LIGO’s new Gravitational Pulse – puzzling for Black-Hole interpretations

The four instances of the inspiral-merger pattern of a gravitation pulse from coalescing binary objects are over-interpreted and raise statistical questions.  The final ring-down is too weak to distinguish exotic ‘Black Hole’ mergers from neutron star mergers.  Why no identifications of the expected closer and smaller merger events?  The extraordinary claim to ‘primordial’ Black Holes is based on uncertain indications of spins.  

The LIGO team has registered a further gravitational wave pulse that shows the inspiral-merger pattern – diminishing wavelength with rising intensity (‘chirp’), then collapse to a low energy tail.  This characterises the coalescence of a pair of compact masses, mutually orbiting ever closer (and faster) as gravitational energy is shed, till they touch and merge. The size at the point of merging at is important, being the Schwarzschild radii for BHs (‘black holes’) but somewhat larger for neutron-star collapsars in general. Spin of the merging objects is a small complicating matter. The shape of the collapse-tail characterises the spin-states and type of coalescing objects.

Detector data (gray), and 90% confidence intervals for wave-forms reconstructed from the morphology-independent wavelet analysis (orange) and binary black hole (BBH) models (blue). “GW170104: Observation of a 50-Solar-Mass Binary Black Hole Coalescence at Redshift 0.2”    B. P. Abbott et al. (LIGO Scientific and Virgo Collaboration) Phys. Rev. Lett. 118, 221101, 2017

The inspiral-merger-collapse signal was recorded both at Hanford and Livingston (Figure shows slight time shift) so it’s real. The LIGO team now use two models to fit the trail (‘Wavelets’ and ‘BBH’) and are able to fit fewer wavelengths (6 rather than ~10 of the discovery pulse).

This fourth detection of merging BHs (the third was only marginal) is close in inferred size (50M rather than the 60M combined mass of the discovery detection). Its post-merger signal has half the signal:noise, so again is too weak to discriminate between types of coalescing objects. This fourth example is inferred – from the energy in the gravitational pulse – to be twice as far away. With uncertainty due to the orbit inclination to the direction of view, the distances of the first three merger events would be similar, ~300-500Mpc.

Two examples of similar mass and one a little smaller is a surprising. BHs were expected to be smaller, ~1-3M, because of mass loss in star evolution, and similar to classical neutron stars. Stars lose mass throughout their lives so have to evolve without significant mass loss in order to leave a 30M BH. Larger stars may exist, having low metallicity or strong magnetisation.  LIGO is sensitive to 4-100M total mass so covers the expected small sizes. The  LIGO network for binary merger events probes to ~70 Mpc for 1.4+1.4 M, 300 Mpc for 10+10 M and 700 Mpc for 30+30 M mergers, showing bias to detecting larger more distant BH mergers (amplitude is inversely proportional to the distance). If the current detection is really twice as far (8 times the space volume) several examples comparable to the new one should be dropping out of the data already accumulated. So why only one?

Why also just one report of  a marginal detection of lower amplitude pulses?   The LIGO team try to match templates of the theoretical inspiral-merger-collapse profile to the noisy signals recorded in the two detectors. A multiplicity of templates covers unknown inclinations and spin states as well as mass ratios. The new signal was so strong as to be detected by eye – yet, where are the smaller events (smaller or more distant mergers)? BBH fitting shows a significant difference beyond 5 wavelengths pre-peak to ‘wavelet’ fitting (Figure above) which implies interpretations including inferences of spin are uncertain.

The LIGO team say there’s a “potentially” significant indication of counter spinning to the orbital spin, which would imply dynamical capture into a binary system rather than co-evolution of the binary pair. This leads them to suggest these ~30M⊙ objects are primordial BHs. Though these hypothetical objects are currently popular, having potential for being the hidden dark matter, this suggestion is problematic as they fail to show in gravitational lensing of light from distant quasars (Mediavilla et al., ApJ Letters 836(2), L18). With such doubts over BH-model fitting, it’s time other models of the compact objects are properly considered.

Merging via gravitational energy loss requires the binary components to be highly dense (neutron matter density) and close to a few times MG/c2 (the gravitational or Schwarzschild scale). The LIGO team should be open to non-BH interpretations. Neutron stars can no longer be excluded as too low in mass, as shell-collapsars can far exceed the supposed limit of 3M.

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