News and Updates

With our latest preprint [arXiv:2412.06887] (led by Marina De Amicis), SXS simulations have for the first time resolved fully nonlinear “tails” from merging black holes. What’s this all about?

We’re so happy that Mark Scheel, one of the senior members of the SXS collaboration, has been elected a 2024 Fellow of the American Physical Society!

We now have the first complete binary black hole inspiral, merger, & ringdown using our next-generation code SpECTRE! Our preprint [arXiv:2410.00265] presenting this first complete BBH simulation was led by Kyle Nelli (grad student at Caltech) and Geoffrey Lovelace (professor at CSU Fullerton).

What are hybrid gravitational waveforms? In meme form:

With our latest preprint [arXiv:2208.07380] (led by Caltech grad student Keefe Mitman), SXS simulations have conclusively shown the presence of nonlinearities in black hole ringdowns. What does it all mean?

On 14 September 2015 at 4:50:45 AM Eastern standard time, LIGO detected its first gravitational waves. The waves descended on Earth from the southern hemisphere, passed through the Earth, and emerged at the Earth’s surface first at the LIGO interferometer in Livingston, Louisiana, and then, 7 milliseconds later, at the LIGO interferometer in Hanford, Washington (shown below).

To detect and characterize gravitational waves from neutron star binaries, LIGO needs good models of all possible signals. Numerical relativity can’t practically be used for every case, but it is needed to test and calibrate the simpler models that LIGO can use. Inspiral waveforms from binaries with neutron stars differ from binary black hole waveforms by the presence of tidal forces. In a recent paper, Tanja Hinderer and collaborators use SXS black hole-neutron star simulations to validate a new model of these tidal forces. They find that tidal effects can be stronger than previously expected when they come close to resonance with a neutron star’s preferred ways of ringing (its normal modes of oscillation).

Matter flung out into space during black hole-neutron star mergers may well be one of the major contributors to the “r-process” heavy-elements in the universe such as gold and lead. To test this idea, we calculated the nuclear reactions taking place in the ejected matter from our black hole-neutron star mergers using the “SkyNet” nucleosynthesis code written by SXS member Jonas Lippuner. Our first studies found that it is very easy for our ejecta to produce the high-mass r-process elements (in abundance ratios not terribly different from what’s in the sun), but very little of the low-mass r-process elements are made, meaning they would have to come from a different source. We then found that this underproduction can be ameliorated (but not removed) by the effect of neutrinos given off by the merger remnant being absorbed by the outgoing matter, which changes its composition. Read the paper here [arXiv:1601.07942].

The SXS collaboration has produced a catalog of about 90 simulations of binary black holes with spins aligned with the orbital angular momentum. We sample systems with both spins co-rotating, one co-rotating and one counter-rotating, or both counter-rotating. We compare these simulations with several waveform models in use by LIGO, and find generally excellent agreement. The papers can be accessed [arXiv:1512.06800] and [arXiv:1601.05396].

CITA Professor Harald Pfeiffer has been awarded a Wilhelm Bessel Research Award of the Alexander von Humboldt Foundation. The award honors Prof. Pfeiffer’s outstanding research record, and invites him to a long-term research stay in Germany. Pfeiffer, who holds the Canada Research Chair in Numerical Relativity and Gravitational Wave Astrophysics, performs research on black holes and Neutron stars. He uses Canadian supercomputers to investigate what happens when such objects collide with each other. Of particular importance is the emission of gravitational waves, ripples in space and time itself, emitted by such collisions. Special-purpose gravitational wave detectors in the U.S., Europe and Japan are searching for these waves, to gain new insights into black holes and Neutron stars that emit gravitational waves, and into any other astrophysical processes that emit gravitational waves. Pfeiffer is also a member of the CITA research group that contributes to analyzing the data of the LIGO, Virgo and GEO gravitational wave detectors, located in the U.S., Italy and Germany, respectively.

The inspiral and merger of binaries composed of two neutron stars are excellent gravitational wave sources and strong candidate explanations for short duration gamma ray bursts. In a series of recent papers, SXS researchers have joined the effort to model these violent events. Nick Tacik and collaborators have been studying the usually-overlooked effect of the spins of the two neutron stars on their final orbits of inspiral. Read the paper here: [arXiv:1508.06986]. Roland Haas, working with many on the SXS team, has carried our first binary neutron star simulation all the way through the collision of the two stars and the collapse of the giant merged neutron star into a black hole. A paper on this simulation is still in the works, but it’s already been used to test a recipe for generating binary neutron star waveforms without full numerical simulations of these events. The paper, by Barkett et al., is available at [arXiv:1509.05782]. The idea is that binary black hole simulations are cheaper than binary neutron star simulations, and one can roughly convert the former into the latter by adding an analytic approximation of the neutron star tidal effects. Lastly, SXS researcher Francois Foucart has carried out merger simulations that study in detail what happens right after two neutron stars merge into a single more-massive neutron star. These post-merger neutron stars ring and give off possibly-detectable gravitational waves. Studying the post-merger mess required us to incorporate realistic nuclear physics and neutrino emission effects into simulations. Read the paper at [arXiv:1510.06398].

The SXS collaboration has produced the first computation of black hole binary coalescence that is capable of following the black holes for over 175 orbits until they collide. Previous computations were limited to only a few dozen orbits. The ability to track many orbits is important for testing the post-Newtonian approximation and for producing waveforms that cover the entire range of frequencies that will be seen by LIGO. See the preprint at [arXiv:1502.04953].

The SXS collaboration can now simulate binaries containing black holes that spin at a rate of 99.4% of their maximum theoretical value, the fastest spins ever simulated. The technical improvements allowing such large spins are reported at [arXiv:1412.1803] and an investigation of the maximum spin during binary coalescence can be found at [arXiv:1411.7297].

Members of the SXS collaboration are pleased to announce new movies [1] and [2] that show how the night sky would look in the presence of a binary black hole merger.

Kate Henriksson and some of her fellow members of the SXS collaboration produced newer and better data for simulations of neutron stars colliding with black holes. Using advanced numerical techniques, the neutron stars they’ve constructed are the densest ever created, which will lead to some of the most extreme simulations in the world. Check out the preprint at [arXiv:1409.7159].

The spins of the black holes are transverse to the infall direction, anti-aligned and of magnitude 0.5. This simulation is described in a publication Phys. Rev. D, also available at [arXiv:gr-qc/0907.0869].

The spins of the black holes are anti-parallel, oriented in the orbital plane, and are of magnitude 0.5. In this configuration, the kick of the final black hole has been observed in simulations by Campanelli et al. to depend on the phase of the binary at the time of merger; more specifically, they found that the kick depended sinusoidally upon the angle between the initial momenta of the BHs and the spins. This simulation will be detailed in a future publication in progress; it is discussed in a paper submitted to Phys. Rev. Lett., available at [arXiv:1012.4869].

The spins have magnitude 0.95, are parallel to each other, but are anti-aligned with the orbital angular momentum. This simulation is described in a publication in Phys. Rev. D., in press. The paper can also be accessed at [arXiv:1010.2777].