Popular science literature often raises questions about a “boundary condition” at the Big Bang, which necessitated a low entropy scenario.

 

In the FLRW model the Universe in its first instant is miniscule (a single quantum, as Lemaître described it). This “Primordial Atom” contained all the matter and energy of the Universe, which completely filled the tiny space it occupied. In that first “quantum” matter/energy occupied all the available space; there was no room for “manoeuvre”; entropy could not have started evolving until more space became available (?).

 

What does it mean to equate this to a low entropy boundary condition? Surely, in that first instant, entropy was at the maximum possible for the conditions, albeit for only the briefest instant.

Well thats definitely a poor way to describe it.

 

Ok lets clarify the above a bit. First throw away the primordial atom.... Although our observable portion of the universe can extrapolated back to that volume the BB theory recognizes that the universe can be infinite.

 

So from the hot dense state of our observable portion history all particles are in a condition called thermal equilibrium. One cannot distinquish any particle from a photon. This means we can describe this condition by strictly its temperature.

 

Now the photon and its antiphoton pair has effectively two degrees of freedom from its spin statistics.

 

This equates to your low entropy (particle degrees of freedom) via Bose-Einstein and Fermi-Dirac statistics. As the universe expands you get a cooling that allows other particles to drop out of thermal equilibrium which adds their effective degrees of freedom. Which equates to greater entropy

OK, that’s a better way of describing it.

 

I was trying to clarify in my own mind the relationship between thermal equilibrium, restricted degrees of freedom and low/high entropy, and to relate that to the Big Bang.