Search for the isotropic stochastic background using data from Advanced LIGO's second observing run (1903.02886v3)

Abstract: The stochastic gravitational-wave background is a superposition of sources that are either too weak or too numerous to detect individually. In this study we present the results from a cross-correlation analysis on data from Advanced LIGO's second observing run (O2), which we combine with the results of the first observing run (O1). We do not find evidence for a stochastic background, so we place upper limits on the normalized energy density in gravitational waves at the 95% credible level of $\Omega_{\rm GW}<6.0\times 10^{-8}$ for a frequency-independent (flat) background and $\Omega_{\rm GW}<4.8\times 10^{-8}$ at 25 Hz for a background of compact binary coalescences. The upper limit improves over the O1 result by a factor of 2.8. Additionally, we place upper limits on the energy density in an isotropic background of scalar- and vector-polarized gravitational waves, and we discuss the implication of these results for models of compact binaries and cosmic string backgrounds. Finally, we present a conservative estimate of the correlated broadband noise due to the magnetic Schumann resonances in O2, based on magnetometer measurements at both the LIGO Hanford and LIGO Livingston observatories. We find that correlated noise is well below the O2 sensitivity.

• The paper established new upper limits on the stochastic gravitational-wave background using cross-correlation analysis of LIGO’s O2 data.
• It employed a Fourier-transformed methodology with 1/32 Hz resolution over a frequency band from 20 to 1726 Hz to isolate weak signals.
• The study underscores the importance of enhanced detector sensitivity and noise mitigation strategies for future gravitational-wave observations.

Analysis of the Isotropic Stochastic Background Search Using Advanced LIGO's Second Observing Run

The paper "Search for the isotropic stochastic background using data from Advanced LIGO’s second observing run" delineates the rigorous analysis undertaken by The LIGO Scientific Collaboration and The Virgo Collaboration to identify evidence of a stochastic gravitational-wave background (SGWB) using data from Advanced LIGO's second observing run (O2). By amalgamating data from both the first (O1) and second observing runs, this paper sought to place constraints on the SGWB, which embodies a confluence of weak or numerous gravitational-wave sources too faint to be detected individually.

Core Methodology

The fundamental technique employed involved a cross-correlation analysis of strain data from the Advanced LIGO detectors in Hanford and Livingston. The research framework capitalized on the optimal search method for an isotropic, stationary, Gaussian, unpolarized stochastic background, embodied in a frequency-dependent cross-correlation statistic $C(f)$. The analysis spanned a frequency band from 20 to 1726 Hz, using a Fourier-transformed segment with a 1/32 Hz frequency resolution.

Results

Despite the comprehensive analytical efforts, the paper did not discern convincing evidence for the presence of the SGWB. Instead, upper limits were established on the normalized energy density of gravitational waves:

• For a frequency-independent background (flat), the 95% credible upper limit is $\Omega_{\text{GW}} < 6.0 \times 10^{-8}$.
• For a background associated with compact binary coalescences at 25 Hz, the upper limit is $\Omega_{\text{GW}} < 4.8 \times 10^{-8}$.

These constraints refine upon the outcomes from O1. Specifically, the results exceeded previous limits by a factor of approximately 2.8 without the inclusion of Virgo data, due to its lesser contribution to sensitivity arising from higher noise levels and minimal coincident integration time.

Discussion and Implications

This paper has profound implications for theoretical models and future observational strategies:

1. Theoretical Implications: Stringent upper limits on the SGWB energy density inform the parameters and viability of different cosmic models, including compact binaries and cosmic strings. Any detection of alternative gravitational-wave polarizations, beyond the two predicted by General Relativity, could fundamentally alter our understanding of gravitational-wave physics.
2. Practical Considerations: Ascertainment of correlated noise, particularly from magnetic Schumann resonances, remains below the sensitivity threshold of O2, yet it signals potential concerns for future operations. Enhancements like calibrated magnetometer installations and Wiener filtering applications are pertinent to mitigate noise interference.
3. Future Prospects: The paper underscores the necessity for continued improvements in detector sensitivity and data analysis methodologies. Future observing runs employing further noise suppression could push the boundaries on detectable SGWB amplitudes.

Conclusion

While this paper did not yield a detection of the stochastic gravitational-wave background, it constitutes an integral step in refining SGWB limits and enhancing gravitational-wave detectors’ ability to scrutinize faint astrophysical signals. As the LIGO and Virgo collaborations advance, the pursuit of SGWB detections is augmented by both instrumental enhancements and methodological innovations, holding the potential to unravel new dimensions of the universe’s gravitational narrative.