GW170817: Implications for the Stochastic Gravitational-Wave Background from Compact Binary Coalescences (1710.05837v2)

Abstract: The LIGO Scientific and Virgo Collaborations have announced the first detection of gravitational waves from the coalescence of two neutron stars. The merger rate of binary neutron stars estimated from this event suggests that distant, unresolvable binary neutron stars create a significant astrophysical stochastic gravitational-wave background. The binary neutron star background will add to the background from binary black holes, increasing the amplitude of the total astrophysical background relative to previous expectations. In the Advanced LIGO-Virgo frequency band most sensitive to stochastic backgrounds (near 25 Hz), we predict a total astrophysical background with amplitude $\Omega_{\rm GW} (f=25 \text{Hz}) = 1.8_{-1.3}^{+2.7} \times 10^{-9}$ with $90\%$ confidence, compared with $\Omega_{\rm GW} (f=25 \text{Hz}) = 1.1_{-0.7}^{+1.2} \times 10^{-9}$ from binary black holes alone. Assuming the most probable rate for compact binary mergers, we find that the total background may be detectable with a signal-to-noise-ratio of 3 after 40 months of total observation time, based on the expected timeline for Advanced LIGO and Virgo to reach their design sensitivity.

• The paper uses the GW170817 detection of a binary neutron star merger to estimate its contribution to the stochastic gravitational-wave background.
• Findings suggest unresolved binary neutron star and binary black hole mergers combined yield a stronger background, with a predicted amplitude of 1.8e-9 at 25 Hz.
• This augmented background could be detectable by Advanced LIGO/Virgo after ~40 months of observation time at design sensitivity.

Implications of GW170817 on the Stochastic Gravitational-Wave Background

The paper "GW170817: Implications for the Stochastic Gravitational-Wave Background from Compact Binary Coalescences," published by the LIGO Scientific Collaboration and the Virgo Collaboration, marks a significant advancement in our understanding of the astrophysical stochastic gravitational-wave background. The detection of GW170817 represents the first observational evidence of gravitational waves emanating from the merger of a binary neutron star (BNS) system, providing new insights into the potential contributions of unresolved binary neutron star events to the gravitational-wave background.

Key Findings and Numerical Predictions

The detection of GW170817 offers empirical data to estimate the merger rates of BNS systems, suggesting that these occurrences are not negligible and should be factored into calculations of the total astrophysical stochastic gravitational-wave background. The paper projects that combined contributions from unresolved BNS and binary black hole (BBH) events result in a stronger background compared to BBH alone. Specifically, the predicted amplitude of the stochastic background is GW(f = 25 Hz) = 1.8_-1.3^+2.7 ×10^-9 with 90% confidence, which is a considerable increase from the previously estimated BBH-only contribution.

Theoretical and Practical Implications

This augmented background amplitude carries both theoretical and practical implications. Theoretically, the findings suggest a need to rethink and possibly recalibrate models of the stochastic gravitational-wave background to include substantial BNS contributions. This incorporation allows for more refined interpretations of the cosmological and astrophysical processes shaping our universe. On a practical level, the paper predicts that this augmented background could be within detectability thresholds of the Advanced LIGO and Virgo observatories, notably predicting a potential detection with a signal-to-noise ratio (SNR) of 3, occurring after approximately 40 months of cumulative observation time—presuming the detectors reach their designed sensitivities.

Statistical Uncertainties and Sensitivities

The findings presented in the paper are statistically robust, but they acknowledge sources of uncertainty, primarily stemming from the limited BNS detections. While only a single BNS event has been observed so far, the statistical methods utilize predictions based on median merger rates. The introduced uncertainties are delineated via Poisson statistics, giving a range that accommodates the variability inherent in astrophysical phenomena. Moreover, the background energy density was compared with power-law integrated sensitivity curves for different observational stages, enhancing confidence in the validation of these predictions.

Future Prospects

Looking ahead, the paper hints at additional observational runs (O3 and beyond) and a heightened probability of early detection or setting constraining upper limits on the evolution of binary merger rates with redshift, which would be instrumental in refining the models of the origin of gravitational-wave backgrounds. Furthermore, the discussion of the potential contribution of black hole-neutron star (BHNS) mergers in the stochastic background emphasizes the continued need for expansive data collection and analysis to account for these facets. The development and application of specialized non-Gaussian analysis methods could also result in enhanced sensitivity to detect the BBH background, presenting a promising avenue for further research in this domain.

Conclusion

The research encapsulated in this paper underlines the impact and significance of neutron star mergers—a newfound component in the stochastic gravitational-wave background assessment. As detectors advance and more BNS events are captured, the potential for distinguishing between BNS and BBH contributions stands to enrich our comprehension of gravitational physics, offering a distinct avenue for evaluating the cosmological implications of these extraordinary astrophysical phenomena. As such, the collaborative efforts of LIGO and Virgo to deepen the reach and sensitivity of gravitational-wave detection continue to be paramount in the quest for understanding the universe's gravitational dynamics.