On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-Wave Observatory simultaneously observed a transient gravitational-wave signal. The signal sweeps upwards in frequency from 35 to 250 Hz with a peak gravitational-wave strain of 1.0×10(-21). It matches the waveform predicted by general relativity for the inspiral and merger of a pair of black holes and the ringdown of the resulting single black hole. The signal was observed with a matched-filter signal-to-noise ratio of 24 and a false alarm rate estimated to be less than 1 event per 203,000 years, equivalent to a significance greater than 5.1σ. The source lies at a luminosity distance of 410(-180)(+160)  Mpc corresponding to a redshift z=0.09(-0.04)(+0.03). In the source frame, the initial black hole masses are 36(-4)(+5)M⊙ and 29(-4)(+4)M⊙, and the final black hole mass is 62(-4)(+4)M⊙, with 3.0(-0.5)(+0.5)M⊙c(2) radiated in gravitational waves. All uncertainties define 90% credible intervals. These observations demonstrate the existence of binary stellar-mass black hole systems. This is the first direct detection of gravitational waves and the first observation of a binary black hole merger.

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The Observation of Gravitational Waves from a Binary Black Hole Merger

The status of experimental tests of general relativity and of theoretical frameworks for analyzing them is reviewed and updated. Einstein’s equivalence principle (EEP) is well supported by experiments such as the Eotvos experiment, tests of local Lorentz invariance and clock experiments. Ongoing tests of EEP and of the inverse square law are searching for new interactions arising from unification or quantum gravity. Tests of general relativity at the post-Newtonian level have reached high precision, including the light deflection, the Shapiro time delay, the perihelion advance of Mercury, the Nordtvedt effect in lunar motion, and frame-dragging. Gravitational wave damping has been detected in an amount that agrees with general relativity to better than half a percent using the Hulse-Taylor binary pulsar, and a growing family of other binary pulsar systems is yielding new tests, especially of strong-field effects. Current and future tests of relativity will center on strong gravity and gravitational waves.

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The Confrontation Between General Relativity and Experiment

Over the past two decades, an avalanche of data from multiwavelength imaging and spectroscopic surveys has revolutionized our view of galaxy formation and evolution. Here we review the range of complementary techniques and theoretical tools that allow astronomers to map the cosmic history of star formation, heavy element production, and reionization of the Universe from the cosmic "dark ages" to the present epoch. A consistent picture is emerging, whereby the star-formation rate density peaked approximately 3.5 Gyr after the Big Bang, at z~1.9, and declined exponentially at later times, with an e-folding timescale of 3.9 Gyr. Half of the stellar mass observed today was formed before a redshift z = 1.3. About 25% formed before the peak of the cosmic star-formation rate density, and another 25% formed after z = 0.7. Less than ~1% of today's stars formed during the epoch of reionization. Under the assumption of a universal initial mass function, the global stellar mass density inferred at any epoch matches reasonably well the time integral of all the preceding star-formation activity. The comoving rates of star formation and central black hole accretion follow a similar rise and fall, offering evidence for co-evolution of black holes and their host galaxies. The rise of the mean metallicity of the Universe to about 0.001 solar by z = 6, one Gyr after the Big Bang, appears to have been accompanied by the production of fewer than ten hydrogen Lyman-continuum photons per baryon, a rather tight budget for cosmological reionization.

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Cosmic Star-Formation History

Supermassive black holes (BHs) have been found in 85 galaxies by dynamical modeling of spatially resolved kinematics. The Hubble Space Telescope revolutionized BH research by advancing the subject from its proof-of-concept phase into quantitative studies of BH demographics. Most influential was the discovery of a tight correlation between BH mass and the velocity dispersion σ of the bulge component of the host galaxy. Together with similar correlations with bulge luminosity and mass, this led to the widespread belief that BHs and bulges coevolve by regulating each other's growth. Conclusions based on one set of correlations from in brightest cluster ellipticals to in the smallest galaxies dominated BH work for more than a decade. New results are now replacing this simple story with a richer and more plausible picture in which BHs correlate differently with different galaxy components. A reasonable aim is to use this progress to refine our understanding of BH-galaxy coevolution. BHs with masses of 105−106M...

