We have already discussed in Section 3.6 the effects of adding a potential to the evolution of the scalar field. An alternative generalization is a modification of the kinetic term in the Lagrangian, with the general formN-dimensional vector field with , and where is a fixed nonlinear function acting as a metric on the so-called target space of the fields . Such a system is called a nonlinear model, harmonic map or wave map. The fields and metric are dimensionless, and this allows the introduction of dimensionless parameters in the system, which cannot be asymptotically neglected using the arguments of Section 2.6. (Compare with the potential , which has dimensions (length)–2, and hence requires dimensionful parameters.)
The case N = 1 gives nothing new, and so Hirschmann and Eardly (HE from now on) studied the N = 2 case with a target manifold with constant curvature , proportional to a real dimensionless constant . Using a single complex coordinate the action of the system can be written as have shown that there is a smooth transition in the BD system from DSS criticality for low (the flat target space case = 0 is equivalent to a self-gravitating complex massless scalar field, whose critical solution is the original DSS spacetime found by Choptuik) to CSS criticality for larger (the case = 1 is equivalent to the axion-dilaton system, and has been shown to display CSS criticality in ).
Generalizing their previous results for =0 [123, 122], HE constructed for each a CSS solution based on the ansatz for the critical scalar field. Studying its perturbations HE concluded that this solution is critical for > 0.0754, but has three unstable modes for < 0.0754 and even more for < –0.28. Below 0.0754 a DSS solution takes over, as shown in the simulations of Liebling and Choptuik , and HE conjectured that the transition is a Hopf bifurcation, such that the DSS cycle smoothly shrinks with growing , collapsing onto the CSS solution at the transition and then disappearing with a finite value of the echoing period .
The close relation between the CSS and DSS critical solutions is also manifest in the construction of their global structures. In particular, the results of  and  for the CSS =0 and =1 solutions respectively show that the Cauchy horizon of the singularity is almost but not quite flat, exactly as was the case with the Choptuik DSS spacetime (see Section 3.3).
This is an N = 3 sigma model, and it also displays a transition between CSS and DSS criticality, but this is a totally different type of transition, in particular showing a divergence in the echoing period .
In a reduction to spherical symmetry, the effective action isr is the area radius, and the coupling constant is dimensionless. It has been shown (numerically  and then analytically ) that for , there is an infinite sequence of CSS solutions labelled by a nodal number n, and having n growing modes. (The case , in which the sigma field decouples from gravity, will be revisited below.) The n-th solution is always regular in the past light cone of the singularity, but is regular up to the future light cone only for where , , and . For larger couplings an apparent horizon develops and the solution cannot be smoothly continued. These results suggest that is a stable naked singularity for , and acts as a critical solution between naked singularity formation and dispersal for and between black hole formation and dispersal for . The numerical experiments agree with this scenario in the range . Other CSS solutions of this system are investigated in , and the possibility of chaos in .
Aichelburg and collaborators [131, 144] have shown that for there is clear DSS type II criticality at the black hole threshold. The period depends on , monotonically decreasing towards an asymptotic value for . Interesting new behavior occurs in the intermediate range that lies between clear CSS and clear DSS. With decreasing the overall DSS includes episodes of approximate CSS , of increasing length (measured in the log-scale time ). As from above the duration of the CSS epochs, and hence the overall DSS period diverges. For time evolutions of initial data near the black hole threshold no longer show overal DSS, but they still show CSS episodes. Black hole mass scaling is unclear in this regime.
It has been conjectured that this transition from CSS to DSS can be interpreted, in the language of the theory of dynamical systems, as the infinite-dimensional analogue of a 3-dimensional Shil’nikov bifurcation . High-precision numerics in  further supports this picture: For a codimension-1 CSS solution coexists in phase space with a codimension-1 DSS attractor such that the (1-dimensional) unstable manifold of the DSS solution lies on the stable manifold of the CSS solution. For close to the two solutions are close and the orbits around the DSS solution become slower because they spend more time in the neighbourhood of the CSS attractor. A linear stability analysis predicts a law for some constant , where is the Lyapunov exponent of the CSS solution. For both solutions touch and the DSS cycle dissapears.
