All Questions
494 questions with no upvoted or accepted answers
12
votes
2
answers
384
views
What's the cause of this gap in this simulation of the Nice model?
A previous question brought me to this video (which has a spectacular change at about 0:34). It shows the orbits of Jupiter, Saturn, Uranus, Neptune, and what appear to be trans-Neptunian objects.
...
- 10.9k
11
votes
1
answer
164
views
Entropy and equilibrium concepts at astronomic scales
I am always puzzled to read here and there discussions dealing with thermodynamic concepts applied to astronomic scales where gravity matters. To my opinion, there is a certain carelessness to go into ...
11
votes
0
answers
458
views
View of the sky from inside a black hole
Consider an observer located at radius $r_o$ from a Schwarzschild black hole of radius $r_s$. The observer may be inside the event horizon ($r_o < r_s$).
Suppose the observer receives a light ray ...
- 1,398
10
votes
0
answers
566
views
What is the Chandrasekhar-Friedman-Schutz (CFS) instability, exactly?
I am confused as to what the Chandrasekhar-Friedman-Schutz (CFS) instability is, exactly.
It seems to refer to this paper by Chandrasekhar, but I do not think this paper covers the full instability. ...
- 11.8k
10
votes
0
answers
345
views
Gauss-bonnet gravity constraints from string theory
Recently there have been advances in observational constraints of gravity theories that contain scalars coupled to the gauss-bonnet topological term:
http://arxiv.org/abs/0704.0175
http://arxiv.org/...
- 14.7k
9
votes
1
answer
242
views
How does adiabatic cooling make a nebula colder than the CMB?
According to measurements, the gas from the Boomerang Nebula is expanding so fast that it's colder than the cosmic microwave background radiation... how does such a simple mechanic work on such a ...
user158288
7
votes
2
answers
446
views
Is there linear 'frame dragging'?
Very massive objects cause the so called 'frame dragging' that can increase the speed of a beam of light to a total aggregate speed faster than the speed of light in normal circumstances so my ...
- 1,835
7
votes
0
answers
209
views
Relativistic rotational squeezing?
I would like to consider a sphere rotating at very high angular speeds, such that the speed in its equator would be relativistic.
This is very similar to Ehrenfest paradox situation, except that ...
- 3,805
6
votes
0
answers
84
views
Improving physics simulation of black hole accretion disk?
I have recently been working on software which uses ray tracing/marching to render a black hole in the Schwarzchild metric. I've implemented most everything that I originally set out to do, and I am ...
6
votes
0
answers
134
views
Could the recently discovered "Dancing Black Holes" be the experimental signature of Hawking Radiation?
Science just reported two gravitationally bound massive black holes that are slowly receding from each other. Under normal circumstances two such bound black holes would be approaching each other as ...
- 6,112
6
votes
0
answers
115
views
are alignment of star systems' orbital planes with ours better than random?
It obviously helps our observation of exoplanets if they transit their star from our point-of-view. I would guess that the chances of this alignment are better than if their orbital plane was randomly ...
- 161
6
votes
0
answers
92
views
Experimental tests of Cluster Decmposition
How tight are experimental and astrophysical tests on whether Cluster Decomposition is satisfied at various space-like separations?
Is there a review paper or a standard reference on the question?
I ...
- 9,966
6
votes
0
answers
98
views
Status of large-scale structure formation within cosmology today
Since the CMB results of the past decade, would it be fair to say that the consensus among cosmologists is that cosmic strings are no longer considered as a (major) source for density perturbations?
...
EB95
5
votes
0
answers
57
views
Gravothermal catastrophe in 2+1 and 4+1 dimensions?
Before I go do a week of numerical simulations...
In our 3+1D spacetime, gravitationally-bound systems of large numbers of particles, like stars in globular clusters, can be stable for long periods, ...
- 2,286
5
votes
0
answers
66
views
Do stellar model luminosities include neutrino losses?
I have had a sudden crisis in my understanding of the published outputs from stellar evolutionary model calculations.
Usually these models output a "luminosity" that one can then use, along ...
- 137k
5
votes
0
answers
144
views
Is the average temperature of a big accretion disk colder than a small one?
Suppose we have a Shakura and Sunyaev style accretion disk between $r_1$ and $r_2$. What is the average temperature? We can calculate it by integrating the local temperature $$T(r)=\left [ \frac{3GM\...
- 33.4k
5
votes
0
answers
309
views
What are the applications of hyperbolic $3$-manifold theory to cosmology?
I am a pure mathematician specialized in hyperbolic $3$-manifold topology. That has been an incredibly active field of research in the past few decades due to the seminal work of Thurston, as many of ...
