# I The laws of thermodynamics and the Universe

#### PhysicsNoob00

Summary
the laws of thermodynamics as applied to the universe's creation and existence
How would the first and second laws of thermodynamics apply to the creation and existence of the universe? I'm not a physicist (and unfortunately, do not remember a lot that I learnt in Physics class in school and college about Thermodynamics). I did some searching and I have come across an article on Wikipedia that the first law of thermodynamics doesn't apply to the universe. I'm stumped as to why this is the case. Is it because its an isolated system (the argument being that the universe as a whole, not just the observable universe, has no surroundings)? However, the second law of thermodynamics is applicable only to closed systems (entropy, measure of disorder in a closed system). How would the other laws apply? If someone could give me an answer bearing in mind that I'm a layman, it would be great. Here's the wiki link:

https://en.wikipedia.org/wiki/Thermodynamics_of_the_universe

Thanks.

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#### Drakkith

Staff Emeritus
2018 Award
How would the first and second laws of thermodynamics apply to the creation and existence of the universe?
No one knows. We don't even know how the universe came into existence, or if it has always existed in some form.

I did some searching and I have come across an article on Wikipedia that the first law of thermodynamics doesn't apply to the universe. I'm stumped as to why this is the case.
That's a problematic question to answer. The issue is that energy is not necessarily conserved in cosmology because it isn't necessarily conserved in general relativity. See this article: http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html

#### Gigaz

Thermodynamics becomes really weird when gravitation is involved.

For example, a planet in thermal equilibrium with the universe will be warmer at the center, because of the gravitational energy shift of photons.
Then there are black holes, which have a negative heat capacity. That does not sound particularly interesting, but it means that a system that seems to be in a thermal equilibrium with constant temperature can spontaneously transition to a state that contains a black hole, and that is very much not in a thermal equilibrium.

#### PhysicsNoob00

@Gigaz Thank you for your reply. I did find some interesting opinions about black holes with respect to their temperature, but I suppose that discussion would belong in an entirely new thread.

#### Mordred

While it is unclear if the conservation law applies as mentioned by others above. The universe is typically modelled as an adiabatic system. Adiabatic meaning no net inflow or outflow of energy.

#### Erk

If the universe has no surroundings then it is a closed system. It has no environment in which anything can exit into or enter from. Therefore, in that case the first law applies.

#### PhysicsNoob00

@Erk I think you are talking about the universe as an isolated system, where no energy and mass is exchanged with the surroundings.

#### Erk

Yes. You mentioned the notion of a universe with no surroundings, and that was what I was responding to. The absolute absence of an environment, including (and not except for) absolutely nothing. This speaks to the first law and also a perpetual universe. Without absolutely nothing there isn't anything for the universe to come from. Assuming absolutely nothing as ever existing has always been a mistake. After all, if absolutely nothing existed there wouldn't be absolutely nothing.

#### PhysicsNoob00

@Erk and @Mordred Aren't all isolated systems also adiabatic systems? Then that would imply that the modeling of the universe would be incorrect (since something cannot exist from absolutely nothing). However, the universe is usually modeled as an adiabatic system. What am I missing here, w.r.t. to your replies to my question? Sorry if I sound stupid.

#### Erk

Since an adiabatic system is within a surrounding the notion that it's the same as a closed system is highly suspicious. It has never been determined there is a scale at which an impenetrable surface actually exists.

#### Mordred

Another way to think of it is that with regards to the universe. The terms homogeneous and isotropy are important in terms of the cosmological principle. We can only determine the thermodynamic state of our observable universe. Which according to the datasets appear strongly homogeneous and isotropic.
From this we can surmise some educated guesses that the region's immediately outside our observable portion must also be of a similiar thermodynamic state. As a significant differences would induce some form of flow which would not be isotropic. An easy example is pressure differentials induce flow. We see no signs of a flow so region's immediately (within causal range) must be in the same state. This then means that no net flow of energy either enters or leaves our observable portion due to thermodynamic differentials.

One must ignore the container walls applications when applying the gas laws as there is no such barrier with regards to our universe. Instead one must switch to applying causality region's. Obviously nothing flows faster than c, however each medium has its own flow rate depending on such factors such as density.

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#### Mordred

In essence we can treat our observable universe as an adiabatic system while not necessarily a closed system

#### Mordred

You can how the gas laws applies to the adiabatic expansion on the above link on terms of formula usage
From the formulas in the first link one can better understand how the equations of state cosmology were derived

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#### ahiddenvariable

Your question can be answered in the same way that Thomas Aquinas' "first cause" proof of God is invalidated. His argument is that, since we always observe that any given event has a cause before it, it's necessarily true that there must be a First Cause that started the whole chain of events. The fallacy is that, even though we experience that every event has a preceding cause, we have no experience observing entire chains of events with first causes. Thus, to your question, we observe that the First and Second laws are valid for "normal kinds" of observable systems (excluding black holes and other issues in GR), but we have no experience observing the behavior of entire Universes. The above comment that these laws are messed up by black holes and GR is instructive, indicating that if such observable systems can be so baffling, certainly entire Universes can be at least incomprehensible.

#### PhysicsNoob00

"The laws of thermodynamics and the Universe"

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