- #1
Kamikaze_951
- 7
- 0
Hi everyone,
It's not a real homework problem, but something I am trying to do that I haven't found in the literature. I am still stating the problem as if it was a homework
Consider a FRW Universe. That is, ℝ x M, where M is a maximally symmetric 3-manifold, with a RW metric: ds[itex]^{2}[/itex] = dt[itex]^{2}[/itex] - R[itex]^{2}[/itex](t) dσ[itex]^{2}[/itex], where
dσ[itex]^{2}[/itex] = dχ[itex]^{2}[/itex] + f(χ)[itex]^{2}[/itex](dθ[itex]^{2}[/itex] + sin[itex]^{2}[/itex]θd[itex]\varphi^2[/itex])
Consider also a system that is homogeneous and isotropic in space.
Prove that :
1) T[itex]^{t}_{r = sinχ}[/itex] = T[itex]^{t}_{θ}[/itex] = T[itex]^{t}_{\varphi}[/itex] = 0
2) T[itex]^{r}_{θ}[/itex] = T[itex]^{r}_{\varphi}[/itex] = T[itex]^{θ}_{\varphi}[/itex] = 0
3) T[itex]^{r}_{r}[/itex] = T[itex]^{θ}_{θ}[/itex] = T[itex]^{\varphi}_{\varphi}[/itex]
where T is the energy-momentum tensor.
1) We can use known Killing vectors related to underlying symmetries without having to prove that they indeed are Killing vectors (ex : the 3 Killing vectors related to spherical symmetry).
For instance, spherical symmetry implies that there are 3 Killing vectors R, S, T satisfying :
i) [R,S] = T
ii) [S,T] = R
iii) [T,R] = S
and in polar coordinates (θ,[itex]\varphi[/itex]), these 3 Killing vectors can be written as follow :
R = [itex]\partial_{\phi}[/itex]
S = [itex]\cos\phi\partial_{\theta} - \cot\theta\sin\phi\partial_\phi[/itex]
T = [itex]-\sin\phi\partial_{\theta} - \cot\theta\cos\phi\partial_\phi[/itex]
We can also use the Christoffel symbols for the RW metric (that can be found in most GR textbooks).
Finally, the equation that defines a conserved charge :
K is a Killing vector implies [itex]T_{\mu\nu}K^\nu = P_\mu[/itex] and
[itex]\nabla_\mu P^\mu = \nabla_\mu( T^{\mu\nu}K_{\nu}) = 0 [/itex]
I considered only R for the moment (if I succeed for the simplest Killing vector, I should succeed for the others). The components of R are :[itex]R^{\nu} = \delta^{\nu}_\phi[/itex].
By plugging this into [itex]\nabla_\mu T^{\mu\nu}K_{\nu} = 0[/itex], I got that
[itex]\nabla_\mu T^{\mu\phi} = 0[/itex].
By direct calculation of the divergence of that vector (using Christoffel symbols), I found :
[itex]0 = \nabla_\mu T^{\mu\phi} = (\partial_t + 3\frac{\dot{R}}{R})T^{t\phi} +
(\partial_\chi + 2\frac{f'(\chi)}{f(\chi)})T^{\chi\phi}
+ (\partial_\theta + \cot\theta)T^{\theta\phi} + \partial_\phi T^{\phi\phi}
[/itex]
This equation should probably imply that some components of T are zero. However, I don't see any way of stating this from that equation. It probably implies (from the problem statement) that [itex]T^{t\phi} = 0[/itex]. Do I have the right methodology to solve this problem? Could someone help me JUST with the R Killing vector (since I will probably be able to do the others when the R one is solved...I just need an example of how to do)?
Thank you a lot for considering my request.
Kami
It's not a real homework problem, but something I am trying to do that I haven't found in the literature. I am still stating the problem as if it was a homework
Homework Statement
Consider a FRW Universe. That is, ℝ x M, where M is a maximally symmetric 3-manifold, with a RW metric: ds[itex]^{2}[/itex] = dt[itex]^{2}[/itex] - R[itex]^{2}[/itex](t) dσ[itex]^{2}[/itex], where
dσ[itex]^{2}[/itex] = dχ[itex]^{2}[/itex] + f(χ)[itex]^{2}[/itex](dθ[itex]^{2}[/itex] + sin[itex]^{2}[/itex]θd[itex]\varphi^2[/itex])
Consider also a system that is homogeneous and isotropic in space.
Prove that :
1) T[itex]^{t}_{r = sinχ}[/itex] = T[itex]^{t}_{θ}[/itex] = T[itex]^{t}_{\varphi}[/itex] = 0
2) T[itex]^{r}_{θ}[/itex] = T[itex]^{r}_{\varphi}[/itex] = T[itex]^{θ}_{\varphi}[/itex] = 0
3) T[itex]^{r}_{r}[/itex] = T[itex]^{θ}_{θ}[/itex] = T[itex]^{\varphi}_{\varphi}[/itex]
where T is the energy-momentum tensor.
Homework Equations
1) We can use known Killing vectors related to underlying symmetries without having to prove that they indeed are Killing vectors (ex : the 3 Killing vectors related to spherical symmetry).
For instance, spherical symmetry implies that there are 3 Killing vectors R, S, T satisfying :
i) [R,S] = T
ii) [S,T] = R
iii) [T,R] = S
and in polar coordinates (θ,[itex]\varphi[/itex]), these 3 Killing vectors can be written as follow :
R = [itex]\partial_{\phi}[/itex]
S = [itex]\cos\phi\partial_{\theta} - \cot\theta\sin\phi\partial_\phi[/itex]
T = [itex]-\sin\phi\partial_{\theta} - \cot\theta\cos\phi\partial_\phi[/itex]
We can also use the Christoffel symbols for the RW metric (that can be found in most GR textbooks).
Finally, the equation that defines a conserved charge :
K is a Killing vector implies [itex]T_{\mu\nu}K^\nu = P_\mu[/itex] and
[itex]\nabla_\mu P^\mu = \nabla_\mu( T^{\mu\nu}K_{\nu}) = 0 [/itex]
The Attempt at a Solution
I considered only R for the moment (if I succeed for the simplest Killing vector, I should succeed for the others). The components of R are :[itex]R^{\nu} = \delta^{\nu}_\phi[/itex].
By plugging this into [itex]\nabla_\mu T^{\mu\nu}K_{\nu} = 0[/itex], I got that
[itex]\nabla_\mu T^{\mu\phi} = 0[/itex].
By direct calculation of the divergence of that vector (using Christoffel symbols), I found :
[itex]0 = \nabla_\mu T^{\mu\phi} = (\partial_t + 3\frac{\dot{R}}{R})T^{t\phi} +
(\partial_\chi + 2\frac{f'(\chi)}{f(\chi)})T^{\chi\phi}
+ (\partial_\theta + \cot\theta)T^{\theta\phi} + \partial_\phi T^{\phi\phi}
[/itex]
This equation should probably imply that some components of T are zero. However, I don't see any way of stating this from that equation. It probably implies (from the problem statement) that [itex]T^{t\phi} = 0[/itex]. Do I have the right methodology to solve this problem? Could someone help me JUST with the R Killing vector (since I will probably be able to do the others when the R one is solved...I just need an example of how to do)?
Thank you a lot for considering my request.
Kami