What is entropy of the universe?

Question

What is entropy of the universe? What is the change in entropy of our universe? Is this change increases or decreases? If the entropy of universe keeps on increasing then what are the consequences? Is there any limiting factor to this increase? One practical application of entropy where it is applied in day to day life?

Answer 1

The current entropy in the Universe is all stored in photons. The first reference by Qmechanic gives you the precise value. Since the photons of the CMBR do not at present interact with anything, the entropy of the Universe is very close to being a constant. What evolution there is, is all due to non-reversible processes in baryonic matter, but it amounts to very little compared to that in photons.

In Friedmann-Robertson-Walker models of the Universe, it can easily be shown that the total entropy density (in comoving coordinates) in photons is conserved. It is very easy to understand why: there are basically no irreversible processes in the Universe, so that the entropy per unit mass must be conserved.

Basically, this means that General Relativity has nothing meaningful to say about entropy: GR-models of the whole Universe could have been built with twice as much, or half as much entropy density than observed, and the models would have been perfectly legitimate. In other words: something, very early in the history of the Universe determines the total entropy content of the Universe, and GR's only task is that of keeping this value a constant.

So this beckons the question: what determined this initial value? Until 1980, the basic answer that could be given was shock waves, which are the most powerful generators of entropy known on cosmological scale.

But since perhaps the late seventies-early eighties, the à la mode answer is that this is a consequence of the fact that very large number of both baryons and antibaryons are produced in the early phases of the Big-Bang, to annihilate later into many photons (among other particles) leaving behind a small left-over which constitutes the present population of baryons in the Universe. While no one can compute with any reliability the entropy per baryon which derives from these processes, they are generally thought to be inescapable, and much more effective than shock waves in producing entropy.

So, while this is still a highly speculative and unsettled chapter of cosmology, it is true that phase transitions provide the greates hope in explaining this otherwise mysterious number.

Answer 2

If we assume our universe as an isolated system, then its entropy can only increase. It cannot decrease because of the second law of thermodynamics. It cannot stay unchanged because the universe is undergoing all kinds of irreversible processes.