36 research outputs found
Genetic Programming + Proof Search = Automatic Improvement
Search Based Software Engineering techniques are emerging as important tools for software maintenance. Foremost among these is Genetic Improvement, which has historically applied the stochastic techniques of Genetic Programming to optimize pre-existing program code. Previous work in this area has not generally preserved program semantics and this article describes an alternative to the traditional mutation operators used, employing deterministic proof search in the sequent calculus to yield semantics-preserving transformations on algebraic data types. Two case studies are described, both of which are applicable to the recently-introduced `grow and graft' technique of Genetic Improvement: the first extends the expressiveness of the `grafting' phase and the second transforms the representation of a list data type to yield an asymptotic efficiency improvement
Pre-Merger Localization of Gravitational-Wave Standard Sirens With LISA: Triggered Search for an Electromagnetic Counterpart
Electromagnetic (EM) counterparts to supermassive black hole binary mergers
observed by LISA can be localized to within the field of view of astronomical
instruments ~10 deg^2 hours to weeks prior to coalescence. The temporal
coincidence of any prompt EM counterpart with a gravitationally-timed merger
may offer the best chance of identifying a unique host galaxy. We discuss the
challenges posed by searches for prompt EM counterparts and propose novel
observational strategies to address them. In particular, we discuss the size
and shape evolution of the LISA localization error ellipses on the sky, and
quantify the requirements for dedicated EM surveys of the area prior to
coalescence. A triggered EM counterpart search campaign will require monitoring
a several-square degree area. It could aim for variability at the 24-27 mag
level in optical bands, for example, which corresponds to 1-10% of the
Eddington luminosity of the prime LISA sources of 10^6-10^7 Msun BHs at z=1-2,
on time-scales of minutes to hours, the orbital time-scale of the binary in the
last 2-4 weeks. A cross-correlation of the period of any variable EM signal
with the quasi-periodic gravitational waveform over 10-1000 cycles may aid the
detection. Alternatively, EM searches can detect a transient signal
accompanying the coalescence. We highlight the measurement of differences in
the arrival times of photons and gravitons from the same cosmological source as
a valuable independent test of the massive character of gravity, and of
possible violations of Lorentz invariance in the gravity sector.Comment: 17 pages, 9 figures, submitted to Ap
Formation and Evolution of Compact Object Binaries in AGN Disks
The astrophysical origin of gravitational wave (GW) events discovered by
LIGO/VIRGO remains an outstanding puzzle. In active galactic nuclei (AGN),
compact-object binaries form, evolve, and interact with a dense star cluster
and a gas disk. An important question is whether and how binaries merge in
these environments. To address this question, we have performed one-dimensional
-body simulations combined with a semi-analytical model which includes the
formation, disruption, and evolution of binaries self-consistently. We point
out that binaries can form in single-single interactions by the dissipation of
kinetic energy in a gaseous medium. This ``gas capture'' binary formation
channel contributes up to of gas-driven mergers and leads to a high
merger rate in AGN disks even without pre-existing binaries. We find the merger
rate to be in the range . The results
are insensitive to the assumptions on gaseous hardening processes: we find that
once they are formed, binaries merge efficiently via binary-single interactions
even if these gaseous processes are neglected. We find that the average number
of mergers per BH is , and the probability for repeated mergers in 30 Myr
is . High BH masses due to repeated mergers, high
eccentricities, and a significant Doppler drift of GWs are promising signatures
which distinguish this merger channel from others. Furthermore, we find that
gas-capture binaries reproduce the distribution of LMXBs in the Galactic
center, including an outer cutoff at pc due to the competition between
migration and hardening by gas torques.Comment: 39 pages, 20 figures, accepted in Ap
Making a supermassive star by stellar bombardment
Approximately two hundred supermassive black holes (SMBHs) have been
discovered within the first Gyr after the Big Bang. One pathway for the
formation of SMBHs is through the collapse of supermassive stars (SMSs). A
possible obstacle to this scenario is that the collapsing gas fragments and
forms a cluster of main-sequence stars. Here we raise the possibility that
stellar collisions may be sufficiently frequent and energetic to inhibit the
contraction of the massive protostar, avoiding strong UV radiation driven
outflows, and allowing it to continue growing into an SMS. We investigate this
scenario with semianalytic models incorporating star formation, gas accretion,
dynamical friction from stars and gas, stellar collisions, and gas ejection. We
find that when the collapsing gas fragments at a density of , the central protostar contracts due to infrequent
stellar mergers, and in turn photoevaporates the remaining collapsing gas,
resulting in the formation of a object. On the
other hand, when the collapsing gas fragments at higher densities (expected for
a metal-poor cloud with with suppressed abundance) the central protostar avoids contraction and keeps growing via
frequent stellar mergers, reaching masses as high as . We conclude that frequent stellar mergers represent a possible
pathway to form massive BHs in the early universe.Comment: 20 pages, 8 figures, accepted in Ap
Conflict-free routing of multi-stop warehouse trucks
The recent interest in greater vehicular autonomy for factory and warehouse automation has stimulated research in conflict-free routing: a challenging network routing problem in which vehicles may not pass each other. Motivated by a real-world case study, we consider one such application: truck movements in a tightly constrained warehouse. We propose an extension of an existing conflict-free routing algorithm to consider multiple stopping points per route. A high level metaheuristic is applied to determine the route construction and assignment of vehicles to routes
Spin Evolution of Stellar-mass Black Hole Binaries in Active Galactic Nuclei
The astrophysical origin of gravitational wave (GW) events is one of the most
timely problems in the wake of the LIGO/Virgo discoveries. In active galactic
nuclei (AGN), binaries form and evolve efficiently by dynamical interactions
and gaseous dissipation. Previous studies have suggested that binary black hole
(BBH) mergers in AGN disks can contribute significantly to BBH mergers observed
by GW interferometers. Here we examine the distribution of the effective spin
parameter of this GW source population. We extend our
semi-analytical model of binary formation and evolution in AGN disks by
following the evolution of the binary orbital angular momenta and black hole
(BH) spins. BH spins change due to gas accretion and BH mergers, while the
binary orbital angular momenta evolve due to gas accretion and binary-single
interactions. We find that the distribution of predicted by
our AGN model is similar to the distribution observed during LIGO/Virgo O1 and
O2. On the other hand, if radial migration of BHs is inefficient,
is skewed toward higher values compared with the observed
distribution, because of the paucity of scattering events that would randomize
spin directions relative to the orbital plane. We suggest that high binary
masses and the positive correlation between binary mass and the standard
deviation of for chirp masses up to
, can be possible signatures for mergers originating in AGN
disks. Finally, hierarchical mergers in AGN disks naturally produce properties
of the recent GW event GW190412, including a low mass ratio, a high primary BH
spin, and a significant spin component in the orbital plane.Comment: 20 pages, 11 figures, accepted in Ap