Topics in Relativistic Astrophysics
Topics in Relativistic Astrophysics
John Friedman
ICTP/SAIFR Advanced School in General Relativity
Parker Center for Gravitation, Cosmology, and Astrophysics
Part I: General relativistic perfect fluids Mathematical prerequisites: Symmetries and Killing vectors
Symmetry under translations
A function f on flat space is symmetric under time translations if
∂=t f 0 The equation can be written in the form α , where α is the tf∇=α 0 t vector field with components t µ = (,,,)1000
Mathematical prerequisites: Symmetries and Killing vectors
Symmetry under translations
A function f on flat space is symmetric under time translations if
∂=t f 0 The equation can be written in the form α , where α is the tf∇=α 0 t vector field with components t µ = (,,,)1000
P
tα generates a family of translations T(t) : It is tangent at each point P to the orbit T(t)P of P. Tt()(,, t0000 rθφ ,)(= t 0 + tr ,, 000 θφ ,) Symmetry under rotations
A function f on flat space is symmetric under rotations if
∂=φ f 0 α α The equation can be written in the form φ ∇= α f 0 , where φ is the vector field whose components in coordinates (t,r,θ,φ) are µµ µ φδ = φ or( φ ) = (,,,) 0001 .
In terms of a Cartesian basis, φ = xy j − i and ∂=∂−∂ φ xy yx .
Symmetry under rotations
A function f on flat space is symmetric under rotations if
∂=φ f 0 α α The equation can be written in the form φ ∇= α f 0 , where φ is the vector field whose components in coordinates (t,r,θ,φ) are µµ µ φδ = φ or( φ ) = (,,,) 0001 .
In terms of a Cartesian basis, φ = xy j − i and ∂=∂−∂ φ xy yx .
P
φ generates a family of rotations R(φ) : It is tangent at each point P to the circular orbit R(φ) P of P R()(,,φ tr00 θφ 0 , 0 )= (,, tr 00 θφ 0 , 0 + φ ) The Euler equation
The Euler equation is just F=ma written for a fluid element, shown here as a small box with density ρ and velocity v.
The pressure on the left face is P(x); the pressure on the right face is P(x+∆x) .
With ∆A the area of each face of the box, The net force in the x-direction is
Fx = P (x)∆A − P (x + ∆x)∆A ∂P = − ∆V, ∂x where ∆V = ∆x∆A is the volume of the fluid element. Thus
F = −∇P ∆V.
John Friedman Relativistic Astrophysics We want to write F = ma or
−∇P ∆V = ρ∆V a, and we need to find a in terms of the velocity field v(t, x). The vector field v(t, x) has the meaning that at time t the fluid element at x has velocity v(t, x). Thus at time t + ∆t that same fluid element is at x + v(t, x)∆t and has velocity v(t + ∆t, x + v(t, x)∆t). The fluid element has changed its velocity by
∆v = v(t + ∆t, x + v(t, x)∆t) − v(t, x) ∂v = + v · ∇v ∆t ∂t in time ∆t, and its acceleration is therefore
a = (∂t + v · ∇) v. (1)
John Friedman Relativistic Astrophysics In this way we obtain Euler’s equation of motion