# Entropy and the universe

1. Apr 7, 2013

### capcom1983

I was thinking maybe the universe isn't in the chaotic state its meant be in. Rather a more organised state and the universe is still in it's younger years. This is just a thought any insight would be great.

2. Apr 7, 2013

### Drakkith

Staff Emeritus
It would help very much if you were a little clearer with what you are asking. When you say "the chaotic state its meant to be in", what exactly do you mean by this? What reasons are there for the universe to be in any other state than its current one?

3. Apr 7, 2013

### Mordred

Entropy is a handy term however care must applied in its usage. Entropy is in one usage a descriptive of thermodynamic flows. In the ideal gas law it describes pressure flows. In the usage of information it describes information losses in communication. However in your usage its a statistical measure. Entropy can be used to describe the tendency to disorder with times arrow however its normally applied in isolated systems.

I'll assume the latter case as you did not describe which usage of entropy your referring to. Though it does apply to thermodynamics, and pressure in cosmology.

First lets take one ordered state, for purpose of my reply we will use the inflationary model in its early form of false vacuum developed by A.Guth.
In this form the lowest energy state is given the term false vacuum. This vacuum cannot stay as a pure state. Heisenburg uncertainties occur.
in other words according to QM quantum fluctuations will occur creating virtual particles that tunnel from the false vacuum to the true vacuum. This creates disordered regions or perturbations. Those perturbations create further regions of disorder causing further quantum effects. So in this sense we can safely infer an ordered system moving toward a more disordered state.
If one applies it to mass densities. An ordered mass density would be one of even distribution throughout its volume. However as one region of mass density accumulates through means such as gravity. Then you now have a disordered state. So in a sense the formation of stars and planets could be inferred as the tendency towards disorder.
Ideally energy density should also be evenly distributed however forces of attraction replusion will by a similar manner as described above will have a tendency to diorder.
So yes its reasonably safe to say the tendency of order to disorder
does apply to the universe in
various forms.

However entropy is applied to closed or isolated systems

Last edited: Apr 7, 2013
4. Apr 7, 2013

### capcom1983

What mordred said
thanks mordred.
I was thinking maybe the universe is relativly new. That's why the universe expansion is accelerating rather than deccelerating. Sorry entropy was a bad choice.

5. Apr 7, 2013

### Mordred

As we dont know how long the universe will last old or new universe is a meaningless description. The Guth false vacuum model has an inherent problem called "runaway expansion" this is one of the primary reasons for later inflationary models which attempt to solve or use runaway expansion. Such later models include bubble universes or pocket universes. Eternal inflationary model etc. Even some string theory models derive from the false vacuum model.

The current best fit model is lambdaCDM. This is a hot big bang model with cold dark matter. Cold dark matter is nonrelativistic matter. In terms of this model lambda is the driving force of expansion. Referred to as dark energy or vaccuum energy. We still dont fully understand that mechanism however we can still account for it with current data. Is it from false vacuum or other proposed models is still under debate.

6. Apr 7, 2013

7. Apr 7, 2013

### Naty1

A different universe might well behave very differently from this one.

Inflation was very rapid and brief early initial expansion, then expansion slowed under the effects of matter [gravity] and from about 7B years onward to now has been accelerating....that's when we shifted from a matter to an [cosmological] energy dominated period.

In addition, I suspect the expansion is not quite what you think it is....

If you were to watch a series of different galaxies pass at some fixed distance over time, later galaxies would pass that same distance mark at a smaller velocity than earlier ones. Not what one might naively think is happening from popular descriptions.

8. Apr 8, 2013

### capcom1983

can you ellaborate.

9. Apr 8, 2013

### Chalnoth

While there is some relationship between chaos/disorder and entropy, the problem is that the former terms are colloquial terms that are very non-specific, and may lead people to incorrect conclusions.

For example, consider a room with air in it, with the air in two different configurations. In each configuration, the air has the exact same amount of total energy, and there are the exact same number of molecules. The difference is on where the molecules are:

1. In the first configuration, the air molecules are spread out almost evenly. If you pick any two cubic centimeters of air in the room, the number of molecules in each cubic centimeter is likely to vary by less than one in a billion.

2. In the other configuration, the air molecules are gathered in clumps, so that one cubic centimeter of air might have a million times as many molecules than another.

Which of the two configuration seems to be more chaotic and disordered? Presumably you would answer configuration 2. But it is configuration 1 that has the higher entropy (much higher, as it turns out, as evidenced by the fact that we never run into a room like number 2).

As for the universe as a whole, the eventual, maximal-entropy configuration appears to be empty space.

10. Apr 8, 2013

### Mordred

Entropy in cosmology is also confusing, in your example you are describing closed systems, which is the more classical form of entropy. In open cosmology systems its more tricky and in a sense what defines order or disorder.

http://philsci-archive.pitt.edu/4744/1/gravent_archive.pdf

the above article discusses some of the misconceptions of what ppl might think of as low entropy or high entropy in cosmology usage.

Its claim:
the assumption that uniformity equates to high entropy ignores the existence
of gravitation. Given the attractive nature of gravity (it is claimed), a uniform
state is actually much lower-entropy than a much more clumped state. Various
forms of argument are given for this; frequently, it is argued that when attractive
long-range forces are present, matter has a higher entropy when highly concentrated
than when diffuse.

[T]he attractive nature of the gravitational interaction is such that
gravitating matter tends to clump, clumped states having larger
entropy. . . . For an ordinary gas, increasing entropy tends to make
the distribution more uniform. For a system of gravitating bodies the
reverse is true. High entropy is achieved by gravitational clumping

there are numerous examples of some of the confusions with regards to entropy usages in cosmology.

I would would be curious on how some of the views of this paper have been resolved into more current cosmology entropy usage and definition. For example entropy in the second law of thermodynamics is a closed system. So how is it used in cosmology which could be an open or closed system globally ?

11. Apr 8, 2013

### Chalnoth

Entropy can be used equally-well in both open and closed systems. The main difficulty with using entropy in cosmology in any sort of detailed manner is the fact that we don't know how to write down the entropy for a general self-gravitating system. But we do know the entropy of a few specific self-gravitating systems. For example, we know the entropy of a black hole, and the entropy of an empty de Sitter universe. These are small, special cases, but they still give us some understanding of entropy in our universe.

For example, we know that the highest-entropy configuration of a bunch of matter is for it to all be in a single black hole. From this we can conclude that all matter will eventually be in black holes. It will take a long time, of course, but it will happen. And our current understanding of how matter works supports this general statement, and gives us more specifics on when (probably approximately $10^{40}$ years).

We also know from analyses of entropy that a universe with a black hole and a positive cosmological constant has less entropy than a universe with just a positive cosmological constant (of the same value). And indeed, we expect those black holes to eventually decay, producing the highest-entropy universe of all: a completely empty universe.

12. Apr 8, 2013

### Mordred

That makes sense, also provides me with a couple of good directions for further research. I'm used to calculaing entropy of closed systems in regards to thermodynamics and the ideal gas law as they apply to automated systems. Or more specifically the classical usages.
When I tried researching the cosmology usages I ran into a lot of ambiquity, your explanation covers that ambiquity.