Choptuik, Chmaj and Bizoń  have found both type I and type II critical collapse in the spherical Einstein–Yang–Mills system with SU(2) gauge potential, restricting to the purely magnetic case, in which the matter is described by a single real scalar field. The situation is very similar to that of the massive scalar field, and now the critical solutions are the well-known static n = 1 Bartnik–McKinnon solution  for type I and a DSS solution (later constructed in ) for type II. In both cases the black holes produced in the supercritical regime are Schwarzschild black holes with zero Yang–Mills field strength, but the final states (and the dynamics leading to them) can be distinguished by the value of the Yang–Mills final gauge potential at infinity, which can take two values, corresponding to two distinct vacuum states.
Choptuik, Hirschmann and Marsa  have investigated the boundary in phase space between formation of those two types of black holes, using a code that can follow the time evolutions for long after the black hole has formed. This is a new “type III” phase transition whose critical solution is an unstable static black hole with Yang–Mills hair [14, 202], which collapses to a hairless Schwarzschild black hole with either vacuum state of the Yang–Mills field, depending on the sign of its one growing perturbation mode. This “coloured” black hole is actually a member of a 1-parameter family parameterized by its apparent horizon radius and outside the horizon it approaches the corresponding BM solution. When the horizon radius approaches zero the three critical solutions meet at a “triple point”. What happens there deserves further investigation.
Millward and Hirschmann  have further coupled a Higgs field to the Einstein–Yang–Mills system. New possible end states appear: regular static solutions, and stable hairy black holes (different from the coloured black holes referred to above). Again there are type I or type II critical phenomena depending on the initial conditions.
It is known that the spherical critical solutions within the magnetic ansatz become more unstable when other components of the gauge field are taken into account, and so they will not be critical in the general case.
Bizoń, Chmaj and Schmidt  have found a way of constructing asymptotically flat vacuum spacetimes in 4+1 dimensions which are spherically symmetric while containing gravitational waves (Birkhoff’s theorem does not hold in more than 3+1 dimensions). Recall that in (3+1 dimensional) Bianchi IX cosmology the manifold is M1 × S3 where the S3 is equipped with an SU(2) invariant (homogeneous but anisotropic) metricM2 × S3 where the now depend only on r and t. This gives rise to nontrivial dynamics, including a threshold between dispersion and black hole formation. With the additional U(1) symmetry (biaxial solutions) there is only one dynamical degree of freedom. At the black hole threshold, type II critical phenomena are seen with and .
In evolutions with the general ansatz where all are different (triaxial solutions) , the U(1) symmetry is recovered dynamically in the approach to the critical surface. However, each biaxial solution, and in particular the critical solution, exists in three copies obtained by permutation of the . Therefore, in the triaxial case, the critical surface contains three critical solutions. The boundaries, within the critical surface, between their basins of attraction contain in turn codimension-two DSS attractors. It is conjectured that there is in fact a countable family of DSS solutions with n unstable modes.
Szybka and Chmaj  give numerical evidence that these boundaries within the critical surface are fractal (in contrast to the critical surface itself, which is smooth as in all other known systems.) Update
A similar ansatz can be made in other odd spacetime dimensions, and in 8+1 dimensions type II critical behaviour is again observed .
Spacetime in 2+1 dimensions is flat everywhere where there is no matter, so that gravity is not acting at a distance in the usual way. There are no gravitational waves, and black holes can only be formed in the presence of a negative cosmological constant (see  for a review).
Scalar field collapse in circular symmetry was investigated numerically by Pretorius and Choptuik , and Husain and Olivier . In a regime where the cosmological constant is small compared to spacetime curvature they find type II critical phenomena with a universal CSS critical solution, and . The value  appears to be less accurate.
Looking for the critical solution in closed form, Garfinkle  found a countable family of exact spherically symmetric CSS solutions for a massless scalar field with , but his results remain inconclusive. The q = 4 solution appears to match the numerical evolutions inside the past light cone, but its past light cone is also an apparent horizon. The q = 4 solution has three growing modes although the top one would give if only the other two could be ruled out . An attempt at this  seems unmotivated. At the same time, it is possible to embed the solutions into a family of ones [64, 65, 43], which can be constructed along the lines of Section 2.6, so that Garfinkle’s solution could be the leading term in an expansion in .
