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Gravitational Waves from Binary Black Hole Mergers Gravitational Waves from Binary Black Hole Mergers Insights from a Rapidly Growing Observational Field David Keitel (Universitat de les Illes Balears) for the LIGO Scientific Collaboration and Virgo Collaboration XII. Jornada CPAN, 2020-10-22 LIGO-G2001811-v3 Gravitational Waves - the long road to observational reality ● 100 years from Einstein’s prediction to the first LIGO-Virgo detection announcement in 2016 ● General relativity: “Spacetime tells matter how to move, and matter tells spacetime how to curve.” ● Time-varying quadrupolar mass distributions lead to propagating ripples in spacetime: GWs. ● Spacetime is “a very stiff fabric”→ only extreme astrophysical sources yield detectable GWs. ● Experimental search started with J. Weber’s bar antennas, later on larger scales, also in Europe (Glasgow, Italy, …). ● Laser interferometers first proposed in 1960s. ● Large US and European projects began in 1980s. ● Initial LIGO & Virgo took data until 2010, set upper limits on compact binary merger rates and on GWs from galactic neutron stars, supernovae, stochastic backgrounds, ... ● Advanced LIGO observing since 2015 (two detectors in US), joined by Virgo (Italy) in 2017 and KAGRA (Japan) in 2020. 2 [LVC, PRL 116,061102] GW150914: the chirp heard around the world ● Twin Advanced LIGO detectors in Hanford (Washington) and Livingston (Louisiana) started observing in September 2015. ● First detection: GWs from two ~30 solar mass black holes, merging ~400 Mpc from Earth (z~0.1). 56 ● ~3 M⊙ radiated in GWs, peak luminosity ~3.6x10 erg/s, -21 peak strain at the detectors: 10 (relative length change of the 4km arms) ● Identified as highly significant by both matched-filter and unmodelled analysis pipelines. ● Observed waveform beautifully matches full Numerical Relativity solutions of Einstein’s equations. 3 Gravitational Waves from Binary Black Holes: the basics ● Higher masses mean mergers at lower frequencies. ● Stellar-mass BHs O(3--100 M⊙): with current ground-based detectors, we observe the final moments of the inspiral, the merger, and the ringdown. ● Analytical, perturbative and numerical relativity together yield the efficient calibrated waveform models used in most analyses. ● Source parameters extracted by Bayesian inference. ● Besides masses, we can also infer spins, distance, sky location, inclination, and final state (merger remnant) properties. [LVC PRL 116,061102 / PRL 116,241102] 4 Advanced LIGO O1: 3 detections, first hints of diversity ● GW150914: the first detection, loudest signal in O1 and most massive source [LVC, PRX 6,041015 (2016)] ● GW151012 (née LVT151012): much weaker (half the mass and ~1Gpc away) ● GW151226: even lower masses, but similar distance as GW150914; also gold-plated from the start; first hints of measurable spin ● First 3 events already established that we were uncovering a diverse population of stellar-mass BBHs in the universe. 5 GWTC-1: LIGO-Virgo transient catalog from O1+O2 [LVC, PRX 9,031040 (2019)] O1: 20150912-20160119; O2: 20161130-20170825 (Virgo joined 20170801) 10 binary black holes, 1 binary neutron star 6 GWTC-1: LIGO-Virgo transient catalog from O1+O2 [LVC, PRX 9,031040 (2019)] The 10 BBHs span a range of masses, but all “fairly vanilla”: ● consistent with equal masses ● weak imprints of spin ● no evidence for spin precession ● fully consistent with GR 7 GWTC-1: LIGO-Virgo transient catalog from O1+O2 ● We have clearly started to uncover a population of heavy BHs not seen in EM before. ● So what have we learned from the catalog as a whole? 8 [LVC, ApJL 882:L24 (2019)] GWTC-1: population inference ● BBH merger rate: ● mass distribution of merging BBHs: consistent with different parameterized models, some support for sharp drop past ~45 M⊙ ● spin distribution: large aligned spins disfavoured, no strong constraints in favour of or against precession ● rate evolution with redshift: flat or increasing ● This was from only 10 detections. ● With larger catalogs, we will be able to better constrain models and compare with predictions for different formation channels. ● E.g. grossly simplifying: isolated binary evolution tends to produce more equal masses and aligned spins, while dynamic capture in dense environments can produce more generic BBHs. But details are very subtle and models are flexible, see ApJL 882:L24 (2019) for a balanced account. 