Conservation laws in a curve spacetime

[Magister]

Homework Statement

Given the energy-momentum tensor for a perfect fluid, what is the conservation laws that I can compute from

[tex]
\nabla_b T^{ab}=0
[/tex]

in a curve space-time.

Homework Equations

[tex]
T^{ab}=(\rho + p) u^a u^b - p g^{ab}
[/tex]

where p is the pressure and [itex]\rho[/itex] is the density.

The Attempt at a Solution

I have already compute the geodesic equation, the equivalent to the equation of motion in a flat space-time.
I would like to know if there is any other conservation law to get. I supose that might be another one equivalent to the Navie-Strokes in flat space-time...

Thanks in advance

 

[Dick]

Just write [tex]\nabla_b T^{ab}=0[/tex] explicitly in components (in terms of density, pressure and scale factor). You will get a conservation type eqn.

 

[Magister]

Just write [tex]\nabla_b T^{ab}=0[/tex] explicitly in components (in terms of density, pressure and scale factor). You will get a conservation type eqn.

In terms of scale factor? The problem is that they don't give me a metric. I don't know what to with the [itex]g^{ab}[/itex] term...

 

[Dick]

I guess I was thinking of the problem in FRW background. Can you simplify anything w/o a metric? Not sure...

 

[Magister]

Well. I have already compute the equation of motion, in a curve space, from [itex]\nabla_b T^{ab}=0[/itex]. So I suppose that there may be another one...

 

[Barnak]

In general, there isn't any other conservation law, besides the covariant divergence of the energy-momentum tensor. However, if you have a space-time with a continuous symetry, there's a Killing field defined on the space-time, associated to that symettry. It can then serves to define another conservation law, in a similar way as a Noether current.

 

[Magister]

What about this:

[tex]

\nabla_b T^{ab}=\nabla_b((\rho + p) u^a u^b) - g^{ab} \nabla_b p = 0

[/tex]

where I have used [itex]\nabla_b g^{ab} = 0[/itex]

[tex]
u^a \nabla_b((\rho + p) u^b) + (\rho + p) u^b \nabla_b u^a -g^{ab} \nabla_b p = 0
[/tex]

[tex]
u_a u^a \nabla_b((\rho + p) u^b) + (\rho + p) u^b u_a \nabla_bu^a -g^{ab} u_a \nabla_b p = 0
[/tex]

using the fact that [itex]\nabla_b (u_a u^a) = \nabla_b 1 \Leftrightarrow 2 u_a \nabla_b u^a = 0[/itex], I get

[tex]
\nabla_b((\rho + p) u^b) = g^{ab} u_a \nabla_b p
[/tex]

using this in the second equation I get

[tex]
(\rho + p) u^b \nabla_b u^a = 0
[/tex]

or

[tex]
\nabla_u u^a = 0
[/tex]

So I can saw that the equation of motion (a conservation law) can be derived from [itex]\nabla_b T^{ab}=0[/itex].
Am I wrong?

In general, there isn't any other conservation law, besides the covariant divergence of the energy-momentum tensor. However, if you have a space-time with a continuous symetry, there's a Killing field defined on the space-time, associated to that symettry. It can then serves to define another conservation law, in a similar way as a Noether current.

Just has I said above, they don't give me any metric, so I can't compute the Killing vectors.

 

[Barnak]

From equation

[tex]

\nabla_b T^{ab}=\nabla_b((\rho + p) u^a u^b) - g^{ab} \nabla_b p = 0,

[/tex]

you get the following :

(1) [tex]
\nabla_a ((\rho + p) u^a) = u^a \nabla_a p,
[/tex]

AND :

(2) [tex]
(\rho + p) u^a \nabla_a ( u^b) = (g^{ab} - u^a u^b) \nabla_a p.
[/tex]

In the case of a pressureless gaz, p = 0, and you get the conservation of "matter" AND the geodesics equation :

(3) [tex]
\nabla_a (\rho u^a) = 0,
[/tex]

AND :

(4) [tex]
u^a \nabla_a ( u^b) = 0.
[/tex]There's no need of any particular metric to define conserved quantites with the Killing fields.