# A Entropy and derivations - is my logic faulty?

1. Dec 27, 2017

### Nick Prince

Do you think that entropy does not increase in our universe?

2. Dec 27, 2017

### Staff: Mentor

dS=dQ/T only for a reversible process. The processes taking place in the universe are not reversible. So entropy is being generated in the universe even though dQ is equal to zero.

3. Dec 27, 2017

### Nick Prince

Do you think that entropy doesn't increase in the universe?

4. Dec 27, 2017

### Nick Prince

Can you elaborate?

5. Dec 27, 2017

### Staff: Mentor

Are you not aware that you can determine the entropy change between two states of a system only by determining the integral of dq/T for a reversible path?

6. Dec 27, 2017

### Nick Prince

The entropy of sub systems is additive so the overall entropy of the universe must be greater than zero. This must be equal to T dS hence dS of the whole universe must be positive not zero?

7. Dec 27, 2017

### Staff: Mentor

I have no idea what you're saying here. Here is a link to a Physics Forums Insights article I wrote a couple of years ago on Entropy and the 2nd Law of Thermodynamics: https://www.physicsforums.com/insights/understanding-entropy-2nd-law-thermodynamics/

Hope this helps.

8. Dec 27, 2017

### Nick Prince

9. Dec 27, 2017

### Staff: Mentor

Yes, I agree that is the terminology she is using.

10. Dec 27, 2017

### Staff: Mentor

No. But that does not mean I think entropy is increasing in the idealized models of a homogeneous, isotropic universe based on the Friedmann equations. Your references make it obvious that it is not. So, if entropy is in fact increasing in our universe, it is obviously doing so as a result of some process that is not included in those idealized models.

11. Dec 27, 2017

### Staff: Mentor

For an isolated system like the universe, the Clausius inequality reduces to $$\Delta S\gt\int{\frac{dq}{T}}=0$$This means that, even with no heat transfer to the system (dq=0), entropy is generated with the system itself, and thereby increases.

12. Dec 27, 2017

### Staff: Mentor

There are no "subsystems" in the Friedmann models, or in the thermodynamics based on them that you referenced. There is just one homogeneous, isotropic universe.

For our actual universe, yes, I agree. For the idealized universe in the Friedmann models, however, you might want to rethink this statement.

No, $T dS$ is the change in entropy as a result of some process. If the process is adiabatic, as the expansion of the homogeneous, isotropic universe is in the Friedmann models, then $dS = 0$. Your references make that clear. What they do not discuss at all is what other processes might be taking place in the actual universe (as opposed to the idealized universe in the Friedmann models) that might increase entropy.

13. Dec 27, 2017

### Nick Prince

So the implicit starting assumptions are wrong and hence the friedmann's accn and continuity equation are not complete?

14. Dec 27, 2017

### Staff: Mentor

Do you think the equations in the references you gave are complete models of everything that happens in the universe?

15. Dec 27, 2017

### Arman777

Thats nice then

Well I mean to radiation energy density. And radiation energy density has a relationship with tempature.

I mean the CMB.