In this talk I will reflect on the beginnings of the
ASGRG, including the early people involved, the first
meeting in Canberra at which the Society was formed, and
the aspirations for the Society at that time. I will
detail some of the key events for ASGRG along its 30 year
journey. 2024 is also the 30th anniversary of the
introduction of the Abstract Boundary construction for
space-time by Scott and Szekeres. I will discuss important
results related to the boundary which have been developed
over those years. In 1998 I initiated Australian
activities in gravitational wave theory and data
analysis. I will describe our early activities, the long
road leading up to gravitational wave detection, the night
of detection, and some of our groundbreaking detections by
the LIGO and Virgo collaborations over the last
decade.
I will review the discoveries made since the first
detection of gravitational waves in 2015, and what further
discoveries we hope to make in the coming years.
Although gravitational waves were postulated within
Einstein's theory of relativity way back in 1917, their
corresponding scattering problem is still completely
open. One major difficulty is incorporating past and
future infinity, where the in and out-states are well
defined respectively. Furthermore, fundamental hindrances
within the conformal field equations -- the Einstein
equations regularly extended to include the points at
infinity -- make global propagation from the past to
future difficult. This talk discusses a novel mathematical
and numerical framework for attacking the global
gravitational wave scattering problem, current result and
future plans.
Over a hundred gravitational-wave signals have now been
detected from the mergers of black holes and neutron
stars, but other sources of gravitational waves have not
yet been discovered. Some of the most violent explosive
events in the Universe are predicted to emit bursts of
gravitational waves, and may result in the next big
multi-messenger discovery. Potential new sources of
gravitational-wave bursts include core-collapse
supernovae, cosmic strings, fast radio bursts, magnetars,
eccentric compact binary systems and pulsar
glitches. Gravitational-wave burst signals often have an
unknown waveform shape, and unknown gravitational-wave
energy, due to unknown or very complicated progenitor
astrophysics. In this talk, I will describe the detection
prospects for gravitational-wave bursts from new sources,
and discuss the challenges of searching for
gravitational-wave burst signals and interpreting the
astrophysics of the source.
Supermassive black holes, the largest singularities in the
Universe, seem to reside at the centres of most
galaxies. Over a million of them are known with masses
that reach certainly millions of solar masses and
sometimes presumably tens of billions of solar
masses. Just recently, their emergence has become more
mysterious than ever. While their mass scale is still
uncertain and their mass growth via accretion is not
well-understood, their origin may even be best explained
by primordial black holes. In this talk, I will review
what we believed to know about them, how we estimate their
masses at present, and what opportunities we have in the
future to improve on the work of the past.
Discovery of the thermodynamic nature of black holes
insinuated that they have a microscopic structure which
can be obtained from a few macroscopic parameters
characterizing them. Moreover, the studies showing
particle creation by black holes gave insights into black
hole thermodynamics and also possibly pointing to a theory
of quantum gravity. Usual derivations of Hawking radiation
are performed in a semiclassical framework where the
spacetime geometry is treated classically and is coupled
to quantum mechanical matter fields. Using the method of
Bogolubov transformations, it was found that a vacuum
state at early times transforms into an outgoing flux of
particles with a thermal spectrum according to a distant
observer. In such calculations, the backreaction effects
of the outgoing radiation to the black hole geometry is
usually neglected or treated in the ad hoc fashion. We
look at the phenomena of particle production and
thermodynamics for the simplest generalization to dynamic
spacetimes in order to uncover unique aspects which are
absent in a static background. The near-horizon behaviour
of the spherically-symmetric black hole system was fully
specified by considering the trapping of light as the
defining feature of a black hole, assuming regularity at
the horizon and seeking the finite-time formation of the
trapped region according to a distant observer. The
evaporating Vaidya metric in advanced null coordinates is
the simplest model which admits such behaviour. It is
because of such desired properties, the evaporating
ingoing Vaidya metric is usually considered to
approximately capture the backreaction effects in the
leading order, and using it for the calculation of Hawking
radiation renders it inherently self-consistent. In
particular, for the linearly evaporating ingoing Vaidya
spacetime, we solve the underlying wave equation using
conformal symmetry, and employ Bogolubov transformations
to comment on the thermal spectrum of the emitted
particles. Taking advantage of the existence of a
conformal Killing vector for dynamic spacetimes which are
conformal to static spacetimes, we derive a relation which
closely resembles the first law of black hole
mechanics. We also argue that it is the event horizon
which admits thermodynamic behaviour.
