Once Before Time - Martin Bojowald [120]
From the cosmological perspective, this leads to an oft-discussed problem: If entropy is ever-increasing, a long time ago it must have been much less than it is now. At one time, it should have had the minimally possible value, in a state of perfect order. How could such a state have emerged—one capable of starting the world but also dooming it never to be perfect?
We formulated this problem by applying our earthly experience to the whole universe. But in doing so, we overlooked a crucial ingredient: gravity, which is unavoidable in the cosmos. Gravity is able to increase order by virtue of its attractive nature, just one of its many special properties. If we view a uniformly distributed gas, such as matter as it was shortly after the big bang (as seen via the cosmic microwave background), it has decidedly low order and structure and thus relatively high entropy. Without gravity, that distribution is not going to change much; entropy does not change and remains at its high level.
But gravity inevitably acts and slowly leads to a concentration of overdense regions where chance (embodied by quantum fluctuations) has made the gas denser in the first place. Over time, ordered structures emerge in the universe and culminate in the formation of galaxies and stars. The entirety of all these structures is much more highly ordered than the initial state; by gravity’s action, entropy was diminished. Every once in a while stars form, shining their light on a planet to continuously provide it with energy from the nuclear fusion taking place in their gravitationally compressed and heated-up interiors. Under suitable conditions, such as those on Earth, much more strongly ordered structures can arise, maintaining themselves as life-forms through consumption of the donated energy.
A better picture than ever-increasing entropy is indeed, as has been admitted by its initiator Ludwig Boltzmann, a nearly constant level, locally reduced by some physical processes or just by mere chance. In these oases of order, further processes play out to slowly increase entropy and bring it back to the initial level. And yet, imagining the universe from the viewpoint of an external, metaphysical observer positioned not in the universe itself but overseeing everything in it and its entire temporal extension, the present state of cosmological parameters indeed seems to be one of enormously high entropy. There are, after all, very many possibilities for what a universe of a certain size, expansion rate, and acceleration could look like. All details of the exact distribution of galaxies, stars, and planets, or even of the atoms in them, are irrelevant in this context, and thus the present universe more closely resembles a vessel broken into the finest pieces than an intact, precisely constructed one. The universe as we happen to find it appears as but one of many possibilities, which could all have the same form of cosmological collective quantities, such as the cosmic background radiation or the distribution of galaxies on large scales. Seen as a whole, the universe does seem to be in agreement with our experience of everyday systems, which has accustomed us to a continuous increase in entropy.
Cosmologists often see a problem here, since one assumes that entropy must have been ever smaller the earlier back in time we go. This becomes especially problematic if the universe did not start a finite time ago but existed at all times—possibly in a long succession of repeating cycles, sometimes expanding and sometimes collapsing, as suggested if quantum gravity removes