9 [LVC, PRD 100,104036 (2019)] GWTC-1: tests of GR ● All events detected so far fit very well to waveforms predicted by GR. ● But how well can the observations constrain deviations from GR if it’s not the true theory of gravity? ● Residual tests: any significant signal power after subtracting the best-fit GR template? No. ● Inspiral-merger-ringdown consistency: do remnant properties inferred from the low- and high-frequency parts of each signal match? Yes. ● Parametrized tests of GW generation and GW dispersion relation → theory-independent upper bounds, complementary to tests with binary pulsars, GW170817, solar system ● Polarization tests: GWs in GR are pure tensor modes, other theories allow for scalar and vector modes. No evidence for those. ● Generally, LVC focuses on theory-independent tests, but wider community has also used constraints (and GW open data) for testing specific alternatives to GR. 10 ...and one Binary Neutron Star Merger, of course [A.Simonnet/LVC] [LVC+ PRL 119,161101 / ApJL 848:L12 / ApJL 848:L13] ● HLV rapid alert and GRB coincidence → spectacular multimessenger campaign ● Hubble constant standard siren measurement ● GW constraints on tidal deformability → neutron star maximum mass, equation of state of high-density nuclear matter [LVC2019] ● And so much more... ● ...but enough said: back to black holes. [LVC+2019] 11 O1, O2 and beyond: the world of GW open data ● gw-openscience.org ● full strain data of observing runs through O2 [arXiv:1912.11716] ● short data stretches for published O3 events ● posterior samples for all significant events just two examples: ● full O1&O2 open data reanalysed by external groups, several more BBH candidates reported with varying significance [Nitz+2020] [Venumadhav+2020] ● Many other works from the wider community: reanalysis of individual events, tests of GR, search for signatures of exotic compact objects, search for gravitational lensing signatures, ... 12 The present: O3 ● third observing run: 20190401-20200327 (cut 1 month short by Covid), break in 2019/10 ● sensitivity significantly improved over O2 ● Virgo part of whole run ● Public alerts: gracedb.ligo.org | chirp.sr.bham.ac.uk emfollow.docs.ligo.org/userguide ● threshold: false-alarm rate > ½ months for CBC alerts ● O3a: 41 alerts (33 not retracted); O3b: 39 alerts (23 not retracted) ● alerts contained 3D localization, source classifications ● no promising low-latency electromagnetic counterparts ● 4 “special events” published so far ● full results to be reported in two catalog updates (up first: O3a-only) 13 GW190425 ● Only a brief honorable mention - low-mass event, by all standards not a BBH! ● No promising EM counterpart; search hindered by large distance and bad sky localization (LV data only). ● No significant tidal constraints from GW data. ● But masses make this event exciting: both components in range of known neutron stars, but total mass a clear outlier compared to known galactic BNSs. systems. ● Possible formation channels under debate. [LVC, ApJL 892:L3] 14 GW190412: the discovery ● HLV joint observation ● m1, m2 squarely within GWTC-1 population ● But: first BBH with clearly unequal masses! [LVC, PRD102, 043015 (2020)] 15 GW190412: the implications ● For O1/O2 events, waveform models including only the dominant quadrupolar mode had been largely sufficient. ● Unequal masses allow first clear measurement of higher multipoles. (Bayes factors > 1000 for their presence, with multiple waveform models.) ● Once again, fully consistent with GR predictions. ● HMs help break distance-inclination degeneracy. ● Unequal masses also an interesting challenge for formation models: ○ Still consistent with both isolated binary evolution or dynamic capture in dense environments. ○ But discovery triggered lots of work on detailed models and rates predictions... [LVC, PRD102, 043015 (2020)] ○ ...and on more exotic alternatives. 16 GW190814: the discovery [LVC, ApJL 896:L44 (2020)] ● HLV observation, excellent sky localization. ● Even more extreme mass ratio than GW190412, lighter object inside the expected “lower mass gap”: extremely heavy for a NS, lighter than expected for any BHs. ● Another clear detection of higher multipoles. 17 GW190814: the implications [LVC, ApJL 896:L44 (2020)] ● lighter object in the “mass gap” expected between NSs and BHs -3 -1 ● inferred rate of similar events, 1-23 Gpc yr , challenges all standard [R.Hurt/Caltech] formation models (isolated binaries, dynamical capture, hierarchical mergers) ● exotic possibilities for the nature of the secondary ● tests of GR in a new regime, best yet “dark siren” H0 measurement 18 GW190521: the discovery ● HLV observation, extremely short signal ending at low frequencies → high masses! ● Largest component masses ever measured in a GW event, remnant can be considered an Intermediate-Mass Black Hole (IMBH). ● higher-mode content [LVC, PRL 125,101102 (2020)] ● hints of spin precession ● ringdown-only measurement of remnant properties consistent with GR expectaction from full IMR 19 GW190521: the implications [LVC, ApJL 900:L13 (2020)] ● Likely at least one component BH in the “upper mass gap” expected due to pair instability supernovae. ● How to form this system? ○ modified stellar evolution models
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