B Is Entropy the Key to Understanding the Big Bang and the Fate of the Universe?

[C_Nordquist]

I am a software trainer and know about as much Physics as I've been able to pick up from the BBC‘s occasional documentaries.
I have never even taken a physics course in high school or college so I am an absolute lay-person here with what’s probably a very lay-person question…

I was watching the Wonders of the Universe with Brian Cox documentary, and he was talking about entropy and about how “high entropy” is a way of describing a disordered arrangement of materials. Like low entropy would be a sandcastle and high entropy would be all of that sand having been scattered across the beach by wind and waves… do I have that basically right?

And he was describing something about the tendency of low entropy to high indicates the arrow of time, and the breakdown of all materials, whether it be a derelict building or a star slowly burning out. Still on point here?

And I was wondering about the universe itself- and our model of Big Bang, to what we have now, to eventual big freeze….

And then I confused myself because the period after the big bang, where energy is supposedly swirling and coalescing, seems like a high entropy state… then moving into a low entropy state through the forces of gravity (?) bringing materials together and ordering them into spheres that orbit others spheres??

Can someone set me straight here and clear up my misunderstanding of how entropy works with cosmic materials like nebulas into stars and dust into planets? If it’s all gravity - then …(scratches head like cartoonish ape)

 

[Dale]

energy is supposedly swirling and coalescing, seems like a high entropy state

It is hard to address the root cause of the question since you don’t explain why you believe this. However, in simplistic terms entropy is low when energy is concentrated into a small place and is thus a high temperature. Both of those conditions were found slightly after the Big Bang. The universe was very small and hot, meaning low entropy.

It is now much more spread out and cool, so entropy is higher.

 

[haushofer]

Maybe you like this paper:

https://arxiv.org/abs/0907.0659

 

[pbuk]

The universe was very small and hot.

I think you mean very dense and hot.

 

[sophiecentaur]

I think you mean very dense and hot.

Very small as well - so the entropy is low because you 'know' the position of everything - so accurately that being 'jumbled up' is less relevant to the measure of entropy(?)

 

[Dale]

I think you mean very dense and hot.

I meant small and hot for people thinking about entropy in general. But dense and hot is a better description of the early universe since it may have been infinite which is not exactly small.

 

[Paul Thatcher]

One key piece of evidence for the Big Bang, the microwave background radiation, indicates that in the beginning matter was in thermal equilibrium, ie a state of maximum entropy. And yet since then, with the expansion of the universe, entropy has been increasing.
'Hang on', we might say, 'How can entropy start at the maximum, and then still keep increasing?'
How to resolve this paradox?
First, let's consider the injection of a coloured gas into a closed box full of air. At the start, its distribution is not uniform, there is a coloured cloud distinct from the clear air, and entropy is minimal. With time, diffusion blurs then eradicates the distinction between the coloured cloud and the air, entropy increases to its maximum, and we're left with a tinted 'fog'. The progression here is, then, towards uniformity.
This however, is not a useful model for the early universe - and the reason is gravity.
At the the Big Bang, matter-energy which had been in thermal equilibrium, was now in an expanding space-time. However, not even the 'smoothing' effects of cosmic inflation could prevent the gravitational clumping together of matter into clouds, then stars and galaxies. This, then, represents a progression away from uniformity.
So, now we still have a paradox, right?
Apparently, yes - except that the universe may well have started, but it is far from finished...
What will happen to those stars and galaxies? Gravity, which played such a role in their coming into being, will also ensure their ultimate fate - as black holes.
The black holes will, in turn, fade away in Hawking radiation.
The result? The eternal exponential expansion of space-time filled with a thin 'mist' of radiation and elementary particles towards heat death, towards maximum uniformity - that is, maximum entropy.
So, if we hang on a few trillion trillion trillion trillion trillion... years, the paradox is resolved.
And seeing a paradox resolved has got to be worth the gloomy prospect of heat death, hasn't it?

 

[sophiecentaur]

infinite which is not exactly small.

I couldn't disagree with that.

 

[Dale]

in the beginning matter was in thermal equilibrium, ie a state of maximum entropy. And yet since then, with the expansion of the universe, entropy has been increasing.
'Hang on', we might say, 'How can entropy start at the maximum, and then still keep increasing?'
How to resolve this paradox?

