Gravitational wave observations of binary black hole mergers probe their
astrophysical origins via the binary spin, namely the spin magnitudes and
directions of each component black hole, together described by six degrees of
freedom. However, the emitted signals primarily depend on two effective spin
parameters that condense the spin degrees of freedom to those parallel and
those perpendicular to the orbital plane. Given this reduction in
dimensionality between the physically relevant problem and what is typically
measurable, we revisit the question of whether information about the component
spin magnitudes and directions can successfully be recovered via
gravitational-wave observations, or if we simply extrapolate information about
the distributions of effective spin parameters.To this end, we simulate three
astrophysical populations with the same underlying effective-spin distribution
but different spin magnitude and tilt distributions, on which we conduct full
individual-event and population-level parameter estimation. We find that
parameterized population models can indeed qualitatively distinguish between
populations with different spin magnitude and tilt distributions at current
sensitivity. However, it remains challenging to either accurately recover the
true distribution or to diagnose biases due to model misspecification. We
attribute the former to practical challenges of dealing with high-dimensional
posterior distributions, and the latter to the fact that each individual event
carries very little information about the full six spin degrees of freedom.
Examining the Conclusions
The article examines the ability to recover information about the spin magnitudes and directions of binary black holes using gravitational wave observations. It finds that while current sensitivity can qualitatively distinguish between populations with different spin magnitude and tilt distributions, accurately recovering the true distribution or diagnosing biases due to model misspecification remains challenging.
Future Roadmap
Potential Challenges:
- Dealing with high-dimensional posterior distributions: Recovering the true distribution of spin magnitudes and directions is hindered by the practical challenges of working with high-dimensional posterior distributions.
- Limited information from individual events: Each individual event carries very little information about the full six spin degrees of freedom, making it difficult to diagnose biases or extract precise information about component spin magnitudes and directions.
Potential Opportunities:
- Qualitative differentiation between populations: Current sensitivity allows for qualitative differentiation between populations with different spin magnitude and tilt distributions. This indicates that significant information about the astrophysical origins of binary black holes can be obtained through gravitational wave observations.
Future Actions:
- Improving analysis techniques: Developing more advanced techniques for handling high-dimensional posterior distributions could help in accurately recovering the true distribution of spin magnitudes and directions.
- Gathering more data: Increasing the number of observed events and improving the sensitivity of gravitational wave detectors could provide more informative data for better diagnosing biases and extracting precise information about component spin parameters.
Conclusion:
While there are challenges to overcome in accurately recovering information about spin magnitudes and directions of binary black holes, the ability to qualitatively differentiate between populations with different spin distributions is a promising avenue for understanding the astrophysical origins of these objects. Advancements in analysis techniques and data gathering can pave the way for further insights into the nature of binary black hole mergers.