A couple of days ago, we showed that the Einstein equations are essentially hyperbolic under the harmonic gauge. To be precise, the solvability of the equations

is equivalent to solving the following hyperbolic system

provided for all . Later on, when we consider the Cauchy problem for the Einstein equations on some appropriate framework, the first and second fundamental forms need to verify some constraint equations, [here and here].

By tracing the above equation, we obtain

which helps us to write

In the vacuum case, the above equation is nothing but

The spacetime we choose will be the product . What we really need is to construct a metric on such that the Einstein equations fulfill. In fact, thanks to the hyperbolic system showed above, we aim to find a suitable initial condition such that

- The gauge condition can propagate in time;
- The hyperbolic system is solvable for small time .

While the latter is standard (we shall touch this issue later), the former needs to study and this is the main point of this notes.

In this entry, given a solution of the constraint equations on a manifold of the dimension , we shall construct an appropriate spacetime of the dimension . Recall that

Initially, we set

As a consequence of the choice above, we also have for non-zero . To fully construct the initial condition, we still need to find .

First, in view of the gauge condition, , we find that

Therefore, at , we get that

Hence, in order to guarantee that at for all , we first select such that at . Once this task is done, we can determine at such that at .

In other words, the initial data that preserves the gauge condition can be found in this way. Within a small time, the hyperbolic system mentioned above always admits a solution . That is to say that the reduced Einstein equations have solution, i.e.,

where the -tensor is given by

However, it is not necessary to have that solves the Einstein equations unless the gauge condition remains valid within the small time. We shall prove this affirmatively.

We now consider how could the gauge condition propagate in time so long as the metric solves the reduced equations. As we shall see, all satisfy a homogeneous linear wave equation which is a consequence of the Bianchi identities with vanishing initial time derivatives which come from the constraint equations.

First, since the metric solves

and thanks to the Einstein equation for the curvature, we immediately have

Let us write

Using this, we have

where is the trace of . Since the Einstein tensor is divergence free, we obtain

Keep in mind that . A simple calculation shows that

Thanks to

we have proved that

The above long calculation also shows that

In view of the vacuum setting and the constraint equations [here and here], there holds

at . Hence, at , we get

By using , i.e. the Hamiltonian constraint, we obtain . We still need to prove that for all . Indeed, thanks to for any , we know that

Since this is true for all and the matrix is invertible, we have that all vanish at for all . Since satisfy a homogeneous linear hyperbolic system with vanishing initial data, vanish identically by the uniqueness for solutions of the Cauchy problem for the hyperbolic evolutions.

See also:

- Why do the Einsteins equations describe the propagation of wavelike phenomena?
- Derivation of the Einstein constraint equations: The Hamitonian constraint
- Derivation of the Einstein constraint equations: The momentum constraint

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