The idea of Littlewood-Paley theory is as follows (see [1] for a good exposition of this). Suppose that we
want to describe the regularity of a function (or, more generally, a tempered distribution) on
. Differentiability properties of
correspond, roughly speaking, to fall-off properties of
its Fourier transform
. This is because the Fourier transform converts differentiation into
multiplication. The Fourier transform is decomposed as
, where
is a dyadic partition of
unity. The statement that it is dyadic means that all the
except one are obtained from
each other by scaling the argument by a factor which is a power of two. Transforming back we
get the decomposition
, where
is the inverse Fourier transform of
. The
component
of
contains only frequencies of the order
. In studying rough solutions of the
Einstein equations, the Littlewood-Paley decomposition is applied to the metric itself. The high
frequencies are discarded to obtain a smoothed metric which plays an important role in the
arguments.
Another important element of the proofs is to rescale the solution by a factor depending on the cut-off
applied in the Littlewood-Paley decomposition. Proving the desired estimates then comes down to
proving the existence of the rescaled solutions on a time interval depending on
in a particular way. The
rescaled data are small in some sense and so a connection is established to the question of long-time
existence of solutions of the Einstein equations for small initial data. In this way, techniques from the work
of Christodoulou and Klainerman on the stability of Minkowski space (see Section 5.2) are brought
in.
What is finally proved? In general, there is a close connection between proving local existence for data in
a certain space and showing that the time of existence of smooth solutions depends only on
the norm of the data in the given space. Klainerman and Rodnianski [214] demonstrate that
the time of existence of solutions of the reduced Einstein equations in harmonic coordinates
depends only on the norm of the initial data for any
. Combining this with the
results of [243] gives an existence result in the same space. It is of interest to try to push the
existence theorem to the limiting case
or even to the slightly weaker assumption on the
data that the curvature is square integrable. This
curvature conjecture (local existence in
this setting) is extremely difficult but interesting progress has been made by Klainerman and
Rodnianski in [215] where the structure of null hypersurfaces was analysed under very weak
hypotheses. For this purpose the authors developed an invariant form of Littlewood-Paley
theory [216]. This uses the asymptotics of solutions of the heat equation on a manifold and is
coordinate-independent.
The techniques discussed in this section, which have been stimulated by the desire to understand the Einstein equations, are also helpful in understanding other nonlinear wave equations. Thus, this is an example where information can flow from general relativity to the theory of partial differential equations.
It may be that the technique of using parabolic equations as a tool to better understand hyperbolic equations can be carried much further. In [333] Tao presents ideas how the harmonic map heat flow could be used to define a high quality gauge for the study of wave maps.
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