All Questions
254
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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: ...
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50
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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
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2
answers
271
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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
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1
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353
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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 ...
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votes
2
answers
44
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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
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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
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0
answers
42
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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
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2
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199
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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
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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
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0
answers
58
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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
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votes
1
answer
70
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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
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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
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1
answer
98
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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
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0
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61
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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
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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 ...
