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Trust10
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Can the universe be treated as a thermodynamic isolated system? And why?
Yes, but given that it is possibly infinite in extent, I'm not clear how that can even matter in any practical sense.Trust10 said:I agree with you on that, i also personally don't think there is anything out there besides our universe. So do you think it can be treated as an isolated system?
Trust10 said:I have gone through some works relating to this topic and have found out that some people are still skeptical about the universe being thermodynamically isolated or closed.
Trust10 said:The universe being a closed system was based on the fact that, it could allow energy exchange between other universes in the multiverse system.
In the derivation of the Fluid equation, from thermodynamics,Trust10 said:Can the universe be treated as a thermodynamically isolated system? And why?
Trust10 said:why call it closed and not isolated system
Trust10 said:For a closed system the condition to be satisfied is that only heat and not matter can be exchange between a system and a surrounding. But for an isolated system neither matter nor heat is exchanged with the surrounding
Arman777 said:If the universe was "gaining" heat then we could have seen this effect on the CMBR. Since CMBR is highly homogeneous
PeterDonis said:Why do you think that the CMBR being homogeneous rules out the universe gaining heat? Couldn't the heat gain be homogeneous?
Arman777 said:I think by studying the recombination era and photon decoupling, we can deduce a theoretical temperature for the CMBR.
Arman777 said:we know the current CMBR temperature, 2.75K. We also know that scale factor is inversely proportional to temperature so we can deduce the temperature of the CMBR.
Arman777 said:any heat exchange would change the photon-baryon ratio.
PeterDonis said:How? We don't directly measure the scale factor. We deduce it from other observations. But those other observations include assumptions, one of which is, basically, that the universe is not gaining heat from somewhere we can't observe.
PeterDonis said:Also, the argument you are making here is not that the universe gaining heat from somewhere we can't observe would make it not homogeneous. It is that the universe gaining heat from somewhere we can't observe would change the temperature of the CMBR--which could happen in a homogeneous fashion. So you appear to have shifted your ground from the post of yours that I originally responded to.
Arman777 said:The scale factor comes from the fluid equation
Arman777 said:We can derive the ##a∝T^{-1}##, by using the thermodynamics
Arman777 said:if see that, theoretical CMBR temperature derived from the at the time of photon decoupling (using particle physics etc.) and the temperature derived from the ##a∝T^{-1}## (that is derived by setting ##dQ=0##) matches
Arman777 said:I did not understand why you said scale factor is observable
Arman777 said:If something affects the CMBR its already part of the our universe.
Arman777 said:I discussed this question before on this site.
With the assumptions of homogeneity and isotropy, yes, definitely. Every co-moving volume will have as much heat leaving the volume as entering it, so it can be treated as isolated. Though this is often done by considering it to have periodic boundary conditions instead.Trust10 said:Can the universe be treated as a thermodynamic isolated system? And why?
kimbyd said:With the assumptions of homogeneity and isotropy, yes, definitely. Every co-moving volume will have as much heat leaving the volume as entering it
To Solve the Friedmann Equations (or to find the scale factor), we need a fluid equation, acceleration equation and the normal Friedmann equation that is derived from GR.PeterDonis said:What do you mean by this?
Okay well,PeterDonis said:Can you be more specific?
PeterDonis said:Matches what? How can you tell, observationally, that it matches?
Oh sorry, I misread it I think.PeterDonis said:I said the scale factor is not observable.
Arman777 said:So you agreed with me that we can calculate the CMBR temperature by using the particle physics etc.
Arman777 said:we can look at the CMBR and we can see that it has a temperature of 2.75K
Arman777 said:by using the Friedmann Equations, we can derive the scale factor
Arman777 said:these are two different ways of obtaining the temperature of the CMBR.
Arman777 said:there's no another way of deriving the CMBR temperature at the last scattering beside the particle physics ?
Hmm okay then, thanksPeterDonis said:No.
To be a little bit more precise, the scale factor itself is not a physical quantity at all. The scale factor at anyone time is whatever we choose it to be. Usually it's convenient to define the scale factor today as being equal to 1.PeterDonis said:We have no way of actually measuring the scale factor now or the scale factor when the CMBR was emitted,
kimbyd said:The scale factor today is measured at roughly 1090 times larger than it was at CMBR emission.
Assumptions have to be made with any measurement. It's the nature of the beast. The CMBR is on pretty solid footing with regard to that, especially as the detailed nature of the statistics of the hot and cold spots provides such rich, testable information about the nature of our universe (such as the ratio of dark matter to normal matter).PeterDonis said:Based on the temperature ratio, yes. Strictly speaking, though, this temperature ratio assumes that no other heat source has produced homogeneous black-body radiation since the CMBR was emitted. I agree that is extremely likely to be the case, but it is still, strictly speaking, an assumption that has to be made in order to deduce the scale factor ratio.
An isolated system is a theoretical concept in physics that describes a system that is completely isolated from its surroundings and does not exchange matter or energy with the outside world. In the context of the universe, it refers to the idea that the universe as a whole is an isolated system, meaning that it does not interact with anything outside of itself.
The universe is considered an isolated system because it is believed to be the only existing system in which all matter and energy is contained. It is thought to be self-contained and not influenced by anything outside of itself, making it an isolated system.
In theory, an isolated system can exist in reality, but it is difficult to create and observe in practice. In order for a system to be truly isolated, it would have to be completely cut off from any external influences, which is nearly impossible to achieve. However, certain systems can be considered isolated for practical purposes, such as a closed ecosystem or a thermally isolated container.
If the universe is indeed an isolated system, it means that everything within it is governed by the laws of thermodynamics, which state that energy cannot be created or destroyed, only transferred or transformed. This has implications for the eventual fate of the universe, as well as the behavior of matter and energy within it.
The Big Bang theory, which is the prevailing scientific explanation for the origin and evolution of the universe, suggests that the universe began as a singularity and has been expanding ever since. In this sense, the universe can be seen as an isolated system, as it is believed to be self-contained and not influenced by anything outside of itself. However, this is still a topic of debate and further research is needed to fully understand the nature of the universe as an isolated system.