All Questions
7
questions
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
2
votes
1
answer
529
views
What was the mean distance photons travelled before colliding with electrons in the matter plasma before recombination?
What was the mean distance photons travelled before colliding with electrons in the matter plasma before recombination?
I have checked other answers close to this but they only mention a mean ...
- 21
1
vote
1
answer
238
views
Modification of Gravitational force's law and related problem
I'm facing a problem which states the following:
Suppose that it is discovered that Newton's law of gravitation is incorrect and that the force $F$ on test particle of mass $m$ due to a body of ...
- 3,705
0
votes
0
answers
113
views
Expansion of Homogeneous Universe using Fluid Dynamics
In the MIT OCW course The Early Universe Prof. Alan Guth justifies the use of Newtonian gravity to approximately describe the homogeneous expansion of the universe, (at least a matter dominated one) ...
- 1,189
0
votes
1
answer
88
views
Modified gravitational model
In an f(R) gravity model
$$
f(R)=R+\lambda{R_{0}}\left(\left(1+\frac{R^{2}}{R_{0}^{2}}\right)^{-n}-1\right).
$$
What are the units of $\lambda$ and $R_{0}$?
- 789
6
votes
3
answers
5k
views
How to obtain the field equations in Brans-Dicke theory from the action?
The action for the Brans-Dicke-Jordan theory of gravity is
$$
\\S =\int d^4x\sqrt{-g} \;
\left(\frac{\phi R - \omega\frac{\partial_a\phi\partial^a\phi}{\phi}}{16\pi} + \mathcal{L}_\mathrm{M}\right).
...
- 431
2
votes
1
answer
2k
views
Photon on null geodesic
If given an FRW metric
$ds^2 = -dt^2 + a^2(t)[dx^2+dy^2+dz^2]$
and for the trajectory followed by a photon (null geodesic; $ds^2=0$) with affine parameter $\lambda$, know that
$g_{\mu\nu}\,\frac{...
- 397