will be the ones who will construct the image of the Universe.
It may then be that the observable universe changes radically for us and that we have to change everything we have learned. However, physicists assume that these entities will appear in the extremely distant future, when the Universe will have become completely cold and dark, then Man will certainly no longer be around.
Physicists now consider that the entropic fluctuations described by Boltzmann are in fact quantum fluctuations of the vacuum. We know, thanks to Heisenberg's principle of indeterminacy, whose time-energy relation ΔE · Δt> h/2, that the vacuum in fact contains an average residual energy and that energy fluctuations around this energy average can generate pairs of virtual particles. A virtual particle is the most archaic structure that can appear from a fluctuation, but nothing prevents more organized structures from appearing from these fluctuations if the energy increases.
Related:What are quantum vacuum fluctuations?
This is the hypothesis supported by cosmologist and theoretical physicist Andréi Linde of Stanford University. For him and his team, although the odds are very low, Boltzmann brains may appear. Thus, over time, complex objects could emerge from quantum fluctuations.
For Linde, if the Universe continues to expand indefinitely, it is very likely that organized entities will emerge and shape the Universe through their observation of it. However, it cannot be ruled out that Boltzmann brains may have already appeared in regions that are inaccessible to us.
As we have seen, as long as a quantum system, a fortiori a particle, is not observed, it remains in superposition; it is only during the measurement that the quantum states are chosen and finally fixed, which is called wave function collapse. However, to solve some problems, physicists have introduced a new mechanism:quantum decoherence.
This explains that for the wave function to collapse, there is no need for direct observation, the interaction of the particle with its environment alone can be enough to make it choose its states (however this generally takes a long time). Our Universe could then have appeared according to this mechanism of decoherence. From an intense primordial vacuum fluctuation, a virtual particle would have passed into the state of a real particle and would have stabilized.
Then, through its role as an indirect observer, i.e. its interaction with its environment, it would have allowed the virtual particles to see their wave function reduced by quantum decoherence. And, by a chain reaction (each newly fixed particle acting in turn as an observer), billions of virtual particles would have become material, then would have given birth to the primordial Universe.
In this scenario, the first particle acts as a Boltzmann brain. However, according to Don Page, it could very well have been, instead of said particle, a real brain of Boltzmann.
Physicists then wondered why the primordial quantum systems would have arranged themselves specifically in such a way as to create the structured universe that we know. From there, appears the principle of quantum Darwinism which postulates that, by quantum decoherence, the environment itself determines what is necessary for its durability. The environment would then select, through its interference, the quantum properties of the particles in such a way that its evolution is favorable.
This partly solves the weak anthropic principle:we can observe the Universe because it is itself the result of favorable interactions that gave birth to it. We are observers only because prior environmental interactions on primordial quantum systems allowed it. Thus, the initial Boltzmann brain would have interacted with the first particles in such a way as to ensure a stable evolution of the Universe identical to its own.
In his article Spooks in Space published in The New Scientist in 2007, Mason Inman (theoretical physicist) postulated that baby-universes or bubble-universes could emerge in parallel with ours.
Indeed, from the initial primordial particle, the mechanism of decoherence could have taken place simultaneously in several different ways, giving rise to universes which for us are not real, that is to say inaccessible to the reality that we have. constructed by our observer-observed relationship.