An easier way to resolve it is to look at this “maximum” and realize that it isn’t actually the maximum. It is only a “maximum” if you hold some other quantities constant. And those quantities are not actually constant.

 

[Paul Thatcher]

Yes, true. Since writing, another perspective has occurred to me.
If physics as it stands can only deal with the universe after the Big Bang, ie with a dynamic universe, in which space-time itself does not remain constant, so it is not possible to speak of maximum entropy at the 'beginning' - and if quantum uncertainty means absolute zero cannot be reached, the expanding universe will only ever approach maximum entropy at the 'end', which never comes - then aren't we dealing with an illusion in speaking of a maximum?
So isn't this another way of expressing the need for a unified theory - or am I, a non-scientist who loves science, lost in space-time?

 

[Paul Thatcher]

I couldn't disagree with that.

"If God had intended us to use analogies, he would not have released Mathematics for our use."
This may have been a light-hearted quip, but reflect on its effect:
I'm a linguist and writer. For one reason or another, I did not learn higher mathematics. Should I therefore not think about physics, not be curious? If the dialogue is confined to mathematics, I, and of course millions of others, are excluded.
Further, should popular science, which uses so many analogies (not always accurate, it is true), be junked? I can think of plenty of politicians who would be only too glad to see science kept away from people and people kept away from science - and they are not the kind of people I want to see running the world, nor, I suspect, are they the kind who like to see engagement in science by the general population, for example, in fora like this.
Such a comment is off-putting and exclusive, not to mention hurtful.
Also, "he"? Really? Surely God's preferred pronoun is "she".

 

[sophiecentaur]

Should I therefore not think about physics,

That's a difficult question to answer. I would say that you should think about Physics and feast on its riches but hesitate to make ground breaking pronouncements about it, based just on extrapolation of analogy. From what you write, I wouldn't have you down as a BS merchant and, if you were to pronounce on Physics, you would probably quote a higher authority. (That's a compliment, btw.)
Any higher authority would not have used just analogies but done things thoroughly. I don't know of a more thorough and reliable approach to Science than to use the language of Mathematics (in the widest sense).

 

[Dale]

"If God had intended us to use analogies, he would not have released Mathematics for our use."
This may have been a light-hearted quip, but reflect on its effect:
I'm a linguist and writer. For one reason or another, I did not learn higher mathematics. Should I therefore not think about physics, not be curious? If the dialogue is confined to mathematics, I, and of course millions of others, are excluded.
Further, should popular science, which uses so many analogies (not always accurate, it is true), be junked? I can think of plenty of politicians who would be only too glad to see science kept away from people and people kept away from science - and they are not the kind of people I want to see running the world, nor, I suspect, are they the kind who like to see engagement in science by the general population, for example, in fora like this.
Such a comment is off-putting and exclusive, not to mention hurtful.
Also, "he"? Really? Surely God's preferred pronoun is "she".

What is going on here? You quote a post but then nothing in your post seems at all connected with or responsive to the post you quoted.

And please don’t make religious comments here at all

 

[Paul Thatcher]

What is going on here? You quote a post but then nothing in your post seems at all connected with or responsive to the post you quoted.

And please don’t make religious comments here at all

Excuse me, but I was replying to the line that appears under sophiecentaur's posts.
My mistake, I'm sure, if that wasn't clear - possibly I clicked in the wrong place.
I'm an atheist, so hardly inclined to bring God into it at all, but I don't think I am to blame if I reply to a post containing a reference to God.
Just in case any of this requires further illumination, please see below sophiecentaur's post, in which I received an interesting and considerate reply to my post, though I do not fully agree with it; sophiecentaur did at least understand what is going on here.
Your response, however, rather conveys the message that 'outsiders' are not really welcome in this forum.

That's a difficult question to answer. I would say that you should think about Physics and feast on its riches but hesitate to make ground breaking pronouncements about it, based just on extrapolation of analogy. From what you write, I wouldn't have you down as a BS merchant and, if you were to pronounce on Physics, you would probably quote a higher authority. (That's a compliment, btw.)
Any higher authority would not have used just analogies but done things thoroughly. I don't know of a more thorough and reliable approach to Science than to use the language of Mathematics (in the widest sense).

