Yes, it is constant everywhere but has changed over time.

Where is masque's serious post? Must be #-a

Posted before #1 ?

Or could it be a hidden #a.x ?

A bit like there can be a 13th floor in American buildings. Hidden...everybody knows 14th follows 12th.

i'd argue, no. there was a singularity. infinite density in an infinitely small volume. inf/(1/inf) does not = 0.

therefore, it was LOW entropy, but not zero.

i think people are making some entropy mistakes somewhere b/c as i understand it (please correct me if i'm wrong), every moment in time that has passed since the big bang, there are more ways to order the universe as there were in the previous moment.

so S = KlogW necessarily increases every moment since the singularity, including what started inflation and during inflation.

inflation supposedly started due to quantum fluctuations in the "skin" of the universe (i.e. imagine a balloon being blown up, small distances/differences become larger as an easier way to think about it), thus there are some small discrepancies in the CMB while it remains globally homogeneous. prior to the start of inflation, there were still fewer and fewer ways to order the universe.

at no point in my understanding did we go from a high entropy state to a low entropy state.

second, if we did assume at first there was literally nothing, that still goes from S = 0, to S>0 from one moment before the existence of the universe to the moment the universe began.

so can we clarify where the entropy issue with the origin of the universe came from? to me it seems that we've always gone from S->S+x, where x>0

Like I said.

PairTheBoard

Have you heard Adele's latest hit song. It's titled, "Just A Little Bit of Entropy" from the hook line;

"The difference between nothing and something is just a little bit of entropy."


PairTheBoard

This is correct, but not the problem being discussed. The question is why was the entropy so low. This assumes the philosophical view that high entropy states are natural compared to low ones.

I guess as an easier way to state this is to ask why the entropy of the universe is so low right now. As I mentioned before the black hole at the center of the galaxy has more entropy than the rest of the visible universe. It looks highly unnatural for the visible universe to have less entropy than a random region of space smaller than the orbit of mercury. It also means the question of why entropy so low is equivalent to why so little of the universe is in black holes. If we take this back to near the big bang, it means that the patch of spacetime that inflated to the observable universe was unimaginably smooth. Generic states are far more lumpy and would have lead to gigantic black holes very early. So the question is why were we in such a seemingly smooth, unnatural state right before inflation.

That's independent of why the singularity having 0 entropy leads to alot of problems. If singlarities are generically 0 entropy, it would mean that if the universe was observed to be contracting (which is allowed by physical laws) we would be approaching a 0 entropy boundary condition and would either observe the opposite of the 2nd law (as Hawking thought at one point) or have some sort of bizarrely fine tuned deflation episode/massive jump discontinuity in entropy near the final states of the big crunch. None of that really makes any sense.

i don't think i'm understanding you. the universe now has SUPER SUPER HIGH entropy. each galaxy has a gigabazilliongilliontrillion ways to order its contents and each local cluster has a gigabazilliongilliontrillion ways to order its galaxies etc. the more condensed the universe, is, the fewer ways there are to order it.

imagine calculating S = KlogW now for the ENTIRE UNIVERSE, nevermind gas in a box. it'd be incalculable. it'd be numbers we hadn't thought of other than in high level number theory.

how is entropy LOW now?

All I would have to do is find the total mass of the universe in black holes. You can completely ignore all the stars, planets, galaxies etc. The entropy of the visible universe is ~10^104. (For comparison, the entropy of visible matter is around ~10^90, which is ~.00000000000001% of the total). Holographic bounds place the limit of the total entropy of the universe at around 10^123. So the entropy of the universe is very very low compared to what it could be and what it probably will be in the distant future.

https://arxiv.org/pdf/0909.3983.pdf

These numbers are all fairly low....they don't even require stacked exponentials to describe, much less something more esoteric (Ackermann function etc).

The covariant entropy boundary based on the holographic principle is based on the assumption that quantum gravity holds. We don't know that yet. So why can't the limit be higher?

Now, that doesn't seem to affect the calcs in that cool paper much, but I still don't see the problem. Why does this level of entropy matter if relativity holds? If physics are the same everywhere, and we know f(S) is an increasing function, then we know the arrow of time held since the low entropy state of the Big Bang.

There are alot of detailed calculations that imply it, so I think its fine to assume its true for our purposes. But if the limit is higher the entropy of the universe now is even lower by comparison.

There is no problem if you are ok with an incredibly fine tuned pre inflation universe. Its a problem if you want the early stages of the universe to look natural.

I should point out that this works right now, because the entropy of the universe is black hole dominated. But this wasn't true in the past (before black holes had formed) and won't be true in the distant future (when black holes have radiated away)

are these calculations also evidence that loop quantum gravity holds? are there testable predictions generated by these calculations? i guess can you link me to them?

i'm a deist so i definitely have no issues with a perfectly fine tuned preinflation universe. i mean in general, how else do you explain the insanely fine tuned physical constants that led to there being....anything nevermind us?

the way i see it it's either a) some deity did it at some point, or b) infinite universes, which is just as untestable.

Going back a bit, you said that you don't think that the arrow of time is a real problem.

Do you mean it is not a problem for physics, or that it just isn't a problem at all?

I'm only wondering because the arrow of time does seem to be pointing in a direction; I am fairly consistent in my failure to become younger, for instance. To me, it is a problem that physics equations have nothing to say about that. It just seems incomplete without something about the directionality of time.

I'm not really the right person to ask about LQG, but the answer is no imo. The godfather of all these holographic conjectures is Bekenstein and Hawking's original black hole entropy calculation that showed

1. Entropy is proportional to the surface area of black holes
2. The constant of proportionality is 1/4 (in natural units).

This was a semi classical calculation that had no statistical mechanic style counting of microstates. Strominger and Vafa produced this exact result by counting miscrostates with string theory. Afaik no other theory of quantum gravity has managed to uncontroversially do the same. I consider this a "testable" prediction.

A third option is a natural explanation of the early universe that doesn't need to be fine tuned.

I don't see the difference between the 2 options.

The laws of physics do say something about time asymmetry (ds/dt>0). This is true in classical mechanics, quantum field theory etc. The fact that the standard model doesn't differentiate between the past and future is a feature not a bug since particle physics experiments themselves are time symmetric

It's available here. It's called "Pop Goes the Universe"

https://www.cfa.harvard.edu/~loeb/

Thanks!

Some cliffs: For the time theme this would be interesting, because there would not be a start of time in the same way as anticipated.

OP, how do you think entropy and the arrow of time would behave with the bounce? The question still is at which point in all of this does entropy decrease?

As much as I admire Avi, I think that article (and the actual paper) don't really make sense. I don't really see any justification for whatever unspecified probability measure they are using to say "likely" versions of inflation don't match data.

I don't really understand the bounce theories, but that could very well be ignorance on my part. I don't get how you can just wait a billion years and get a really, really smooth universe. What prevents it from being clumpy with black holes like we expect would happen to our universe if it was contracting?