- 361
4
votes
0
answers
72
views
What is the time-averaged gravitational wave power of the Milky Way
I would like to learn, if possible, an estimate of the total time-averaged power of gravitational wave emission at frequencies below GHz by the Milky Way. For example the value might be something like
...
- 61.8k
4
votes
0
answers
130
views
On the physical effects and intuitive interpretation of the NUT parameter (as in the Taub-NUT or Kerr-Taub-NUT solutions)
Various papers¹ have been written about the Taub-NUT or Kerr-Taub-NUT exact solutions to Einstein's equations, but while they describe mathematically precise (local or global) properties of that ...
- 850
4
votes
1
answer
111
views
Is the amount of dark matter per galaxy the same as you look back through time (further away from earth)?
In the hope that it may inform us about the development/evolution (if any) of dark matter over time, are there any differences (eg. in structure or concentration) in the dark matter at large radial ...
- 351
4
votes
0
answers
118
views
Hills Mechanism
The Hills mechanism postulates that when a stellar binary system is perturbed by a supermassive black hole (SMBH), the tidal forces at play result in the capture of one star while simultaneously ...
- 1,327
4
votes
0
answers
451
views
What is the current status or resolution of Greisen–Zatsepin–Kuzmin (GZK) cosmic-ray paradox?
The Greisen–Zatsepin–Kuzmin limit (GZK limit) is a theoretical upper limit on the energy of cosmic ray protons traveling from other galaxies through the intergalactic medium to our galaxy.
A number of ...
- 4,376
4
votes
0
answers
108
views
What happens in the event that the cooling radius is shorter than the virial radius of a Cold Dark Matter Halo?
The cooling radius of a cold dark matter halo is defined to be the time at which the cooling time $t_{cool} = t_{free fall}$
where
$$t_{cool}=\frac{\rho \varepsilon }{\Lambda \left ( T \right )n_{H}...
- 889
4
votes
1
answer
384
views
Mass-luminosity relation for a fully convective star
I'm trying to show that the mass-luminosity for a fully convective star is
$$L \propto M^{\frac{113}{66}}$$
I know that the energy generation is via pp-chain and the opacity is due to $H^{-}$ ions ...
- 521
4
votes
0
answers
102
views
Why do some stars actually produce "Gamma ray bursts"?
I looked it up but I haven't found any explanation as to why some stars produce them, I understand that collapsing and merging stars produce them, but my question is why is the energy concentrated in ...
- 1,137
4
votes
0
answers
325
views
What is the theoretical geometry of bubble universes?
My research has led me to look into the idea of bubble universes which I don't know very much about. The first thing that I am looking for is understanding or visualising how could many bubbles co-...
- 321
4
votes
0
answers
74
views
squeezed radiation astronomy
Squeezed electromagnetic vacuum does have a renormalized energy density smaller than the vacuum. So it makes it in my opinion a inconspicuous candidate for a dark energy carrier.
Are there ...
- 2,299
4
votes
1
answer
2k
views
Relationship between oscillator strength and cross-section
In the context of absorption of photons by atoms, I have come across two seemingly very related quantities, cross section and oscillator strength. In the book Physics of the Interstellar and ...
- 5,452
3
votes
0
answers
224
views
How do you build a Coherent Elastic Neutrino-Nucleus Scattering (CEvNS) detector?
*Milkjug-sized neutrino detector (the COHERENT neutrino detector):
https://news.uchicago.edu/story/worlds-smallest-neutrino-detector-observes-elusive-interactions-particles#:~:text=The%204%2Dinch%...
- 47
3
votes
0
answers
67
views
What if you ejected a cone of material from the Earth out into space?
For calculation purposes, let's say it's a cone with cone angle (at Earth's center) of 30 degrees, that extends down to the outer surface of Earth's solid inner core, and the entire mass is ejected ...
- 131
3
votes
1
answer
112
views
How does absorbing a free thermal neutron convert nitrogen-14 into carbon-14? (Radiocarbon dating, etc.)
Why doesn't a nuclide of N-14 simply absorb the neutron created by cosmic rays or solar wind interacting with the atmosphere and become N-15, which is stable?
How or why does the resulting Nitrogen-15 ...
- 4,709
3
votes
1
answer
198
views
Understanding comoving frame in an expanding universe?
A particle is moving in a universe that is expanding with a constant acceleration (i.e. expansion of universe is accelerating). I am considering 2 cases where the particle is (1) constantly ...
- 73
3
votes
1
answer
71
views
Resources on Post-Einsteinian Results in GR
What are some good books, lecture notes, articles, etc. that can be used as introduction to the landscape of major results in general relativity since Einstein? In terms of the timeline, I'm thinking ...