https://www.annualreviews.org/doi/suppl/10.1146/annurev-astro-082708-101811/suppl_file/aa51_kormendy_supmat.pdf

Coevolution (Or Not) of Supermassive Black Holes and Host Galaxies

We substantially update the capabilities of the open source software package Modules for Experiments in Stellar Astrophysics (MESA), and its one-dimensional stellar evolution module, MESA star. Improvements in MESA star's ability to model the evolution of giant planets now extends its applicability down to masses as low as one-tenth that of Jupiter. The dramatic improvement in asteroseismology enabled by the space-based Kepler and CoRoT missions motivates our full coupling of the ADIPLS adiabatic pulsation code with MESA star. This also motivates a numerical recasting of the Ledoux criterion that is more easily implemented when many nuclei are present at non-negligible abundances. This impacts the way in which MESA star calculates semi-convective and thermohaline mixing. We exhibit the evolution of 3-8 M ? stars through the end of core He burning, the onset of He thermal pulses, and arrival on the white dwarf cooling sequence. We implement diffusion of angular momentum and chemical abundances that enable calculations of rotating-star models, which we compare thoroughly with earlier work. We introduce a new treatment of radiation-dominated envelopes that allows the uninterrupted evolution of massive stars to core collapse. This enables the generation of new sets of supernovae, long gamma-ray burst, and pair-instability progenitor models. We substantially modify the way in which MESA star solves the fully coupled stellar structure and composition equations, and we show how this has improved the scaling of MESA's calculational speed on multi-core processors. Updates to the modules for equation of state, opacity, nuclear reaction rates, and atmospheric boundary conditions are also provided. We describe the MESA Software Development Kit that packages all the required components needed to form a unified, maintained, and well-validated build environment for MESA. We also highlight a few tools developed by the community for rapid visualization of MESA star results.

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Modules for Experiments in Stellar Astrophysics (MESA): Planets, Oscillations, Rotation, and Massive Stars

The neutron star (NS) merger GW170817 was followed over several days by optical-wavelength ("blue") kilonova (KN) emission likely powered by the radioactive decay of light r-process nuclei synthesized by ejecta with a low neutron abundance (electron fraction Ye ~ 0.25-0.35). While the composition and high velocities of the blue KN ejecta are consistent with shock-heated dynamical material, the large quantity is in tension with the results of numerical simulations. We propose an alternative ejecta source: the neutrino-heated, magnetically-accelerated wind from the strongly-magnetized hypermassive NS (HMNS) remnant. A rapidly-spinning HMNS with an ordered surface magnetic field of strength B ~ 1-3e14 G and lifetime t_rem ~ 0.1-1 s can simultaneously explain the velocity, total mass, and electron fraction of the blue KN ejecta. The inferred HMNS lifetime is close to its Alfven crossing time, suggesting global magnetic torques could be responsible for bringing the HMNS into solid body rotation and instigating its gravitational collapse. Different origins for the KN ejecta may be distinguished by their predictions for the emission in the first hours after the merger, when the luminosity is enhanced by heating from internal shocks; the latter are likely generic to any temporally-extended ejecta source (e.g. magnetar or accretion disk wind) and are not unique to the emergence of a relativistic jet. The same shocks could mix and homogenizes the composition to a low but non-zero lanthanide mass fraction, X_La ~ 1e-3, as advocated by some authors, but only if the mixing occurs after neutrons are consumed in the r-process on a timescale >~ 1 s.

A Magnetar Origin for the Kilonova Ejecta in GW170817

M81 and NGC 4579 are two of the few low-luminosity active galactic nuclei that have an estimated mass for the central black hole, detected hard X-ray emission, and detected optical/UV emission. In contrast to the canonical "big blue bump," both have optical/UV spectra that decrease with increasing frequency in a nuLnu plot. Barring significant reddening by dust and/or large errors in the black hole mass estimates, the optical/UV spectra of these systems require that the inner edge of a geometrically thin, optically thick accretion disk lies at approximately 100 Schwarzschild radii. The observed X-ray radiation can be explained by an optically thin, two-temperature, advection-dominated accretion flow at smaller radii.

https://iopscience.iop.org/article/10.1086/312353/pdf

Possible Evidence for Truncated Thin Disks in the Low-Luminosity Active Galactic Nuclei M81 and NGC 4579.

