After the Great Discovery:


Valued Senior Member

The discovery of the gravitational-wave (GW) source GW150914 with the Advanced LIGO detectors provides the first observational evidence for the existence of binary black hole (BH) systems that inspiral and merge within the age of the universe. Such BH mergers have been predicted in two main types of formation models, involving isolated binaries in galactic fields or dynamical interactions in young and old dense stellar environments. The measured masses robustly demonstrate that relatively "heavy" BHs (
) can form in nature. This discovery implies relatively weak massive-star winds and thus the formation of GW150914 in an environment with a metallicity lower than about 1/2 of the solar value. The rate of binary-BH (BBH) mergers inferred from the observation of GW150914 is consistent with the higher end of rate predictions (
Gpc−3yr−1) from both types of formation models. The low measured redshift (
) of GW150914 and the low inferred metallicity of the stellar progenitor imply either BBH formation in a low-mass galaxy in the local universe and a prompt merger, or formation at high redshift with a time delay between formation and merger of several Gyr. This discovery motivates further studies of binary-BH formation astrophysics. It also has implications for future detections and studies by Advanced LIGO and Advanced Virgo, and GW detectors in space.

We have examined the implications of the GW discovery of a BBH merger in the context of the existing literature on the formation of BBHs in isolated binaries and in dense stellar environments. Despite the fact that we have only one firm detection, we can draw several astrophysical conclusions. For the first time we have observational evidence that BBH systems actually form in nature, with properties such that they merge in the local universe. This is a unique confirmation of numerous theoretical predictions over the past 40 years that merging BBHs can form, from both isolated binaries in galactic fields and from dense stellar environments. Notably, the measured BH masses in the merging binary are higher than any of the BH masses dynamically measured reliably from XRBs. Such “heavy” BHs require that they were formed from massive stars in low-metallicity environments (1/2 Z or lower), given our current understanding of massive-star winds and their dependence on metallicity. Model rate predictions from both formation mechanisms are broadly consistent with the BBH merger rate implied by the GW150914 discovery. The relatively extreme models that either abort the formation of merging BBHs or predict rates lower than ;1 Gpc yr - - 3 1 are now excluded. Apart from weaker winds at low metallicities, a significant fraction of BHs must receive low kicks; survival through common-envelope phases or high rotation in massive stars may be necessary. We note that the majority of recent model predictions survive this constraint. Targeted simulations and additional GW merger detections will be needed to quantify the balance between BBH formation rate, delay times until merger, and hence, BBH merger rates as a function of redshift. This first BBH discovery already has implications for a stochastic GW background and for the potential of observations with a future eLISA-like space mission. These are the key conclusions we can derive based on the GW150914 properties and the existing DCO astrophysics literature. Final analysis of this first aLIGO observational run may provide additional rate constraints from additional detections of BBHs or NS binaries, or in their absence interesting upper limits on merger rates of NS binaries. These combined rate constraints will provide the most stringent quantitative limits on model predictions. An increased source sample resulting from future GW data will of course better constrain the merger rates, but will also allow us to probe the mass distributions and any dependence on redshift. To go beyond the current, mostly qualitative discussion, and move toward comprehensive model constraints, it will be important to develop frameworks that account for observational biases and for appropriate sampling of the model parameter space including relevant parameter degeneracies. In closing, we are looking forward to the development of GW astronomy as a new way of probing the universe. The authors gratefully acknowledge the support of the United States National Science Foundation (NSF) for the construction and operation of the LIGO Laboratory and Advanced LIGO, as well as the Science and Technology Facilities Council (STFC) of the United Kingdom, the MaxPlanck-Society (MPS), and the State of Niedersachsen/ Germany for support of the construction of Advanced LIGO and construction and operation of the GEO600 detector. Additional support for Advanced LIGO was provided by the Australian Research Council. The authors gratefully acknowledge the Italian Istituto Nazionale di Fisica Nucleare (INFN), the French Centre National de la Recherche Scientifique (CNRS), and the Foundation for Fundamental Research on Matter supported by the Netherlands Organisation for Scientific Research for the construction and operation of the Virgo detector and the creation and support of the EGO consortium. The authors also gratefully acknowledge research support from these agencies as well as by the Council of Scientific and Industrial Research of India, Department of Science and Technology, India, Science & Engineering Research Board (SERB), India, Ministry of Human Resource Development, India, the Spanish Ministerio de Economía y Competitividad, the Conselleria d’Economia i Competitivitat and Conselleria d’Educació Cultura i Universitats of the Govern de les Illes Balears, the National Science Centre of Poland, the European Union, the Royal Society, the Scottish Funding Council, the Scottish Universities Physics Alliance, the Lyon Institute of Origins (LIO), the National Research Foundation of Korea, Industry Canada and the Province of Ontario through the Ministry of Economic Development and Innovation, the National Science and Engineering Research Council Canada, the Brazilian Ministry of Science, Technology, and Innovation, the Leverhulme Trust, the Research Corporation, Ministry of Science and Technology (MOST), Taiwan, and the Kavli Foundation. The authors gratefully acknowledge the support of the NSF, STFC, MPS, INFN, CNRS, and the State of Niedersachsen/Germany for provision of computational resources.