All Questions
Tagged with cosmology statistical-mechanics
77
questions
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
33
votes
7
answers
22k
views
Why was the universe in an extraordinarily low-entropy state right after the big bang?
Let me start by saying that I have no scientific background whatsoever. I am very interested in science though and I'm currently enjoying Brian Greene's The Fabric of the Cosmos. I'm at chapter 7 and ...
Kristof Claes
15
votes
4
answers
3k
views
If dark matter can't lose kinetic energy, then why is it not traveling at relativistic speeds?
I have read this question:
The only way you can do this is to remove kinetic energy from the system.
With normal matter this is done through electromagnetic interactions, which turn the kinetic ...
- 29k
13
votes
2
answers
580
views
Can $10^{23}$ stars be treated with methods of statistical mechanics?
Statistical mechanics is used to describe systems with large number of particles ~$10^{23}$.
The observable universe contains between $10^{22}$ to $10^{24}$ stars. Can we treat those many stars as a ...
- 17.1k
12
votes
1
answer
10k
views
Number $g(T)$ of relativistic degrees of freedom as a function of temperature $T$
Let us consider the total number of relativistic degrees of freedom $g(T)$ for particle species in our universe:
$$g(T)=\left(\sum_Bg_B\right)+\frac{7}{8}\left(\sum_Fg_F\right)$$
Where the sums are ...
- 1,517
12
votes
1
answer
748
views
Integrated Boltzmann equation for dark matter
In Dodelson's book, the equation for a scattering process $ a + b \Leftrightarrow c + d $ is given as
\begin{align}
a^{-3} \frac{\mathrm d (n_a a^3)}{\mathrm d t}&=-n^{\text{eq}}_a n^{\text{eq}}...
- 301
10
votes
10
answers
11k
views
If we were able to prove that the universe is infinite, wouldn't that statistically prove that there is no other forms of life?
I want to begin my explanation using abstract mathematical explanation to repetition possibility by taking independent samples $X_n$ from some continuous probability distribution: https://math....
- 368
10
votes
7
answers
2k
views
Mathematically possible vs physically probable outcomes
A good buddy of mine and I have had a friendly debate about the origins of the current state of our universe (namely; Earth and life on Earth) and have fundamentally disagreed in our stances with ...
- 201
10
votes
4
answers
3k
views
Why aren't we Boltzmann brains in an infinite universe?
Either space is finite or it is infinite.
a) - If space is infinite in extent, either it is thermal over an infinite volume, or it is in the vacuum state for most of it. If it is thermal, infinity ...
- 109
9
votes
4
answers
4k
views
Is heat death reversible by thermal or quantum fluctuations given an infinite time?
I'm new here, so apologies if the question doesn't sound meaningful considering what physics is supposed to answer. I don't have a physics or mathematics background, but I did learn a few things about ...
- 452
9
votes
1
answer
446
views
How to understand that correlators measure physical correlation?
Background In physics, we always come across $n$-point correlators (e.g. 2-point correlators). For instance, in phase transitions, one is interested in finding the correlation function between order ...
- 26.8k
8
votes
2
answers
3k
views
Do Boltzmann brain thought experiments suggest literally anything can form randomly?
Do Boltzmann brain thought experiments suggest literally anything can form randomly?
What are the limitations to what random fluctuations can form? Literally any physical, material object?
Lastly, I ...
user353810
8
votes
0
answers
245
views
Does quark color contribute to "spin degeneracy" for QGP calculations?
Like the title say, does quark color matter in counting contributions in a early universe plasma (QGP), as when adding up the total plasma energy density, or is it just spin? The book I have (Pathria) ...
- 397
8
votes
0
answers
145
views
Does the Standard Model plasma develop a spontaneous magnetisation at finite temperature?
Reference: arXiv:1204.3604v1 [hep-ph] Long-range magnetic fields in the ground state of the Standard Model plasma.
Alexey Boyarsky, Oleg Ruchayskiy, Mikhail Shaposhnikov.
The authors of this paper ...
- 16.6k
7
votes
5
answers
556
views
Can the "Boltzmann brain" concept inform discussions of cosmology?
The Boltzmann brain was originally discussed as a sort of thought-experiment or aid
to reflection on what might possibly happen in the universe. Its first discussion was
in the context of thermal ...
- 59.6k