We solve the one-dimensional neutrino-heated nonrelativistic magnetohydrodynamic (MHD) wind problem for conditions that range from slowly rotating (spin period P 10 ms) proto-neutron stars (PNSs) with field strengths typical of radio pulsars (B 1013 G) to "protomagnetars" with B ≈ 1014-1015 G in their hypothesized rapidly rotating initial states (P ≈ 1 ms). We use the relativistic axisymmetric simulations of Bucciantini and coworkers to map our split-monopole results onto a more physical dipole geometry and to estimate the spin-down of PNSs when their winds are relativistic. We then quantify the effects of rotation and magnetic fields on the mass loss, energy loss, and r-process nucleosynthesis in PNS winds. We describe the evolution of PNS winds through the Kelvin-Helmholtz cooling epoch, emphasizing the transition between (1) thermal neutrino-driven, (2) nonrelativistic magnetically dominated, and (3) relativistic magnetically dominated outflows. In the latter, spin-down is enhanced relative to the canonical force-free rate because of additional open magnetic flux caused by neutrino-driven mass loss. We find that protomagnetars with P ≈ 1 ms and B 1015 G drive winds with luminosities, energies, and Lorentz factors (magnetization σ ~ 0.1-1000) consistent with those required to produce long-duration gamma-ray bursts and hyperenergetic supernovae (SNe). A significant fraction of the rotational energy may be extracted in only a few seconds, sufficiently rapidly to alter the energy of the SN remnant, its morphology, and potentially its nucleosynthesis. We show that winds from protomagnetars with P ≈ 2-10 ms produce conditions more favorable for the r-process than winds from slowly rotating, nonmagnetized PNSs; in addition, energy and momentum deposition by convectively excited waves further increase the likelihood of a successful r-process.

Proto-Neutron Star Winds with Magnetic Fields and Rotation

The coalescence of massive black hole (MBH) binaries following galaxy mergers is one of the main sources of low-frequency gravitational radiation. A higher order relativistic phenomenon, the recoil as a result of the nonzero net linear momentum carried away by gravitational waves, may have interesting consequences for the demography of MBHs at the centers of galaxies. We study the dynamics of recoiling MBHs and its observational consequences. The "gravitational rocket" may (1) deplete MBHs from late-type spiral galaxies, dwarf galaxies, and stellar clusters; (2) produce off-nuclear quasars, including unusual radio morphologies during the recoil of a radio-loud source; and (3) give rise to a population of interstellar and intergalactic MBHs.

https://iopscience.iop.org/article/10.1086/421017/pdf

The Effect of Gravitational-Wave Recoil on the Demography of Massive Black Holes

The inner moons of Jupiter and Saturn migrate outwards due to tidal energy dissipation within the planets, the details of which remain poorly understood. We demonstrate that resonance locking between moons and internal oscillation modes of the planet can produce rapid tidal migration. Resonance locking arises due to the internal structural evolution of the planet and typically produces an outward migration rate comparable to the age of the Solar system. Resonance locking predicts a similar migration time-scale but a different effective tidal quality factor Q governing the migration of each moon. The theory also predicts nearly constant migration time-scales a function of semimajor axis, such that effective Q values were larger in the past. Recent measurements of Jupiter and Saturn's moon systems find effective Q values that are smaller than expected (and are different between moons), and which correspond to migration time-scales of ∼10 Gyr. If confirmed, the measurements are broadly consistent with resonance locking as the dominant source of tidal dissipation in Jupiter and Saturn. Resonance locking also provides solutions to several problems posed by current measurements: it naturally explains the exceptionally small Q governing Rhea's migration, it allows the large heating rate of Enceladus to be achieved in an equilibrium eccentricity configuration, and it resolves evolutionary problems arising from present-day migration/heating rates.

Eliot Quataert

Papers

A Magnetar Origin for the Kilonova Ejecta in GW170817

Possible Evidence for Truncated Thin Disks in the Low-Luminosity Active Galactic Nuclei M81 and NGC 4579.

Proto-Neutron Star Winds with Magnetic Fields and Rotation

The Effect of Gravitational-Wave Recoil on the Demography of Massive Black Holes

Resonance locking as the source of rapid tidal migration in the Jupiter and Saturn moon systems