Yukawa International Seminar

John Friedman, University of Wisconsin, Milwaukee

friedman@thales.phys.uwm.edu

friedman@thales.phys.uwm.edu

About 150 people gathered under cloudy skies in the old imperial
capital, Kyoto, June 28 - July 2, to listen to six days of talks and
view an extraordinarily good set of posters, all summarizing
theoretical, observational, and experimental work on gravitational
waves, black-hole physics, and numerical gravity during the Yukawa
International Symposium (YKIS99) on black holes and gravitational
waves. Unfortunately, a summary of posters would make this already
long review too long to be useful.

In a first session on black holes in a quantum context, Ted Jacobson
summarized his work with Corley and Mattingly (e.g., hep-th/9908099).
In the usual semiclassical computation of Hawking radiation, the
late-time flux arises from modes that, prior to the black hole's
formation, are vastly shorter than the Planck length; and one can worry
that black-hole radiance would not survive if the universe's
small-scale structure does not allow one to make sense of such
ultra-high frequency fields. Jacobson and his coworkers address the
problem by placing a lattice on a black-hole background, and looking at
field theory on the lattice - considering, in effect, quantum field
theory on a discrete spacetime. Satisfyingly, the lattice reproduces
the Hawking effect with an accuracy that depends on the ratio of the
lattice spacing to the black hole radius. A lattice regular at the
horizon is not static, and the lattice used is falling inward. The
scattering by the lattice of a ingoing to outgoing wave is formally
analogous to the Bloch oscillation of an electron in in a crystal with
a uniform electric field.

Following this discussion, Gary Gibbons spoke on black holes in unified
theories. He noted that classical solutions are important in quantum
theories if their quantum corrections vanish, and that commonly
requires supersymmetry to cancel the fluctuations of bosons against
those of fermions. The Breckenridge-Myers-Peet-Vafa solution provides
an example of a BPS black hole with nonvanishing angular momentum, a
solution that was used to count states using D-brane techniques (and
gave agreement with black-hole entropy). Gibbons and Herdeiro
(hep-th/9906098) have completed a substantial study of the solution,
finding its geodesics and computing the scattering of a scalar field
off this extreme black hole. The solution includes an example of a ``naked
stable time machine,'' with spatially unbound geodesics that can travel
back in time; but Gibbons and Herdeiro argue that chronology protection
may be enforced by the third law of thermodynamics, preventing the
formation of an extreme black hole by means of a finite process.

Describing the work led by Israel's group on the nature of the
interiors of black holes formed in collapse, Patrick Brady reviewed
the linear instability of the Cauchy horizon and the spherical
models of black-hole interiors and then turned to more recent work
that appears to confirm key features of the earlier models.
In particular, Ori and Flanagan have used the Cauchy-Kovalevsky theorem to
show that ``there exist functionally generic solutions of Einstein's
equations containing a null and weak scalar curvature singularity,''
and work by Barack and Ori and by Israel, Brady, Chambers, Droz and
Morsink characterizes more precisely the Weyl curvature near these null
Cauchy-horizon singularities.

Andrzej Krolak continued the discussion of the nature of singularities
in gravitational collapse, summarizing theorems that characterize
Cauchy horizons or restrict the
occurence of naked singularities. Here are a few.

Chrusciel and Galloway and Budzynski, Kondracki, and Krolak have shown
the existence of a large class of nowhere differentiable Cauchy
horizons. Harada, Iguchi, and Nakao showed that generic
counterrotation prevents central shell-focusing formation. This is
consistent with Rendall's result with cylindrical symmetry that a
regular distribution function in phase space prevents naked
singularities of the kind apparently seen by Shapiro and Teukolsky,
using a singular distribution function for collisionless matter; and
consistent with the conjecture that matter described by smooth
distribution functions obeys cosmic censorship - that, as in the
Newtonian theory, velocity dispersion dissolves naked singularities.