Dark matter remains one of the most intriguing mysteries
in our understanding of the universe. Recent theoretical
advancements have motivated new experimental strategies to
detect ultralight dark matter particles. This talk
explores innovative methods for searching for these
elusive particles using gravitational wave detectors. I
will discuss how ultralight dark matter may generate
detectable signals, either through direct interactions
with detectors or through the emission of gravitational
waves. Focusing on gravitational wave observatories such
as LIGO, Virgo, and KAGRA, I will summarize recent
observational efforts and highlight emerging possibilities
for uncovering the nature of this fundamental component of
the universe.
The dark matter hypothesis, namely the existence of
non-baryonic matter dominating the mass content of the
Universe, has been successful in interpreting a plethora
of different astrophysical observations, from the rotation
curves of disc galaxies to the formation of structures
from initial matter inhomogeneities. However, after more
than three decades of research, a direct detection of
particulate dark matter is still missing. The late
Universe estimates of particulate cold dark matter are
derived from the use of additive Newtonian gravitational
potentials on a fixed Minkowskian background in the study
of astrophysical systems with subrelativistic orbital
velocities. Novel results, based on new solutions to
Einstein's equations, show that such an approach neglects
the important quasilocal energy, and angular momentum
content of the regional backgrounds defined by the
time--averaged motion of matter sources. The Newtonian
limit of general relativity has been found to not be the
only possible weak field approximation. The novel
quasilocal Newtonian limit, which has already been applied
to the modelling of realistic disc galaxies, point to a
possible, complete recalibration of the cold particulate
dark matter content in the dark matter halos. Thus, this
carries heavy consequences for astrophysics, cosmology,
and high--energy particle physics.
The 30-plus years since the launches of COBE and the
Hubble Space Telescope in 1989/90 are often regarded as a
golden age for cosmology. From an observational viewpoint
this is certainly true, as the parameters describing the
large-scale properties of the universe have been measured
with unprecedented and ever-increasing precision. But,
from the perspective of cosmological theory, there has
been very little progress on the problems raised by the
discovery of the apparent accelerated expansion of the
universe in the late 1990s. If anything, the inadequacies
of the most popular models have been underlined by the
hardening of the Hubble and S₈ "tensions", and by the
realisation that large galaxies formed much earlier than
was previously expected from cosmogonic theory. Since
1998, literally thousands of proposals have been published
that aim to resolve the Hubble tension by tweaking the
standard ΛCDM model, or (more ambitiously) by dispensing
with dark energy altogether, but none has gained
widespread acceptance. In this talk, I will focus
primarily on the work that has been done in just one area,
inhomogeneous cosmologies, partly because this is where
Australasian cosmologists have contributed most, and
partly because it is (in my opinion) the approach that is
most likely to break the current impasse.
The observations of gravitational waves emitted by compact
binary mergers offer an avenue to test predictions of
general relativity (GR). The post-merger part of the
signal, known as ringdown, is (eventually) described by
linear perturbation theory as the remnant black hole
relaxes to equilibrium. GR predicts the signal is composed
of so-called “quasi-normal modes”, with the frequencies
and damping times of the modes determined by the remnant
black hole’s mass and spin. The observed ringdown signals
allow us to check for violations of GR and test beyond-GR
theories which may alter the black hole ringdown. I will
describe the background of ringdown analysis, present a
method for studying the quasi-normal modes, and discuss
the future possibilities offered by next-generation
gravitational-wave detectors.
The discovery of the memory effect more than 30 years ago
has influenced several recent developments in mathematical
general relativity. Following my own memory of some of the
highlights in the field, this talk gives my perspective on
several open problems for the years to come, in the
analysis of emitted gravitational waves from isolated
self-gravitating systems, and their relation to scattering
problems for systems of nonlinear wave equations, and
kinetic equations.
Quantum gravity candidate theories are generally expected
to cure the singularities inherent to the mathematical
(e.g. Schwarzschild, Kerr) black holes predicted by
general relativity. In the absence of such a theory,
singularity-free models have become a popular alternative
to avoid the nontrivial causal structures typically
associated with mathematical black holes. These
alternatives fall under the category of ultracompact
objects, which may or may not possess a horizon, but have
the ability to form light rings. We review the methods
used to generate such nonsingular geometries based on
nonlinear electrodynamics. In this case, the magnetic
charge introduces a minimal length scale that regularizes
the spacetime. We find that the phenomenon of
birefringence is present, resulting in the splitting of
circular light rays with different polarizations. One
polarization propagates through the background geometry,
while the other travels in an effective (optical)
metric. We compare the light rings in both geometries and
demonstrate that, for certain values of the minimal length
scale parameter, a single light ring is possible for
horizonless ultracompact objects. We explore how this
result is intertwined with the necessity of violating the
null energy condition during gravitational collapse for
the formation of such objects, the established finding
that horizonless ultracompact objects possess two light
rings, and the viability of nonlinear electrodynamics as
an effective description of these objects. Finally, we
compare the phase velocities of different polarizations
with those in the singular Schwarzschild geometry and
investigate regions where superluminal velocities and
causality violations may occur across various models.
