Nevertheless, critical collapse is an interesting test of cosmic censorship. First of all, the set of data is of codimension one, certainly in the space of spherical asymptotically flat data, and apparently [77] also in the space of all asymptotically flat data. This means that one can fine-tune any generic parameter, whichever comes to hand, as long as it parameterizes a smooth curve in the space of initial data. Secondly, critical phenomena seem to be generic with respect to matter models, including realistic matter models with intrinsic scales. These two features together mean that, in a hypothetical experiment to create a Planck-sized black hole in the laboratory through a strong explosion, one could fine-tune any one design parameter of the bomb, without requiring control over its detailed effects on the explosion.

The metric of the critical spacetime is of the form times a regular metric. From this general form alone, one can conclude that is a curvature singularity, where Riemann and Ricci invariants blow up like , and which is at finite proper time from regular points. The Weyl tensor with index position is conformally invariant, so that components with this index position remain finite as . In this property it resembles the initial singularity in Penrose's Weyl tensor conjecture rather than the final singularity in generic gravitational collapse. This type of singularity is called ``conformally compactifiable'' [116] or ``isotropic'' [69]. Is the singularity naked, and is it timelike, null or a ``point''? The answer to these questions remains confused, partly because of coordinate complications, partly because of the difficulty of investigating the singular behavior of solutions numerically.

Choptuik's, and Evans and Coleman's numerical codes were
limited to the region
*t*
<0 in the Schwarzschild-like coordinates (4), with the origin of
*t*
adjusted so that the singularity is at
*t*
=0. Evans and Coleman conjectured that the singularity is
shrouded in an infinite redshift based on the fact that
grows as a small power of
*r*
at constant
*t*
. This is directly related to the fact that
*a*
goes to a constant
as
at constant
*t*, as one can see from the Einstein equation (8). This in turn means simply that the critical spacetime is not
asymptotically flat, but asymptotically conical at spacelike
infinity, with the Hawking mass proportional to
*r*
.

Hamadé and Stewart [81] evolved near-critical scalar field spacetimes on a double null grid, which allowed them to follow the time evolution up to close to the future light cone of the singularity. They found evidence that this light cone is not preceded by an apparent horizon, that it is not itself a (null) curvature singularity, and that there is only a finite redshift along outgoing null geodesics slightly preceding it. (All spherically symmetric critical spacetimes appear to be qualitatively alike as far as the singularity structure is concerned, so that what we say about one is likely to hold for the others.)

Hirschmann and Eardley [84] were the first to continue a critical solution itself right up
to the future light cone. They examined a CSS complex scalar
field solution that they had constructed as a nonlinear ODE
boundary value problem, as discussed in Section
4.4
. (This particular one is not a proper critical solution, but
that should not matter for the global structure.) They continued
the ODE evolution in the self-similar coordinate
*x*
through the coordinate singularity at
*t*
=0 up to the future light cone by introducing a new
self-similarity coordinate
*x*
. The self-similar ansatz reduces the field equations to an ODE
system. The past and future light cones are regular singular
points of the system, at
and
. At these ``points'' one of the two independent solutions is
regular and one singular. The boundary value problem that
originally defines the critical solution corresponds to
completely suppressing the singular solution at
(the past light cone). The solution can be continued through
this point up to
. There it is a mixture of the regular and the singular
solution.

We now state this more mathematically. The ansatz of Hirschmann and Eardley for the self-similar complex scalar field is (we slightly adapt their notation)

with
a real constant. Near the future light cone they find that
*f*
is approximately of the form

with and regular at , and a small positive constant. The singular part of the scalar field oscillates an infinite number of times as , but with decaying amplitude. This means that the scalar field is just differentiable, and that therefore the stress tensor is just continuous. It is crucial that spacetime is not flat, or else would vanish. For this in turn it is crucial that the regular part of the solution does not vanish, as one sees from the field equations.

The only other case in which the critical solution has been continued up to the future light cone is Choptuik's real scalar field solution [74]. Let and be the ingoing and outgoing wave degrees of freedom respectively defined in (54). At the future light cone the solution has the form

where
*C*
is a positive real constant,
,
and
are regular real functions with period
in their second argument, and
is a small positive real constant. (We have again simplified the
original notation.) Again, the singular part of the solution
oscillates an infinite number of times but with decaying
amplitude. Gundlach concludes that the scalar field, the metric
coefficients, all their first derivatives, and the Riemann tensor
exist, but that is as far as differentiability goes. (Not all
second derivatives of the metric exist, but enough to construct
the Riemann tensor.) If either of the regular parts
or
vanished, spacetime would be flat,
would vanish, and the scalar field itself would be singular. In
this sense, gravity regularizes the self-similar matter field
ansatz. In the critical solution, it does this perfectly at the
past lightcone, but only partly at the future lightcone. Perhaps
significantly, spacetime is almost flat at the future horizon in
both the examples, in the sense that the Hawking mass divided by
*r*
is a very small number. In the spacetime of Hirschmann and
Eardley it appears to be as small as
, but not zero according to numerical work by Horne [88].