Matt Choptuik summarized work by about 30 people on critical phenomena
in gravitational collapse that has led to a coherent picture. Critical solutions are unstable by construction, lying on the boundary between two distinct stable endstates of collapse - black hole or no black hole.
Underlying the key features of near-critical collapse is the fact that
the critical solutions are ``minimally unstable intermediate attractors,'' solutions whose linear perturbations have a single unstable mode.
Critical solutions exhibit discrete self-similarity (an oscillation
within a scaling envelope) characterized by a rescaling exponent,
for massless scalar, gravitational, and SU(2) Yang-Mills fields;
while perfect fluids and multiple-scalar field systems are continuously
self-similar. The transition to collapse studied earlier, of, say
neutron stars pushed over their upper mass limit by an addition of
an arbitrarily small mass, exhibits a mass gap, and Choptuik calls
these type I transitions. Examples are massive scalar fields,
and colored black holes (variants with horizons of the Bartnik-Mckinnon
Y-M Einstein solutions); the latter have overlapping regions in
parameter space that correspond to type I and type II solutions.
Choptuik's transparencies, with names and details suppressed here are at
http://laplace.physics.ubc.ca/People/matt/Doc/ykis99.ps.

Jeffery Winicour described the characteristic treatment of black holes,
and the current status of the The Pitt null code, developed by Welling,
Isaacson, Gomez, Papadopoulos, Lehner, Bishop, Maharaj, Szylagyi, and
Husa. In the vacuum case, a 3-D code has been implemented and tested
in a variety of contexts. More recent work (
gr-qc/9901056)
incorporates
a perfect fluid with a 1-parameter equation of state. It has so far
passed tests involving localized distributions of matter around a
Schwarzschild black hole, and the code is found to be stable and
convergent. Modifications are needed to handle shocks, and problems of
astrophysical interest remain to be tackled.

Next morning, Kip Thorne began his talk by prodding the theorists to
intensify their effort to keep up with the rapid progress of the LIGO
experimentalists, and provide an accurate template for inspiral, with
no drift in phase over the time of detection. Theorists particularly
lag in the NS/BH problem, where spin-induced precession is important.
The construction of the two LIGO observatories is essentially complete,
and LIGO I sensitivity is to be reached by November, 2001. A LISA date
of 2008-2010 is likely (i.e., US support is likely). Thorne emphasized
that already for LIGO II with signal recycling, the standard quantum
limit may be reached. To reach greater sensitivity, one cannot rely on
standard position detectors that ignore correlations, and Thorne
reviewed work with Braginsky, Gorodetsky, Matsko, Vyatchanin, Khalili,
Levin, and Kimble on measurements beyond the SQL. Methods rely on
correlations between the photon shot noise and the back-action noise
induced by radiation pressure on the test masses. One can modify the
input or output optics of current interferometers so that measurements
at different times commute and state reduction has no influence on the
noise. Thorne's current estimates for LIGO II event rates:

NS/NS,
a few/yr; NS-BH a few/month (!); BH-BH, unknown.

Other sources
mentioned were r-modes, strongly accreting LMXB's, and accretion induced
collapse of white dwarfs.

Seiji Kawamura spoke on the current status of the Japanese detector,
TAMA300: a mode cleaner was locked successfully 4 months ago, and a
noise spectrum has been obtained. He summarized the recycling
arrangement, and ongoing efforts to reduce shot, seismic and thermal
noise. Great luck would be needed if there were to be a source strong
enough for TAMA300 to detect, but plans are to step up to a much
larger, 3-km cryogenic detector in the Kamgoka tunnel.

Next, two talks, by Ed Seidel and Jorge Pullin, on colliding black
holes. Seidel noted both the long distance to go before inspiral can
be computed and the progress made by the NSF black-hole grand challenge
project, in developing a code that handles a variety of initial data
sets, giving, in particular a stable 3-D evolution of a set of
distorted black holes. The development of the cactus code was
outlined, and a general PDE solver, due for public release in August,
with an open source code, may be of wide interest to readers of MOG.
10 groups are now using it. Seidel emphasized recent methods (initially
due to Shibata and Nakamura) of that give significantly more stable
evolution, by promoting badly behaved quantities to independent
variables. Although evolutions can be very accurate, by *t* = 50 *M*,
every code crashes.

