Upper Limits on the Stochastic Gravitational-Wave Background from Advanced LIGO's First Observing Run (1612.02029v3)

Abstract: A wide variety of astrophysical and cosmological sources are expected to contribute to a stochastic gravitational-wave background. Following the observations of GW150914 and GW151226, the rate and mass of coalescing binary black holes appear to be greater than many previous expectations. As a result, the stochastic background from unresolved compact binary coalescences is expected to be particularly loud. We perform a search for the isotropic stochastic gravitational-wave background using data from Advanced LIGO's first observing run. The data display no evidence of a stochastic gravitational-wave signal. We constrain the dimensionless energy density of gravitational waves to be $\Omega_0<1.7\times 10^{-7}$ with 95% confidence, assuming a flat energy density spectrum in the most sensitive part of the LIGO band (20-86 Hz). This is a factor of ~33 times more sensitive than previous measurements. We also constrain arbitrary power-law spectra. Finally, we investigate the implications of this search for the background of binary black holes using an astrophysical model for the background.

• The paper establishes upper limits on the stochastic gravitational-wave background using Advanced LIGO's first observing run.
• It employs a cross-correlation technique between the Hanford and Livingston observatories to enhance sensitivity to isotropic signals.
• The findings, with no significant detection, improve sensitivity by a factor of 33 over previous measurements and guide future research.

Upper Limits on the Stochastic Gravitational-Wave Background from Advanced LIGO's First Observing Run

The paper "Upper Limits on the Stochastic Gravitational-Wave Background from Advanced LIGO's First Observing Run" presents an analysis of data collected from Advanced LIGO's initial observational campaign, focused on the search for a stochastic gravitational-wave background (SGWB). Such a background represents a collective signal from a multitude of unresolved astrophysical and cosmological events, each too faint to be individually distinguished. Understanding this background has the potential to provide insights into a variety of astrophysical processes, including binary coalescences—particularly binary black holes (BBHs) and neutron stars—as well as phenomena such as rotating neutron stars, cosmic strings, and early universe phase transitions.

Methodology

The investigation employs data from the two LIGO observatories, located in Hanford and Livingston, during their first operational phase from September 18, 2015, to January 12, 2016. The researchers used a cross-correlation technique optimized for detecting isotropic SGWB signals by analyzing the coherent data streams from these geographically separated observatories. This approach enhances sensitivity to potential gravitational-wave signatures while mitigating the impact of localized noise and interference.

Results

No statistically significant evidence of a stochastic signal was detected. Consequently, the upper limit on the dimensionless energy density of gravitational waves, denoted as $\Omega_0$, was established at less than $1.7 \times 10^{-7}$ with $95\%$ confidence for a flat energy spectrum in the range of 20 to 86 Hz. This represents an enhancement in sensitivity by a factor of approximately 33 compared to previous measurements. Furthermore, the paper imposes constraints on both cosmologically and astrophysically motivated power-law spectra.

Implications and Future Prospects

These findings have several important implications for our understanding of the universe. The results provide valuable constraints on the rate and population density of coalescing compact binaries throughout the cosmos, contributing to our broader comprehension of cosmic phenomena such as star formation rates and the initial mass function of stars. The research anticipates future detection capabilities, particularly as LIGO and Virgo improve in sensitivity, potentially identifying SGWB owing to BBHs.

Furthermore, this analysis lays groundwork for future gravitational-wave astronomy that could refine understanding of the cosmological parameters through indirect measurements of various early universe processes. As the network of gravitational wave detectors expands and incorporates new technologies, additional runs are likely to further push the sensitivity frontiers, potentially enabling the detection of weaker SGWB signals, thereby offering deeper insights into the fabric of spacetime and the early universe.

In summary, the paper makes significant strides in advancing the upper limits on stochastic gravitational-wave signals and sets the stage for future explorations in the gravitational-wave domain. The constraints outlined form a crucial step in refining theoretical models predicting the composite gravitational-wave signal arising from countless, unresolved sources distributed across the universe.