An entirely symmetrical universe would dissipate symmetrically, foreclosing any sort of self-organizing capacities. The exact source(s) of cosmic asymmetry are hotly debated, but it's these asymmetries which account for self-organizing tendencies which do not violate the second law. — StreetlightX
I suppose the (I guess we must think universal?) operation of entropy is itself the most basic asymmetry, since it would seem to produce temporal directionality. You say that a symmetrical universe would dissipate symmetrically.Thinking about this the question that comes to mind is whether in an absolutely homogeneous, that is absolutely symmetrical, universe any dissipation of energy would occur at all. — John
SX makes the critical points very nicely. I will add a few thoughts.
The baseline state of the Universe was set up by the symmetry breaking that was the hot Big Bang. The Universe started out as a simple spreading/cooling bath of radiation. So from one perspective, it was an entropic gradient - the Universe was running down the hill from the Planck temperature towards absolute zero. But then because the Universe was effectively its own heat sink - it cooled by metric expansion - you could say that this creation of "new space" was a matching negentropic order.
So from a global perspective - one that counts degrees of freedom or microstates - it is difficult to say the entropy count actually changes. The essential change - the symmetry breaking represented by the Big Bang - already created a maximum entropy state. The radiative contents were already as messy as they could be. The now locked in story of a constant c rate radiative expansion and cooling had been "paid for" in terms of the phase transition that resulted in such a world with its orderly Planck scale structure and three dimensional, radiation dissipating, geometry.
So if we ask the usual question of how the Universe started in a state of high negentropy - an initial orderliness which could then be the fuel for a second law trajectory towards messiness - one answer is that the Big Bang was itself a mathematical-strength structural asymmetry just waiting to happen.
Before the Big Bang was a vagueness or quantum roil - a state of unbounded fluctuation or infinite dimensionality. There was action happening in any direction and so no actual global geometry or real dissipation. For structural reasons, limiting this wild chaos by constraining the action to a 3D heat sink - grabbing a chunk of this primal energy and spinning it into a cool/expanding fabric of radiative events - was a way to make a world. It created a realm of distinct pathways - the three dimensions that allowed powerlaw dilution of thermal action - that could then roll downhill towards a maximum separation between the complementary things of position and momentum, the container that is spacetime and the contents of this expanding box which was its gas of particles or thermalising events.
So before the Big Bang, things would (logically) have been so symmetrical as to be vague. Action was unbounded and so nothing existed to say that anything was happening in some direction. The Big Bang was then the dualised creation of the very split by which negentropy and entropy could even be distinguished. The emergence of an expanding spacetime dimensionality as the organised container was what made possible a matching story of spreading and so cooling particles or thermalising events. Structurally, it locked in a trajectory in which particles could make symmetric exchanges of energy among themselves - there was no trajectory of change at the individual level. But then emergently, statistically, the particles would find themselves behaving asymmetrically, the hotter particles always on the whole yielding to the probability they would make radiative exchanges with cooler particles.
This very simple initial universe - a spreading/cooling gas - then hit further symmetry breakings as its temperature dropped. Like a tide going out, suddenly a rocky deeper structure was exposed and rock pools of trapped negentropy formed.
The critical one was the electroweak symmetry breaking that saw the Higgs mechanism switched on and particles becoming gravitationally massive. This happened all at once at a critical temperature and so represented a sudden entropic deceleration everywhere in the Universe. There was a shift from the steady entropification rate where radiation was spreading as fast as it could - the speed of light - to a Universe where a good chunk of its hot contents was now dragging along at sub-light speed. The balance of the Universe was suddenly out of equilibrium, setting up the need (the telos) for a new level of dissipative mechanism. The Universe was spreading/cooling at a sub-optimal rate now. And so that paid for any further negentropic structure that would help it catch up, re-accelerate the entropification.
Hence stars. Mass clumped gravitationally. But then as a further twist of fate, it caught fire and started turning mass into radiation.
That then left its own negentropic residue in the form of heavy elements and rocky planets. And so afresh, you have the negentropic platform for life to emerge and add its (fantastically tiny) contribution to the universal cause.
So my point is that second law entropy thinking explains a heck of a lot. But metaphysically, we then have to recognise how entropy and negentropy are two faces of the same coin in some deep way. And rather than chasing some chicken and egg question of which comes first - the symmetry or its breaking - we need to have a story where they both co-arise in synergistic fashion from an even more primal state - the state that can be dubbed a chaos, an apeiron, a roil, a vagueness, an unbounded dimensionality of fluctuation.
But it is still the case that the Big Bang looks to represent a properly crisp symmetry breaking. That was the instant when a strictly limited dimensionality clicked into place. And from there, with spreading/cooling as a locked in story, further mathematical outcomes become a historical inevitability. Once action was confined to the point where it had highly constrained properties - once it was playing out in a world in which crisp dimensionality underwrote definite symmetries like those of translation and rotation - then the structural mathematics of those definite symmetries became an inevitable emergent fact. As the Universe cooled enough, it would have to go through the symmetry breakings that are represented by gauge symmetries or lie groups in particular, and so result in the Standard Model family of fundamental particles.
Long-term of course, all matter should be returned to pure radiation even if it has to be swept up into black holes first. At the Heat Death, following a history of sudden global decelerations and subsequent slowly catching up local re-accelerations, the Universe will get back to being a homogenous entropic equilibrium. It will become just the lingering black body fizzle of cosmic event horizon radiation.
But that of course is a steady-state fate that is itself underwritten by the new thing of dark energy or the cosmological constant. Everywhere the spatial fabric of the Universe is undergoing a further faint acceleration for some reason.
This is the reason we can now say the Universe will coast to a halt in terms of cosmic event horizons and so - in third law of thermodynamic fashion - actually arrive at a minimum entropy condition (rather than cooling endlessly). In Red Queen style, the Universe will still be expanding/cooling at c. But that will become running on the spot for event horizons as the underlying spacetime will be continuing to accelerate away at superluminal speed.
Yet while this negentropic dark energy acceleration is a further energy that makes certain the general entropic tale of the Universe is drawn to a close, it is of course now a new source of mystery. The hope is that a better understanding of the symmetry breaking that was the Big Bang will reveal how dark energy is again the negentropic flip-side of some larger entropic symmetry breaking. It must be another tiny source of order that paid for a lot of extra mess in some fashion.
So our explanatory instinct is always to try to arrange existence into a temporal order of causes and effects. If we are talking about entropy and negentropy, mess and order, spacetime and material contents, symmetries and symmetry-breakings, we want to decide which is chicken, which is egg. We want to impose a temporal linearity that conforms with our metaphysical prejudices.
But while that is indeed a useful way of looking at things, and even a true way of looking at things once a state of crisp organisation has developed, there is then a deeper way of looking at things which is dependent on seeing symmetry and symmetry-breaking as itself the two sides of one coin. As each other's dichotomous "other", each has to arise in the presence of its opposite even to be crisply actual.
Four causes thinking can get at this by treating finality as "lurking structure awaiting its inevitable expression".
Who knew that the entropic cooling/spreading of a 3D bath of radiation would have to get interrupted by a cascade of further negentropic symmetry-breakings as it passed critical temperatures? Well those breakings already lurked in the future due to the necessity of structural mathematics. The path to ultimate simplicity was always going to be a bumpy ride as it jolted over these hidden symmetry features that define the Standard Model family of particle species - all the ways that spin in particular can have a complexity, an intrinsic asymmetry, in its directions.