Pullin described work by a number of people on colliding black holes in
the close limit (
gr-qc/9905081).
From a computational viewpoint,
binary black hole coalescence has three stages: a post-Newtian
inspiral, of about 10^{4} orbits, ending at roughly 10-12 *M*; the
plunge; and the ringdown. The key to a perturbative treatment of the
ringdown stage is that realization that it is *is* the ringdown
stage, that a common, distorted horizon surrounds what were two black
holes well before their apparent horizons merge. The evolution of the
distorted horizon is what Pullin and his collaborators have treated
with remarkable success as a perturbation of a Schwarzschild black
hole. The work has included the development of a second-order
perturbation formalism and its application to the ringdown problem.
The approach serves as a code check for numerical relativity and
a way to allow dying codes to run longer; and the ringdown serves in
its own right as a source for LIGO. An analogous close limit for
neutron stars, with the merged stars regarded as a perturbation
of a spherical star has also been recently developed (
gr-qc/9903100).

Yasufumi Kojima and I then gave two talks on the recently discovered
r-mode variant of the nonaxisymmetric instability that besets rapidly
rotating relativistic stars. First noticed by Nils Andersson, in a
numerical study, the instability of these axial-parity modes may
dominate the spin-down of neutron stars that are rapidly rotating at
birth, and the gravitational waves they produce may be detectable by
LIGO II with narrow banding from sources out to somewhere between 4 and
20 Mpc. These dramatic implications have led to papers by about 35
authors in the past two years, but because my talk is now on the Web in
a version written with Keith Lockitch (
gr-qc/9808083),
and each of our
talks at the ITP can be seen and heard live at
http://www.itp.ucsb.edu/online/gravity_c99/,
I'll leave it at that.
Caveats are the assumption that nonlinear effects will allow the mode
to grow until perturbed velocities are of order of the background
velocity - that the mode does not transfer its energy to turbulence or
to a magnetic field, while its amplitude is small.

Kojima emphasized research he has done, partly with Hosonuma, on the
r-modes of of relativistic stars, reporting work that showed a
continuous spectrum for nonisentropic stars in a slow-rotation
approximation, and Beyer and Kokkotas make the claim
precise. In addition, Kojima and Hosonuma have studied the
mixing of axial and polar perturbations to order
in rotating
relativistic stars, again finding a continuous mode spectrum. Kojima
obtained a single, second-order equation for the radial behavior of the
modes. If the continuous spectrum is a genuine feature of relativistic
stars, it would be remarkable, but it may well be an artifact of
approximations that force the frequency to be real: In the slow-motion
approximation, the continuous spectrum arises from the vanishing of the
highest derivative term in the ODE, found by Kojima, that describes the
axial-parity modes.

A session devoted to the post-Newtonian computations reported
progress on calculations
that must provide the highly accurate templates for binary
inspiral that are needed to use gravitational-wave detectors for astronomy.
Both Cliff Will and Luc Blanchet spoke on the post-Newtonian description
of binary inspiral. Luc Blanchet spoke on the post-Newtonian description of
gravitational radiation, developed by Damour and colleagues. He
emphasized the use of the Hadamard expansion to regularize the
infinities that arise at the position of point-particles, when one
expresses the multipole moments of the source as integrals extending
over the distribution of stress-energy. One's confidence in using this
renormalization method relies on a combination of its success and its
elegance.

