All Questions
24
questions
4
votes
2
answers
139
views
Why can photon be treated like gas?
In Cosmology, especially when studying Cosmic Dynamic, sometime we will treat photons as gas to calculate its pressure, but according to my understanding, photon and gas are nothing alike. Why can ...
- 445
5
votes
1
answer
81
views
Net particle number density for relativistic particles at finite chemical potential (tricky integral)
Question: How does one show that the chemical potential of relativistic fermions is negligible at high energies? In particular, I would like to show that the difference between the particle density $n$...
- 1,198
0
votes
0
answers
32
views
Poincaré recurrence in a closed universe? [duplicate]
Is it possible that the Poincaré recurrence applies to a closed universe (with a finite spacetime)? If it is, would this mean that a closed universe could eventually reach the same state as its ...
- 2,472
1
vote
0
answers
36
views
Why are two assumptions for the approximation from the BE, FD to MB distribution and $T$ invariance in the Boltzmann equation reasonable?
As the title says, I can not understand whether the assumptions
are reasonable.
If an interaction $1+2\leftrightarrow 3+4$ is taken into account, the number variance in time is proportional to
$$|\...
0
votes
0
answers
40
views
Is the 2nd law of thermodynamics more of a likelihood than a law? [duplicate]
Given enough time, even extremely unlikely events are certain to happen. This includes spontaneous reduction in disorder.
As an example, given enough time, all the particles of a gas in a container ...
- 1,421
3
votes
1
answer
1k
views
Conservation of entropy in cosmology
I've been trying to follow the procedure that some books give in order to prove that the entropy of the universe is conserved (S is constant).
It usually goes like this:
Consider the second law of ...
1
vote
0
answers
192
views
Energy density of particle species in thermal equilibrium
I am reading the book Kolb and Turner "The Early Universe". In the thermodynamics section they mention that the total energy density of different species in equilibrium is
$$\rho=T^4\sum_{\...
user255856
1
vote
1
answer
53
views
Number of interaction of early universe
I am reading An introduction to particle dark matter by Stefano Profumo. I am struggling on a derivation.
As given from the text, in the early universe $\frac{\dot{T}}{T}=-\frac{\dot{a}}{a}=-H$, ...
- 567
5
votes
2
answers
797
views
Can we really apply the second law to the entire universe?
I do not doubt the second law in general, just if it rigorously applied to the entire universe. Here's why I ask this
2nd law - restricted to isolated systems: "The second law may be formulated ...
- 601
60
votes
9
answers
8k
views
How do different definitions of entropy connect with each other?
In many places over the Internet, I have tried to understand entropy.
Many definitions are presented, among which I can formulate three (please correct me if any definition is wrong):
Entropy = ...
- 1,137
4
votes
1
answer
280
views
What happens here when the Second Law of Thermodynamics doesn't hold true?
In the 1920s, theoretical physicists, most notably Albert Einstein, considered the possibility of a cyclic model for the universe as an (everlasting) alternative to the model of an expanding universe. ...
- 1,764
2
votes
0
answers
87
views
How is the entropy of the universe defined?
My understanding is that to define the entropy of a system what you have to do is as follows:
Define the boundaries of your system.
Define a set of "microstates" of the system.
Define a partition of ...
- 225
-1
votes
2
answers
153
views
How much energy is required by a planet like Earth to support and maintain life as we know it?
How much energy is required by a planet like Earth to support and maintain life as we know it and could this energy be provided by a star the size of the moon, located where the moon is without ...
-1
votes
3
answers
93
views
If the moon produced energy like the Sun would it radiate the same energy and light on Earth as the Sun? [closed]
I would like to know if the moon where to shine like the sun would the Earth get the same energy and light as it is getting now from the Sun?
2
votes
0
answers
158
views
How could the universe's expansion "remove the mean gravitational potential"?
I'm citing W. C. Saslaw's The Distribution of the Galaxies: Gravitational Clustering in Cosmology, chapter 25, where he adresses (what seems to be a Newtonian approximation of) the thermodynamic ...
- 2,463