All Questions
254
questions
-2
votes
3
answers
100
views
What is the gravitational field of a hole in an infinite perfect crystal?
Or equivalently and more interestingly: In the early universe when there was uniform H/He gas everywhere, gravitational field was close to 0 everywhere. Every test particle was pulled from all sides ...
2
votes
0
answers
90
views
Gravitational halos made of neutrinos...?
I have been recently interested in how halos made of standard model particles could be formed and behave.
After asking some questions in this site, I was told about how neutrinos could form such halos....
- 2,462
11
votes
2
answers
513
views
Why is nonzero net charge density incompatible with the cosmological principle?
In an answer to a question about the overall charge-neutrality of the universe, benrg writes,
A nonzero net charge density is incompatible with the cosmological
principle. Unlike the gravitational ...
rob♦
- 91.5k
5
votes
3
answers
414
views
Why is the universe charge-neutral?
The positive charges (such as from the protons) of the universe are almost neutralized by the negative charges (such as from the electrons).
Is there an explanation for this neutrality? Does it ...
- 4,452
0
votes
0
answers
27
views
Detailed derivation of ESCK gravity and Extended Friedmann Equations with Torsion
Do you know a textbook on the Einstein-Cartan-Sciama-Kibble theory of Gravitation and its application to derive Extended Friedmann Equations with Torsion, which shows the calculations in detail?
0
votes
1
answer
128
views
Do we really know the universal gravitational constant?
We've all heard $$F_g=\frac{gm_1m_2}{r^2}.$$ However, since I took physics, we've discovered "dark energy," which if I have any concept of the current thinking is caused by space being ...
0
votes
0
answers
53
views
Can there be structures made from neutrinos that can have angular momentum?
Would it be possible to eventually have structures made from neutrinos somewhere in the universe, as it is indicated in this question (Are neutrino stars theoretically possible?), like halos of ...
- 2,462
1
vote
1
answer
68
views
Does dark energy work on the principle of anti-gravity, i.e. repulsive gravity?
Our universe is made up of 95% dark energy+ dark matter (of which most is dark energy), and this dark energy is considered to be the main reason for the expansion of our universe. But, anything that ...
0
votes
0
answers
33
views
Turnaround radius being increased in fast collapsing regions?
In the context of formation of large-scale structures, there would be a turnaround radius where expanding matter would detach from the Hubble flow and start to collapse (https://www.aanda.org/articles/...
- 2,462
1
vote
1
answer
85
views
Tests for general relativity
As a theorist, I’d guess that in Newtonian gravity we can check for proportionality to mass, and inverse square proportionality to distance, by measuring the ratios of gravitational forces.
Is there ...
1
vote
2
answers
59
views
Interplay between gravity and cosmic expansion causing collapse of large-scale structures?
As far as I understand it, in the context of large structure formation, the interplay between gravity and cosmic expansion can cause certain anisotropies in voids that can make them collapse (https://...
- 2,462
1
vote
1
answer
30
views
Questions about cosmic sheets/walls/filaments
I had a couple of questions about cosmic filaments in the context of the cosmic web structure formation:
In this thesis (https://www.imprs-hd.mpg.de/51939/thesis\_cpenzo.pdf), the author indicates in ...
- 2,462
0
votes
0
answers
47
views
Transition to Neutron Star
If we're asked to believe that the explosive aspect of a supernova results from a rebound of residual free falling matter off of the stellar core, shouldn't we also be asked to believe that transition ...
user394592
0
votes
0
answers
32
views
Can the integrated Sachs-Wolfe and the Rees-Sciama effects have any influence on matter?
CMB photons can be affected by the expansion of the universe through the linear integrated Sachs-Wolfe effect (ISW) 1 and the non-linear ISW effect or also called Rees-Sciama effect 1.
In particular, ...
- 2,462
1
vote
0
answers
54
views
Conformal transformation to Einstein frame for a Non-minimally coupled Ricci and Maxwell term
I am currently working on a modified gravity theory which has non-minimal coupling between Ricci scalar and Maxwell term. The precise action is
$$\int d^4x\sqrt{-g} \left(R + \alpha R^2 + (1 + \beta R)...
- 23
1
vote
1
answer
125
views
What if gravitation was the only force? [closed]
This is a follow up of Interactions within constituents of dark matter .
I wonder about dark matter, and, naturally, compare it with our observable world.
If gravitation would be the only force acting ...
- 1,028
3
votes
0
answers
126
views
Is Brans-Dicke theory really ruled out by solar system tests?
Brans-Dicke theory with small values of parameter $w$ are said to be ruled out by solar system general relativity tests like the Shapiro time delay test and the deflection of starlight by the sun.