Cliff Will summarized a method called DIRE (Direct Integration of the
Relaxed Einstein Equations), based on a framework developed by Epstein
and Wagoner and extended by Will, Wiseman and Pati. Like the
Blanchet-Damour-Iyer approach, DIRE begins with integrals over source
and field. The integrals are finite when one restricts the use of the
slow-motion approximation to the near zone and observes that the
far-zone integral is bounded for a source that is well-behaved in the
distant past. Equations up to 3.5 PN order are obtained, within the
assumption that the orbiting bodies are sufficiently small, by
isolating terms that neither vanish nor blow up as the size *D* of a
body shrinks to zero, with the remaining terms absorbed into
renormalized masses of the bodies. The resulting procedure is
well-defined, although the assumption has been checked completely only
at 1PN order.

In an enthusiastic update on the prospects for GEO600, Bernard Schutz
emphasized the expected performance with signal recycling in both a
narrow-band and broad-band mode: a maximum sensitivity of
*h* < 10^{-22} at minimum noise, for frequencies between 100 and 1000 Hz.
The state of the project: The vacuum system is complete, and the first
mode cleaner is locked and working; interferometry and test optics are
expected to be ready by mid-2000, and full sensitivity is to be reached
by mid-2001. There has been close collaboration with LIGO, and a
memorandum of understanding has been signed for full data exchange
between LIGO I and GEO600.

The next day saw two talks on black-hole astrophysics. Nils Andersson
summarized work on oscillations of rotating black holes, mentioning his
work with Krivan, Laguna, and Papadopoulos on a 2-D code that evolves
perturbations on a Kerr background and produces waveforms for
black-hole ringing. Andersson emphasized two regimes of outgoing-mode
ringing for a given value of *l*, say *l*=2, corresponding to the
different imaginary parts of the frequencies for .
It
remiains to be seen whether, in the extreme Kerr limit both the
retrograde, more quickly dying mode and the prograde, slowly dying mode
both have comparable amplitudes (Mashoon and Ferrari suggested that the
prograde mode was suppressed). Preliminary work suggests a curious
feature of near-extreme Kerr: a sum over harmonics appears to give an
oscillation whose damping is described by an envelope with a power-law
fall-off that is *slower* than the Price tail. If true, one might
never see the Price power law (for )
in the late-time ringing
of a near-extreme black hole.

Shin Mineshige spoke on the dramatic success of the ADAF
(advection-dominated accretion flow) model of accretion disks that
started with the 1977 work of Ichimaru. In this model, heat is
dominantly transported by radial gas motion, and the spectrum is
broad-band. Mineshige emphasized the fact that three models of
accretion - the standard thin and thick disk models and the ADAF model
are all solutions to the same set of equations with different values of
optical depth. He reiterated the evidence for a central black hole
in ADAF disks, arising from the fact that the emitted power is much
smaller (as a fraction of the mass accretion rate) than it should be if
matter were falling on a solid surface. And he discussed the observational
tests of disk models and a recent model for X-ray novae.

Interspersed with these talks, and continuing through the next
morning's sessions were a series of talks on the binary coalescence
problem for neutron stars. Eriguchi and Gourgoulhon began these with a
discussion of numerical work on Darwin Riemann problems in Newtonian
gravity and in GR. The classical Roche, Darwin, and Riemann problems
refer respectively to (Roche) the tidal forces on a finite mass
orbiting a point mass with its spin and orbital frequencies identical;
(Darwin) both masses are finite perfect fluids; (Riemann) the masses
can have internal vorticity. The Darwin-Riemann problem that Eriguchi
and Gourgoulhon consider has two masses whose spin frequencies are
arbitrary, but for which the planes of rotation are aligned with the
orbital plane. A series of papers by Uryu and Eriguchi have
numerically solved the exact problem for Newtonian polytropes; and
Usui, Eriguchi, and Uryu have begun a program to construct
quasi-equilibrium models in GR, starting with spacetimes having an
exact Killing vector of the form
,
using
a truncated set of field equations that allows a non-radiative field
and a smaller set of potentials than one would need for the exact
binary system. They thus replace the approximation of spatial conformal flatness by one in which the metric has only a
off-diagonal term.
Bonazzola, Gourgoulhon, and Marck repeat the Mathews-Maronetti-Wilson
computation for an *n*=1 polytrope (adiabatic index ),
using a multi-spectral method and obtaining agreement with the new MMW
code to better than 2%. No innermost stable circular orbit is found
for this value of .
A useful summary of our knowledge of
the ISCO was given (see
gr-qc/9904040
and Uryu-Eriguchi).

