# entropy and expansion of universe

Discussion in 'Physics & Math' started by ash64449, Mar 31, 2013.

1. ### ash64449Registered Senior Member

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hello friends,
what is entropy and why entropy doesn't decrease when universe expands?

3. ### originIn a democracy you deserve the leaders you elect.Valued Senior Member

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If you don't know what entropy is then it is quite difficult to answer the second question.

Entropy is a bit confusing and subtle. There are people in the forum that have been struggling with the concept for years. I suspect one of those individuals will chime into this thread before too long.

Try looking at this site and then ask specific questions if you have any.

5. ### hansdaValued Senior Member

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Entropy basically is order of disorder. For any system as the disorder increases entropy increases and as order increases entropy decreases.

As the time increases or universe expands, disorder increases; so entropy increases.

7. ### ash64449Registered Senior Member

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i know basics of entropy origin.. That it is measure of disorderness and comparing how much amount of energy of it can be used to do work.. But i am a bit confused when i consider universe.. Can you explain me the second question?

8. ### ash64449Registered Senior Member

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as time increases how disorder increases?? Universe looks the same everywhere.. So as a result it should be more ordered.. So entropy should decrease.. And also why scientists say that universe will get cold as universe expands? Doesn't coldness indicate less entropy?? And i am confused again. Because in some book i heard that total entropy never decrease.. So which is right?

9. ### hansdaValued Senior Member

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Time can be seen as "past+present+future". So at any moment in the sysytem, Present is becoming Past, Future is becoming Present and Past is remaining Past. So in a sysytem present disorders are added with past disorders. So, entropy increases with time.

Perhaps they are finding similarity with air-conditioning principle. That is work done to expand, heat is absorbed.

Entropy depends on many factors. So, sum-total of entropy may be increasing. It is energy which remains constant.

10. ### originIn a democracy you deserve the leaders you elect.Valued Senior Member

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Entropy was developed as a thermodynamic principle. The equation for entropy is:

$\Delta S = \frac{Q}{T}$

This is basically telling you how much the temperature changes for a reversable heat engine (Q).

The implications are pretty deep. This is one way of stating the second law of thermodynamics. The equation is saying that a heat engine cannot be more than 100% efficient in theory and in practice it is always less than 100% efficient. You could have a 100% efficient heat engine if the final temperature was absolute zero.

For me the easiest way to look at entropy is to use this definition and to realize that the overall usable energy of every closed decreases over time. If you only use the order and disorder definiton (which is fine if you want to use that one) you can confuse yourself.

So why would an expanding universe increase the entropy of the universe or decrease the usable energy of the universe. Well as the universe expands we know that photons are red shifted (the frequency decreases), in other words the photons decrease in energy which means the usable energy decreases - entropy increases.

Another way to look at it is that the space between the photons (energy) is increasing so the concentration of energy per unit volume is decreasing meaning the temperature of the universe is decreasing and the entropy of the universe is increasing.

11. ### quantum_waveContemplating the "as yet" unknownValued Senior Member

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You have the concept of entropy as change, but it looks like before reviewing and grasping the links and comments you might have been confused about the meaning of the direction of entropy. Am I right?

If you have that straight, then the discussion of entropy and cosmology can take place without the confusion between what is meant by increasing and decreasing entropy. Cosmologically, most theories view the universe as a closed system, finite in content and extent, in which case the link about entropy and the discussion so far would support the concept that the expanding universe is headed toward complete entropy.

But of course Origin was right, some of us don't consider that to be the only possibility; there is the concept of an infinite universe where entropy is maintained within thresholds of maximum and minimum entropy. My so called model is one such view of cosmology, though it doesn't rise to the level of science theory.

12. ### eramSciengineerValued Senior Member

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In Carnot's theory the maximum efficiency assumes constant entropy, and is already less than 100%. The fact that entropy increases means our world is less efficient than Carnot's ideal.

13. ### ash64449Registered Senior Member

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So Energy is constant but Entropy is increasing? Right?
But this conclusion comes in conflict with the book that i have read which said the entropy doesn't decrease. Based on this fact(that entropy doesn't decrease) he proved that Black holes have entropy.

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What book?

15. ### ash64449Registered Senior Member

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The Trouble With Physics by Lee Smolin

16. ### arfa branecall me arfValued Senior Member

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I'm reading The Arrow of TIme by Coveney and Highfield. In it they say that thermodynamic processes are irreversible; more exactly, the Second Law tells us that all physical processes involving transformations of energy are irreversible.
Another point they make is about the common notion that entropy and disorder are equivalent. But in a gas at equilibrium, every particle has the same average energy--the particles aren't disordered. Furthermore in systems far from equilibrium, order can arise spontaneously (around chaotic attractors).
The 'maximally disordered' viewpoint isn't that useful, on analysis.

17. ### ash64449Registered Senior Member

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In the book i mentioned,it said that entropy of a closed system can never decrease. Bekenstein worried that if he had a box filled with hot gas-which has lots of entropy,because the motion of gas molecules was random and disordered-and threw it into the black hole,the entropy of the universe would decrease,because the gas could never be recovered.To save the second law,he proposed that black hole must have entropy,which would increase when the box of gas fell in,so the total entropy of the universe would not decrease.

18. ### Q-reeusValued Senior Member

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Really? Where do you get that idea from?

19. ### James RJust this guy, you know?Staff Member

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Particles in a gas at equilibrium have a range of energies, following a Maxwellian distribution.

20. ### Q-reeusValued Senior Member

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Quite so. What's more, the gas entropy is a maximum when the system is at equilibrium, so just where 'the particles aren't disordered' comes from is a mystery. :wtf:

21. ### Motor Daddy☼☼☼☼☼☼☼☼☼☼☼Valued Senior Member

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The density order would be most dense at the center and least dense at the outer edge, like this pic:

22. ### Q-reeusValued Senior Member

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I hesitate to engage, but on the assumption your #18 is a response to my #17 (please - always quote!):
What is that pic supposed to mean exactly? Are you throwing in gravity here in some undefined manner? If so - define.

23. ### ash64449Registered Senior Member

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So what is right? Entropy increasing,decreasing,constant... What is right?? I mean about the universe which is a closed system that is expanding.