Critical collapse of a massless scalar field in spherical symmetry in 5+1 spacetime dimensions was investigated in . Results are similar to 3+1 dimensions, with a DSS critical solution and mass scaling with . Birukou et al [13, 133] have developed a code for arbitrary spacetime dimension. They confirm known results in 3+1 () and 5+1 () dimensions, and investigate 4+1 dimensions. Without a cosmological constant they find mass scaling with for one family of initial data and for another. They see wiggles in the versus plot that indicate a DSS critical solution, but have not investigated the critical solution directly. With a negative cosmological constant and the second family, they find = 0.49. Bland and Kunstatter  have made a more precise determination: = 0.4131 ± 0.0001. This was motivated by an attempt to explain this exponent using an holographic duality between the strong coupling regime of 4+1 gravity and the weak coupling regime of 3+1 QCD , which had predicted = 0.409552.
Kol  relates a solution that is related to the Choptuik solution to a variant of the critical solution in the black-string black hole transition, and claims to obtain analytic estimates for and . This has motivated a numerical determination of and for the spherical massless scalar field in noninteger dimension up to 14 [190, 30].
Choptuik, Hirschmann and Liebling  have presented perturbative indications that the static solutions found by van Putten  in the vacuum Brans–Dicke system are critical solutions. They have also performed full numerical simulations, but only starting from small deviations with respect to those solutions.
Ventrella and Choptuik  have performed numerical simulations of collapse of a massless Dirac field in a special state: an incoherent sum of two independent left-handed zero-spin fields having opposite orbital angular momentum. This is prepared so that the total distribution of energy-momentum is spherically symmetric. The freedom in the system is then contained in a single complex scalar field obeying a modified linear wave equation in spherical symmetry. There are clear signs of CSS criticality in the metric variables, and the critical complex field exhibits a phase of the form for a definite (the Hirschmann and Eardly ansatz for the complex scalar field critical solutions), which can be considered as a trivial form of DSS.
Garfinkle, Mann and Vuille  have found coexistence of types I and II criticality in the spherical collapse of a massive vector field (the Proca system), the scenario being almost identical to that of a massive scalar field. In the self-similar phase the collapse amplifies the longitudinal mode of the Proca field with respect to its transverse modes, which become negligible, and the critical solution is simply the gradient of the Choptuik DSS spacetime.
Sarbach and Lehner  find type I critical behaviour in q+3-dimensional spacetimes with U(1) × SO(q+1) symmetry in Einstein–Maxwell theory at the threshold between dispersion and formation of a black string.
|Matter||Type||Collapse||Critical solution||Perturbations of|
|Perfect fluid,||II||[72, 161]||CSS [72, 153, 161]||[153, 139, 101, 103]|
|– in 4+1, 5+1, 6+1||II||CSS |||
|Real scalar field:|
|– massless, minimally coupled||II||[47, 48, 49]||DSS ||[98, 154]|
|– massive||I||||oscillating |
|II||||DSS [112, 106]||[112, 106]|
|– conformally coupled||II||||DSS|
|Massive complex scalar field||I, II|||||||
|Massless scalar electrodynamics||II||||DSS |||
|Massive vector field||II||||DSS |||
|Massless Dirac||II||||CSS |
|Vacuum Brans–Dicke||I||||static |||
|2-d sigma model:|
|– complex scalar ( = 0)||II||||DSS |||
|– axion-dilaton ( = 1)||II||||CSS [69, 110]|||
|– scalar-Brans–Dicke ( > 0)||II||[150, 147]||CSS, DSS|
|– general including < 0||II||CSS, DSS |||
|SU(2) Yang–Mills||I||||static |||
|“III”||||coloured BH [14, 202]||[193, 201, 18]|
|SU(2) Skyrme model||I||||static |||
|SO(3) Mexican hat||II||||DSS|
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