Wai-Mo Suen and Ken-ichi Oohara summarized progress on the numerical
simulation of coalescing neutron stars by the NASA Neutron Star
Grand-Challenge group and by the Japanese group. Suen's discussion
emphasized the extensive code testing that is underway and successes in
meeting the Grand-Challenge milestones. A long enough time evolution
to model coalescence is still in the future, but Suen reported a
computation of head-on collisions using the coupled Einstein and
hydrodynamic equations. These runs tested a conjecture of Shapiro that
the shock-heating generated by infall, at least for stars falling from
infinity, is enough to support the star until neutino cooling sets in
(a time long compared to the dynamical timescale). If true, this would
give an early cutoff to gravitational-wave emission. However, Suen
argued that the dynamical time scale of infall was so short that the
shock heating effect might not be important. He showed a simulation of
the head-on collision of two 1.4 solar mass neutron stars, and reported
finding an apparent horizon in the infalling time scale. Oohara
summarized the longer history of the Japanese program, with a full GR
code completed in '94. Test runs of on the order of one revolution
have been run on a 201^{3} grid, with a 10-hr CPU time on a VPP300.
Finally, Masaru Shibata has completed and tested on sample problems a
related 3-D code for binary coalescence of neutron stars
(gr-qc/9908027).

Following the binary-coalescence series, Max Ruffert and Peter Mezaros
presented different views on the possible origins of -ray
bursts. BATSE has revealed about one burst/galaxy/10^{6} years,
distributed isotropically at distances of order
10^{28}-10^{29} cm.,
implying luminosities of order 10^{5}3 *erg*/s. The duration of bursts
varies greatly, ranging from milliseconds to hours. Evidence for a
fireball as a common source is good, but fireballs may be produced by
mergers of NS-NS, NS-BH, WD-NS, WD-BH, or by a collapsar, a rotating,
collapsing ``failed'' supernova (or possibly a neutron star pushed over
its upper mass limit by accretion). Evidence for the collapsar comes
from the identification of -ray bursts with galaxies that
suggest the bursts probably occur in star-forming regions more often
than would be expected for the old NS-NS or NS-BH systems. Ruffert and
his collaborators (Janka, Eberl, and Fryer, astro-ph/9908290) have run
a series of Newtonian simulations of NS-BH and NS-NS mergers
incorporating back-reaction of gravitational waves. Using a
Lattimer-Swesty equation of state and carefully taking account of
neutrino sources and sinks, they confirm BH-NS mergers as a possible
source of -ray bursts, but find an energy of 10^{5}1 erg
requiring Lorentz beaming at the upper end of the possible. Mezaros'
even-handed summary reviewed evidence for the fireball model he
developed with Rees and others. Fireballs from all of the mechanisms
have similar energies, with 10^{54} erg possible via MHD, while less
than 10^{erg} is likely if only neutrino annihilation is used (as in
the simulations discussed by Ruffert). This leaves coalescence clearly
still in the game.

The final two talks, by Bernard Carr and Jun'ichi Yokoyama, concerned
primordial black holes. Carr delineated ways that PBH's can be used as
a probe of the early universe; in particular, the limit on PBH's set by
the absence of observed evaporating black-holes limits the spectral
index during inflation. Both Carr and Yokoyama emphasized the
possibility that MACHOS are PBH's of mass 0.5 ,
the right mass
for their having been created in a quark-hadron transition. Should the
LMC MACHOS be PBH's, work by Yokoyama and collaborators sharply
constrains parameters of inflationary models; and nearby BH-BH
coalescence would be frequent, with possibly observable gravitational
waves (Nakamura, Sasaki, Tanaka, Thorne). Carr considered a
PBH-related test of whether *G* varies.