But ...
- 5,925
0
votes
1
answer
112
views
Sean Carroll, can I skip to chapter 8 after chapter 4? [closed]
For anyone who has studied the book 'An Introduction to General Relativity Spacetime and Geometry' by Sean Carroll, can I study chapters 1 to 4 (which do differential geometry & field equations it ...
- 35
0
votes
0
answers
37
views
Is there a formula for gravitational time dilation at the cosmological scale?
There is a standard formula for gravitational time dilation used in eg the operation of the global positioning system. It is based on the integration of g(h) where g is a function relating ...
- 371
1
vote
2
answers
163
views
“Negative energy” definition of “gravitationally bound” in expanding cosmology
First off, I realize that there is already a number of questions relating to gravitational binding in cosmology:
Gravitationally bound systems in an expanding universe
Gravitationally bound systems ...
- 486
1
vote
0
answers
44
views
Effects of dark energy in the kinetic energy of a body?
If I launch a ball into the sky it would reach a distance after which it would return into the ground transforming the potential energy into kinetic energy as it hits the ground
This is similar to ...
- 2,462
0
votes
1
answer
134
views
Theoretically, is it feasible for the dark matter density to be constant and homogeneous, as dark energy is, and the two to be related?
I know that currently dark matter and dark energy are separate things, not related and one not deriving from the other. But if both are included in a generalized gravitation theory, the picture can ...
- 1
12
votes
1
answer
1k
views
Is there a distance from a gravitational source where the influence of gravity and dark energy are balanced out?
While gravity is a force that attracts
objects with mass, dark energy (or, alternatively, the accelerated expansion of the universe) is not.
However, I have found numerous articles, forums, questions ...
- 2,462
1
vote
1
answer
97
views
Dark energy contributing to, or modifying, mass estimates?
I have found some papers (like this one: https://www.aanda.org/articles/aa/full_html/2009/45/aa12762-09/aa12762-09.html) which say that dark energy increases the potential energy in a system of a ...
- 2,462
1
vote
0
answers
51
views
Zel'dovich pancake derivation
I am looking for a derivation of the Zel'dovich pancake. Does anyone have a reference to the derivation or a link to the original paper?
Y. Zel'dovich, Gravitational instability: An Approximate theory ...
-7
votes
1
answer
100
views
Contradiction of propagation speed of gravitational waves with non-locality of gravity?
Although LIGO at 2016 verified the speed of a traverse gravitational wave being the speed of light in a vacuum $c$ there is also the fact that gravity is a non-local phenomenon, meaning that it is not ...
- 4,170
1
vote
1
answer
121
views
Dark matter, MOND or flattened gravitational fields? [closed]
Could there not be a third variant to explain why e.g. long-distance multistar systems rotate faster than Newton's law of gravity suggests?
In addition to the Dark matter hypothesis and MOND then, ...
- 521
1
vote
2
answers
189
views
What is the gravitational field intensity of a uniformly distributed mass content in Newtonian gravity?
In an infinite universe composed of single point masses which can be simplified as a uniformly distributed mass density, what is the equation for the gravitational field intensity in Newtonian gravity?...
- 476
0
votes
0
answers
33
views
How does the gravitational potential change when using physical instead of comoving coordinates?
If we consider the universe as filled by a self-gravitating fluid, then there will exist an associated gravitational potential $\phi$ and the Poisson equation $\nabla^2=4\pi Ga^2\rho$ will hold, where ...
- 293
1
vote
1
answer
239
views
Poisson equation in Cosmology at first order in perturbation theory
The book Cosmology by Daniel Baumann states that the Poisson equation for a universe where we consider the effects of both gravity and expansion, expressed in physical coordinates $\vec{r}=a\vec{x}$, ...
- 293
0
votes
0
answers
65
views
Hydrogen atom in void
We have a hydrogen atom, in a gravitationally bound system nothing interesting happens to it. Let's put it in an empty world where there are only an electron and a proton. Several forces act on it: ...
- 1
0
votes
0
answers
50
views
Is the $σ8$ tension in the ΛCDM cosmology evidence that the amount of Dark Matter (DM) in the universe is increasing?
Strong evidence is provided by recent cosmological studies that “clumpiness” in the large scale structure (LSS), as measured by $σ8$, is decreasing (for example, “Hyper Suprime-Cam Year 3 Results…”, X....
- 11
4
votes
2
answers
271
views
How did the big bang's low entropy (which comes from gravity) get converted to sunlight?
In many places you will read that just after the big bang, the universe was filled with very hot gas which appeared to be in thermal equilibrium. You will then read that this is actually quite a low ...
