Hoop conjecture in five dimensions: Violation of cosmic censorship
ABSTRACT We study the condition of black hole formation in five-dimensional space-time. We analytically solve the constraint equations of five-dimensional Einstein equations for momentarily static and conformally flat initial data of a spheroidal mass. We numer-ically search for an apparent horizon in various initial hypersurfaces and find both necessary and sufficient conditions for the horizon formation in terms of inequalities relating a geometric quantity and a mass defined in an appropriate manner. In the case of infinitely thin spheroid, our results suggest a possibility of naked singularity formation by the spindle gravitational collapse in five-dimensional space-time.
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ABSTRACT: Classical bremsstrahlung of a massless scalar field $\Phi$ is studied in gravity mediated ultra-relativistic collisions with impact parameter $b$ of two massive point particles in the presence of $d$ non-compact or toroidal extra dimensions. The spectral and angular distribution of the scalar radiation are analyzed, while the total emitted $\Phi-$energy is found to be strongly enhanced by a $d-$dependent power of the Lorentz factor $\gamma$. The direct radiation amplitude from the accelerated particles is shown to interfere destructively (in the first two leading ultra-relativistic orders) with the one due to the $\Phi-\Phi-graviton$ interaction in the frequency regime $\gamma/b\lesssim \omega \lesssim \gamma^2/b$ in all dimensions.Journal of High Energy Physics 06/2011; 11. · 5.62 Impact Factor
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ABSTRACT: We investigate the validity of the hyperhoop conjecture, which claims to determine a necessary and sufficient condition for the formation of black hole horizons in higher-dimensional space-times. Here we consider momentarily static, conformally flat initial data sets each describing a gravitational field of uniform massive k-sphere sources, for k=1,2, on the five-dimensional Cauchy surface. The numerical result shows the validity of the hyperhoop conjecture for a wide range of model parameters. We also confirm for the first time the existence of an apparent horizon homeomorphism to S**2 x S**2 or S**1 x S**3, which is a higher-dimensional generalization of the black ring.Physical review D: Particles and fields 02/2012; 85(4).
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ABSTRACT: We numerically investigate the gravitational collapse of collisionless particles in spheroidal configurations both in four and five-dimensional (5D) space-time. We repeat the simulation performed by Shapiro and Teukolsky (1991) that announced an appearance of a naked singularity, and also find that the similar results in 5D version. That is, in a collapse of a highly prolate spindle, the Kretschmann invariant blows up outside the matter and no apparent horizon forms. We also find that the collapses in 5D proceed rapidly than in 4D, and the critical prolateness for appearance of apparent horizon in 5D is loosened compared to 4D cases. We also show how collapses differ with spatial symmetries comparing 5D evolutions in single-axisymmetry, SO(3), and those in double-axisymmetry, U(1)$\times$U(1).Physical review D: Particles and fields 02/2011; 83(6).
arXiv:gr-qc/0503008v2 13 May 2005
Department of Mathematics and Physics
Osaka City University
Hoop Conjecture in Five-dimensions
-Violation of Cosmic Censorship-
Chul-Moon Yoo∗, Ken-ichi Nakao†
Department of Mathematics and Physics, Graduate School of Science,
Osaka City University, Osaka 558-8585, Japan
Department of Physics, Gakushuin University, Tokyo 171-8588, Japan
We study the condition of black hole formation in five-dimensional space-time. We
analytically solve the constraint equations of five-dimensional Einstein equations for
momentarily static and conformally flat initial data of a spheroidal mass. We numer-
ically search for an apparent horizon in various initial hypersurfaces and find both
necessary and sufficient conditions for the horizon formation in terms of inequalities
relating a geometric quantity and a mass defined in an appropriate manner. In the
case of infinitely thin spheroid, our results suggest a possibility of naked singularity
formation by the spindle gravitational collapse in five-dimensional space-time.
PACS numbers: 04.50.+h, 04.70.Bw
∗E-mail:c m firstname.lastname@example.org
In an attempt to unify fundamental forces including gravity, the possibility that the space-
time dimensions of our universe is higher than four has been much discussed. Such higher-
dimensional theories need mechanism to reduce the space-time dimensions down to four, for
example via Kaluza-Klein type compactifications of extra dimensions, so as to be consistent
with the observed world. The brane world scenario is another attractive idea of dimensional
reduction. In this scenario, the standard model particles are confined to the boundary of
a higher-dimensional space-time and only gravity can propagate in the extra dimensions.
Models of the brane world scenario with large extra dimensions compared to the four-
dimensional Planck scale (≈ 1.6 × 10−33cm) have been considered in some recent works .
According to these models, the fundamental (namely, higher-dimensional) Planck scale may
be set to rather low energy scale, even to 1TeV, of which low energy effects just alter
the short distance behaviour of classical gravitational interactions. The discrepancy in the
gravitational interaction between the four and higher-dimensional theories arises only at the
length scale below 0.1mm so that it is consistent with the gravitational experiments . In
such TeV gravity models, it is suggested that small black holes are produced in accelerators,
such as the CERN Large Hadron Collider  or in high energy cosmic ray events .
In order to understand physical phenomena caused by strong gravitational fields, the
criterion for black hole formation is very crucial. In the case of four-dimensional Einstein
gravity, such a criterion is well known as the hoop conjecture . Hoop conjecture claims that
the necessary and sufficient condition for black hole formation is given by the following; Black
holes with horizons form when and only when a mass M gets compacted into a region whose
circumference in every direction is C ? 4πG4M, where G4is the gravitational constant in
four-dimensional theory of gravity. It is remarkable that no serious counterexample against
hoop conjecture has been presented. However, at first glance, hoop conjecture is not valid
in higher-dimensional Einstein gravity ; there is black string solutions in five or higher-
dimensions, which have infinitely long event horizons, while hoop conjecture claims that
any length scale characterizing black hole should be less than the gravitational length scale
determined by the Schwarzschild radius.