The idea of the apparent paradox and its "resolution" does indeed come from a higher authority (but not a Higher Authority), namely, Penrose; as far as I am aware, however, this is not an idea exclusive to him, therefore I did not see the need to reference that particular lecture (in which, BTW, he uses non-mathematical language to explain his point).
Far from being a pronouncement, it was a test of myself, to see whether I had grasped the essential point. Where better to do so than such a forum?
The compliment did not pass unnoticed or unappreciated!

 

[sophiecentaur]

I don't think I am to blame if I reply to a post containing a reference to God.

Some people can find references to such things when none were intended.
As for the term "higher authority" I don't need to look very high to find a higher one than I.

You quote a post but then nothing in your post seems at all connected with or responsive to the post you quoted.

He was quoting my signature. Even I had to read his post twice before I got the message.
@Paul Thatcher I think you may be over thinking all this. Go with popular Science but be careful where you get it from. If you want reliable sources then I'd suggest PF would be a safe source for references. You may have experienced a bit of the PF 'ton of bricks' effect. It keeps us all up to scratch though.

 

[Paul Thatcher]

Thanky you, sophiecentaur, for your replies. Scratched by falling bricks, but not wounded to the quick.

 

[Dale]

I was replying to the line that appears under sophiecentaur's posts.
My mistake, I'm sure, if that wasn't clear

Ah, that makes sense. That is a signature and it doesn’t show up for people on a mobile (like me). So for me it came out of nowhere.

Your response, however, rather conveys the message that 'outsiders' are not really welcome in this forum.

My apologies there. I just had no idea what you were talking about.

 

[phinds]

I was watching the Wonders of the Universe with Brian Cox documentary ...

Well, see, there's your problem right there. Pop science "documentaries" are entertainment and not science education. They get a LOT of stuff right but they get stuff wrong as well and if you aren't already versed in the topics you can't tell what they get right and what they get wrong.

 

[Paul Thatcher]

I imagine that, like me, C_Nordquist came here to clarify things see and heard in popular science documentaries - and in any case, doesn't the need to be well-informed in a topic before one starts to look at it imply a break with causality?

 

[phinds]

I imagine that, like me, C_Nordquist came here to clarify things see and heard in popular science documentaries ...

Perfectly reasonable and a good use of PF, but I just like for folks to be aware that when they watch pop-science presentations they are not necessarily being educated and in fact may be being mislead.

 

[Paul Thatcher]

Very true - I see goodness knows how many examples of misleading explanations and theories (actually, I'm being polite - they're totally wrong) in the equivalent of popular science in my own field (language and linguistics), namely coursebooks and textbooks...

 

[Paul Thatcher]

Ah, that makes sense. That is a signature and it doesn’t show up for people on a mobile (like me). So for me it came out of nowhere.

My apologies there. I just had no idea what you were talking about.

All is cool, friction overcome, pleased to be part of the debate.

 

[pinball1970]

Thanky you, sophiecentaur, for your replies. Scratched by falling bricks, but not wounded to the quick.

Seven years here and I still think twice before posting! The strict rules mean you get high quality responses and solid references.

 

[phinds]

Seven years here and I still think twice before posting! The strict rules mean you get high quality responses and solid references.

Well, not always. Sometimes they let me post

 

[pinball1970]

Well, not always. Sometimes they let me post

9413 reaction points, a 'star' verified expert in science and maths, insights author...

I think we are safe.

 

[phinds]

I think we are safe.

Oh, you optimist, you.

 

[Vanadium 50]

Entropy is an extensive quantity - i.e. the amount of entropy depends on the volume being considered. "Entropy of the Big Bang" does not seem well-defined. What exactly is meant here?

 

[Paul Thatcher]

Oh, you optimist, you.

Entropy is an extensive quantity - i.e. the amount of entropy depends on the volume being considered. "Entropy of the Big Bang" does not seem well-defined. What exactly is meant here?

Thanks. Wow, I think this was the question I needed to be asked. Time to think - which will, given my hotheadedness, contribute a little extra to the entropy...