General Relativity Physics Honours 2009

General RelativityPhysics Honours 2009 Florian Girelli Rm 364, A28 girelli@physics.usyd.edu.au Lecture Notes 5

Courses on the 16th September are cancelled due to a university strike. Instead they will be on the 23rd September, from 10am to 12pm • The third assignment will be given on the 21st September (it was initially planed to be given the 16th). • It should be handed out the 5th October.

Einstein eq. and applications Up to now, you have seen: • Space-time defined by a metric, (global) symmetries given by Killing vectors. • Propagation of matter (particle) given by geodesic equation Geometry tells how mattershouldpropagate. Matter tells how geometryshouldcurve. What is the analog of the Poisson equation?

Source of gravity = mass density =M/V ~ energy/volume. needs generalization to the 4d case. 3d volume: 4d volume: 3d volume embedded in 4d: Energy momentum To obtain a momentum from a 3d volume embedded in 4d, we need a tensor encoding the “momentum density”. Stress-energy-momentum tensor

Stress-Energy-Momentum To understand what this means, consider flat spacetime at a constant time; this is a 3-d space with n=(1,0,0,0). energy density momentum density stress tensor: measures internal forces that part of the medium exert on the others (Cauchy 1822) force per unit of area ~ pressure Chapter 22

Examples Fluid: If we are at rest with respect to a perfect fluid then In flat spacetime, we can extend this to a moving fluid so It should be clear that in the rest frame, this becomes the above rest frame expression. Particle: it is not difficult to see that we have From a lagrangian:

Gravity as curvature Einstein’s key idea is to understand that gravity is encoded in the geometric properties of spacetime: curvature. Define the parallell transport of vectors Define curvature Check that curvature is related to the gravitational potential

Covariant Derivative We now need to look at the mathematical structure behind general relativity. This begins with the concept of the covariant derivative. Let’s start with flat, Minkowski spacetime; Where the second expression gives us the derivative of the vector in the t direction. To compare the vectors at two points, we have had to parallel transport one vector back along the path to the other. In Cartesian coordinates, this is no problem as the components of the vector do not change. This is not true in general. Chapter 20.4

Covariant Derivative Remember, in general curvilinear coordinates the basis vector change over the plane. This change of basis vectors needs to be considered when calculating the derivative. Hence, the Christoffel symbol can be seen to represent a correction to the derivative due to the change in the basis vectors over the plane. For Cartesian, these are zero, but for polar coordinates, they are not. A vector field v is parallely transported along t if

Example: Geodesics Imagine we have a straight line path though space time, parameterized by , and this path have a unit tangent vector u then Hence, geodesics are paths that parallel transport their own tangent vector along them (i.e. there is no change to the tangent vector along the path). Think about a straight line path through polar coordinates! Geodesics in curved spacetime are just a generalization of the above.

Example: Free Falling Frames Orthonormal bases are parallel transported by definition This allows to construct freely falling frames. If we have someone falling from infinity radially inwards in the Schwarzschild metric, then Where the first component is the 4-velocity, but you should check for yourself that the other components are parallel transported.

Covariant Derivatives for tensors We can generalize the covariant derivative for general tensors (This is straight forward to see if we remember that t = v wand remember the Leibniz’s rules). What about downstairs (covariant) component?

Covariant Derivative for tensors One of the fundamental properties of the Christoffel symbols is In Special Relativity, the stress-energy tensor is conserved. This is naturally generalized to the curved case using the covariant derivative: This is not anymore properly a conservation of energy since if spacetime is dynamical matter can exchange energy with it (« local conservation of energy » cf Example 22.7)

Introducing curvature To measure curvature we analyze the parallell transport of the vector along a closed loop: curvature measures the failure of coming back to the same vector. Mathematically it can be obtained from is the Riemann tensor.

Riemann Tensor Writing this in a local inertial frame, it becomes Leading to some immediate symmetries (true in general); Instead of 256 independent components, this tensor really only has 20 (Phew!)