- 2,070
0
votes
1
answer
353
views
Has this random youtube crackpot trying to disprove GR inadvertedly stumbled on a genuine prediction of it? [duplicate]
I'm an Astrophysics major. I was watching strange fringe physics crackpots on Youtube to make fun of them, because I'm an acollierastro, planarwalk kinda gal. I came across this guy who thinks Black ...
0
votes
2
answers
44
views
Age of the universe due to expansion
"If gravity working on matter is the only force at work on large scales, then the attactive force of gravity will act to slow the expansion. In this case, the universe was expanding more rapidly ...
3
votes
1
answer
115
views
Understanding calculation in Feynman's Lecture on Theory of Gravitation about age of universe and gravitational constant
I am reading Feynman's Lectures. In the chapter entitled "The Theory of Gravitation", there is the following part
If we take, in some natural units, the repulsion of two electrons
(nature’s ...
- 311
2
votes
0
answers
42
views
Did galaxies spin differently 4, 5, 6 billion years ago?
Today I read that the Matter-dominated era ended 4 billion years ago, and now we're in the Dark Energy dominated era.
Obviously, light from galaxies far enough away would be from earlier in the ...
- 121
4
votes
2
answers
199
views
Is Hubbles law due to Gravity?
Hubble's law states that Distance is proportional to Velocity. A ScienceDirect article states that Classical Hubble expansion is characterized by a proportional increase in the rate of expansion ...
user348071
1
vote
1
answer
49
views
Do expanding mass shells gain energy?
I've found an article which says in section 2
an expanding gravitational system of given mass gains energy
Does an expanding gravitational system or an expanding shell of a given mass gain energy as ...
- 2,462
2
votes
0
answers
58
views
The gravitational field of all the distant mass exactly cancels the positive mass-energy in the universe. Why do they think so? [duplicate]
It is often said that the gravitational field has negative energy, and that this negative gravitational energy of all the distant mass exactly cancels the positive mass-energy in the universe.
Why do ...
- 278
0
votes
1
answer
70
views
According to the standard cosmological model, would an expanding gravitational field produce particles?
I found this article 1 about the production of particles in a time-dependent gravitational field. I have a few questions about it:
According to the standard model of cosmology and our current ...
- 2,462
2
votes
2
answers
87
views
Why the megaastrophysical objects do not collapse due to their gravitational selfattraction?
At the largest scale in the universe exist formations called filaments which form something like a spider web along the whole universe. But they seem to be quite static. So my question is about the ...
- 2,255
2
votes
1
answer
98
views
On the implementation of the spherical collapse model in cosmology
Lecture notes on the spherical collapse model found online (https://www.uio.no/studier/emner/matnat/astro/AST4320/h12/undervisningsmateriale/spherecollapse.pdf is one of them) consider a spherical top ...
- 579
1
vote
0
answers
61
views
Non-minimally coupled inflation — expansion
In the Wikipedia article on "Inflaton" there appears the following formula:
$$S=\int d^{4}x \sqrt{-g} \left[\frac{1}{2}m^2_{P}R-\frac{1}{2}\partial^\mu\Phi\partial_{ \mu }\Phi-V(\Phi)-\frac{ ...
- 11
2
votes
1
answer
71
views
Can empty space 'press' galaxies?
If gravity slows the effects of time, then empty space will see greater expansion than space inhabited by massive objects. So the space within a galaxy will be expanding more slowly than the space ...
3
votes
1
answer
120
views
Energy conservation in the Sachs-Wolfe effect
When energetic photons climb out of a potential well (eg Sachs-Wolfe effect) they lose energy. According to conservation of energy, where does this energy go? Into the gravitational potential making ...
- 928
1
vote
1
answer
47
views
Based on structure formation and the lifetime of the universe why is there an upper bound on the cosmological constant?
I understand that significantly greater values than the cosmological constant would result in difficulty for the formation of large gravitationally bound structures within the lifetime of the universe....
- 51
0
votes
2
answers
488
views
Is most of the energy in the universe potential energy?
So I asked a question about what would happen in regards to gravitational potential if I left earth and then vaporized it. The answer I got was that the Mass would still remain the same and even if ...
9
votes
1
answer
356
views
Why are galaxies much closer spaced (relative to their size) than stars?
I have read this question:
So the average spacing is somewhere in the range of 10 - 100 times the size of the biggest galaxies. The peas I had for lunch today were (at a guess - I didn't measure them!...
- 29k
5
votes
2
answers
2k
views
Is it the mass that bends spacetime, or is it the gravity?
I had understood that mass bends spacetime and that curvature generates gravity, but I have recently read that what bends spacetime is gravity.
Which comes first? Does the mass generate gravity, and ...
- 173