Recently, two of the present authors, DI and KN, proposed a higher-dimensional version
of hoop conjecture . Here we call it the hyperhoop conjecture in the sense that it is a
possible generalization of the original hoop conjecture; Black holes with horizons form when
and only when a mass M gets compacted into a region whose (D−3)-dimensional area VD−3
in every direction is
where GDis the gravitational constant in D-dimensional theory of gravity, and the (D−3)-
dimensional area means the volume of (D−3)-dimensional closed submanifold of a spacelike
hypersurface. Hereafter we call this (D −3)-dimensional closed submanifold the hyperhoop.
The necessity of the condition (1) was confirmed in the case of momentarily static and
conformally flat initial data sets of an axially symmetric line, disk and thin ring source
for the five-dimensional Einstein equations  and for the system of point-particles .
Consistent results with the previous ones were obtained by Barrab´ es et al . They derived
two inequalities for (D−3)-dimensional volume as the necessary and sufficient conditions for
apparent horizon formation in the case of a (D−2)-dimensional convex thin shell collapsing
with the speed of light in a D-dimensional space-time.
The purpose of the present paper is to study both the necessity and in particular suffi-
ciency of the inequality (1) for the horizon formation in different situations from the case
treated in Ref. . We consider the momentarily static and conformally flat four-dimensional
initial hypersurfaces in which a four-dimensional homogeneous spheroid is put as a gravita-
tional source. This procedure has been implemented by Nakamura et al. . We apply their
method to higher-dimensional case. Then, we analytically solve the constraint equations for
five-dimensional Einstein equations. In order to investigate the validity of hyperhoop con-
jecture, we numerically search for an apparent horizon and calculate the ratio V2/G5M for
substantially various hyperhoops.
This paper is organized as follows. In Sec. II, assuming five-dimensional Einstein gravity,
we derive the constraint equations for conformally flat initial hypersurfaces and then give
analytic solutions of these equations for a homogeneous mass of a spheroidal shape. In
Sec. III, we search for an apparent horizon in initial hypersurfaces with various shapes of a
homogeneous spheroid including infinitely thin case by numerically solving a second order
ordinary differential equation. This equation corresponds to the minimum volume condition
for a three-dimensional closed submanifold of an initial hypersurface. The suggestion of the
naked singularity formation is given in this section. In Sec. IV, we define V2/G5M in a
reasonable manner and then give a procedure to select the hyperhoop with minimal value
of V2/G5M. In Sec. V, we show numerical results and their implication to the necessary
and sufficient condition for the horizon formation. Finally, Sec. VI is devoted to summary.
In Appendix A, we derive analytic solutions for the Newtonian gravitational potential of an
ellipsoid in arbitrary space dimension. In Appendix B, the necessary condition of black hole
formation based on Ref. is derived.
In this paper, we adopt the unit of c = 1. We basically follow the notations and sign
conventions in Ref..
II. A MOMENTARILY STATIC SPHEROID IN FIVE-DIMENSIONAL
Let us consider an initial data set (hab,Kab) in a four-dimensional spacelike hypersurface
Σ, where habis the induced metric in Σ and Kabis the extrinsic curvature which represents
how Σ is embedded in the five-dimensional space-time. Denoting the unit normal vector to
Σ by na, haband Kabare, respectively, written as
hab = gab+ nanb, (2)
Kab = −hc
where ∇cis the covariant derivative in the five-dimensional space-time.
The initial data set (hab,Kab) has to satisfy the Hamiltonian and momentum constraints
R − KabKab+ K2= 24π2G5ρ(4)
?Kab− habK?= 12π2G5Ja, (5)
where ρ and Jaare the energy density and energy flux for normal line observers to Σ, Da
and R are the covariant derivative within Σ and the scalar curvature of hab, and G5 is
the gravitational constant in five-dimensional theory of gravity. In this paper, we focus on
momentarily static and conformally flat initial hypersurfaces:
Kab = 0(6)
hab = f2δab,(7)
where δabis the metric tensor of four-dimensional Euclidean space. We also require the axial
symmetry in the sense that the metric on Σ have the form
dl2= f2(R,z)?dR2+ R2?dϑ2+ sin2ϑdϕ2?+ dz2?,
where 0 ≤ R < +∞ and −∞ < z < +∞ while ϑ and ϕ are the round coordinates. Then the
momentum constraint leads to zero flux condition Ja= 0, and the Hamiltonian constraint
Here we note that the Hamiltonian constraint (9) is equivalent to the Poisson equation
for axi-symmetric Newtonian gravitational potential. Let us consider the density profile
respecting the axial symmetry given by
2M/π2a3b for R2/a2+ z2/b2≤1,
where a, b and M are constant parameters.
We consider the gravitational field of an isolated body, so that we assume the asymptotic
condition given by
f → 1for r → ∞,(11)
The regular solution is then obtained as
f = 1 −4G5M [b(2a + b)R2+ 3a2z2− 3a2b(a + b)]
3a3b(a + b)2
b2≤ 1, (13)
f = 1 −4G5M
2R2− 6z2+ 3e2b2+
(F − e2b2)2−2R2− 3z2+ 3e2b2
F − e2b2
F + e2b2