3
votes
0
answers
77
views
Spherical harmonics with 2 cosmological probes : considering $a_{\ell m, photo}$ as a constant and $\hat{a}_{\ell m, spectro}$ as an estimator
I am in cosmological context where the survey on which I am working has 2 probes : a photometric galaxy clustering ($GC_{ph}$) probe and a spectroscopic galaxy clustering ($GC_{sp}$ probe).
We use an ...
user87745
3
votes
0
answers
136
views
How do bars in barred spiral galaxies form?
There is this same question in astronomy stackexchange. But its only answer is unfortunately a rather unsatisfying quote which I could not even find within the provided link. Also, that question is ...
- 401
3
votes
0
answers
176
views
Using the Virial Theorem to determine where the most massive stars reside in a star cluster
I have come across a questions which I am unsure of. It asks to use the virial theorem to determine where the most massive stars reside in a star cluster. My approach is as follows:
Virial theorm:
$$...
- 31
3
votes
0
answers
75
views
A formula that seems to involve the variation coefficient
A friend walking in Minsk, Belarus, found this mural from the Soviet period and sent me an image (see below) of it, asking me what the formula in it was referring to. From my naive reading (which ...
3
votes
0
answers
33
views
Evaporation of a gravitationally-bound spherical cloud of a warm ideal gas in a vacuum
It seems that for any gravitationally bound system with a very large number of atoms/particles, there will always be some particles that manage to achieve escape speed and ultimately leave the cloud. ...
- 2,413
3
votes
0
answers
62
views
How efficient can an astrophysical jet be when it's driven by a black hole of maximal spin?
Some astrophysical jets appear to be driven by black holes. As far as I know, a detailed understanding of how black holes produce jets is still under development, but one general possibility is that ...
- 55k
3
votes
1
answer
95
views
Will pyrolytic carbon/graphite repel the Sun's solar wind?
Since a moving charged particle has a magnetic field, as well as an electric field, and pyrolytic carbon/graphite repels an external magnetic field, would this mean that an object made out of ...
user217618
3
votes
0
answers
71
views
Can mass still be observed in our universe after moving from a black-hole to a white-hole?
This question is based on the recent papers on LQG black-holes by Abhay Ashtekar. Quantum extension of the Kruskal spacetime and Quantum Transfiguration by Kruskal Black Holes
When mass moves from ...
3
votes
0
answers
103
views
On the conversion of neutron stars in deconfined quark stars
Imagine a neutron star, constituted of nucleons ($p$, $n$) and charged leptons (say $e$ and $\mu$), can decay into a deconfined quark star through some process. Given the Bodmer-Witten hypothesis, the ...
- 31
3
votes
0
answers
204
views
What is the density profile of Sagittarius A*'s accretion disk?
It is thought that Sagittarius A*, the supermassive black hole at the center of the Milky Way, is surrounded by an accretion disk.
Can anyone provide a resource for the general density profile of ...
3
votes
0
answers
74
views
Which theory do we use when dealing with the center of a star?
The center of a black hole is known as being one of the locations in spacetime that needs an unification theory between quantum physics and general relativity, because of having a large mass in a ...
- 1,743
3
votes
0
answers
82
views
What is the acceleration of expansion?
If me and my pet duck, who weighs next to nothing, were placed far away from any gravity source and in such a way that our mutual attraction was negligible, would I see my duck accelerate away from me?...
user32023
3
votes
0
answers
77
views
Neutron star merger: large scale dynamics
Yesterday was announced the first detection of a neutron star merger via gravitational and electromagnetic waves.
On the announcement one of the speakers is showing a simulation of the dynamics of ...
- 3,805
3
votes
0
answers
538
views
How to plot spectral indices $n_s$ and $r$ from Planck data?
From a certain inflationary model I have obtained the theoretical values for the spectral indices $n_s$ and $r$ as functions of $N$, the e-folds number of inflation. At this point it would be ...
- 322
3
votes
0
answers
375
views
Pressure inside a typical white dwarf
Does any one know the order of magnitude of pressure inside a typical white dwarf (better with reference)? Thanks!
I think it should be $m_e^4c^5/h^3$ (may be multiplied by $\pi$), which is $10^{22} \...
- 4,034
3
votes
0
answers
181
views
Dependence of the neutron-freeze-out temperature
What is the physical explanation for the dependence for the freeze-out temperature $T_\text{F0}$ of the neutrons as a function of the number of degrees of freedom $g^\star$?
$$T_\text{F0}=\left(\frac{...
- 3,902
3
votes
0
answers
91
views
Modeling the formation of a stellar system and matter accretion
I am trying to figure out what do I need to know to properly simulate the creation of a solar system from a particle cloud with random distribution of hydrogen atoms.
Being more of a programming ...
- 131