In summary, the future light cone (or Cauchy horizon) of these
two critical spacetimes is not a curvature singularity, but it is
singular in the sense that differentiability is lower than
elsewhere in the solution. Locally, one can continue the solution
through the future light cone to an almost flat spacetime (the
solution is of course not unique). It is not clear, however, if
such a continuation can have a regular center
*r*
=0 (for
*t*
>0), although this seems to have been assumed in [84]. A priori, one should expect a conical singularity, with a
(small) defect angle at
*r*
=0.

The results just discussed were hampered by the fact that they are investigations of singular spacetimes that are only known in numerical form, with a limited precision. As an exact toy model we consider an exact spherically symmetric, CSS solution for a massless real scalar field that was apparently first discovered by Roberts [112] and then re-discovered in the context of critical collapse by Brady [18] and Oshiro et al. [106]. We use the notation of Oshiro et al. The solution can be given in double null coordinates as

with
*p*
a constant parameter. (Units
*G*
=
*c*
=1.) Two important curvature indicators, the Ricci scalar and the
Hawking mass, are

The center
*r*
=0 has two branches,
*u*
=(1+
*p*)
*v*
in the past of
*u*
=
*v*
=0, and
*u*
=(1-
*p*)
*v*
in the future. For 0<
*p*
<1 these are timelike curvature singularities. The
singularities have negative mass, and the Hawking mass is
negative in the past and future light cones. One can cut these
regions out and replace them by Minkowski space, not smoothly of
course, but without creating a
-function in the stress-energy tensor. The resulting spacetime
resembles the critical spacetimes arising in gravitational
collapse in some respects: It is self-similar, has a regular
center
*r*
=0 at the past of the curvature singularity
*u*
=
*v*
=0 and is continuous at the past light cone. It is also
continuous at the future light cone, and the future branch of
*r*
=0 is again regular.

It is interesting to compare this with the genuine critical
solutions that arise as attractors in critical collapse. They are
as regular as the Roberts solution (analytic) at the past
*r*
=0, more regular (analytic versus continuous) at the past light
cone, as regular (continuous) at the future light cone and, it is
to be feared, less regular at the future branch of
*r*
=0: In contrary to previous claims [84,
72] there may be no continuation through the future sound or light
cone that does not have a conical singularity at the future
branch of
*r*
=0. The global structure still needs to be clarified for all
known critical solutions.

In summary, the critical spacetimes that arise asymptotically
in the fine-tuning of gravitational collapse to the black hole
threshold have a curvature singularity that is visible at
infinity with a finite redshift. The Cauchy horizon of the
singularity is mildly singular (low differentiability), but the
curvature is finite there. It is unclear at present if the
singularity is timelike or if there exists a continuation beyond
the Cauchy horizon with a regular center, so that the singularity
is limited, loosely speaking, to a point. Further work should be
able to clarify this. In any case, the singularity is naked and
the critical solutions therefore provide counter-examples to any
formulation of cosmic censorship which states only that naked
singularities cannot arise from smooth initial data in reasonable
matter models. The statement must be that there is no
*open ball*
of smooth initial data for naked singularities.

Recent analytic work by Christodoulou on the spherical scalar field [49] is not directly relevant to the smooth (analytic or ) initial data discussed here. Christodoulou considers a larger space of initial data that are not . He shows that for any data set in this class that forms a naked singularity there are data and such that the data sets do not contain a naked singularity, for any and except zero. Here is data of bounded variation, and is absolutely continuous data. Therefore, the set of naked singularity data is at least codimension two in the space of data of bounded variation, and of codimension at least one in the space of absolutely continuous data. The semi-numerical result of Gundlach claims that it is codimension exactly one in the set of smooth data. The result of Christodoulou holds for any , including initial data for the Choptuik solution. The apparent contradiction is resolved if one notes that the and of Christodoulou are not smooth in (at least) one point, namely where the initial data surface is intersected by the past light cone of the singularity in . The data are therefore not smooth.

Critical Phenomena in Gravitational Collapse
Carsten Gundlach
http://www.livingreviews.org/lrr-1999-4
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