All Questions
Tagged with cosmology statistical-mechanics
77
questions
2
votes
2
answers
286
views
Why can't primordial fluctuations be super-"horizon" without inflation?
I am trying to understand why, in cosmology, it is said that the presence of fluctuations at scales above the Hubble distance would not be expected in the absence of inflation or something like it.
We ...
- 59.6k
5
votes
1
answer
98
views
Decoupled Temperature for photons: why is it 0.25 $\rm eV$ rather than 13.6 $\rm eV$?
When calculating the decoupled temperature of photons using Saha' equation for the following process:
\begin{equation}
e^- p\longleftrightarrow H\gamma
\end{equation}
we find that $T_{dec}=3000$ K$=0....
- 699
3
votes
2
answers
557
views
On Boltzmann brain formation
As a popular thought conundrum it is said that if our universe can randomly fluctuate into a self-conscious formation (called Boltzmann brain), then given enough time it will do so and hence the ...
- 79
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
0
votes
1
answer
218
views
Chemical equilibrium for a $2-2$ scattering and Boltzmann equation in cosmology and astroparticle physics
Definition of chemical equilibrium According to the definition of chemical equilibrium in Wikipedia, it is a situation where the rate of the forward reaction is same as the rate of backward reaction ...
- 26.8k
0
votes
1
answer
303
views
Energy density, pressure and temperature for massive neutrinos in cosmology
I want to be able to numerically compute the mean energy density and pressure for a massive neutrino species in cosmology, at any given scale factor $a$. These are given in terms of the distribution ...
- 121
6
votes
1
answer
1k
views
Derivation of relativistic pressure
As you can find in many cosmology textbooks, the relativistic pressure in quantum statistical mechanics can be witten as below:
$$p=g \int \frac{d^3P}{(2 \pi \hbar)^3} \frac{c^2 |\mathbf{P}|^2}{3E(\...
- 83
-1
votes
2
answers
153
views
How much energy is required by a planet like Earth to support and maintain life as we know it?
How much energy is required by a planet like Earth to support and maintain life as we know it and could this energy be provided by a star the size of the moon, located where the moon is without ...
-1
votes
3
answers
93
views
If the moon produced energy like the Sun would it radiate the same energy and light on Earth as the Sun? [closed]
I would like to know if the moon where to shine like the sun would the Earth get the same energy and light as it is getting now from the Sun?
2
votes
0
answers
158
views
How could the universe's expansion "remove the mean gravitational potential"?
I'm citing W. C. Saslaw's The Distribution of the Galaxies: Gravitational Clustering in Cosmology, chapter 25, where he adresses (what seems to be a Newtonian approximation of) the thermodynamic ...
- 2,463
9
votes
1
answer
446
views
How to understand that correlators measure physical correlation?
Background In physics, we always come across $n$-point correlators (e.g. 2-point correlators). For instance, in phase transitions, one is interested in finding the correlation function between order ...
- 26.8k
2
votes
1
answer
91
views
No Temperature in an Expanding Universe?
The statistical definition of temperature as $\bigg(\dfrac{\partial \ln \Omega}{\partial E}\bigg)^{-1}$ inherently assumes the existence of a well-defined energy. To my understanding, a well-defined ...
user87745
3
votes
1
answer
312
views
How to compute Coulomb scattering rate(in early universe)?
I want to compare Coulomb scattering rate with Hubble's expansion rate.
To compute Coulomb scattering rate, I know that
$$ \Gamma = n_p\sigma v_{rel} $$
Here, $\sigma = \frac{e^{4}}{(4\pi\...
- 624
1
vote
0
answers
32
views
What might be the equillibrium Distribution Function of Dark Matter?
Can we assign a thermal distribution function to Dark Matter(not necessarily known ones)?
Is it possible that they won't even follow thermal distribution function and maybe a function of something ...
- 624