Riemann Tensor Notation: Bianchi Identity (proof written in inertial frame to neglect Christoffel terms)

Contractions Contracting the Riemann tensor gives firstly the Ricci tensor Contracting again gives the Ricci scalar We also have the Kretschmann scalar, which is the measure of the underlying curvature If this is not zero, the spacetime is not flat! For the Schwarzschild metric, we have

Curvature and gravity Curvature also measures the failure of initially parallell geodesic to remain parallell. We consider two particles in a gravitational potential, first in the Newtonian formalism, then in the GR case and compare.

Newton deviation In the Newonian picture, we have the Newton equation. If we consider two nearby particles separated by a vector  Taking the leading terms, we get the Newtonian deviation between two trajectories Chapter 21

Geodesic Deviation The separation of two free falling objects gives a measure of the underlying curvature. We need to consider the paths of two nearby geodesics, with a 4-space separation as a function of the proper time along both curves. Where the Riemann Curvature tensor is

Geodesic Deviation This is easier to understand in the free falling frame. We need project the deviation vector into the orthonormal frame Remembering what the 4-velocity is in the free falling frame, then Where the Riemann tensor has been projected onto the orthonormal basis.

Geodesic Deviation In the weak field limit If we assume that our objects fall slowly (non-relativistic) along the geodesics, then the orthonormal and coordinate frame are approximately the same, so We can calculate the Christoffel symbols from the metric and then calculate the components of the Riemann tensor.

Geodesic Deviation Keeping only the lowest order terms, we find In the weak field limit, non-relativistic limit, we recover the result from Newtonian physics. Remember, the Riemann tensor is something that describes geometry, but here it is related to something physical, the gravitational potential. Gravityisthereforerelated to the geometry of spacetime, ieitscurvature.

Looking for Einstein Equation We have now the tools to determine some equations of motion intertwining gravity and matter. The source should be expressed in terms of the stress energy tensor. Between 1910 and 1913, Nordstrom proposed to consider generalizations of the Poisson equation (arXiv:gr-qc/0611100): Wrong for many reasons.Eg gravitiational field carry energy and should self gravitate: non-linear effects Einstein and Fokker showed that this latter proposition can be put into a geometric shape This is the first geometrical candidate for gravity (scalar gravity). Not physical: no bending of light (metric is conformally flat), wrong precession…

Looking for Einstein Equation Gravitational degrees of freedom are encoded by curvature (Riemann tensor, Ricci tensor, Ricci scalar), we look for the equations which give the Poisson equation in the Newtonian limit and are second order in derivatives in the metric. Einstein first tried But this fails to work since we have also the conservation of the stress energy tensor and Instead we have (thanks to the Bianchi identities)

Einstein Equation We (finally) get the Einstein equation (for zero cosmological constant): matter tells how spacetime to curve.

GR is not alone… General Relativity is the « simplest » theory, but one can generate other theories compatible with experiments by adding scalar, vector fields, higher order contributions in curvature... Introduce Parametrized Post-Newtonian parameters: they determine how the gravitational theory candidate is different from Newtonian gravity (in the weak field limit). Experiments put constraints on the value of these coefficients. See for example arXiv:gr-qc/9903058. Tiny modifications of Schwarzschild metric Chapter 10.2

GR is not alone…

Example 20.8 Hence, we can have a general form for the acceleration of an object due to an applied force In flat Minkowski space time (or local inertial frame) this is So, what acceleration is required to hover at a distance r from a Schwarzschild black hole? The 4-velocity is given by

Example 20.8 What acceleration do you need to remain at r? As the 4-velocity is independent of time, we find Only non-zero Christoffel symbol is As expected, the force is in the radial direction, but it appears to be finite at the horizon!

Example 20.8 Of course, this is in the coordinate frame and we need to project this vector onto the observers orthonormal basis to determine how much acceleration they feel. However, we can simply calculate the magnitude of the vector which is Hence, the acceleration diverges as we try and hover closer and closer to the horizon (which is what we expect).

General Relativity Physics Honours 2009