With about a hundred binary black hole (BBH) mergers detected by
LIGO-Virgo-KAGRA, and with several hundreds expected in the current O4 run, GWs
are revolutionizing our understanding of the universe. Some BBH sources are too
faint to be individually detected, but collectively they may give rise to a
stochastic GW background (SGWB). In this paper, we calculate the SGWB
associated with BBH mergers dynamically assembled in dense star clusters, using
state-of-the-art numerical models. We discuss the role of modeling the
evolution of the mass distribution of BBH mergers, which has significant
implications for model selection and parameter estimation, and could be used to
distinguish between different channels of BBH formation. We demonstrate how the
birth properties of star clusters affect the amplitude and frequency spectrum
of the SGWB, and show that upcoming observation runs of ground-based GW
detectors may be sensitive enough to detect it. Even in the case of a
non-detection, we find that GW data can be used to constrain the highly
uncertain cluster birth properties, which can complement direct observations of
young massive clusters and proto-star clusters in the early universe by JWST.
With the increasing number of binary black hole (BBH) mergers detected by LIGO-Virgo-KAGRA, and even more expected in the current O4 run, gravitational waves (GWs) are revolutionizing our understanding of the universe. However, some BBH sources are too faint to be individually detected, leading to the concept of a stochastic GW background (SGWB) formed by the collective merger of these sources. In this paper, we have used advanced numerical models to calculate the SGWB associated with BBH mergers dynamically assembled in dense star clusters.
Importance of Modeling the Mass Distribution
One of the key aspects that we have focused on is modeling the evolution of the mass distribution of BBH mergers. This is crucial for model selection and parameter estimation, as it allows us to distinguish between different channels of BBH formation. By studying the birth properties of star clusters, we have been able to determine how they affect the amplitude and frequency spectrum of the SGWB.
Potential Detection of SGWB
We find that upcoming observation runs of ground-based GW detectors may have enough sensitivity to detect the SGWB. This would be a significant milestone in our understanding of the universe, as it would provide further evidence for the existence of faint BBH sources that cannot be individually detected. The detection of the SGWB would confirm the importance of stochastic processes in the formation and evolution of black holes.
Using GW Data to Constrain Cluster Birth Properties
Even in the case of a non-detection, our study shows that GW data can still be valuable in constraining the highly uncertain cluster birth properties. This information can complement direct observations of young massive clusters and proto-star clusters in the early universe, such as those provided by the James Webb Space Telescope (JWST). Therefore, even without a detection, our research has implications for a wide range of astrophysical studies.
Roadmap for the Future
Looking ahead, there are several challenges and opportunities on the horizon. Here is a roadmap for future research:
- Refine Numerical Models: Continuously improve numerical models used to calculate the SGWB associated with dynamically assembled BBH mergers in star clusters.
- Increase Sensitivity of GW Detectors: Work towards enhancing the sensitivity of ground-based GW detectors, allowing for more accurate detection of the SGWB.
- Explore Other Sources: Investigate the possibility of stochastic backgrounds from other gravitational wave sources, such as neutron star mergers or cosmic string networks.
- Combine Multiple Observational Approaches: Further integrate GW data with direct observations from telescopes like JWST to better understand the formation and evolution of star clusters and black holes.
- Interpret Non-Detections: Analyze non-detection scenarios to extract valuable information about cluster birth properties and improve our understanding of BBH formation channels.
In conclusion, our research on the SGWB associated with BBH mergers in star clusters has shed light on the importance of modeling their mass distribution and has shown promise for future detections using ground-based GW detectors. The valuable insights gained from this study, whether through a detection or non-detection, can contribute to advancing our understanding of the universe and its astrophysical processes.