GW150914: The Advanced LIGO Detectors in the Era of First Discoveries (1602.03838v1)

Abstract: Following a major upgrade, the two advanced detectors of the Laser Interferometer Gravitational-wave Observatory (LIGO) held their first observation run between September 2015 and January 2016. With a strain sensitivity of $10^{-23}/\sqrt{\mathrm{Hz}}$ at 100 Hz, the product of observable volume and measurement time exceeded that of all previous runs within the first 16 days of coincident observation. On September 14th, 2015 the Advanced LIGO detectors observed a transient gravitational-wave signal determined to be the coalescence of two black holes [Phys. Rev. Lett. 116, 061102 (2016)], launching the era of gravitational-wave astronomy. The event, GW150914, was observed with a combined signal-to-noise ratio of 24 in coincidence by the two detectors. Here we present the main features of the detectors that enabled this observation. At full sensitivity, the Advanced LIGO detectors are designed to deliver another factor of three improvement in the signal-to-noise ratio for binary black hole systems similar in masses to GW150914.

• The paper demonstrates that Advanced LIGO achieved a tenfold sensitivity improvement, enabling the first detection of gravitational waves from a black hole merger.
• It details the use of advanced interferometric techniques, reaching strain sensitivities near 10⁻²³/√Hz and incorporating robust noise reduction strategies.
• The findings validate Einstein's general relativity and open prospects for future gravitational-wave astronomy through enhanced detector networks and expanded observational reach.

Overview of the Advanced LIGO Detectors and GW150914 Observation

The paper "GW150914: The Advanced LIGO Detectors in the Era of First Discoveries" provides a comprehensive insight into the first successful observation of gravitational waves by the Advanced LIGO detectors. This observation marked a significant milestone in astrophysics, confirming the existence of gravitational waves and opening new possibilities for observing the universe.

The Advanced LIGO detectors, which became operational in September 2015 following major upgrades, were designed with a goal of replicating a tenfold improvement in sensitivity over the initial detectors. During the first observation run, spanning from September 2015 to January 2016, the detectors achieved a strain sensitivity of $10^{-23}/\sqrt{\text{Hz}}$ at 100 Hz. Notably, in the early days of this observation run, on September 14, 2015, the detectors identified a transient gravitational-wave signal, denoted as GW150914. This signal was attributed to the merger of two black holes, each with masses approximately 36 $M_\odot$ and 29 $M_\odot$, resulting in a final black hole mass of around 62 $M_\odot$. The energy radiated as gravitational waves was about 3.0 solar masses, or roughly $5.4\times10^{47}$ J.

Detector Features and Achievements

The paper emphasizes the significant advancements made in the detector's technology, such as:

1. Sensitivity Improvements: During the initial observation run, the Advanced LIGO detectors surpassed the sensitivity of all previous detectors, specifically in the range of 100 Hz to 300 Hz. At lower frequencies around 50 Hz, they considerably outperformed earlier capabilities by nearly two orders of magnitude.
2. Astrophysical Reach and Detection Volume: The improved sensitivity directly translated to an expanded astrophysical reach. For instance, the ability of Advanced LIGO to measure events similar to GW150914 means a capability of detecting such events with a combined signal-to-noise ratio (SNR) of 24 at a redshift $z = 0.09$.
3. Interferometric Transducer and Quantum Noise: The paper underscores the use of a Michelson interferometer, augmented by optical cavities to enhance circulating power and sensitivity. During the initial run, circulating power was around 100 kW per arm, with future aims to increase this up to 750 kW to further reduce noise contributions.
4. Seismic and Thermal Noise Reduction: To address environmental noise, the detectors employ intricate multi-stage pendulum suspensions and reactive seismic isolation platforms, which significantly mitigate ground vibrations and thermal noise.

Implications and Future Prospects

The detection of GW150914 not only demonstrated the advanced capabilities of LIGO but also marked the inception of gravitational-wave astronomy. This observation confirmed the theoretical predictions of Einstein's general relativity in strong field conditions. The paper discusses the potential advancements and future upgrades, suggesting that enhanced sensitivity and inclusion of other observatories, such as Virgo and KAGRA, would enhance localization accuracy and expand the observable volume.

Furthermore, the paper indicates that future technological innovations could lead to another doubling of sensitivity beyond the design goals of Advanced LIGO. Such improvements could facilitate the detection of gravitational-wave events with significantly higher SNRs, providing deeper insights into the dynamics of black hole mergers and potentially other exotic astrophysical phenomena.

In conclusion, the GW150914 observation exemplifies a major scientific achievement facilitated by the cutting-edge engineering and scientific capabilities of the Advanced LIGO structures. It signals the beginning of a new exploratory era in observing the cosmos through gravitational waves, promising to complement and expand the astrophysical knowledge derived from electromagnetic observations.