With gravitational-wave astronomy flourishing, many
countries are working towards the next generation of
observatories, hopefully to come online in the late
2030s. I will discuss Australia's role in this, including
prospects and opportunities for hosting our own
next-generation observatory.
In the 1990's Robert Bartnik found a null quasi-spherical
(NQS) metric for which the Einstein equations become a
particularly simple characteristic transport system
coupled to a time evolution equation. Robert employed me
as a postdoc/programmer, and together we turned his NQS
metric into a pseudospectral code for solving the full
Einstein equations for radiating black hole spacetimes
[1]. A conference on the past and future 30 years of
gravity research in Australasia seems like an appropriate
time to reveal Robert's other metric, which due to
Robert's illness, was impractical to follow up on or
publish on at the time. It remains an unexplored
opportunity for future research in numerical relativity.
[1] Robert Bartnik and Andrew H. Norton, Numerical
methods for the Einstein equations in null quasi-spherical
coordinates, SIAM J SCI. COMPUT., (2000), Vol 22, 917–950.
Matthew Bailes (Swinburne University of Technology / OzGrav)
In this review talk I will describe the discovery and
timing of millisecond pulsars over the past few decades and how they have been
used in tests of relativity.
The Quantum Technologies and Dark Matter research
laboratory has a rich history of developing precision
tools for testing fundamental physics at low
energies. This includes the efforts to discover “Beyond
Standard Model” physics, including the nature of Dark
Matter and the unification of Quantum Mechanics with
General Relativity to help uncover a unified theory of
everything. In particular, our work includes searches for
Lorentz invariance violations in the photon, phonon and
gravity sectors, possible variations in fundamental
constants, searches for wave-like dark matter, test of
quantum gravity and the determination of temporal
geometric phases. This includes experiments that take
advantage of axion-photon coupling and axion-spin coupling
to search for axion dark matter. High acoustic Q phonon
systems to search for Lorentz violations, high frequency
gravity waves, scalar dark matter, tests of quantum
gravity from the possible modification of the Heisenberg
uncertainty principle and the new proposal to undertake a
temporal Pound-Rebka experiment as gravitational
Aharonov-Bohm effect.
In the late 1990s Australians played a major role in the
discovery that the expansion of the Universe is
accelerating - the Nobel Prize winning discovery of "dark
energy". A quarter of a century on we are continuing the
tradition with the recent supernova results from the Dark
Energy Survey (DES) suggesting that the properties of dark
energy might change with time -- supported by the even
more recent large-scale-structure results from the Dark
Energy Spectroscopic Instrument (DESI). Australian
researchers play a major role in both surveys. This talk
will reflect on Australia's contributions to the dark
energy puzzle, and peer toward our plans for the future.
Thippayawis (Tong) Cheunchitra (University of Melbourne)
The luminosity distance-redshift ($D_L$--$z$) relation of
Type Ia supernovae (SNe Ia) yields evidence for a nonzero
cosmological constant, i.e. "dark energy". SNe Ia analyses
typically involve fitting the $D_L$ and $z$ to the
functional form derived theoretically from the homogeneous
and isotropic Friedmann-Lemaitre-Robertson-Walker (FLRW)
metric. However, the metric in the epoch relevant to SNIa
measurements deviates appreciably from FLRW due to
gravitational clumping of mass into large-scale structures
like filaments and voids, whose size distribution spans
many orders-of-magnitude. Each line of sight to a SNe Ia
passes through a random sequence of structures, so $D_L$
differs stochastically from one line of sight to the
next. Such dispersion in $D_L$ may be dominated by a few
large voids or many small voids, partly depending on the
probability density function of the void size. In this
work, we calculate the $D_L$ dispersion in a
Lemaitre-Tolman-Bondi Swiss-cheese universe with a
power-law hole size distribution, as a function of the
lower cut-off and logarithmic slope.
